ISRM Suggested Methods for Land Geophysics in Rock Engineering

International Journal of Rock Mechanics & Mining Sciences 41 (2004) 885–914

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1. INTROD

2. SEISMI

3. SHALLO

4. ELECTR

5. ELECTR

6. GROUN

7. GRAVIT

8. RADIO

ACKNOWL

REFERENC

1365-1609/$ - see

doi:10.1016/j.ijrm

INTERNATIONAL SOCIETY FOR ROCK MECHANICS

COMMISSION ON APPLICATION OF GEOPHYSICS TOROCK ENGINEERING

SUGGESTED METHODS FORLAND GEOPHYSICS IN ROCK ENGINEERING

CONTENTS

UCTION............................................................................................................................................ 886

C REFRACTION................................................................................................................................ 887

W SEISMIC REFLECTION............................................................................................................ 891

ICAL.................................................................................................................................................. 895

OMAGNETIC................................................................................................................................... 899

D PENETRATING RADAR............................................................................................................ 903

Y........................................................................................................................................................ 906

METRIC.............................................................................................................................................. 911

EDGEMENT....................................................................................................................................... 913

ES........................................................................................................................................................ 914

Co-ordinatorT. Takahashi (Japan)

front matter r 2004 Elsevier Ltd. All rights reserved.

ms.2004.02.009

International Journal of Rock Mechanics & Mining Sciences 41 (2004) 885–914

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Methods to Pr

on Application

Nakabayashi, K

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ISRM Suggested Methods for land geophysics in rock engineering

T. Takahashi*,1

Tsukuba Technical Research and Development Center, OYO Corporation, 43 Miyukigaoka, Tsukuba, Ibaraki 305-0841, Japan

Accepted 19 February 2004

1. Introduction

In recent years, geophysical applications have beenexpanding from natural resources exploration to thefields of civil engineering, disaster prevention andenvironmental preservation. Their applications to rockengineering are also growing significantly. Geophysicaltechnology itself has been advancing very rapidly. Newtechnologies have been developed and new improve-ments have been accomplished in many geophysicalmethods. As a result, accuracy and reliability ofgeophysical images of underground have been increased.As advances are made in geophysics, it becomes more

important to make an optimal survey plan and interpretits result correctly. There are many procedures ingeophysical applications such as planning, data acquisi-tion, data processing, interpretation and reporting.Therefore, to use geophysical methods more effecti-vely in rock engineering projects, suggestions orrecommendations for the procedures of geophysicalapplications are needed.To promote geophysical methods in rock engineering,

the International Society for Rock Mechanics (ISRM)has, therefore, organized and published the followingsuggested methods for geophysics:

(1)
‘‘Suggested Methods for Geophysical Logging ofBorehole’’ by Commission on Standardization ofLaboratory and Field Tests (1981),
(2)
‘‘Suggested Methods for Seismic Testing within andbetween boreholes’’ by Commission on TestingMethods (1988).
8-51-6621; fax: +81-298-51-5450.

ss: [email protected] (T. Takahashi).

any written comments on this ISRM Suggested

of. K. Sassa, President of the ISRM Commission

of Geophysics to Rock Engineering, 41 Ohmiya

ita-Ku, Kyoto 603-8404, Japan.

front matter r 2004 Elsevier Ltd. All rights reserved.

ms.2004.02.009

In ‘‘Recommendations of site investigation techni-ques’’ published in 1975 by the ISRM, generalrecommendations on the use of geophysical methodswere described in Chapter 4. This recommendationdescribes the general use of geophysics in various stagesof the investigation, such as planning, design, construc-tion and maintenance in rock engineering projects.Geophysical methods are divided into two categories,

land geophysics and borehole geophysics, by theobservation geometry used in the method. In landgeophysics, the ground surface is used for its observa-tion, while borehole geophysics uses the borehole(s) forits observation. Existing suggested methods abovedescribed the methods and the procedures used inborehole geophysics. Recently, many land geophysicalsurveys have also been used at various stages in civilengineering applications. Therefore, the suggestedmethods describing land geophysical methods andprocedures used in rock engineering applications areneeded for their practical and effective uses.These suggested methods describe the following seven

land geophysical methods: seismic refraction, shallowseismic reflection, electrical, electromagnetic, groundpenetrating radar, gravity and radiometric. Althoughseismic refraction, electrical and radiometric methodswere already described in the Recommendations men-tioned above, new computer-aided technologies havebeen developed and used in rock engineering applica-tions. These new technologies are included in the newsuggested methods. Seismic reflection, electromagnetic,ground penetrating radar and gravity methods have beenrecently employed in rock engineering applications suchas characterization of rock mass and detection of shallowsubsurface cavities. There are some other land geophy-sical methods such as the magnetic method. Since theyhave not been applied very much to rock engineeringinvestigations as compared to seven methods above, theyare not included in this version of the suggested methods.

ARTICLE IN PRESST. Takahashi / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 885–914 887

These suggested methods are intended to assistgeophysicists or rock engineers to use geophysicsproperly and obtain solutions required in rock engineer-ing projects. Therefore, suggestions for all proceduresfrom planning to reporting in geophysical applicationsare described in the suggested methods. Suggestions forthe planning stage are especially emphasized, because itis most important how to select and integrate geophy-sical methods for obtaining optimal solutions to meetengineering requirements.The suggested methods are each composed of the

following seven sections:(1) General: The principle and general features of the

method are described. Standard techniques and theirvariations are also described. A schematic diagramexplaining the principle and measurement configura-tions of the method is presented.(2) Applicability: General applicability of the method

is described. Various limitations and conditions inapplication of the method are described in general.Typical exploration targets and recent applications ofthe method are also included.(3) Planning: Key items to be studied in planning a

survey are listed up and notes for each item aredescribed. Key items commonly described in eachmethod are as follows:

1. Study of existing information and data: Informationand data to be reviewed and studied in making asurvey plan are recommended. It is also explainedhow to analyse the data.

2. Selection of the optimal technique and equipment: Itis suggested how to select the optimal technique andequipment. Although it depends on the objectivesand the requirements of the survey, guidelines aredescribed.

3. Design of a survey line layout and data acquisitionparameters: It is described in general how to design asurvey line layout and data acquisition parameters.Standard layout and parameters are also presented.

4. The depth of investigation and resolution of themethod: The depth of investigation, resolution andaccuracy expected by the method are described ingeneral. Their relationships to the method andequipment used are also outlined.

(4) Field operation: In this section, the following keyitems to be considered in a field operation are described:

1. Types of equipment: Types of equipment commonlyused in the method and how to check them inadvance of the measurement are described. There aresome remarks in selection of equipment to be used.

2. Positioning of the measurement locations: There aresome remarks to be noted in setting up the measure-ment points and lines.

3. Field parameter tests and quality control (QC) of theacquired data: There are remarks on what parametersand data should be tested before, during and aftermeasurements in the field.

4. Measurement procedures: Standard and commonlyused measurement procedures in the method aredescribed. There are also some remarks about fieldtechniques for acquiring higher quality data.

(5) Data processing: Standard and commonly useddata processing procedures for the method and a briefexplanation are described. A schematic diagram is alsopresented for its better understanding. There are someremarks on data processing and the QC of the processeddata.(6) Interpretation: Techniques and procedures for

interpretation of the processed data are briefly de-scribed. There are some remarks on key items ininterpretation of the data. An example of the interpretedprofile is also presented for demonstrating an exampleof the output of a survey.(7) Output and report: Standard outputs of the

application of the method and minimum requirementsfor the survey report are listed.There are many factors and options to be examined in

application of geophysical methods to rock engineering.Therefore, the suggested methods should not beautomatically employed as a manual for geophysicalapplications. They should be used as a guideline forgeophysicists and rock engineers to optimally applygeophysical methods to rock engineering and toobtain useful and valuable outputs for rock engineeringprojects.

2. Seismic refraction

2.1. General

The seismic refraction method is a geophysicalmethod to determine the subsurface velocity structurethrough an analysis of the seismic waves that return tothe ground surface after refraction at the boundaries ofsubsurface layers with different seismic velocities. It hasbeen widely used for many years in civil engineeringapplications. Although there are several types of seismicrefraction methods depending on the survey objectivesor targets, the most common methods are based on thefirst arrivals of P-waves. The digital measuring equip-ment for seismic refraction surveying is becomingincreasingly more compact and offers multi-channelrecording capability. Data processing techniques in-creasingly employ automated analysis. In addition,seismic tomographic data processing techniques havebeen recently applied to seismic refraction data in orderto derive more detailed velocity structures. Fig. 1 shows

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Fig. 1. Schematic diagram of the seismic refraction method (SEG

Japan, 2000) [1].

T. Takahashi / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 885–914888

the schematic diagram of the seismic refraction methodusing dynamite as a seismic source.

2.2. Applicability

The seismic refraction method is applicable insituations where the P-wave velocity increases withdepth. Since this is the usual situation in the near surfacein a rock site, the method has been used widely for sitecharacterization in road construction, dam constructionand tunneling projects. The depths to the various layerscan be determined and the seismic velocities estimatedby the method can be utilized to determine lithology,rock strength, crack density, degree of weathering ormetamorphism, and locations of fault zones. Themethod may not be useful in situations when the seismicvelocity does not increase with depth (velocity inver-sions) and when the velocity and thickness of somelayers are such that they do not give rise to seismic firstarrivals (the blind zone problem).

2.3. Planning

2.3.1. Planning the survey

1. The survey needs to be planned in consultationwith the client so that any issues concerning landaccess and environmental restrictions are properlymanaged.

2. The client also needs to understand what is involvedin conducting the survey and the likely results.Responsibilities for different aspects of the surveyneed to be clearly understood between the client andcontractor.

3. Reporting requirements and time frames need to beestablished.

4. The supply of supplementary information that willassist in interpretation and validation of resultsshould also be considered. This information cancome from the use of other geophysical methods andfrom existing or additional drilling.

2.3.2. Study of existing information

In planning a survey, a review should be made ofexisting information including borehole data, thetopography, the geology, the depth to the water table,the degree of weathering, the possibility of thin layersand layers associated with velocity inversions, and dipsof possible faults at the survey site.

2.3.3. Arrangement of the survey line(s)
1. The survey lines should be prepared in considerationof the survey objectives, the depth of investigation,the geological conditions and the topography. Thelocation and length of the survey lines, the source andreceiver intervals and the maximum offset distancebetween sources and receivers are basic parameters inplanning a seismic refraction survey.
2. The seismic refraction method can accommodaterough topography but it is desirable to arrangesurvey line(s) to avoid extremely rough terrain.

3. Because the seismic refraction method derivestwo-dimensional (2-D) depth profiles, the surveylines should be arranged perpendicular to thestrike of the target geological structures and bound-aries. Setting up a survey grid will provide tie-lines and facilitate delineation of targets in threedimensions.

4. The minimum length of the survey line is deter-mined by the depth of the expected targets and thevelocity structure. As a rule of thumb, this isgenerally around 5–10 times of the depth ofinvestigation. If the survey line cannot be made tohave sufficient length, then it is desirable to compen-sate for this by arranging remote shot points alongthe extension of the survey line or by arranging shotpoints in deep boreholes at the end of the survey line.

2.3.4. Intervals of source and receiver points
1. In most civil engineering applications for the seismicrefraction method, the depth of investigation iswithin several tens of metres. In these cases, 10-mgeophone intervals are usually adequate but forshallower targets, this interval can be reduced to5m or less.
2. During the survey, the geophones will be arranged inspreads of typically 12 or 24 geophones. These will beused to simultaneously measure the seismic wavesarriving from a single source. For long lines, spreadsshould be run end to end.

3. Within each spread, the location of sources andthe remote (far) source points should be plannedso that there will be first arrivals from each ofthe layers present along as much of the spreadas possible. For the deepest layer (the main refractor),this coverage is mainly achieved by the remoteshots. For the intermediate layers, the coverage isobtained using sources within the spread. It is


generally recommended that these source pointsshould be at intervals of 30–60m. Additional sourcepoints can be considered when the structure iscomplex. In all cases, coverage needs to be obtainedin forward and reverse directions along the spread.

2.3.5. Types of seismic sources

1. Dynamite is an excellent seismic source, especiallyfor deeper exploration. Shots need to be buriedto ensure maximum coupling of energy intothe ground and to ensure that there is noblow-out causing surface damage and creatingsafety issues. As the timing of seismic signals needsto be accurate to millisecond accuracies, non-delayelectric detonators of the ‘‘instantaneous’’ type arerequired.

2. During last two decades, powerful mechanical seismicenergy devices like weight drops accelerated byrubber bands or vacuum and mini-vibrators havebeen developed. These devises may be used forsurveys where the depth of investigation is up toseveral hundreds of metres.

3. In case of shallow surveys where the depth ofinvestigation is less than 20m, hammers and weightdrops can be used as alternate, non-explosive sources.With such sources it is possible to repeat the impactand sum (stack) the results to build up signalstrength.

2.4. Field operation

2.4.1. Equipment

1. Survey equipment generally consists of geophones(receivers), geophone cables with connecting take-outs, extension cables, a data acquisition system(including amplifiers, display and recording facilities),and a detonating box (blaster). Geophones typicallyhave a natural frequency of 30Hz or less and aredamped to ensure that there is not a strong resonanceat the natural frequency. For a high-resolution surveywith a short measurement line in a hard rock, highernatural frequency geophones or accelerometersshould be used for high precision travel timemeasurements.

2. All instruments should undergo routine checksprior to use. Checks on the level and waveformsof geophone output, cable continuity and leak-age, safety of the detonator, and functions ofthe data acquisition system are particularly impor-tant.

2.4.2. Positioning of the survey line

1. The locations and elevations of the survey line, thegeophones and shot points need to be determined byappropriate surveying.

2. If there is more than one spread of geophones ina line, the ends of each spread should overlap sothat continuity in the travel time data can bepreserved.

2.4.3. Preparation for the measurement
1. To obtain good signals, all geophones should beplanted firmly into the ground. They should beconnected via take-outs to the geophone cable so thatthere is the same polarity for all geophones.
2. The instant of shot detonation starts the recordingprocess. This time can be transmitted from the shotpoint via radio or via a cable. Prior to givingthe shot fire command to detonate the shot, theoperator of the recording equipment (the seismicobserver) needs to monitor the background noiselevel and to ensure that noise from wind, fromroad traffic, from drills, from aircraft, etc., is at aminimum.

2.4.4. Measurement
1. The observer needs to maintain an observer’s logdetailing the locations of all geophones within aspread, the locations of the shots, the shot recordnumbers and the depth of each shot hole.
2. The observer needs to monitor amplifier gains and/orfiltering parameters to ensure the quality of therecorded data. If necessary, repeated measurementsshould be made to obtain better quality data.

2.4.5. Completion of survey

At completion of the survey, all equipment needs tobe retrieved and cleaned. The site needs to berehabilitated in accordance with the client’s require-ments. The observer needs to ensure that all seismicdata, observer’s logs and relevant survey information isproperly archived and available for data processing andinterpretation.

2.5. Data processing

2.5.1. Arrangement of the field data

All field data—seismic recordings (shot records),observer’s logs and survey information need to beorganized and compiled for a processing sequence of thetype illustrated in Fig. 2. This is a standard processingsequence. There are other ways of processing andinterpreting data, for example by using ray tracingtechniques and through tomographic inversion. Thesealternative techniques are not yet widely practised andare not described in this document.

2.5.2. Picking first arrivals

First of all, the first arrival times of the refractedP-waves are picked on the shot records in order toconstruct travel time curves. Usually, first arrival times

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Observed waveform records

Preparation of waveforms

First arrival time picking

Preparation of travel time curves

Check and adjustment of travel time curves

Selection of travel time curves

Calculation of velocity - travel time curves

Calculation of depth - travel time curves

Velocity profile/depth section

Correction for layer dips (migration)

Verification with ray path calculations

Fig. 2. Flow chart for the standard data processing of seismic

refraction data (SEG Japan, 2000) [1].


are picked with a time resolution of around 1ms. For ahigh precision survey, time resolution is often less than0.1ms. Picking can be done manually on printed seismicrecords or using automatic and interactive computertechniques. An uphole correction is required to com-pensate for the burial of the shot.

2.5.3. Construction of the travel time curves

Based on the distance along the survey line, thereceiver intervals (the geophone spacing) and the firstarrival times, travel time curves are plotted with thehorizontal axis being distance and the vertical axis traveltime. For hard copy travel time curves, typical scales are1500or 1

1000for the distance axis and 5 or 10ms to the cm

for the vertical axis, see Fig. 3.

2.5.4. Checking of the travel time curves

The travel time curves should be checked andcorrected if necessary, on the basis of the following:

1. Reciprocity of the travel times. (Travel times betweena pair of shot points should be equal.)

2. At each shot point, coincidence of intercept timesfrom each of the refractors.

3. Parallel travel time curves when different shotsprovide travel times to the same refractor at thesame locations.

Bulk shifts in travel times can be used if it is decidedthat there is a constant delay, due to errors in the upholecorrection. Individual travel times may need to beadjusted after further consideration of the shot records.

2.5.5. Derivation and verification of the velocity profile

The number of refractors present in the travel times isdetermined on the basis of the number of changes inslope and the degree of parallel behaviour observed onthe travel time curves. The 2-D velocity profile (depthsection) under the survey line is then obtained byanalysing the travel time curves using techniques such asthe generalized reciprocal method (GRM) and Hagi-wara’s method. An intermediate step for these methodsinvolves determining the velocities of the P-waves ineach of the refracting layers present.Verification of the depth section using ray tracing to

calculate synthetic travel time curves is desirable. Thesecan be plotted on the corresponding observed traveltime curves.

2.6. Interpretation

1. The depth section thus obtained is generally inter-preted in consideration of the survey objectives,existing data and additional or supplemental profilesif available.

2. If the travel time curves can be interpreted by two ormore different models, it is desirable to report on allpossible interpretations. These situations typically arisewhen hidden layers and velocity inversions are present.

3. The P-wave velocities obtained with a seismicrefraction survey can be used as an indicator of rockquality for designing a construction such as a tunneland a dam in rock engineering applications.

4. Fig. 3 shows an example of travel time curves and theresultant depth section.

2.7. Output and report

Outputs of a seismic refraction survey should includeat least the following:

1. location map of the survey site;2. layout of survey lines;

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Fig. 3. Example of travel time curves and the resultant velocity/depth profile (SEG Japan, 2000) [1].

T. Takahashi / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 885–914 891

3. observer’s logs;4. shot records (in digital form);5. travel time curves;6. velocity profile verified with ray paths.

The survey report should describe at least thefollowing items:

1.
outline of the survey; 2. field operations (including equipment used); 3. data processing; 4. results:
J
waveforms; J travel time curves; J velocity profile;
5.
interpretation and discussion of the profile; 6. references.
3. Shallow seismic reflection

3.1. General

The shallow seismic reflection method is a seismicmethod that delineates subsurface structures usingseismic waves reflected back to the ground surface fromgeological boundaries in the subsurface. Seismic reflec-tion profiles provide high-resolution images of thesubsurface that can easily be understood and inter-preted. Fig. 4 shows a schematic diagram of the surveymethod. Seismic reflection waves are recorded andprocessed to obtain a seismic reflection profile. P- orS- (SH-) waves are used depending on the surveyobjectives. For shallow seismic reflection, a 2-D surveyis commonly used.

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Fig. 4. Schematic diagram of the shallow seismic reflection method (SEG Japan, 2000) [1].


3.2. Applicability

1. The shallow seismic reflection method is best used inareas where the subsurface layers are nearly hor-izontal and where the terrain is not too rough.Therefore, the method is well suited for alluvial/diluvial deposits and/or sedimentary rocks. Themethod is usually employed to characterize thesubsurface stratigraphy and faults (especially buriedfaults), and to delineate bedrock topography under-lying unconsolidated sediments. The method issometimes used for detection of subsurface cavitiessuch as karsts and old mine workings.

2. The depth of investigation for shallow seismicreflection surveys is usually from several tensto several hundreds of metres. The method canbe used from preliminary to detailed investi-gations stages in civil engineering and miningprojects.

3.3. Planning

3.3.1. Study of existing information

In planning the survey, all available existing informa-tion about the site should be collected and studied. Itshould include surface topography, geological maps andwell log data if available, from which subsurface velocitydistribution should be predicted. Land ownership andenvironmental factors also need to be investigated.Potential sources of noise should be identified.

3.3.2. Positioning of the survey lines

The survey line(s) should be set up as straight aspossible and oriented parallel to the dip direction andperpendicular to any cross cutting targets. Severetopographic features and areas where there is nosurface access should be avoided if possible. A series

of parallel lines with occasional tie-lines assists withinterpretation.

3.3.3. Source and receiver intervals

Source and receiver intervals should be determined inconsideration of the survey objectives and the depth ofinvestigation. One or 2m source and receiver intervalsare often used for a very shallow survey and 5 or 10mintervals are often used for a deeper survey. Once thesource and receiver intervals have been determined for aparticular survey, in general, they should remainconstant throughout the survey so that the trace spacingand the fold can be maintained.

3.3.4. Number of recording channels

The number of data recording channel is alsodetermined in consideration of the survey objectives,the depth of investigation and accuracy of the reflectionimage as well as the cost and efficiency of the survey.The number is usually a multiple of 12. Twenty-four or48 channel data for a very shallow survey and 48, 96 ormore channel data for a deeper survey are acquired ingeneral.

3.3.5. Type of seismic waves and sources used

For very shallow surveys with targets less than 50mdepth, both P- and S-waves are often used. For deeperdepths of investigation, P-waves are generally used. Fora very shallow survey, the sources are commonly asledge hammer blow for the P-waves and side-onhammering of a plank for the S-waves. For deepersurveys, a hydraulic vibrator, a weight drop or explosivesources are commonly used to generate the P-waves.

3.3.6. Instruments
1. The instruments for data acquisition consist ofgeophones, take-out cables (CDP cables), extensioncables, a roll-along switch and a recording system.


2. The instruments should be tested with routineprocedures prior to the field operation. The testsshould include measurements of the output levelsand the waveforms of the test signals, cable con-tinuity and leakage, and functions of the recordingsystem.

3. Moving coil geophones similar to those used in deepseismic reflection surveys in the oil and gas explora-tion field are commonly used. However, for shallowseismic reflection, higher natural frequency geo-phones are preferably used in order to obtainhigher-resolution reflection images compared to thosefor deep seismic reflection surveys.

4. The data recording system should include a digitaldata acquisition unit, a display unit, a digitalmemory and a printer in the standard systemconfiguration.

3.3.7. Recording format

It is preferable to record the data in the field in one ofthe SEG standard formats such as SEG-2 and SEG-Y sothat the field data can later be easily input to standardprocessing systems.


3.4.1. Preparation of the survey lines

In accordance with the survey plan, the seismic linesshould be pegged out on the ground surface and with thelocations of the shot and geophone points accuratelysurveyed. Vegetation may need to be cleared to facilitatefield operations but this should be done in accordance

Fig. 5. Examples of field records of P- a

with environmental factors and any rehabilitationrequirements.

3.4.2. Field acquisition parameter tests

Prior to making measurements in the field, theacquisition parameters should be tested. Theseparameters include the sampling rate, the recordinglength, the field acquisition filters, the minimumand maximum source–receiver offsets and the stackingfold.

3.4.3. Measurement

1. Following the survey plan, the receivers (geophones)should be laid out on the ground and connected tothe recording system through the CDP cables. Theseismic source is then employed at each source pointsequentially to generate seismic waves. The dataacquisition operator (seismic observer) should moni-tor the recorded signals and judge the quality of therecords by observing if reflections correlate across therecords. Noise, on the other hand, should beincoherent. If the data quality is not good then themeasurements should be repeated at the same sourcelocation until better data are obtained. Verticalstacking is one way to improve the signal-to-noiseratio at the field. Careful observers logs need to bemaintained to ensure that an accurate log ofrecording parameters, source and geophone locationsis maintained.

2. The recorded digital data are usually stored on amagnetic tape, a floppy disk or a hard disk of therecording system. Fig. 5 shows examples of the fieldrecords.

nd S-waves (SEG Japan, 2000) [1].

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Original data



3.5.1. Preparation for data processing
Format
conversion

Data editing

CDP sorting

Bandpass filter

Gain recovery

Deconvolution filter

Static correction

NMO correction

Mute

Residual statics

Amplitude balancing

CDP stack

Coherency and

bandpass filters

Statics analysis

Velocity analysis

Survey data

Migration Depth conversion

Time section Depth section

Deconvolution filter

Fig. 6. A standard data processing flow of shallow seismic reflection

data (SEG Japan, 2000) [1].

1. The coordinates and elevations for all source andreceiver locations have to be determined from thesurveyor’s notes.

2. The file numbers of all shot records and their sourceand receiver locations have to be confirmed andedited.

3. Additional information such as drilling logs, VSPdata and/or seismic refraction data collected atthe same time as the reflection data may need to beused as reference data in the processing of thereflection data.

3.5.2. Data processing

Shallow seismic reflection data are generallyprocessed using a standard CDP (or CMP) stackingmethod as devised for oil and gas exploration.Fig. 6 shows a standard data processing flow ofshallow seismic reflection data. Processing is usuallyundertaken by specialists of seismic data processingcompanies.

3.6. Interpretation

3.6.1. Interpretation of the shallow seismic reflection

profile

In principle, a seismic reflection profile provides avisualization of subsurface boundaries where acousticimpedance (or velocity) contrasts occur as the reflectionevents. In order to obtain the geological section, theseismic reflection profile should be interpreted incomparison with surface and borehole geologicalinformation as well as well logs and VSP if available.Fig. 7 shows an example of the interpreted depth sectionindicating buried faults. Interpretation can be under-taken on printed seismic profiles or using interactivecomputing facilities.

3.6.2. Resolution of the seismic reflection profile

The resolution of the seismic reflection methoddepends on the parameters such as the source frequency,the velocities of the layers and the depth of thereflections. Vertical resolution depends on the wave-length and horizontal resolution is controlled by theradius of the first Fresnel zone, which is determined bythe wavelength and the depth of the reflector. Verticalresolution of a thin layer is ideally around 1

4of the

wavelength, but practically around a wavelength.However, it is well known that the thin layer can bedetected even if its thickness is 1

10or less of the

wavelength of the reflection wave.


Outputs of a shallow seismic reflection survey shouldinclude at least the following:

1. location map of the measurement lines;2. examples of field records;3. examples of processed CDP or shot gathers;4. NMO velocity chart with statics;5. seismic reflection time section on paper and in digitalformat (with or without migration);

6. seismic reflection depth section on paper and indigital format;

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Fig. 7. Example of an interpreted depth section (SEG Japan, 2000) [1].

C1, C2: Current electrode P1, P2: Potential electrode

I

V

Transmitter

Receiver

C1 C2 P1 P2

Traces of current flow

Equipotential surfaces

Ground surface


7. interpreted time or depth section;8. original survey recordings and other relevant infor-mation need to be archived.

The survey report should describe at least thefollowing:

1. outline of the survey;2. field operation (including the equipment used);3. data processing procedures;4. reflection time and depth sections;5. interpretation and discussion of the sections;6. references.

Fig. 8. A schematic diagram of measurements involved in the

resistivity method (SEG Japan, 2000) [1].

4. Electrical

4.1. General

The electrical method delineates the subsurfacestructure and anomalies through the distribution of theirelectrical properties. There are several techniques andvariations in the electrical method: the (direct current)resistivity method, self-potential method, induced polar-ization method, etc. Among them, the resistivity methodis the most commonly used in civil engineering applica-tions. The resistivity method includes some variationssuch as horizontal profiling, vertical sounding and 2-Dprofiling. Application of 2-D profiling has increased inrecent years. Fig. 8 shows a schematic diagram of themeasurements involved in the resistivity method. Themethod is based on transmitting current into the groundthrough electrodes C1 and C2, and measuring the

electrical potential with electrodes P1 and P2 todetermine the electrical resistivity of the underground.

4.2. Applicability

1. The resistivity method is applicable to variousinvestigations in the civil engineering field, such asgroundwater detection, landslide characterization,constructions of tunnels and dams, cavity detectionand delineation of subsurface geological structure.Furthermore, it can be used in various stages of acivil engineering project from reconnaissance throughsite investigation to maintenance.

2. In characterization of geological structure, the meth-od is used for delineation of fractured zonesaccompanied by faults, classification of weatheringand alteration of rocks, and groundwater exploration.

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4.3. Planning

4.3.1. Review of existing information

It is recommended to study as much backgroundmaterials as possible on underground electrical proper-ties as well as geological information in and around thesurvey area.

4.3.2. Layout of survey line(s)

The layout of the survey line(s) is determined inconsideration of the objectives of the investigation,topographical and geological conditions, the depth ofinvestigation and resolution required, obstacles tomeasurement, cost and operational efficiency, etc.Supplemental lines are planned in addition to the mainlines if necessary. In the layout of the survey lines, thefollowing points should be noted:

1. It is desirable to set up the survey lines on a site withminimal topographic change, and ideally lay the lineson a horizontal or gently sloping ground.

2. The survey lines should be laid out far from obstacles,especially artificial metal structures such as metalfences and grounded power lines, because they maycause short circuits of the current and generateanomalous voltages around potential electrodes.

3. For 2-D profiling, the survey line should be set up asperpendicular as possible to the strike of subsurfacelayers and faults. If the line cannot be set up in suchway due to various restrictions in the survey area,supplemental lines should be arranged for delineatingsubsurface structure as correctly as possible.

4. In the layout of the survey line, the relationshipbetween the depth of investigation and the survey linelength should be taken into consideration (refer toSection 4.3.5).

5. Even for a survey along a curved line such as a tunnelroute, it is desirable to arrange the survey line asstraight as possible.

4.3.3. Selection of the survey method

1. The most appropriate method should be selectedfrom vertical sounding, horizontal profiling, 2-Dprofiling and other available methods taking intoconsideration the scope and the stage of thesurvey. In the selection of the survey method,topographical and geological conditions, cost andoperational efficiency should also be taken intoconsideration.

2. The vertical sounding method is cost-effective for theinvestigation of horizontally stratified structure. Thehorizontal profiling method is effective for mappingnear-surface geological boundaries, buried objects,and fracture zones bearing groundwater. The 2-Dprofiling method, combining vertical sounding andhorizontal profiling, is applied when the subsurface

geology is more complex. The three-dimensional(3-D) survey techniques, which have become practicalin these years, can be applied at sites of verycomplicated geology, considering the cost and thespecial requirements.

4.3.4. Electrode configuration
1. Optimum electrode configuration is chosen in con-sideration of the method used, geological and surfaceconditions, and the desired output (see Table 1).
2. The Schlumberger array and the Wenner array arecommonly used for vertical sounding.

3. The Wenner array, the pole–pole array, the pole–dipole array and the dipole–dipole array are suitablefor horizontal profiling.

4. The same electrode configurations as for horizontalprofiling are commonly employed for 2-D profiling.

4.3.5. The depth of investigation and length of the survey

line

The depth of investigation of the resistivity methodmostly depends on the electrode configuration andelectrode spacing. As a first approximation, the depthof investigation is a little smaller than the longestseparation between the current electrodes and potentialelectrodes. A survey line should be much longer thanthe estimated depth of the survey target in order to coverthe investigated area not only horizontally but alsovertically.

4.3.6. Electrode spacing

The maximum electrode spacing is determined fromthe target depth and the electrode configuration used.The minimum spacing is determined from the requiredspatial resolution. In horizontal profiling and 2-Dprofiling, requirements for both depth of investigationand resolution cannot be satisfied simultaneously.Therefore, they are usually determined bearing inmind the survey objectives, cost and operationalefficiency.

4.3.7. Remote electrode
1. Remote electrodes are required for the pole–polearray and the pole–dipole array. In order to improvethe field measurement efficiency, it is suggested toselect the location of the remote electrodes inconsidering access from the survey lines prior to thesurvey.
2. It is desirable to separate the remote electrodes fromthe survey line by at least 10 times the maximumelectrode spacing.


1. Field equipment for the measurements consistsmainly of the resistivity meter (usually consisting of

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Table 1

Electrode configuration factors for various electrode arrays

Array type Image point of apparent resistivity Electrode configuration factor

2-electrodes Pole–polea

a

P1C∞ P∞C1 2pa

3-electrodes CPP

a a

C1

a

P1C∞ P2 4pa

Pole–dipole

a

C1

naC∞ P2P1

a(n +0.5)2

2n(n+1)pa

4-electrodes Wenner C1

aaa

a

C2P1 P2 2pa

Dipole–dipole P2a ana

P1C2C1

a(n+1)

2

2n(n+1)(n+2)pa

Schlumberger

L

L/2

C1(A) P1 (M) P2

1

(N) C2(B) p(L2–l2)/4 l (L>5 l)


a power supply, a transmitter and a voltage receiver),electrodes, and cables.

2. It is important to check whether the cables are not cutand there is proper connection between electrodesand the resistivity meter. Furthermore, it is recom-mended to check the ground contact resistance ofevery electrode. If the ground contact resistance of anelectrode is extremely high, it should be lowered byinserting the electrode sticks more deeply into theground, adding electrode sticks at the station,sprinkling water around the electrode sticks, ormoving the electrode position slightly. Also, it isimportant to check whether any cable is directlyleaking current into the ground.

3. It is important to check repeatability of the measure-ments with the same electrode settings to ensuredata quality. The amplitude of the measured voltagemust be larger than the noise level of the resistivitymeter.

4. Another important issue concerns safety during thefield measurement. Since high-voltage current isapplied to the cables and the current electrodes, it isvery important to avoid any accident or injury.

5. In the vertical sounding method, resistivity measure-ments are repeated using a set of electrodes with the

same midpoint but with different intervals. Since alarger electrode interval corresponds to a deeperpenetration depth, a one-dimensional (1-D) resistivitymodel at the measurement location is obtained withthe vertical sounding method.

6. In the horizontal profiling method, a set of electrodeswith a fixed spacing is moved along the survey line toimage the horizontal variation of subsurface resistiv-ity at a similar penetration depth.

7. In the 2-D profiling method, horizontal profiling isrepeated with various electrode spacings along thesurvey line. A modern resistivity meter controlled bya computer can automatically select a set of electro-des on the survey line, transmit a current and measurethe potential data.


1. Apparent resistivity, ra (Om), is estimated using thefollowing equation for the measured electrical poten-tial V (V) due to the injected electrical current I (A):

ra ¼ GV=I ;

where G is an electrode configuration factorwhich is calculated from the electrode spacing and

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Observed potential data

Set up an initial 2-D resistivity model

Calculate theoretical potential data

Calculate residual (observed – calculated) potential data

Residual is small ?

Update the 2-D resistivity model

Final 2-D resistivity model

NO

YES

Fig. 9. A standard inversion process of the 2-D resistivity profiling data (SEG Japan, 2000) [1].


configuration shown in Table 1. Apparent resistivityis considered to represent average resistivity from thesurface to the penetration depth.

2. For vertical sounding (VES), it is common to createan apparent resistivity versus electrode spacing curve(a VES curve; ra–a curve for the Wenner array) bydrawing apparent resistivity values against thecorresponding electrode spacing on a full logarithmicsection paper. For horizontal profiling, it is mostcommon to create an apparent resistivity curve orsection as a function of horizontal position. For 2-Dprofiling, it is most common to create an apparentresistivity pseudo-section by taking the position asthe horizontal axis and the electrode spacing as thevertical axis.

3. For vertical sounding, it is possible to estimateresistivity and depths of boundaries of subsurfaceformations from the VES curve. Data processing waspreviously conducted by the graphical analysismethod (the curve fitting method) using mastercurves. However, it is now commonly performed by1-D inversion on a computer.

4. In 2-D profiling, the data are inverted with a non-linear inversion method to obtain a 2-D subsurfaceresistivity model. Fig. 9 shows a standard dataprocessing flow of the 2-D resistivity profiling data.

4.6. Interpretation

1. To avoid misinterpretation of the processed resistiv-ity image, it is important to understand restrictionsand limitations of the inverse method. It is recom-mended to refer to additional or other informationabout the survey site.

2. Resistivity varies with various factors such as rocktype, porosity, water saturation, groundwater resis-tivity, clay content associated with weathering oralteration, and temperature. Lower resistivity gen-erally indicates higher clay content or higher watercontent (i.e., the porosity multiplied by watersaturation).

3. Geotechnical parameters such us hydraulic conduc-tivity and porosity of the ground can be estimatedfrom the resistivity values using empirical or experi-mental equations.

4. Fig. 10 shows an example of the 2-D resistivity imagealong a planned tunnel route in a granitic rockobtained with 2-D resistivity profiling. Low resistivityportions were interpreted as high water contentzones.


Outputs of an electrical (resistivity) survey shouldinclude at least the following:

1. location map;2. survey line layout;3. apparent resistivity curves, profiles, or pseudo-sec-tions;

4. resistivity models;5. list of measured data.


1. outline of the survey;2. measurement method;3. field equipment;

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Fig. 10. Example of a 2-D resistivity model obtained by a 2-D inversion (SEG Japan, 2000) [1].


4. field notes describing the survey layouts and theconditions of the survey area;

5. data processing procedures;6. apparent resistivity curves, profiles, or pseudo-sec-tions;

7. resistivity models;8. interpretation and discussion of the resistivity models;9. references.

5. Electromagnetic

5.1. General

The electromagnetic (EM) method is a geophysicalexploration technique to delineate subsurface resistivitydistribution similar to the electrical method. Thereare many techniques and variations in the EM method,and they can be applied for different purposes in civilengineering. Among them, these suggested methodsdescribe the controlled-source audio-frequency magne-totelluric (CSAMT) method (Fig. 11) and the transientelectromagnetic (TEM) method (Fig. 12), which aremost often used.

5.2. Applicability

1. The EM method can image the subsurface resistivitydistribution, which is similar to the electrical method.

2. As compared with the electrical method, betteroperational efficiency and deeper penetration aremain features of the EM method. It is applicable tosite characterization for planned tunnels and dams, todetection of faults or fractured zones in rocks, etc.

3. The EM method is not suitable for the survey at anarea near power lines, telephone lines, radio stations,factories, railways, power plants, power stations andurban area, because EM waves radiated from theseconstructions and metallic objects strongly interferethe EM measurements.

5.3. Planning


1.
In order to apply the EM method most effectively,it is important to study the following items prior tothe survey using existing information:
J
depth, size and electrical properties of theexploration target;
J
surface topography and available roadsthat affect operational efficiency of thesurvey.
2.
High-voltage power lines and man-made structuresmay generate noise in an EM survey. It isimportant to check potential noise sources in andaround the survey area.
3.
It is desired to examine the location of the fixed EMtransmitter for the CSAMT and Offset TEMmeasurements that may be set up outside thesurvey area.
5.3.2. Selecting the survey method

First of all, the method used should be determinedconsidering (a) the survey objectives and specifications,(b) surface and subsurface conditions, and (c) opera-tional cost and time. Both the CSAMT and TEMmethods are suitable for deeper resistivity profilingas compared to the electrical method. Since theTEM method can use non-contact sources andreceivers such as a loop source and a magneticcoil, it is more suitable when a good contact of anelectrode is difficult at a survey area of high-resistivityunderground, for instance, snow accumulation and arocky area.

5.3.3. Determining the survey lines and station interval

1. There are two types of measurement arrangements.One is the profiling in which measurement stationsare densely arrayed along a survey line. The other isthe mapping in which measurement stations are

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Fig. 12. A schematic diagram of the TEM method (SEG Japan, 2000) [1].

Fig. 11. A schematic diagram of the CSAMT method (SEG Japan, 2000) [1].


spread over the survey area. The profiling and themapping can provide 2-D and 3-D resistivity images,respectively, through representing plural 1-D resis-tivity models or 2-D and 3-D data interpretation.

2. In the profiling, the survey line should be set up asperpendicular to the strike of geological structure tobe investigated. In the investigation for a linearconstruction such as a tunnel and a bridge, the survey


lines are usually set up along the planned construc-tion route. When the survey line is set up parallel tothe subsurface geological structure, some comple-mentary survey lines should be arranged perpendi-cular to the main survey line to obtain more reliablesubsurface images.

3. The length and linearity of survey lines and stationintervals do not affect very much the penetrationdepth and the quality of the data unlike the electricalmethod. Therefore, planned stations can be moved ifthere are unexpected obstacles or noise sources closeto them.

5.3.4. Determining penetration depth

The penetration depth depends on the frequencyused for the measurement and on the resistivity ofthe ground. Therefore, the frequency range used in themeasurement should be determined considering thesurvey objectives, the depth of investigation and theexpected resistivity of the ground. The penetration depthin frequency domain is usually referred to the skin depthof the plane EM wave. Fig. 13 shows the relationshipbetween the skin depth and frequency used in theCSAMT method. The penetration depth in time domainis usually referred to a depth where a magnetic (orelectric) field from a plane impulse source takes themaximum value at a certain time. Fig. 14 shows thepenetration depth as a function of time, which is used inthe TEM method.

5.3.5. Setting the remote EM transmitter

For deeper CSAMT and TEM surveys, a large fixedtransmitter (usually a grounded wire) is used as an EM

Fig. 13. Relationship between the skin depth of a plane wave EM fiel

source (Figs. 11 and 12). The large transmitter should beset up at a distance of more than 3–5 times the skindepth from the receivers to avoid the near field effect inthe CSAMT survey. The location of the transmittershould be planned prior to the survey considering thepenetration depth (the target depth) and surfaceconditions at the site.


5.4.1. Equipment

1. The EM equipment consists of a transmitter,a receiver, sensors, electrodes and cables. Thetransmitter and receiver are special equipmentsdesigned for the methods. They should have specifi-cations that meet the requirements in accuracy andreliability for the measurement.

5.4.2. Preparation

1. Sensors and electrodes at receiving stationsshould be laid out on as flat ground surface aspossible and far away from EM noise sourcessuch as power lines, metal fences and existingconstructions.

2. For the fixed transmitter source, electrode sticksor plates should be installed into the ground veryfirmly such that the contact resistance is lessthan 100O. Since the transmitter is usually kepton the ground for a long time until the measurementis completed, it should be kept safe from accidentssuch as electrical shock and leakage and cuttingof cables.

d and frequency in the CSAMT method (SEG Japan, 2000) [1].

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Fig. 14. Relationship between the penetration depth of a plane EM impulse and time in the TEM method (SEG Japan, 2000) [1].


5.4.3. Measurements
1. In the CSAMT measurement, a pair of a magneticfield and an electric field, which are perpendicular toeach other and both horizontal, are usually measuredat a station in a frequency range from a few Hz toseveral kHz. In the TEM measurement, a verticalmagnetic field is usually measured at a station withtransient time from several microseconds to severalhundreds milliseconds depending on the configura-tion of the measurement.
2. It should be reconfirmed prior to starting themeasurement that the measurement parameters areproperly set up on the equipment. During themeasurement, the measured data are always mon-itored in order to check the data quality. To ensurethe reliability of the data, repeatability of themeasurements should be examined. If the measureddata are not stable, measurement parameters shouldbe checked and changed if necessary. Ambient noiseshould also be checked.

3. During and immediately after the measurement, it isimportant to check the quality of the acquired databy plotting the apparent resistivity curves or voltagedecay curves on site.


1. Fig. 15 schematically illustrates a standard dataprocessing flow of the CSAMT and TEM data.First of all, apparent resistivity values calculated

from measured data are plotted against frequencyor time. Then apparent resistivity sections ormaps are made as fundamental data for furtherprocessing. These apparent resistivity sections ormaps are sometimes presented as the final explorationresults.

2. The apparent resistivity curves are usually used for alayered model inversion to obtain 1-D resistivitymodel beneath each measurement station.

3. When more detailed subsurface resistivity images arerequired, 2-D inversion using finite difference or finiteelement methods is applied to the measured data forobtaining 2-D resistivity models.

5.6. Interpretation

1. Interpretation of the CSAMT and TEM data isusually conducted with 1-D resistivity inversionto obtain layered earth models. A 2-D image isobtained by presenting plural 1-D resistivity modelsalong a measurement line. Fig. 16 shows such anexample.

2. Any inversion technique has a non-uniquenessproblem. The inversion of the EM data has non-uniqueness due to what is called ‘‘the equivalentresistivity layer’’. To avoid misinterpretation ofthe inversion results, a number of inversion trialswith different a priori information should be con-ducted.

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CSAMT TEM

Survey line

Apparent resistivity Apparent resistivity

Horizontal section of1-D model

Survey line

Vertical section ofstitched 1-D model

CSAMT: Each frequencyTEM: Each time

Each depth orelevation

Depth d1

Surface

Survey line

Depth d2

Obtain the resistivityof each cell

Cell n

Cell m

Cell n+1

Cell m+1 Cell m-1

Cell n-1

Survey line

2D resistivitymodel

Measuring pointssurvey line

Survey line

Topographic section Calculation of apparent resistivity

Apparent resistivity curve

Frequency f Time t

CSAMT TEM

App

aren

t res

isti

vity

App

aren

t res

isti

vity

Multi-layer structure

CSAMT · TEM

Depth d1

Layer 1

Layer 2

Depth dn-1Layer n

Surface

Resistivity n2

Resistivity nn

Depth dn

2-D interpretation

1-D interpretation

multi-layer model

Resistivity n1

Fig. 15. A schematic diagram of a standard data processing flow of the EM data (SEG Japan, 2000) [1].



Outputs of an EM survey should include at least thefollowing:

1. location map of the survey site;2. layout of survey lines and measurement stations;3. apparent resistivity curves, voltage decay curves,profiles, or pseudo-sections;

4. resistivity models;5. list of measured data.


1. outline of the survey;2. field operations (including equipments used);3. field notes describing the survey layouts and theconditions of the survey area;

4. data processing procedures;

5. apparent resistivity curves, voltage decay curves,profiles, or pseudo-sections;

6. resistivity models;7. interpretation and discussion of the resistivity models;8. references.

6. Ground penetrating radar

6.1. General

GPR is a geophysical method that uses the transmis-sion of high-frequency EM waves for detecting subsur-face objects. The operating frequency is higher thanseveral MHz. GPR can be used on the ground surfaceand in boreholes. Only surface GPR is described in thisdocument. In a GPR survey, transmitting and receivingantennae are placed on the ground surface and EMwaves are injected into the ground from the transmitter

ARTICLE IN PRESS

Fig. 16. Examples of stitched 1-D resistivity model obtained with the CSAMTmethod at a survey area of a tunnel construction (SEG Japan, 2000) [1].

Fig. 17. A schematic diagram of the surface GPR survey (SEG Japan, 2000) [1].


antenna. Reflected EM waves from subsurface geologi-cal boundaries or objects are then received by thereceiving antenna and processed to image these subsur-face structures (Fig. 17).

6.2. Applicability

GPR is a non-destructive and non-invasive geophy-sical method for rapid and high-resolution imaging ofsubsurface objects. The penetration depth is typicallyseveral metres in soil ground. There are a variety ofapplications including detection of buried pipes, locat-ing reinforcing bars in concrete, mapping voids beneathroad pavements or behind tunnel linings, and inspectionof roads and concrete structures. The method can alsobe used in archaeological investigations, monitoring thespread of contamination in the ground, and mappingfaults and fracture zones in rocks.

6.3. Planning


The feasibility and the output of GPR can often bepredicted by studying existing site information and by

site reconnaissance. The survey plan should be made inconsideration of expected properties of subsurfacematerials (especially resistivity), groundwater condition,depth and characteristics of the targets, and accessibilityof the equipments to the survey areas.

6.3.2. Choice of GPR antenna

The centre frequency of the GPR antenna typicallyranges from 30MHz to 1.5GHz. Resolution andpenetration of GPR waves strongly depend on theantenna frequency used, as shown in Table 2, i.e.,higher-frequency antenna can have higher resolution,but lower penetration. The antenna frequency used inthe survey should be determined considering thefollowing conditions:

(1)
Features of exploration targets such as size, depthand material type, and resolution required.
(2)
Properties of soil, rock and medium in which thetarget is buried.
(3)
Moisture and clay contents of the medium, whichaffect GPR penetration.
(4)
Surface unevenness and vegetation of the site andaccessibility of the antenna.

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Table 2

Relationship of GPR parameters with respect to dielectric constant, electrical conductivity of the ground, and antenna frequency

GPR parameters Dielectrical

constant

Electrical

conductivity

Antenna frequency Remarks

Low High Low High Low High

Propagation velocity Fast Slow

Attenuation High Low Low High Low High Attenuation is influenced strongly by electrical

conductivity

Penetration Short Long Long Short Long Short The lower the attenuation, the greater the penetration

distance

Wavelength Long Short Long Short

Resolution Low High Low High The shorter the wavelength, the higher the resolution


6.3.3. Arrangement for survey lines and their spacings

For linear targets, such as buried pipes, the survey lineis normally set up perpendicularly to the longitudinaldirection of the expected buried pipes. The survey linespacing should be determined considering the targettype, size and depth. Typically it is 1–2m. For metallictargets (e.g., iron pipes), the antenna dipoles are usuallyoriented parallel to the longitudinal axis of the target.For non-metallic targets (e.g., gas pipes), the antennadipoles are normally set up at the right angle to thetarget orientation. The layout of survey line(s) and theline spacing (s) should be determined in consideration ofthe following conditions:

(1)
frequency and moving speed of the antenna; (2) resolution required; (3) surface unevenness and vegetation of the site; (4) cost and acquisition time available.
Additional survey lines or a dense grid-based surveymay be required to detect smaller and multiple targets orif very accurate data processing is necessary.

6.3.4. The depth of investigation

The depth of investigation of a GPR survey dependson the frequency and the power of transmittedEM waves and electrical conductivity of the groundor the medium to be investigated. The depth ofinvestigation varies from 0.1 to 100m, but typicallyfrom 0 to 5m for most of civil engineering applications.Deeper penetration is possible with lower frequencies(30–100MHz) in the ground that has lower conductivity.


6.4.1. Equipment

The GPR equipment consists of a data acquisitionsystem and antennae. Those should be chosen inconsideration of the depth of the target and the accuracyrequired with reference to the objectives of the survey.

Selection of antenna frequency is crucial for success ofthe survey.

6.4.2. Preparation for the survey

1. The locations of the survey lines are marked out atthe site. The start and end positions of the surveylines are normally indicated in the survey plan.

2. For a rough and uneven ground, land readjustmentmay be needed to obtain good contact between theantenna and the ground surface.

3. The antennae are mounted on a sled or a wheeledtrolley to enable them to move smoothly on theground.

6.4.3. Field procedures

1. The GPR data acquisition system is first set up andconnected to a power supply such as a 12V DCbattery. It is recommended to warm it up for a whileto stabilize the zero time position. A preliminary testmeasurement should be conducted prior to the surveyin order to ensure that the GPR system is functioningproperly and to optimize the measurement para-meters. The field acquisition parameters such asthe recording time range (in nanoseconds), thesampling rate, signal gains and band-pass filtersshould be optimally selected in consideration of thetarget depth and the antenna frequency. The measur-ing wheel should be calibrated on site for a knowndistance.

2. The measured data should be monitored on thescreen of the equipment during the survey for dataQC. Unwanted noise or anomalous signals should beidentified to remove their causes.

3. The recording time range should be adjusted for theinvestigation depth by measuring the propagationvelocity of the EM waves in the ground using thewide-angle or the arrayed antenna measurements orby analysis of the hyperbolic diffractions generatedby a discrete target. Alternatively, the velocity can becalculated from the dielectric constant of the ground,which can be measured on site using a portable


dielectric meter or determined in the laboratory testusing soil samples.

6.4.4. QC of the measured data

The quality of the recorded data should be checked onsite during or immediately after the survey by monitor-ing the GPR signals or profiles on the screen of theequipment. Survey information, such as the positions ofthe survey lines, variations in surface conditions, andlocations of metallic objects close to the survey linecausing spurious reflections, should be recorded in thefield observer’s notes for data processing and interpreta-tion later on.


1. The GPR profile (or Radargramme) is normallyrepresented as a 2-D time section (or a depth sectionif velocity is known) with wiggle traces or grey/colourscales.

2. To enhance the target signals, the following digitalsignal processing is usually applied to the measureddata on site or in house after the survey: verticalstacking, deconvolution filter, bandpass filter, mov-ing average/background removal filter, automaticgain control (AGC) or time-variant gain (TVG).

3. In case that the data are recorded on multiple surveylines on a grid, the GPR data can be displayed inthree dimensions using commercial presentationsoftware for making its interpretation much easier.

6.6. Interpretation

1. A GPR profile can be interpreted in a similar way tothe interpretation of a seismic reflection profile.Subsurface anomalies such as buried pipes andcavities can be detected as diffraction patterns inthe profile.

2. It is important to collect as much existing informa-tion as possible to interpret the GPR profile properly.For example, in the application of the GPR todetection of buried pipes, information relating totheir locations, depths, sizes and materials should besearched prior to the interpretation.

3. If an accurate depth section is necessary, informationaffecting the EM wave velocity such as dielectricconstants and moisture contents of the ground shouldbe considered in the interpretation.

4. If other geophysical data such as shallow EMmapping, magnetic or resistivity data are available,they can be very useful for interpretation of the GPRprofile.

5. Fig. 18 shows some examples of interpreted GPRprofiles.


Output of a GPR survey should include at least thefollowing:

1. location map of the measurement lines;2. GPR profiles;3. interpreted GPR profiles.

The survey report should describe at least thefollowing:

1. outline of the survey;2. field operation (including the equipments used);3. data processing procedures;4. GPR profiles;5. interpretation and discussion of the profiles;6. references.

7. Gravity

7.1. General

The gravity method is a geophysical method in whichthe subsurface density distribution (or the densityanomaly) is estimated from the observed gravity field.Gravity is usually measured at the ground surface usinga gravity meter or a gravimeter. Gravity may also bemeasured in the air, at sea, or in a borehole usinggravimeters designed for those environments. In thisdocument, we will focus on and describe in detail theland gravity method. The gravity method has beenwidely used for delineating a base rock for earthquakeresistant construction design, locating buried faults anddetecting underground cavities due to civil engineeringactivities and old mine working. Fig. 19 shows theschematic diagram of a gravity anomaly and an exampleof a gravimeter. The gravity method has normally beenused for large-scale subsurface imaging, for instance, forstudies in Earth Sciences. The micro-gravity methodusing very dense measurements has recently beenemployed for near-surface civil engineering applications.

7.2. Applicability

1.
The gravity method can be used for: (1) detection of subsurface cavities and voids due to
civil engineering activities and old mine working,
(2) delineation of a base rock for earthquake-
resistant construction design,
(3) delineation of buried faults underneath thick
sediments,
(4) evaluation of ground improvement such as
grouting and compaction by comparing gravityanomalies measured before and after the im-provement.

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Fig. 18. Examples of interpreted GPR profiles (SEG Japan, 2000) [1].


2.
In earthquake engineering field, the gravity methodcan often be employed to delineate larger-scalesubsurface structures such as deeper base rocks andlarge faults to predict accurate earthquake groundmotion.
3.
The surface gravity method is used to detectlateral density changes in the ground, but inprinciple, it cannot be used to delineate horizontallystratified structure whose density changes only withdepth.
7.3. Planning


Existing information should be studied from thefollowing points of view: (1) dimension, depth anddensity of the geological or structural features to beinvestigated, (2) surface topography around the surveyarea and (3) accessibility to the site such as roadconditions.

7.3.2. Resolution of detectable subsurface structures or

anomalies

1. Resolution of detectable subsurface structures oranomalies with the micro-gravity method can be

around 1m using high-density measurements underrelatively low ambient noise.

2. Using more widely spaced measurement points in avery wide area, it is possible to delineate large-scalesubsurface structure with the dimension of severalkilometres.

3. Fig. 20 shows the relationship between the detectabledimension of the target and the amount of the gravityanomaly due to it. Since the most modern gravimetercan measure the gravity change of several micro-Gals, subsurface features with the dimension ofaround 1m can be detected by means of the micro-gravity method.

7.3.3. Determination of the survey area and the station

spacing
1. The micro-gravity survey will normally be con-ducted in one of two modes, a profile and an arealsurvey. The former is commonly employed forinvestigation along the linear construction such asa railway and a road, while the latter would beused for an areal survey, for instance, for delineationof a bed rock topography beneath the investigationsite
2. The survey area should be determined considering thedepth of the features of interest in the survey. The

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Fig. 19. Schematic diagram of the gravity anomaly and a gravimeter (SEG Japan, 2000) [1].

Fig. 20. Relationship between dimension of the target and gravity anomaly (SEG Japan, 2000) [1].


dimension of the survey area should be 5 or 6 timesthe maximum depth of investigation. The stationspacing should be comparable to, or smaller than, theminimum depth of the features of interest.

3. It is desirable to keep the station spacing as uniformas possible.


7.4.1. Measurement principle
1. A gravimeter commonly used for a gravity survey canonly measure relative gravity value. Therefore, in a
gravity survey, the difference of the gravity valuebetween a station and a fixed station called as a baseor reference station is measured. The referencestation should be either a station where the absolutegravity value is known or a station connected toanother reference station where the absolute gravityvalue is known.

2. A measurement technique called the closed-loopmethod is commonly used, in which the first and lastmeasurements of successive measurements in a loopare made at the same station to obtain the errorbetween these two measurements, known as the


closure error. The measurement error at each stationcan then be evaluated based on this closure error inthe loop. Successive measurements in a loop shouldbe normally completed within no more than abouthalf a day to keep the error to a minimum. Theclosure error, mainly due to the drift of the spring inthe gravimeter, should be less than 0.2mGal.

3. In a very high accurate survey, such as detection of avery small cavity with a diameter of less than 1m, itshould be necessary to know the drift error asprecisely as possible. In such a case, the succes-sive measurements in a loop should be completedwithin 2 h or less and the measurements be re-peated by visiting stations in the loop in a reverseorder.

7.4.2. Measurement accuracy
1. A gravimeter used for a micro-gravity survey shouldhave an accuracy of a few micro-Gals, while that foran ordinary gravity survey may have an accuracy ofbetter than approximately 0.1mGal. A gravimeter isa very sensitive device and must be handled withextreme care during transportation as well as duringmeasurements.
2. The elevation of each measurement station in amicro-gravity survey must be known to the accuracyof a few millimetres or less and the accuracy of thehorizontal location is required within a few tens ofcentimetres. For ordinary gravity survey, accuraciesof the elevation and the horizontal location should beless than a few tens of centimetres and a few metres,respectively.

3. In a gravity survey over an area more than a fewkilometres squared, the location of each stationcan be determined using a geographical map witha scale of 1:1000 to 1:25 000 corresponding tothe required accuracy of the survey. Benchmarks,trig stations or other points with elevations indi-cated on the map can be used as reference elevationpoints.

7.4.3. Measurement procedures
1. To make the measurement procedure most effi-cient, the route and order of visits to the measure-ment stations should be determined at thebeginning of the survey.
2.
Measurement stations should be arranged at a flatplace without abrupt topographic changes nearbyand not placed on the top or at the foot of a cliff oran embankment.
3.
The measurement should be done on a firm and flatsurface such as a pavement and away from sourcesof vibrations such as traffic noise.
4.
The exact station location should be marked withpaint, a survey nail or a survey peg prior to thesurvey.
5.
It is desirable to repeat measurements several timesat each station to improve the accuracy andreliability of the measurement.
6.
The following items should be recorded in the fieldnote: (1) the model name and serial number of the
gravimeter used;
(2) the height of the instrument from the earth’s
surface at each measurement point;
(3) each reading and its time.
7.
It is desirable to make a sketch of the measurementlocation and its vicinity for the further terraincorrection in data processing.
8.
Re-measurement of any loops should be consid-ered if the closure error is neither negligible noracceptable.

1. The measured gravity value at each station should befirst corrected for the gravity tide variations, thegravimeter drift and the instrument height at themeasurement station. Usually, these corrections areconducted in the field for QC.

2. The gravity anomaly value at each station iscomputed by applying latitude corrections, free-aircorrections, Bouguer corrections, terrain correctionsand atmospheric corrections to the field datacorrected as above. The coordinate system used forthese corrections may be either the geodetic referencesystem (latitude, longitude, height) or a Cartesiansystem ðX ;Y ;ZÞ:

3. Fig. 21 shows a standard data processing flow ofgravity data. The gravity anomaly thus obtained isthe subsurface density anomaly and is the object offurther interpretation.

7.6. Interpretation

1. For qualitative interpretation of the subsurfacedensity distribution, the residual gravity anomalymap is usually used. It is computed by applying thetrend analysis and/or spatial filters to the originalgravity anomaly data to enhance the anomalies to beextracted.

2. Depending on the requirements or the objec-tives of the investigation, quantitative interpreta-tion can be used to delineate bedrock topogra-phy, to calculate the location or the depth ofa cavity, or to estimate the vertical displacementof a fault.

3. The interpretation may be difficult or unreliable nearthe boundary of the survey area. Such problemsmight be avoided by integrating existing gravity data,if available, into the measured data.

ARTICLE IN PRESS

Measured Gravity Data

Gravity tide correction

Correction for instrumental height

Drift correction

Gravity value at measurement point

Topographic correction

Bouger correction

Free-air correction

Latitude correction

Atmospheric correction

Gravity anomaly distribution (on irregular grid)

Data interpolation in space

Gravity anomaly distribution (on regular grid)

Fourier analysis (model calculation)

Subsurface density distribution

Correction for solar and lunar mass attraction

Correction for altitude from a ground surface tothe sense position of a gravity meter

Correction for near field topographic mass attraction

Correction for the drift of gravity meter

Correction for gravity difference due to the

altitude difference from measurement point

Correction for the normal gravity

Correction for near field topographic massattraction

Correction for atmospheric massattraction

Transformation of irregular to regular grid data

To obtain the target underground densitystructure based on spectral analysis, filter operation and model calculation

Fig. 21. Standard data processing flow of gravity data (SEG Japan, 2000) [1].


4. Since the gravity anomaly represents the subsurfacedensity anomalies (density variations), the densitycontrast between the survey target and its surround-ings should be taken into account in interpreting thesurvey result.

5. If available, it is important to integrate existinginformation into the interpretation, such as subsur-face geology, geophysical images and topographyaround the site.

6. Fig. 22 shows an example of an interpreted gravityanomaly map for detection of subsurface cavities.


Outputs of a gravity survey should include at least thefollowing:

1. maps of the station locations and the survey area;


2. gravity (and/or gravity anomaly) maps;3. interpreted gravity maps;4. measurement data.


1. outline of the survey;2. field operations (including the equipment used);3. data processing procedures;4. gravity anomaly maps;5. interpretation and discussion of the gravity anomalymaps;

6. references.

8. Radiometric

8.1. General

The radiometric method can be used to detectsubsurface geological features such as a fault or a shear

Fig. 22. Example of an interpreted gravity anomaly map for detection

of cavities (SEG Japan, 2000) [1].

Fig. 23. Schematic diagram of the radiometric method (radionuclide:

zone by measuring changes in intensity of naturalradiation from the earth’s surface. Gamma rays aregenerally used in the radiometric survey for geologicalapplications, and the term ‘‘radiometric survey’’ usuallytherefore refers to the gamma-ray spectrometric surveyunless explicitly stated otherwise. Fig. 23 shows theschematic diagram of the gamma-ray spectrometricsurvey. Two types of measurement methods can be usedin the gamma-ray survey: the total count method andthe gamma-ray spectrometric method. The total countmethod measures gamma rays at all energies with nodistinction between radionuclides, whereas the gamma-ray spectrometric method measures both the intensityand energy of radiation and can distinguish betweenradionuclides. The gamma-ray spectrometric method ispreferred because it can provide more information thanthe total count method. The survey techniquesare classified based on the survey operation modeinto air-borne, car-borne and man-borne. The surveyequipments are mounted in a helicopter or a fixed-wingaircraft for the air-borne survey and in a motor vehiclefor the car-borne survey. For the man-borne survey (orthe portable gamma-ray survey), the operator carries thesurvey equipments.

8.2. Applicability

1. For civil engineering applications, the radiometricsurvey is applied mainly to detection of subsurfacegeological features such as faults or shear zones,which are often conducted for characterization ofactive faults in earthquake disaster preventionprojects and for groundwater or hot spring develop-ments.

2. The radiometric method is also used to investigate thedistribution of rocks at the earth’s surface bymeasuring changes in gamma-ray intensity due todifferences in the natural abundance of the radio-isotopes in rock forming minerals.

Bi: bismuth, K: potassium, Tl: thallium) (SEG Japan, 2000) [1].


8.3. Planning


Before planning a survey, all available technicalreferences and investigation reports regarding the siteshould be thoroughly studied. The selection of thesurvey area, the survey method and the locations ofmeasurement points or lines should be made after dueconsideration of the purpose of the survey and thesurvey targets as well as the existing information above.It is also essential to preliminarily study the geologicaldistribution of the radioelements, the locations ofpotential fault zones and any possible noise sourcessuch as man-made structures and unfavourable topo-graphic conditions.

8.3.2. Selecting the survey method

The survey method should be selected from amongthe air-borne, the car-borne and the man-borne methodin consideration of survey objectives and requirements,time and cost of the survey, etc. The air-borne method issuitable for a very wide area survey within a short timeand for a survey of the area where it is difficult to access.The car-borne is suitable for a wide area survey and theman-borne is suitable for a limited area survey. Atypical application of the car-borne method would be apreliminary investigation for the selection of a furtherdetailed investigation area in a large area. The man-borne survey is, for example, useful for detection ofaccurate location of a buried fault.

8.3.3. Designing the survey

A survey design consists of positioning the surveylines and the measurement points. The survey shouldbe designed after due consideration of the surveyobjectives and targets. Additionally, the requiredaccuracy, geological and topographic conditions andthe locations of the man-made structures should be alsoconsidered. For locating a buried fault, the survey lineshould be set up as perpendicularly as possible to theexpected strike of the fault. If there is no informationabout geological features of the site, the grid-basedmeasurement lines should be set up for more accurateestimation of them. Survey lines and measurementpoints should be set up so as to avoid the areawith abrupt topography changes and man-made con-structions.


8.4.1. Equipment

The measuring equipment used in the radiometricmethod is generally a scintillation counter with a NaI(Tl) detector. There are two types of equipment,the total count meter and the spectrometer. The totalcount meter measures gamma rays at all energies with

no distinction between radionuclides, whereas thespectrometer measures both intensity and energy ofradiation and can distinguish between radionuclides.The latter is commonly used in a recent radiometricsurvey.

8.4.2. Stationary measurement at a fixed point

In order to correct for time variations of gamma-rayradiation during the survey, stationary measurement ata fixed point should be taken at least once or twice aday. In case of the survey over more than 1 day,stationary measurement at a fixed point must be takenevery day in order to correct for daily changes in themeasurement.

8.4.3. Measurement

1. For the air-borne survey, the special gamma-raydetector designed for an aircraft is used. Thehelicopter is more preferable to civil engineeringapplications than the fixed-wing aircraft because ahelicopter can fly at a lower altitude and lower speedthan a fixed-wing aircraft, which leads to a higher-resolution survey. In the air-borne survey, navigationand altitude data must be recorded together with thegamma-ray measurements.

2. For the car-borne survey, the speed of the vehicleshould be kept as slow and steady as possible toensure good quality data. The vehicle speed should bedetermined considering time and accuracy of mea-surement.

3. For the man-borne survey, measurement should beconducted normally for 300–500 s at a measurementpoint. The intensity of radiation due to each of majorradionuclides should be recorded in the field note foreach measurement.

4. Data QC should be made during or immediately afterthe measurement on site. Additional or repeatedmeasurements should be taken if necessary.


1. The total count method provides the total gamma-ray intensity at each measurement point. The gamma-ray spectrometric method provides the gamma-rayintensity spectrum at each point. From this spectrum,the total intensity and the spectral intensities of threeindexed radionuclides, 40 K (1.46MeV), 214 Bi(1.76MeV) and 208 Tl (2.61MeV) can be calculated.These measurement values are compiled in a table foreach survey line or measurement point.

2. The measured value should be corrected for timevariation and daily change of gamma-ray radiationbased on the measurement values at the fixed point. Ifpossible, climatic or surface topographic correctionsshould be applied. For the air-borne survey data,

ARTICLE IN PRESS

Fig. 24. Example of interpretation of radiometric survey profiles (SEG Japan, 2000) [1].


attenuation correction must be made for removingeffects of altitude changes.

3. The spatial average and standard deviation of thetotal gamma-ray intensities and the spectral inten-sities of 40 K (1.46MeV), 214 Bi (1.76MeV) and208 Tl (2.61MeV) are calculated at first. Anoma-lous intensities are then detected on the basis ofthe mean value and the standard deviation. Forthe spectrometric method, the spectral ratios suchas Bi/K, Tl/K and Bi/Tl are calculated. For the linesurvey, these calculated values are presented as aprofile with distance. In case of the grid-basedmeasurement, the calculated total or spectral inten-sities are normally presented as a contour map ofthem.

8.6. Interpretation

The calculated total or spectral gamma-ray inten-sity profile maps are interpreted in terms of thesurvey target and objectives by referring to existinginformation such as a geological map. If there areother geophysical data, they can be helpful for theinterpretation. Fig. 24 shows an example of interpreta-tion of gamma-ray total intensity and spectral ratioprofiles obtained at a radiometric survey for detection offaults.


Outputs of a radiometric survey should include atleast the following:

1. location map of the measurement lines or points;2. gamma-ray total or spectral intensity or spectral ratioprofile or map;

3. interpreted profile or map;4. measurement data listed in a table.

The survey report should describe at least the following:

1. outline of the survey;2. field operation (including the equipment used);3. data processing;4. gamma-ray total or spectral intensity or spectral ratioprofile or map;

5. interpretation and discussion of the results;6. references.

Acknowledgements

The suggested methods were drafted by the WorkingGroup on Standardization of Geophysical Methodsfor Rock Engineering, consisting of 11 members fromsix countries: P. Hatherly, M.S. King, H. Kusumi,B. Lehmann, L. Myer, L. Pyrak-Nolte, T. Takahashi,


N.M. Toksoz, T. Uchida, T. Watanabe and C. Xu. ToruTakahashi has coordinated the working group. Majorcontributions in drafting and revision were receivedfrom Dr. Peter Hatherly (Australia) and ToshihiroUchida (Japan). We would like to express special thanksto these two members for their efforts. The drafts werealso greatly improved by fruitful comments and sugges-tions from E. Brueckl (Austria), C. Cosma (Finland),B. Lehmann (Germany), G. Borm (Germany), K. Sassa(Japan), C. Lee (Korea) and M.S. King (UK) who arethe ISRM commission members.The first drafts of the suggested methods were drafted

by the sub-working group consisting of following 19members belonging to the Japanese Committee for RockMechanics (JCRM) working group on Applied Geophy-sics for Rock Engineering: T. Aizawa, Y. Ashida, A.Chiba, K. Chida, K. Ikeda, K. Kishida, N. Kurahashi,H. Kusumi, T. Matsuoka, S. Miki, H. Suzuki, K. Suzuki,S. Tanaka, T. Tanaka, T. Takahashi, T Takeuchi, T.

Tokumaru, F. Watanabe and T. Watanabe. We wouldalso like to thank these members for their efforts.

References

[1] SEG Japan. The guidebook of geophysical exploration methods

for civil engineering. Japan: SEG Japan; 2000 [in Japanese].

Further reading

Suggested Readings for details of each geophysical

method

Telford WM, Geldart LP, Sheriff RE. Applied geophysics. 2nd ed.

Cambridge: Cambridge University Press, 1990.

Burger HR. Exploration geophysics of the shallow subsurface.

Englewood Cliffs, NJ: Prentice-Hall, 1992.

Documents

ISRM Suggested Methods for Land Geophysics in Rock Engineering