Surface Geophysical methods

Since 1971, Technos has pio-neered the application of geophysical methods to environmental, geotechnical, and groundwater investigations. Surface geophysical methods allow subsurface features to be located, mapped, and char-acterized by making measurements at the surface that respond to a physical, elec-trical, or chemical property. These non-invasive measurements can be effectively used to provide reconnaissance to de-

tailed geologic information, guide sub-surface sampling and excavation, and provide continuous monitoring. Surface geophysical methods provide data at a variety of scales, from the regional geo-logic setting to site-specific geotechnical forensics. This document summarizes the applications of surface geophysical methods and provides a brief description of the most commonly employed meth-ods.

The single most critical issue we face in site evaluation work is accurately characterizing the site's geology and hydrogeology. If we can achieve an ac-curate understanding of a site's geology and hydrogeology, predicting the engi-neering performance of a site, locating contaminants and predicting their move-ment, or designing a hazardous waste clean up operation will be reasonably straightforward. However, the frequent lack of understanding of site geology and hydrogeology is often responsible for the structural and environmental failures that occur. If all sites were simple (horizontally stratified geology with uni-form properties), site characterization would be easy. Data from just one bor-ing would be sufficient to characterize the site. However, in most geologic set-tings, this will not be the case. Even at sites where the geology appears to be uniform, one must be alert to often-subtle variations that can cause significant changes in structural or hydrological properties.

Traditional approaches to sub-surface field investigations commonly rely only upon the use of direct sampling methods such as: • Borings for soil and rock samples; • Monitoring wells for gathering hy-

drogeologic data and water samples; • Laboratory analysis of discrete soil,

rock and water samples to provide a quantitative assessment of site con-ditions; and

• Extensive interpolation and extrapo-lation from a limited number of data points.

Soil and rock sampling pro-grams and the placement of borings and wells are done mainly by educated guesswork. The accuracy and effective-ness of such an approach is heavily de-pendent upon the assumption that sub-surface conditions are uniform. Numer-ous pitfalls are associated with this ap-proach that can result in an incomplete or even erroneous understanding of site conditions. These oversights are the cause of many structural and environ-mental failures.

Background

The Site Characterization Problem

Surface Geophysical Methods Volume 1, Fall 2004

An insight to the number of discrete samples or borings that are required for accurate site characteriza-tion can be obtained by considering detection probabil-ity. If the area of the site is 10 times that of the target (a site-to-target ratio of 10:1), 10 borings spaced over a regular grid would be required to achieve a detection probability of 90%. The target (whose size and location are usually unknown) could be a waste disposal site, a plume from a chemical spill, an old sinkhole, or a buried stream channel. For smaller targets, such as a narrow sand lens or fracture, the site-to-target ratio can increase significantly to 100:1 or 1000:1, thus 100 to 1,000 bor-ings may be required to achieve the same 90% confi-dence level of detection. Site-to-target ratios of 100:1 to 1000:1 make subsurface investigations with traditional borings and sampling like "looking for a needle in a haystack".

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Drilling and Spatial Sampling Requirements

SURF ACE GEOPHYSICAL ME THODS

In many cases, direct sampling alone is not suf-ficient to accurately characterize site conditions. This is the primary reason for the application of surface geo-physical methods. The geophysical methods encompass a wide range of airborne, surface, marine, and downhole methods that can be used to significantly improve the accuracy of subsurface investigations. Since surface geophysical measurements can be made relatively quickly, they provide a means to significantly increase data density. In some cases, total site coverage is eco-nomically possible. Because of the greater sample den-

sity, anomalous conditions are more likely to be de-tected, resulting in an accurate characterization of sub-surface conditions. Surface geophysical methods, like any other means of measurement, have advantages and limita-tions. There is no single, universally applicable sur-face geophysical method, and some methods are quite site-specific in their performance. The methods must be carefully selected based upon specific site condi-tions and project requirements.

The Application of Surface Geophysics

An Improved Approach

• Assessment of natural geologic and hydrogeologic con-ditions;

• Detection and mapping of contaminant plumes, spills, and leaks;

• Detection and mapping of landfills, trenches, buried wastes and drums, or other underground structures and utilities; and

• Evaluation of soil and rock properties and man-made structures.

A summary of the most commonly employed geo-physical methods for these applications is shown in Table 1, modified from ASTM guidelines (ASTM, D6429-999).

Area of Site = As

Area of Cavity = Ac

As/Ac=10

Probability of Detection

As/AC=10 As/AC=100 AS/AC=1000

100 16 160 1600 98 13 130 1300 90 10 100 1000 75 8 80 800 50 5 50 500 40 4 40 400

Illustration of number of borings needed for a given probability of detection

There are four major areas where surface geophysical methods may be applied to environmental and engineering problems:

VOLUME 1 , I SSUE 1

Soil types along with their spatial extent and thickness can often be assessed. Depth to the water table and rock can be determined as well as the depth and thickness of soil and rock layers. Larger structural features such as synclines, folds and faults can be located and mapped. Smaller localized features such as, the degree of weath-ering, existence of sand and clay lenses, fracture zones, buried relic stream channels, cavities, sinkholes and other geologic hazards can be located, mapped and as-sessed.

Probably the most important task of any site in-vestigation is characterizing the natural geologic and hydrogeologic conditions. The characterization of geo-logical heterogeneity is one of the most difficult prob-lems we face. Understanding the hydrogeologic condi-tions can make the difference between success and fail-ure in site characterization, since anomalous conditions will often control the integrity of the soil and rock mass, ground water flow, and contaminant transport. A variety of surface geophysical methods can be used to characterize natural soil and rock conditions.

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Assessment of Natural Geologic and Hydrogeologic Conditions

Table 1. Summary of Applications for Common Surface Geophysical Techniques

This table provides an overview of the common surface geophysical methods. There are many exceptions to this table and many variables that dictate the applicability of each method. Final selection of the methods and their application should be done by experienced, qualified professionals who can take into account all of the site-specific conditions.

Applications Refraction ReflectionMASW

(Surface Waves)

DC Resistivity

Imaging

Capacitively-Coupled

Resistivity Imaging

Induced Polarization (IP)

Spontaneous Potential (SP)

Frequency Domain

Time Domain

Metal Detectors

VLFGround

Penetrating Radar

Magnetics Microgravity

Natural Geologic and Hydrologic Conditions

• Soil/unconsolidated layers B B A A A B B B A

• Rock layers B A A B B B B B

• Depth to bedrock A A B A A B B B A A

• Depth to water table B A A B B B B

• Fractures and fault zones B B B B B A A A B A A B B

• Voids and sinkholes B B A B B B B A A

• Soil and rock properties A A B B B B B

• Dam and lagoon leakage B B B A B B

Inorganic Contaminants

• Landfill leachate A B B A A B

• Saltwater intrusion A B B A A B

• Soil salinity A B A B

Organic Contaminants

• Light, nonaqueous phase liquids B B B B B

Manmade Buried Objects

• Utilities B A A B

• Drums and USTs A A A A

• UXO A B A

• Abandoned wells B A

• Landfill and trench boundaries B B B B B A B A B

• Archaeological features B B B B B A A A B

*This table has been modifid by Technos, Inc. from ASTM Guidelines

“A” implies a primary choice of method

“B” implies a secondary choice or alternative method

Seismic Electrical Electromagnetics Potential Methods

ble by mapping the biodegradation products of organic compounds. These biodegradation products are acidic, producing high specific conductance values detectable with electrical and electromagnetic techniques. Directly identifying the presence of organic contaminants is difficult using surface geophysics. In cases where the contaminant cannot be detected di-rectly by surface geophysical methods, the methods can be used to assess the natural geologic and hydro-geologic conditions that are controlling groundwater flow. These controlling features may include frac-

tures, buried channels or natural geologic traps that may hold contaminants. Subtle changes in hydro-geologic heterogeneity are often the key subsurface features. Both surface geophysics and borehole geo-physical logging can be used to aid in characterizing these features. Once the contaminant flow pathways have been identified, direct sampling methods can be used to further assess conditions. In all cases, it is rec-ommended that the assessment of natural geologic and hydrologic conditions be carried out as early as possi-ble in the site characterization program. This is par-ticularly important when dealing with organics.

An objective of many site investigations is the location and mapping of inorganic or organic contami-nant plumes. Surface geophysical methods can be em-ployed in two ways to solve this problem, direct detec-tion of the contaminant or mapping features that may be controlling the flow of contaminants. Inorganics Inorganic contaminants from landfills, salt brines, acid spills and natural salt-water intrusions are detectable by electrical and electromagnetic methods because of their high values of specific conductance. Such measurements can be used to map the extent of landfill leachate or inorganic chemical spills and can provide the vertical extent of inorganic contaminant plumes. Time-series measurements can be made to map lateral and vertical movement of inorganic contaminants over time. Even subtle changes in soil salinity can be detected with these methods.

Organics Hydrocarbons and other organic chemical con-taminants are generally not detectable by geophysical methods at the typical ppm or ppb levels of regulatory concern. Furthermore, highly water-soluble organics, such as alcohol, are not detected by geophysical meth-ods even at much higher concentrations. However, indi-rect detection of organic compounds is sometimes possi-

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Detection and Mapping of Contaminant Plumes, Spills, and Leaks


Time series monitoring of landfill leachate plume using resistivity and electromagnetics

Landfill1977

Landfill1981

Landfill1985

Regional Groundwater Flow

Well FieldOn

WellFieldOff

Well FieldOn

Boundary of 60 ohm-m contour

Abandoned mine site with acidic runoff from mine tailings.

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• Verification of foundations and determination of pile lengths;

• Forensic investigation of failed structures Archeological Investigations • Inspection of historic and archeological sites; • Location of burial sites and graves.

Surface geophysical methods can be used to as-sess soil and rock properties and for the non-destructive testing of man-made structures. They are also fre-quently used for archeological investigations. Geo-physical methods can be used for geotechnical forensics in which the cause of a structure failure is investigated.

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Detection and Mapping of Landfills, Trenches, Buried Wastes, Drums, or Other Underground Structures and Utilities

Evaluation of Soil and Rock Properties and Man-Made Structures

An important objective in many environmental projects and real estate transfers, is determining if there is "something" buried at the site. On sites suspected of

containing buried septic tanks, soakage pits, French drains, underground storage tanks and utilities a variety of methods can be used to detect and map their loca-tions and boundaries. Boundaries of buried debris, trenches, landfills and disposal areas can be identified. The depth of a trench or landfill can be estimated by some measurements or as a result of modeling data. In many cases, the trenches associated with utilities can be detected and mapped because they form a permeable pathway for contaminant migration. Knowing that these type of features exist at a site and knowing their location provides a great deal of insight to any property. This information can then be used to focus excavation efforts and plan removal or guide chemical sampling to more representative loca-tions.

Evaluation of concrete pile length below existing structure.

Four large underground storage tanks found in area released for redevelopment at a Brownfield Site

Soil and Rock Properties • Corrosion measurements of soil; • Elastic properties of soil and rock; • Rippability and dredgeability of rock. Non-Destructive Testing • Road and airport runway inspection; • Concrete inspection for delamination and steel rein-

forcement location; • Voids behind tunnels, walls and beneath founda-

tions; • Dynamic response of man-made structures; • Soil piping into sewers and drainage system; • Cavities due to acid spills; • Surface subsidence due to piping and collapse; • Mapping abandoned mines and tunnels;

Page 6 SURF ACE GEOPHYSICAL ME THODS

Seismic measurements involve the measurement of seismic waves traveling through the subsurface. Stratigraphy, structure, and material properties can be assessed with seismic methods. Three of the most use-ful seismic methods for shallow applications include re-fraction, reflection, and surface wave analysis (MASW).

Seismic refraction is a method to determine the P-wave velocity structure of the subsurface (ASTM D5777-00). Seismic P-waves are generated on the sur-face, propagate through the soil and rock, and are re-corded by geophones at known distances from the source. When the seismic waves encounter interfaces separating material of different seismic velocities, the waves are refracted according to Snell’s Law. At the critical angle for each interface (energy refracted 90 de-grees), the seismic wave will travel along the interface with a velocity of the underlying layer. Since P-waves are the fastest portion of the seismic wave, they repre-sent the first arriving energy at each geophone (either direct or refracted). A seismograph is used to record the travel-times of these first arrivals, after which seismic velocities can be derived. Depths to the refracting lay-ers can also be determined. Note that the refraction method assumes that velocity of the layers increases with depth and that layers must be thick enough and have enough velocity contrast to be resolved. Applications: • Primary application for seismic refraction is for de-

termination of depth to bedrock and thickness of geologic strata, structure and identifying anomalous conditions;

• Depths can be calculated under each geophone to produce a detailed two-dimensional top of rock pro-file;

• Rippability and dredgeability can be determined from P-wave velocity measurements;

• If compressional (P-wave) and shear (S-wave) ve-locities are measured, in-situ elastic moduli of soil and rock can be determined.

Advantages: • Provides data to depths of 100 feet or more; • Resolves up to 2 or 3 layers; • Provides a 2D cross-section of P-wave velocity; • The source of seismic energy can be as simple as 8-

pound sledgehammer. Limitations: • The survey line length (source to farthest geophone)

may be 4 to 5 times the desired depth of investiga-tion;

• Sensitive to acoustic noise and vibrations; • Seismic velocity of layers must increase with depth

(will not resolve low velocity layers below high ve-locity layers);

• Will not detect thin layers; • Deep measurements may require explosives as an

energy source.

Seismic

Marine seismic refraction was used to map top of rock and rock

Commonly used energy source is a 16-lb sledge hammer.

VOLUME 1 , I SSUE 1 Page 7

Seismic reflection measures the travel time of seismic waves from the surface downward to geologic contacts where part of the seismic energy is reflected back to hydrophones at the surface. A reflection will occur from geologic strata when the reflection coeffi-cient (derived from density and seismic velocity con-trasts) between strata is sufficient. After the raw data are processed, a cross-sectional picture of subsurface strata and anomalous conditions can be developed. Applications: Primary application is for determination of depth and thickness of geologic strata, structural and anomalous conditions.

Advantages: • Provides a high resolution cross-section of soil/rock

along profile line; • Depth range as shallow as 30 feet to greater than

1,000 feet; • Both P-waves and S-waves can be measured with

the appropriate equipment. Limitations: • Slower production rate than most geophysical meth-

ods; • Requires extensive processing, and • Sensitive to acoustic noise and vibration.

Seismic (cont.)

Elastic weight drop is used for seismic data acquisition. Example of seismic reflection data showing uniform stratigraphy.

Example of marine seismic reflection perpendicular to shoreline showing reefs and sediment deposits

Reefs Areas of Dredging Reef

Sand Covered Reefs

Outermost Reef

Water Depth 40 feet

Sediments 80 ft


Seismic (cont.)

MASW data acquired with a landstreamer over asphalt.

Shear-wave cross-section developed from MASW data showing a paleocollapse feature.

ratio than more traditional SASW methods. • Measurements can also be made in water covered

areas. Limitations: • Maximum depth limited to approximately 80 feet in

typical conditions; • Lateral resolution is typically 25 to 50% of the geo-

phone spread length..

Multichannel Analysis of Surface Waves (MASW) is a geophysical method that uses the disper-sive characteristics of surface waves to determine the variation of shear wave (S-wave) velocity with depth (Park, et al., 1999). Data are acquired by measuring seismic surface waves generated by an impulsive source and received by an array of geophones. A dispersion curve is calculated from the data that shows the velocity of the surface waves as a function of frequency. A shear wave velocity profile (1-D profile of velocity as a func-tion of depth) is then modeled from the dispersion curve. The resulting shear wave profiles from multiple measurement locations along a survey line are combined and contoured into a 2-D cross-section of shear wave velocity. Shear wave velocity is a function of the elastic properties of the soil and rock and is directly related to the hardness (N-values) and stiffness of the materials. Applications: • Determining the depth and thickness of stratigraphic

layers; • Identifying low-velocity (weak) zones such as voids

and sinkholes; • Determining soil and rock elastic properties. Advantages: • Provides a cross-section of shear-wave velocity

along a profile line; • Can identify low-velocity layers below higher ve-

locity layers; • Data can be acquired with a “landstreamer” where

the geophones are towed along the ground for fast acquisition;

• Multiple geophones provide greater signal-to-noise

VOLUME 1 , I SSUE 1

tions along the line to provide information at vari-ous lateral and vertical locations beneath the array.

Electrical resistivity measurements are made by placing four electrodes in contact with the soil or rock (ASTM D6431-99). A current is caused to flow in the earth between one pair of electrodes while the voltage across the other pair of electrodes is measured. The depth of measure-ments is related to the electrode spacing. The resis-tivity measurement represents the apparent resistiv-ity averaged over a volume of the earth determined by the soil, rock, and pore fluid resistivity, along with the electrode geometry and spacing. Resistiv-ity measurements include profiling (with fixed electrode spacing, sounding (1D) by increasing electrode spacing at a fixed location and 2D and 3D imaging using multi-electrode arrays. Electrical resistivity imaging (ERI) is a technique in which many individual resistivity measurements are combined to produce a 2D or 3D resistivity cross-section of the subsurface. A linear array of electrodes is set into the ground and a com-puter system controls which two electrodes act as the potential electrodes and which two electrodes act as the current electrodes. Measurements are ac-quired at various electrode separations and posi-

Page 9

Electrical Resistivity

Electrical resistivity imaging (ERI) cross-section showing highly-weathered top of rock in a karst environment.

Multiple electrodes used along profile line

Resistivity imaging using Sting and Swift System

Applications: • Determine the depth and thickness of geologic

strata; • Detect lateral changes and locate anomalous

geologic conditions; • Measure soil resistivity for estimating metal

corrosion rates and designing grounding grids; • Map saltwater intrusion and contaminant

plumes; • Locate buried wastes (e.g. locate landfill

boundaries); • Azimuthal measurements to determine fracture

orientation; • Create a detailed 3D resistivity model using

data obtained with a grid of electrodes. Advantages: • Good vertical resolution; • Depth range of up to 200 feet; • Various electrode configurations are available

for different applications. Limitations: • Susceptible to interference from nearby metal

fences, buried pipes, cables, etc.; • Effectiveness decreases at very low resistivity

values. Traditional resistivity measurements use electrodes that are in direct contact with the ground to inject a DC current and to measure the resulting voltage difference. This method is effectively used in a wide variety of environments to provide resis-tivity soundings, profiles, and cross-sections of the subsurface. However, in areas where the surface resistivity is high and in areas where driving elec-trode stakes into the ground is not feasible (e.g. concrete, exposed rock, etc.), traditional resistivity measurements are not easily obtained. In these conditions, electromagnetic methods or ca-pacitively-coupled resistivity measurements could be a better choice.

Page 10

Electrical Resistivity (cont.)


Marine resistivity cross-section showing weathered zone

Marine resistivity array towed behind boat

Weathered Zone

• Data acquired continuously by pulling the array along a survey line.

Limitations: • Works in areas of high resistivity only; • Typical maximum depth of 30 feet.

Capacitively-coupled resistivity measure-ments use the capacitance of an antenna to non-invasively couple an AC signal into the ground. Ohm-Mapper, manufactured by Geometrics, Inc., is a ca-pacitively-coupled resistivity system that is configured with coaxial cable in a dipole-dipole array. The conduc-tors in the cable act as one plate of the capacitor and the earth acts as the other plate, with the insulating sheath as the capacitor’s insulator. Since an AC signal can pass between the plates of a capacitor, the transmitter on the OhmMapper system sends an AC signal (16.5 kHz) into the ground using the coaxial cable. The receiver meas-ures the AC voltage at the transmitter frequency. This provides an AC equivalent to traditional DC resistivity measurements (Geometrics, 1999). Applications: • Determine the depth and thickness of geologic

strata; • Detect lateral changes and locate anomalous geo-

logic conditions. Advantages: • Used without electrodes;

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• Produce 2-D cross-sections to detect lateral subsur-face changes;

• Locate buried wastes (e.g. locate landfill bounda-ries).

Advantages: • Measurements can be made to depths of 200 feet or

more; • Responds to certain types of non-metallic debris. Limitations: • Highly sensitive to external electrical noise; • Susceptible to interference from nearby metal

fences, buried pipes, cables, etc.

Induced Polarization IP measurements are made by momentarily injecting a DC current into the ground and measuring the resulting voltage decay at the surface between electrodes. The depth of measurement is related to electrode spacing. Time domain IP meas-ures the “chargeability” of the subsurface material, which is primarily a function of the presence of clay minerals and certain disseminated metals. Applications: • IP is used most commonly for mineral exploration,

but there is a growing number of applications for engineering and environmental problems;

Advantages: • Measurements are relatively easy to make; • Time series measurements can be made to monitor

any changes in seepage. Limitations: • Requires intrusive contact with the ground; • Nonpolarizing electrodes must be used; • Susceptible to interference from cathodic protec-

tion, natural Earth currents, and cultural features.

Spontaneous potential (SP), also known as self-potential, measures the natural voltage at the ground surface between two electrodes. Natural voltage differ-ences are caused by geochemical effects or by the mo-tion of ions (e.g. water flow). Applications: • Assessing seepage from dams; • Locating fracture flow; • Locating caves and recharge zones (i.e. sinkholes,

etc.).

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Electrical Resistivity (cont.)

Capacitively-coupled resistivity measurements (OhmMapper) used in highly-resistive conditions


to detecting and mapping contaminant plumes; • Electromagnetic measurements can be used for

mapping buried wastes, metal drums, tanks, and metal utilities;

• Can be used for azimuthal measurements to deter-mine fracture orientation.

Advantages: • Measurements are relatively easy to make; • Electromagnetic measurements provide excellent

lateral resolution with profiling; • Does not require ground contact; • Provides measurements with depths ranging from a

few feet to 200 feet; • Continuous data may be acquired to depths of 50

feet with hand-carried or vehicle-mounted equip-ment.

Limitations: • Limited vertical resolution; • Susceptible to interference from nearby metal pipes,

cables, fences, vehicles and induced noise from power lines;

• Effectiveness of electromagnetic measurements de-creases at very low conductivities (use resistivity).

Electromagnetic methods provide a means to measure subsurface electrical conductivity and to iden-tify subsurface metal objects. Electrical conductivity is a function of soil and rock type, porosity and permeabil-ity, as well as the composition of fluids that fill the pore spaces (McNeill, 1980 and McNeill and Bosnar, 1986). Conductivity values may be related to groundwater properties such as specific conductance or total dis-solved solids. Electrical conductivity values are given in units of milliSiemens/meter (mS/m). Electromagnetic conductivity values are not necessarily diagnostic in themselves, rather, it is the spatial variations in conduc-tivity values that are significant. Frequency domain electromagnetic methods measure the electrical conductivity of soil and rock by measuring the magnitude and phase of an induced elec-tromagnetic current (ASTM D6639-01). The EM31, EM34, and EM38 manufactured by Geonics, Ltd. are three common frequency domain electromagnetic in-struments. These instruments provide bulk measure-ments of apparent conductivity values integrated over a volume of the subsurface to a maximum depth as shown in the following table. They can also be used as a metal detector to locate ferrous and non-ferrous metallic ob-jects. Applications: • Frequency domain electromagnetic measurements

are primarily used for profiling to detect and map lateral changes in natural geologic and hydro-geologic conditions;

• Electromagnetic measurements are also applicable

Electromagnetics

EM31 system being hand carried to acquire data

Instrument Coil Spac-ing meters (vertical dipole)

Maximum Depth of

Measurement 70% response

Depth of Maximum Response

EM38 1.0 1.5 m (5 ft) 0.4 m (1.3 ft) EM31 3.7 6 m (20 ft) 1.5 m (5 ft) EM34 10 15 m (50 ft) 4 m (13 ft) EM34 20 30 m (100 ft) 8 m (26 ft) EM34 40 60 m (200 ft) 16 m (52 ft)

Approximate depth of bulk conductivity measurement from the Geonics, Ltd. instruments

VOLUME 1 , ISSUE 1 Page 13

exploration. Advantages: • Good lateral and vertical resolution; • Depth range of approximately 20 to 3,000 feet; Limitations: • Deeper measurements require a large transmitter

coil (1,000 x 1,000 feet or more) for which space may not be readily available;

• Susceptible to interference from nearby metal pipes, cables, fences, vehicles and induced noise from power lines;

• Effectiveness of electromagnetic measurements de-creases at very low conductivities (use resistivity).

Time domain electromagnetic (TDEM) meth-ods measure the electrical conductivity of soil and rock by inducing pulsating currents in the ground with a transmitter coil and monitoring the decay of the induced current over time with a separate receiver coil (ASTM D6820-02). The EM47 and EM57 systems manufac-tured by Geonics, Ltd. are common TDEM systems. Applications: • TDEM measurements are primarily used to deter-

mine depth and thickness of natural geologic and hydrologic conditions;

• TDEM measurements can also be applied to detec-tion and mapping of landfill leachate plumes, seep-age from brine pits, saltwater intrusion and mineral

Electromagnetics (continued)

EM31 conductivity contour of an inorganic contaminant plume

EM34 system mounted with a fixed coil spacing of 10 m

Example of TDEM measurement using 10 meter square loop and centered receiver.

Cross-section created using line of TDEM measurements across barrier island.


• Can provide relatively deep measurements (hundred of feet).

Limitations: • Station measurements only; • Does not function well in horizontal layers of soil

and rock with very few fractures or if soil is electri-cally conductive (e.g. clay-rich soils or water cov-ered areas);

• Axis of fracture must be oriented approximately to-ward the VLF transmitter;

• Susceptible to interference from nearby metal pipes, cables, fences, vehicles, induced noise from power lines, and loss of signal from the VLF transmitter.

Very Low Frequency (VLF) measurements are made by measuring the distortions of a VLF wave from a distant transmitter. Distortions of the VLF wave occur due to a local increase in electrical conductivity usually found within fractures. The increase in electri-cal conductivity is a function of the conductive material, such as water, clay or minerals within the fracture. Applications: • VLF measurements are primarily used for location

and mapping of near vertical contacts, fractures and faults containing water, clay, or minerals.

Advantages: • Measurements are easily and rapidly made;

Advantages: • Measurements are relatively easy to make; • Does not require intrusive ground contact; • Provides better spatial definition than magnetics; • Can be hand-carried or vehicle-mounted. Limitations: • Susceptible to interference from nearby fences,

pipes, cables, vehicles, buildings, etc.; • Response to metal decreases rapidly as depth of in-

vestigation increases.

Metal detector measurements respond to the presence of buried ferrous and non-ferrous metals. A metal detector response is a function of the area of the metal object and its depth. Frequency domain and time domain electromagnetic metal detectors are available for a variety of applications. Applications: • Detect the presence of buried metal containers,

drums and tanks; • Trace metallic utilities (pipe and cable locators); • Map unexploded ordinance (UXO).

Electromagnetics (cont.)

Metal Detector

Mapping buried debris and old building foundations at a low-level radioactive waste site using Geonics Ltd. EM61 metal detector.

Mapping utility lines using a pipe and cable locator.

VOLUME 1 , ISSUE 1 Page 15

There are two primary applications for magnetic measurements: (1) locating and mapping buried ferrous metals, and (2) mapping geologic structures. Location and Mapping of Buried Ferrous Metals Magnetic measurements can be used for locat-ing and mapping buried ferrous metals (e.g. waste, drums or underground structures and utilities). The presence of buried ferrous metals creates a local varia-tion in the strength of the earth's magnetic field, permit-ting the detection and mapping of buried ferrous metal. Total field measurements (made with one magnetome-ter) and gradient measurements (made with two magne-tometers) are commonly used. Magnetic gradient meas-urements are made by a gradiometer, which is simply two magnetic sensors separated by a constant offset. Magnetic gradient measurements are often used to en-hance the anomalies produced by shallow buried ob-jects.

• Carried by hand or vehicle-mounted. Limitations: • Susceptible to interference from steel pipes, fences,

vehicles and buildings; • Total field measurements susceptible to natural fluc-

tuations in Earth's magnetic field (a base station must be used to remove natural fluctuations in Earth's field).

Mapping Geologic Structures Magnetic measurements can be used for geo-logic mapping by responding to the magnetic suscepti-bility of soil and rock. Generally total field measure-ments are used for geologic mapping. The primary ap-plication is for mineral exploration and for characteriz-ing geologic structure such as faults. Advantages: • Measurements are relatively easy to make; • Does not require intrusive ground contact;

Magnetics

Gradient magnetic measurements with system mounted on ATV to cover large areas.

Magnetic contour map over municipal landfill showing areas of ferrous targets.

Advantages: • Measurements are relatively easy to make; • Does not require intrusive ground contact; • Detects ferrous metal (iron or steel) only; • Carried by hand or vehicle-mounted; • Gradient measurements can detect a single 55-

gallon drum to a depth of about 8 feet (total field measurements about 18 feet).

Limitations: • Gradient measurements are less sensitive to

deeper objects than total field measurements; • Magnetic measurements are susceptible to inter-

ference from steel pipes, fences, vehicles and buildings.


A microgravity survey provides a measure of change in subsurface density (ASTM D6430-99). Natu-ral variations in subsurface density include lateral changes in soil or rock density, buried channels, large fractures, faults, dissolution-enlarged joints and cavities. The microgravity method comes closest of all the geo-physical methods for allowing a positive statement to be made regarding the presence or absence of subsurface cavities at a site (Butler, 1984). A microgravity survey consists of making sensitive gravity measurements at the microGal (µGal) level (1/1000 of a milliGal or 10-9 of the Earth’s gravitational field) with a gravimeter. Gravity measurements are acquired at discrete points along a profile line or within a grid, and are corrected for instrument drift, tidal effects, elevation changes, and latitude. Gravity anomalies are directly related to lateral variations in subsurface density. Applications: • Identify karst features (caves, voids, sinkholes,

weathered zones); • Map top-of-rock; • Map erosional features (buried channels); • Characterize stratigraphic thickness and structure; • Identify man-made structures (tunnels and mines). Advantages: • Provides a means to characterize conditions in geo-

logic and cultural environments, where other geo-physical methods may fail;

• Data can be interpreted to provide estimates of depth, size, and the nature of the anomaly;

• Can be used inside buildings and structures. Limitations: • Data acquisition is relatively slow compared with

other geophysical methods; • Irregular topography will produce artifacts in the

data unless accounted for in the processing; • Local sources of vibrations, wind, storms, and dis-

tant earthquakes can produce interference.

Microgravity

Microgravity measurements acquired with a Scintrex gravimeter.

Microgravity anomaly over a filled collapse area at an abandoned mine site.

Microgravity contour map identifying low gravity anomalies related to paleocollapse features.


Ground penetrating radar (GPR) uses high fre-quency electromagnetic waves to acquire subsurface in-formation (ASTM D6432-99). Energy is radiated down-ward into the ground from a transmitter and is reflected back to a receiving antenna. The reflected signals are recorded and produce a continuous cross-sectional "picture" or profile of shallow subsurface conditions. Reflections of the radar wave occur where there is a change in the dielectric constant or electrical conductiv-ity between two materials. Changes in conductivity and in dielectric properties are associated with natural hy-drogeologic conditions such as bedding, cementation, moisture, clay content, voids, and fractures. Large changes in dielectric properties often exist between geo-logic materials and manmade structures such as buried utilities or tanks.

and vertical resolution; • Continuous profile measurements are effective for

larger surveys; • Output is a continuous graphic picture-like display; • The antenna may be pulled by hand or vehicle; • Penetration is commonly less than 30 feet but can be

greater than 100 feet in materials having very low conductivity (e.g. freshwater ice).

Limitations: • Depth of penetration is site specific; • Penetration in silts and clays and in materials having

conductivity values above 15 to 20 mS/m is limited.

Applications: • Ground penetrating radar is primarily used to obtain

a high resolution cross-section of natural geologic, hydrogeologic, and anomalous conditions;

• Ground penetrating radar is also used for location and evaluation of man-made structures, including location of utilities, rebar in concrete non-destructive testing of man-made structures;

• Ground penetrating radar has some applications in detecting and mapping contaminant plumes, buried wastes and drums.

Advantages: • Measurements are relatively easy to make; • Ground penetrating radar provides very high lateral

Ground Penetrating Radar

Radar used to map the lateral and vertical extent of fill at a construction site.

Radar data acquired with a100 MHz antenna towed by an ATV.


Ground Penetrating Radar (cont.)

Ground penetrating radar used to map variable clay layer controlling DNAPL flow

Ground penetrating radar used to map void space behind concrete tunnel lining

Ground penetrating radar data acquired using low fre-quency antenna and station

measurements

Void

Rock

Back of Concrete

Depth in Inches

0

12

30

Rebar

0

28

14

42

56

70

Appr

oxim

ate D

epth

in Fe

et

Distance Approximately 220 feet


Some Considerations for Selection and Successful Application of Surface Geophysical Methods

Numerous geophysical application guidelines have been developed by ASTM, as well as federal and state agen-cies. For example, ASTM D6429-99 covers the applications of surface geophysical methods and ASTM D5753-95 covers the borehole methods. Geophysicists have presented a num-ber of papers on selecting appropriate geophysical methods. These papers provide an excellent overview of the state-of-practice geophysical methods available for geologic site char-acterizations. These guidelines and papers are intended for initial planning purposes by personnel not experienced with geophysical methods. However, all to often, they are used as a means of final selection of methods without the benefit of experienced counsel. All guidelines are inherently generalized and cannot possibly consider all of the variables involved in planning a site specific site investigation program. As a result, unique conditions are not considered and may tend to eliminate a method from consideration. For example, ground penetrating radar is well known for its poor performance (limited penetra-tion) in clays. Using radar in a clayey environment is not rec-ommended in the guidelines. Yet it was the method of choice for successfully finding fractures in the runways of the dry clay-covered lakebed of Edwards Air Force Base in Califor-nia (Kratochvil, et al, 1972). Anderson and Ismail (2003), Benson and Yuhr (2002), and Olhoeft (2003) have outlined some of the key steps and considerations for proper selection of the geophysi-cal methods. The following is a brief summary from these papers:

• The first step is to define the problem • What is the area of interest? • What data are presently available? • What are the physical properties of interest and

which geophysical methods will respond to the physical properties of interest?

• What is the depth of interest of the target?

• What is the geometry of the target? What is the re-quired site coverage, spatial sampling and resolution to detect the target and which techniques can provide the desired results?

• What are the site-specific constraints and which geo-

physical tools can perform well under these condi-tions?

• Which techniques can provide complementary data? • What other data (geophysical or non-geophysical)

are required to interpret and or constrain the interpre-tation of acquired geophysical data and when will it be obtained?

• What are the expected results? • What are the likely limitations? • Who will make the decision of the methods to be

used, plan the survey, make the measurements, ana-lyze the data integrate the geophysical data with other data to arrive at a realistic conceptual geologic model and write the report?

• Will the overall program likely be technically suc-cessful and cost-effective?

Note that approximately half of these issues deal with non- geophysical aspects which are as critical to the overall success of the program as are the selection and proper application of the method(s).

Response of a bulk measurement with respect to depth of target

Various survey geometries

A. Uniform Grid of Station

Measurements

B. High density station

measurements along profile

lines

C. Continuous measurements

along profile lines

D. Continuous measurements where access

permits

A. Planar Surface B. Linear Feature C. Discrete Isolated Feature

Example of different target geometries

American Society for Testing and Materials, 2001. Standard guide for using the frequency domain electromagnetic method for subsurface investigation, ASTM D6639-01, Consho-hocken, Pennsylvania.

American Society for Testing and Materials, 2000. Standard guide for using the seismic refraction method for subsurface investigation, ASTM D5777-00, Conshohocken, Pennsyl-vania.

American Society for Testing and Materials, 1999. Standard guide for using the direct current resistivity method for subsur-face investigation, ASTM D6431-99, Conshohocken, Pennsylvania.

American Society for Testing and Materials, 1999. Standard guide for using the gravity method for subsurface investigation, ASTM D6430-99, Conshohocken, Pennsylvania.

American Society for Testing and Materials, 1999. Standard Guide for Selecting Surface Geophysical Methods, ASTM D6429-99, West Conshohocken, Pennsylvania.

Anderson, N. and Ismail, A., 2003. A generalized protocol for se-lecting appropriate geophysical techniques. Geophysical Technologies for Detecting Underground Coal Mine Voids Forum, July 28-30, 2003 – Lexington, Kentucky.

Benson, R. C. and Yuhr, L.; 2002. Site characterization strategies: old and new. Geophysics 2002, Second Annual Confer-ence on the Application of Geophysical and NDT Method-ologies to Transportation Facilities, Federal Highway Ad-ministration, April 15-19, Los Angeles, California.

Technos, Inc. 10430 NW 31st Terrace Miami, Florida 33172

References

Phone: 305-718-9594 Fax: 305-718-9621 Email: [email protected] Website: www.technos-inc.com

Specialists in Geologic Site Characterization

Benson, R.C.; Yuhr, L. and Kaufmann, R., 2003. Some considera-tions for selection and successful application of geo-physical methods. Third International Conference on Applied Geophysics, Geophysics 2003, December 8-12, Orlando, Florida.

Butler, D. K., 1984. Microgravimetric techniques for detection and delineation of subsurface cavities, Proceedings of the National Symposium and Workshops on Abandoned Mine Land Reclamation, Bismark, North Dakota, pp 63-100.

Geometrics, Inc., 1999. OhmMapper TR1 Operation Manual, San Jose, CA, 116p.

Kratochvil, G.; Benson, G.; Fenner, T., 1992. Fishing for Fissures. The Military Engineer, Jan-Feb, Vol. 84 (547).

McNeill, J. D., 1980. Electromagnetic terrain conductivity meas-urement at low induction numbers: Geonics Limited, Technical Note TN-6.

McNeill, J. D., and Bosnar, M., 1986. Surface and Borehole Elec-tromagnetic Groundwater Contamination Surveys, Pit-man Lateral Transect, Nevada, U.S.A, Technical Note TN-22. Geonics Limited, Ontario, Canada. 4 pp.

Olhoeft, G. R., 2003. Quality control in geophysics. Geophysical Technologies for Detecting Underground Coal Mine Voids Forum, July 28-30, 2003 – Lexington, Kentucky.

Park, C.B, Miller, R.D., and Xia, J., 1999. Multichannel analysis of surface waves, Geophysics, Vol. 64, P. 800-808.

Documents

Surface Geophysical methods