IMPACT · Shallow refraction seismics utilizes refraction of the seismic wave on interface between the more slowly and faster environment (e.g. material of embankment against original

$Page 1: IMPACT · Shallow refraction seismics utilizes refraction of the seismic wave on interface between the more slowly and faster environment (e.g. material of embankment against original$
IMPACT

(Investigation of Extreme Flood Processes and Uncertainty)

Deliverable D6.3

Report on the

Implementation of monitoring programme on WP2 test

embankments. Summary and analysis of data. Interpretation against extreme event processes

GEO Group a.s.

(Partner No. 10)

EC CONTRACT NUMBER EVG1-CT2001-00037

IMPACT, Deliverable D6.3 Geophysical Investigation of Flood Dikes, Partner No.10

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Contents

1. INTRODUCTION 3 2. 1-st PHASE OF THE MONITORING PROGEAMME 4 3. LIST OF PICTURES AND ATTACHEMENTS 16 4. 1-st PHASE OF THE MONITORING PROGEAMME 18


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I. Introduction

Originally in the IMPACT DoW, the works on the D6.2 output had to be complemented, within the D6.3, with implementation of monitoring programme on the WP2 test embankments in Norway. On basis of changes in DoW, these very expensive works were discussed at the Workshop in Prague (December 2002) and modified after the 3rd Management report, with the work focusing on embankments within the Czech Republic. As a substitute variant within the Czech Republic, the following possibilities of pilot testing localities were considered: - section of the dam embankment in a part of Odra river catchment, different than the locality used for monitoring works within D 6.2 - embankment in the catchment of the river Morava (in the Morava river catchment, the catastrophic floods came in the year 1997) - embankment of the Velký Bělčický pond damaged during the floods in August 2002. Finally, after discussion with representatives of the catchment and owner of the pond and evaluation of all conditions, advantages, and disadvantages, the pilot site of Velký Bělčický pond was selected for implementation of the monitoring programme. Velký Bělčický pond (artificial lake) is situated near the village Belčice in the Central Bohemian district, in the close proximity of Metelský pond. Embankment of the Velký Bělčický pond was breached and damaged in August 2002, during the catastrophic floods. Reconstruction of the embankment was finished on August 20, 2003 and approved in the following week. Company GEO performed then monitoring of the existing embankment. State of the embankment was observed and documented in 2 phases. 1-st phase was performed in September 2003 after the repair of embankment but before beginning of water filling-up of the reservoir (at water level 0 m) 2-nd phase was performed in April 2004 (the evaluation under process, will be ready in the June 2004) Velký Bělčický pond pilot site characterization: Velky Belcicky pond is located approximately 7 km North-East of Lnáře town. Embankment was reconstructed after the heavy rain-storm, which came during catastrophic floods in August 2002. This rain-storm was finally 35 m wide, and it ditched an erosion crater of the size 40 x 30 x 6 m. During its repair, quarrying and overall reconstruction of embankment in its approximately 90 m long section was performed. Within this section, the lower discharge was shifted, too. In the place of erosion crater on downstream side of embankment, construction was strengthened by massive stabilizing fill. Preserved parts of embankment were considerably strengthened. Upstream slope of the embankment (with declination 1 : 3) was filled up and modified. Original upstream slope was strongly damaged by erosion, at some places it had declination up to 1 : 1 and steeper. Height of embankment was raised by approximately 0.5 m, and path on its crest was strengthened by aggregate. These final modifications of the embankment crest, however, were not finished during the 1st phase of measurement.


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1-st fase of the monitoring programme: Performed geophysical research pursues two following basic targets:

1. Description of embankment state after the repair with focus on evaluation of material homogeneity and compaction in the repaired part and on searching for eventual weakened and washed places in original parts of embankment.

2. Testing electromagnetic methods and, at the same time, testing of frequency methods (CM method) for measurement of conductivity. Just CM method should serve to the fast nondestructive monitoring of state of earth embankments and fast orientation during selection of weakened places within the monitored embankment.

Results: Methodics for measurement

State of embankment, i.e. material homogeneity and compaction in the repaired part of embankment, and searching for more permeable, washed, and broken sections in original parts of embankment, was considered on basis of combination of following geophysical methods:

- electromagnetic measurement of conductivity frequency method (CM); - method of very long waves (VLW); - method of spontaneous polarization (SP); - multielectrode resistivity method (MEM); - microgravimetry; - shallow refraction seismics (SRS).

First 4 methods (CM, VLW, SP and MEM) belong among geoelectric methods. These methods detect changes in electric properties of searched environment, mainly electric resistivity or conductivity. On basis of detected resistivities, borders of individual layers in environment may be interpreted, and their properties evaluated, as e.g. variations of clay and sand fraction content, (i.e. of relative permeability of environment). Tested method CM utilizes induction of electromagnetic field from transmitting dipole to the detecting dipole. Intensity of induced field depends on conductivity (resistivity) of surrounding environment, in our case body of the embankment. Great advantage of this method is its high productivity. Measurement is performed just by one operator. Reading of conductivity (resistivity) in one point takes approximately 1 second (at increased preciseness of measurement - approximately 3 s). It enables to measure up to 10 km of earth embankments per day, without greater problems. Productivity of measurement may be increased even more in case, when for navigation of measurement we use GPS system synchronized with measurement of conductivity. When using various frequencies of primary electromagnetic field and various distances between transmitting and receiving dipole, we can reach various depths of reaches of measurement. There is an advantage that after analysis of results from various depth levels, it can be evaluated, if there is anomaly of conductivity (e.g. seepage or porous section) in the embankment body or on the earth plane. Currently, however, it requires mostly measurement by various types of apparatuses (in our case CM-31 with depth reach of approximately 4 - 6 m, CM-32


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with depth reach of approximately 2 3 m, and CM-138 with the reach of approximately 1 1.5 m). Change of the depth reach of CM apparatus may also be obtained by rotation of horizontal level of dipoles (coils) of apparatus by 90 degrees. After the rotation, the stated depth reach of apparatus will decrease to approximately one half. We have utilized this fact during the testing measurements, because apparatus with smallest depth reach (CM-138) was not available. Measurement for depth level of around 1 m we performed by apparatus CM-032 with rotated dipoles. Step of measurement by the method CM was on profiles during the 1st phase of testing measurement selected 1 m. Another geoelectric method, VLW method, serves to searching for tectonic failures of rock environment in the bed of embankment. Knowledge of tectonic breaks in bedrock of embankment is important because such failure is often permeable and serves to communication of bottom water under the embankment. At elevated water level, strengthening of seepages under the embankment and its consequent failures may occur. This method utilizes deformations of long-wave electromagnetic field from the distant transmitter (during the measurement, transmitters for navigation of submarines were used) in the place of thin conductor (tectonic water-bearing failure). In the conductor, secondary field is induced, which deforms homogeneous field of distant transmitters. Measured are parameters of deformation of the resulting field. Measured profile was situated in the flooded area, approximately 30 m from the embankment, measurement was performed with the step 10 m. The used apparatus was Geonics EM16-R. Method SP watches natural electric field of rock environment, which is most often generated by filtration of bottom water through the porous environment (e.g. seepages through the embankment). It is, however, necessary to differ between this natural electric field and perturbing influences of the field induced on iron underground constructions or influences of industry. Measurement by the method SP was performed in two-electrode potential variant (difference between voltage in the measured points and fixed base) and by non-polarizable electrodes. Step of measurement was 2.5 m. Method SP was measured by apparatuses GEOTER I and II. Method MEM consists of measurement of specific rock resistivities by high amount of electrodes interconnected by special cable. Cable enables measurement in various variants of 4-electrode arrangement, where individual electrodes are gradually used as current and potential electrodes. Measurement runs automatically, everything is controlled by PC. During the measurement, automatic geoelectric system ARS-200E was used; total length of cable was 62 m with 32 connected electrodes. By gradual shifting of 8-electrode section of cable from the beginning to the end of measured section (so called swapping). Marked profiles were gradually measured in the whole length. Depth reach of the method is under the given conditions fluctuating around 12 m (except on edges of profiles, where it gradually decreases). Measured data were finally transferred by interpretation software into the resistivity 2D model cross section in the place of the profile. Results of measurement by the method MEM serve further for checking of the tested method CM.


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Shallow refraction seismics utilizes refraction of the seismic wave on interface between the more slowly and faster environment (e.g. material of embankment against original cover /deluvium, eluvium/, or cover against base rocks). Depth of refraction interface may be determined by interpretation of seismic data. On the seismic coverage (measured part of the profile), 24 geophones were connected, with spacing 2.5 m. Within one coverage, 7 points of shooting were measured. Seismic energy was excited by the hammer. Seismic data were processed by the method t0. Interpreted seismic velocities for the embankment serve to determination of material density, eventually other geotechnical properties. Above the framework of required measurement, seismic coverages were shot from the side, from the downstream toe of embankment. In further phase of research, these measurements will be processed by means of seismic tomography. Tomographic method for processing seismic data utilizes penetration of seismic signal through the environment between the point of impulse and geophones. On basis of combination of various places of points of explosion, model of velocities of environment between the line of geophones and points of explosions may be calculated by the special software. On selected typical places of embankment, data were further measured for frequency analysis of seismic records MASW (Multi-channel Analysis of Surface Wave) with utilization of vibration source. Such measurement serves to precising of calculation of geotechnical properties of embankment material. All seismic measurements were performed by the apparatus OYO McSeis 1600 (Japan). Last used method is microgravimetric method. It is sensitive to changes of volume densities in the place of measurement. Thus, it can directly detect eventual larger cavities or washed sections of embankment and areas of increased porosity. All presented phenomena are manifested by certain dilution of environment, which is manifested in gravitational field as local minimum. Changes in the volume densities of environment may be quantified by gravimetric modeling. Gravimetry was measured by automatic gravimeter SCINTREX CG-3M (Canada). For leveling, apparatus SOKKIA C4.1 (Japan) was used. Selected step of measurement was 2 to 4 m. Geophysical measurement on embankment of Velký Bělčický pond was performed on basic profile in axis of the embankment (profile P1). Yardage of axial profile relates to embankment localization in kilometer spacing, according to project documentation for the repair. Measured section was limited by yardage 22 to yardage 308 m. As mentioned above, method VLW was measured on auxiliary parallel profile in the flood area (profile P3 on the picture 1). Another auxiliary profile (P2) is located on the toe of embankment downstream side. It was measured by the method SP and it served to the side seismic shootings. Besides the profiles, parallel with longitudinal axis of embankment, also the cross profiles were marked on 4 characteristic places, selected on basis of preliminary interpretation of data from the profile P1. These profiles are K90, K130, K182 and K212. In average, cross profiles are 31 m long (16 m directed to upstream side of embankment and 15 m on downstream side of embankment). All cross profiles were measured by the method CM, profiles K130 and K182 were measured by the method MEM. Scheme of all profiles is illustrated on the picture 1.


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Results of measurement 1-st stage of the monitoring programme Results of method CM are presented in the form of graphs of virtual specific resistivities for the measured profiles (see pictures 2a and 2b). Besides the curves measured by the method CM, graphs include, for the checking purposes, curve of virtual specific resistivities for relevant depth level, received by the method MEM. Performed measurement serves as initial data file, which documents resistivity circumstances in the embankment before water filling-up. More detailed analysis of measurement results obtained by the method CM will be performed after further phase of measurement, as it was stated in the Introduction. All data are archived and deposited in the company GEO Group Inc., which is responsible for the field measurements and their evaluation. On the picture 3, graphs of real and imaginary component of VLW are presented. Measurement was performed by two transmitting stations, GBR and ICV. Station GBR is sensitive mainly to NW-SE failures, station ICV shows failures mainly in the direction NS (station GBR was used only orientationally, direction of the station contains with direction of the profile unfavorable angel of approximately 35 degrees.). In the graphs, red quadrate marks the places of characteristic decreases of the Re component of VLW, which localizes thin conductor (tectonic failure). Measurement has proved that geological base of embankment suffers by numerous failures, of which, the most significant ones are located on the yardages 190 and 130 m (in the place of profile P3). Method SP (see the picture 4) is utilized for localization of places of water seepages through the embankment or through its base. Such place should show relative minimum SP on upstream side compared with downstream side. In our case, when during the 1st phase of measurement there was no water in the reservoir and thus no seepages could occur, the performed SP measurement documents initial state of natural electric potentials in the embankment. Results of measurement will be used for comparison with other phases of measurement. Noteworthy is bright, one-point anomaly on the profile P2 on yardage 152 m. It is probably manifestation of connection (keying) of repaired part of embankment to the original embankment. Apparently, at this place equalization of electric potentials between both materials did not occur, yet. Measured anomaly should gradually decrease up to the complete disappearance. Pictures 5a and 5b show resistivity cross sections obtained from interpretation of multielectrode resistivity measurements MEM. These resistivity models offer the most precise information on state of embankment. Generally, it holds that decreased resistivities are limiting fine-grained, less permeable materials, while increased resistivities correspond mostly with sandy to gravelly materials with increased permeability and porosity. In the model for the profile P1, there is a well visible section of repaired rupture (60 to 150 m), where more clayish, i.e. more conducting, material was used. In original parts of embankment, two resistivity layers are apparent. From the surface to the depth of approximately 3 to 4 m, relatively non-homogeneous layer with increased resistivity is apparent (numerous places with values above 300 ohm/m). Sections with anomalously increased resistivities are especially on yardages 40 to 60 m (near the small house), around yardages 164, 182, 200. 222 m, and further on the whole end of embankment from the yardage 235 m to the bridge of overflow. Here in original embankment, it is necessary to count


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with occurrence of material with increased permeability. In the bed of this near-surface layer, a layer of decreased resistivities is apparent, in the cross section. That is probably manifestation of more clayish core of embankment (partially also of shallow argillaceous bedrock) in combination with its residual moisture. Nor side influence of filled-up, more clayish (more conducting) material on the upstream side of embankment can be excluded. In the cross section, two bright anomalies with extremely increased resistivities are further apparent. It is manifestation of construction of lower outlet (yardage 92 m) and bridge of overflow (yardage 282 to 294 m). Two local conductive structures near surface of embankment on the yardages 213 and 230 m are localizing -with high probability- an old technological constructions (troughs, piping). Structure of embankment in the cross section is apparent in resistivity cross section on the picture 5b. Perpendicular cross section K130 is located in completely repaired part. Here, usage of homogeneous material with resistivity around 70 ohm/m with dried thin layer near the surface is apparent. Cross section on yardage 182 m documents original embankment with widening and filled-up material on its upstream side. It is obvious that filling-up was done again by more clayish material with resistivity around 70 ohm/m. Original embankment with extremely steep upstream side shows material with resistivity of around 500 ohm/m. Layer with extremely high resistivities on the downstream side is probably caused by overdrying of material (nor unfavorable influence of topoefect can be excluded). Results of microgravimetry are presented in a form of density model of embankment body (picture 6). In upper part of picture, graph of measured gravimetric values, corrected by some perturbing influences, is shown. This corrected value is called Bouguer anomaly. With simplification we can say that Bouguer anomaly corresponds with earth gravity corrected by tidal forces of Sun and Moon, by influence of Earth rotation, by influence of altitude above sea level and so called Standard Masses. Bouguer anomaly therefore represents gravitational impact of unknown bodies inside the Earth with anomalous specific volume density. Method is sensitive especially to the close, shallow anomalous bodies (in our case to density of non-homogeneities in the embankment), because gravitational potential of anomalous bodies decreases with square of distance. Interpretation of values of Bouguer anomaly and calculation of density model are based on few following basic principles: A) Minimum on Bouguer anomaly graph represents deficit of mass in place of measurement, while maximum represents excess of mass in comparison with standard undisturbed environment. B) For the starting density model, all accessible information is used, especially results of geoelectric measurement and seismics (non-homogeneities in the embankment, extent of its cover, tectonic break of bedrock, etc.) C) Final shape of anomalous bodies (i.e. length and depth to which they reach) and their mean volume density is given by optimization of agreement between measured data and impact of the model. Average volume density of embankment material was determined on the value 1,900 kg/m3 (its absolute value is only derived from the total impact of embankment and from our experiences). For evaluation of material homogeneity and compaction,


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essential are relative differences between densities in anomalous bodies in the embankment and above mentioned average value. In the embankment body, numerous local anomalies are apparent with relative deficit of mass (with difference between local and average density) of -40 to -170 kg/m3. Anomalies correspond mostly with porous (gravelly) materials or materials with decreased consolidation. The most bright anomalies in the original parts of embankment were recorded in the vicinity of the bridge of overflow, at yardage 282 to 294 m (they are probably gravelly backfills of the bridge construction) and further in the sections 40 to 60 m (near the little house), around yardages 160 m, 170 to 200 m, 215 and 235 to 255 m (gravelly-sandy material). In the place of complete repair, deficit of mass was recorded on both sides of keying of the repaired part (yardages 65 to 80 and around 140 m), in the place of the filled-up erosion crater (100 to 120 m), and in the vicinity of new lower outlet (yardage 92 m). Here, it is probably manifestation of weaker intensity of compaction around piping of outlet (because of technological reasons?) or of wet to plastic consistency of material during its depositing. It holds especially for the area of filled-up erosion crater, from which pumping water was technically impossible. With regard to intensity of anomalies of up to 110 kg/m3, these failures are not essential. These sections may be in future manifested by decrease of surface with amplitude of up to approximately 15 cm. Shallow refraction seismics is processed as seismic velocity cross section (picture 7). In the seismic cross section, course of interpreted refraction interfaces is marked by ticked lines. Upper interface copies mostly extent of embankment. Deeper interface catches border between eluvium and already relatively firm rock. Background for the cross section consists of the map of isolines of seismic velocities, arranged on basis of interpreted boundary velocities and calculation of positive velocity gradient of the environment. Embankment material in completely repaired part (60 to 150 m) shows seismic velocities mostly above 500 m/s, what witnesses on sufficient material consolidation (on the level of interpreted interface). Original material has seismic velocities slightly lower (mostly around 400 m/s, only exceptionally under 300 m/s). It again confirms that original material is more sandy and less homogeneous than the material used for the repair. Bedrock of embankment (argillaceous-sandy eluvium) shows usual seismic velocities around 1,500 m/s. Firm rocks on the level of deeper seismic interface have velocities already around 3,000 m/s. At the yardages 60, 100 to 120, and 160 m signs of fractures (of tectonic breaking) were recorded.

Results of performed measurements may be summarized according to the watched targets in the following paragraphs:

1) Original material of embankment is relatively heterogeneous, in some places sandy to gravelly with increased porosity. The mostly weakened sections are located on yardages 20 to 40 m (near the small house), around 160, 175 to 205 and 235 to 282 m (near the bridge). With regard to the fact that embankment was considerably strengthened and sealed by filling-up the upstream slope, these anomalies, however, with high probability, do not represent any serious safety risk.

2) In the embankment, 4 local anomalies were recorded, which we interpret as

manifestation of artificial bodies in the embankment: yardage 92 m (new lower


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outlet), 213 and 230 m (troughs, piping), 282 to 294 m (bridge of overflow). No anomaly was recorded, which would correspond with manifestation of greater washed cavity, whose sagging could endanger stability of embankment.

3) For repair of embankment, appropriate material was used with increased

content of clay component (of specific resistivity mostly between 50 to 70 ohm/m), which is predominantly sufficiently consolidated (seismic velocities above 500 m/s). Exceptional is area around the lower outlet, at the base of embankment and filling of erosion crater in the place of rapture. Here, deficit of mass around 100 kg/m3 was recorded. It is probably manifestation of lower intensity compaction around piping of outlet and a wet to plastic consistency of material during its depositing. These failures are not, however, essential. Construction of embankment is here also strengthened by massive stabilizing fill. In the future, surface of embankment may here slightly decrease with amplitude of up to approximately 15 cm.

4) Testing measurement by CM method has confirmed that close dependence

exists between virtual specific resistivities measured by the method MEM (values of symmetric resistivity profiling SRP for comparable depth reach) and output values of CM apparatuses. It seems, however, that calibration of CM apparatuses is not fully exact. Resulting CM values therefore cannot be called resistivities in a sense of direct current geoelectric methods. It bears problems even under the mutual comparison of CM apparatuses and attempts to derive approximate depth of source of anomaly (e.g. composition of simple pseudo-conductivity cross section). After adjustment of appropriate sensitivity of CM apparatuses, however, measured curves may be interpreted identically with e.g. graphs of the method SRP.

5) Interesting is the finding that in the place of shallow artificial non-homogeneity

(bridge on yardage 282 to 294 m) anomalies measured by both methods (MEM-SRP and CM) show identical character. In case of deep resistivity non-homogeneity (lower outlet on yardage 92 m) the only responding method is MEM-SRP (especially at the most deep variant), while the method CM has no response. It witnesses on the fact that regularly used estimations of depth reaches of both methods are not fully exact, and are influenced by various parameters.

6) Sensitivity of CM methods was confirmed for presence of larger iron objects in

the close vicinity of measurements. Oscillation of CM values around yardage 160 to 180 m is caused by presence of reinforced concrete column of electric power line and passage of cars through the building site.

7) For usability of CM method for research of earth embankments, and especially

during planned repeated checking (monitoring), evaluation of stability of apparatuses is essential, as well as repeatability of measurement. Exact way of calibration and checking functions for apparatuses need to be designed. Only thus, there will be a theoretical chance to observe advancement of embankment saturation by water, and in future e.g. advancement of seepages.


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Results of the 1st stage of monitoring at Velký Bělčický rybník, during comparison of newly tested frequency method CM (apparatuses CM-31 and CM-32) plus results received during measurement by the multi-electrode method ARS-200E have showed that: close dependence exists between character of the course of values of apparent resistances received from both presented methods. Results of the frequency method after correlation with direct-current methods, however, show complexity and necessity of setting (calibration) of these apparatuses. Outputs were given for evaluation to the consultant RNDr. O. Pazdirek, independent geophysical expert with many years of experience just with these tested methods (CM and ARS-200E). This consultant has emphasized justified doubts regarding satisfactory repeated calibration of apparatuses CM-31 and CM-32, and their utilization within the project IMPACT, D6.3:

- Previously used apparatuses of CM series by the Czech producer GF INSTRUMENTS, Ltd. are not stable;

- Repeatability of measured values is insufficient; - Although the basic idea of common application of the frequency methods and

automatic geo-electric system ARS 200-E is correct and very promising, utilization of mentioned apparatuses of series CM is for reaching targets of the project (and for idea of development of new technology enabling fast and economical measurement of inhomogeneities in the linear water-management constructions = protective dikes) probably inconvenient, while even eventual modification of apparatuses does not offer good expectations for reaching the set targets;

- Similar apparatus should be recommended, but by another producer, which enables changing of frequency of EM from 330 Hz to 24 KHz, and at the same time, measurement of several frequencies simultaneously, including selection of undisturbed frequencies before commencement of works at the selected locality.

On the basis of all previous findings, Dr. Pazdirek has recommended changes in the project (respectively changes in conception of D6.3) so that outputs of the Deliverable were suitable as backgrounds for eventual construction prototype of geophysical apparatus, utilizable for measurement of inhomogeneities within the linear water-management constructions with maximal effectiveness. Recommended changes:

- To stop testing of the CM series apparatuses; - To find a new suitable apparatus, recommended by the consultant

(preliminary: apparatus GEM-2); - To rent an apparatus GEM-2 including software for 1 month with possibility of

its later purchase; - To use the apparatus GEM-2 for measurement of the 2nd stage at the Velký

Bělčický pond in April 2004; - To lend the apparatus GEM-2 to the Faculty of Science of Charles University

in Prague, department of Geology, specialization Applied geophysics, for about 14 days. Independent specialists will prepare (on the basis of data received from the 2nd stage of measurement at the Velky Belcicky pond and their own observations) an expertise on possibilities of utilization of the apparatus in the studied area, including evaluation of own quality of GEM-2. This expertise will be concluded by recommendation or rejection of


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recommendation of the purchase and inclusion of the apparatus into the proposal of new technology.

On the basis of recommendation of the consultant, the team of the company GEO Group evaluated and (with coordinator of the project) consulted renting of the apparatus GEM-2 from USA. Apparatus GEM-2 was rented for tests at the pilot localities Albertov, Nizbor, Velky Belcicky pond (and marginally also at localities of the project IMPACT in the catchment of the Odra river) in the time period April 5 April 26, 2004. The results of the GEM-2 testing on the Odra river is involved into the Deliverable D.6.2.C

Professional expertise of quality of the apparatus GEM-2 and suitability of its utilization for the fast diagnostics and monitoring of the state of protective dikes, prepared at Faculty of Science of Charles University in Prague. Authors: Doc. J. Knez, L. Zima. Apparatus GEM-2 by company Geophex Ltd. (USA) represents modern type of universal apparatus for application of the method of electro-magnetic measurement DEMP (dipole electromagnetic profiling). The main advantages of tested apparatus are as follows:

• Large frequency range 330 Hz to 48 kHz (at the supplied type it is 40 kHz); • Simultaneous measurement of up to 15 frequencies (practically utilizable:

max. 5); • Continual measurement of velocity of the walk 25 measurements per

second (with five frequencies it represents 5 measurements per second); • Apparatus GEM-2 may be used also for measurement carried on modified

transport means on the ground, as well as on the water surface; • Possibility of registration of noise within the whole frequency spectrum what

is important for selection of optimal frequencies in the given locality; • Automatic observation of the power network frequency what may be used

for passive searching for cables and pipelines; • Possibility of measurement of signal at the frequencies other than transmitted

ones, what enables observation of so called electro-magnetic smog; • Automatic transformation of measured data of real and imaginary component

at the apparent conductivity and susceptibility; • Small weight and dimensions of the apparatus.

Apparatus GEM-2 was developed specially for searching for and exact

localization of underground metal objects, as ammunition, wild dumps with metal

waste, pipelines, etc. It expects laymen operation, what causes its main

disadvantages for the standard geophysical measurement:

• Small display allowing no imaging and observation of measured data; • Only 3 control push-buttons for controlling and adjustment of measurement

process; • All data are saved into the only block (file);


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• Any change of regime of measurement and transport of data is possible only with help of a computer (notebook),

• Exact localization of measured points/profiles is possible only with connected GPS apparatus or subsequently with help of marks marking was not functioning at the supplied type;

• Imperfect and imprecise manual, some described operations at the supplied type were not functioning or were designated as not yet implemented see marking;

• Possibility of calibration is not expected outside of the production company. Within the given task, an experimental measurement was performed (with the target of confirmation of stability and repeatability of measured data) at the locality Praha 2 Albertov, in an area of grassed plot in front of Institute of pathology of FVL of Charles University in Prague, by the department of Applied geophysics of Faculty of Science of Charles University in Prague. At this locality, seasonal changes of apparent specific resistances were already monitored in the past by classical resistance profiling, as well as by multi-electrode tomography (Novak 2003); measurements obtained by the apparatus GEM-2 are confronted with their results. Other comparative measurements were performed at the locality Nizbor near Beroun, in area of the castle, where the detail geophysical survey was performed for archeological purposes, in the years 2000 2002 (Dohnal et al. 2004). Results of presented experimental and comparative measurements are presented in the Attachments 1 to 5, and discussed in the further text. Besides the above mentioned measurements, also verification measurements were performed by workers of G Impuls Praha, at two localities in an area of protective dikes, which are object of separate report. Locality Albertov a) Determination of optimal methodology for measurement with apparatus

GEM-2 At the marked, 15 m long profile, the series of 68 measurements was performed in the days of April 6 14, 2004, under the various configurations and adjustments of the apparatus. Their purpose was in verification of individual functions of apparatus and selection of optimal procedure for further testing measurements. One of the first tasks was to determine the most suitable frequencies and their number for simultaneous measurement. For selection of frequencies, the whole spectrum of noise was registered, and after its analysis, frequencies 6325 Hz, 16475 Hz, 24725 Hz and 35625 Hz were selected. As a compromise, number of 4 frequencies was selected for the current measurement. Higher number of frequencies of current measurements is possible, but at the cost of decrease of transmitted output, which is distributed among individual transmitted frequencies. The lowest frequency was originally tested in the range of hundreds and first thousands of Hz, but it was discovered that the strong interference practically prohibits measurement in the range under 5 kHz. That does not exclude, however, possibility of application of these low frequencies in distant areas with lower interfering field. In USA, this apparatus is continually set to the frequencies 1170 Hz and 3930 Hz!


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With regard to high level of interference, we have selected number of summations (averaging factor) 10, and later 15, what resulted in gaining relatively continual data. After the useless effort to regenerate function of the markers, we have proposed the only possible way of localization of measurement by push-buttons start/stop, which enable division of one measured profile into a higher number of equally long sections (e.g. 10 m), registered as separate profiles. Incompleteness and imprecision of manual of the apparatus GEM-2 has caused serious complications and delay of verification field works. b) Study of repeatability and stability of measured data After determination of optimal methodology for measurement, total of 15 repeated measurements was performed under the constant conditions and adjustment of the apparatus at the marked 15 m long profile, in the time period from April 7 to 19, 2004. Results of these measurements are shown in the Attachments 1 and 2. In the Attachment 1, the originally measured values from all 15 repeated measurements are shown. Their spread is within the range 3 - 5%, what can be considered as satisfactory with regard to the fact that imprecision of position of individual measured points is given by the way of registration of measurement provided by the apparatus GEM-2. Apparatus will bind each measured value with the position relevant to the given length of profile, obtained by the linear interpolation between the first and last point. With regard to irregularity in walking during the measurement, no equal position can be assured for intermediary measured points, except the beginning and end points of the profile. Therefore, graphs are showing statistic parameters only for the end points of the profile, whose positions were reliably fixed. More favorable results were gained via sliding of measured data by the five-points running average, as it is shown in the Attachment 2. Suitable combination of number of summations made during the own measurement and consequent filtration of measured values can smooth even the data with a lot of noise, as it was practically proved at the locality Nizbor (see the following text). Gained results confirm very good stability of the apparatus and short-term repeatability of measurement, what corresponds with parameters of the apparatuses for common geo-electrical methods. c) Comparison of resistance data by the apparatus GEM-2 with previous resistance measurements Results of multi-electrode resistance measurement (selected 4 spacings) and measurement by apparatus GEM-2 are shown in the Attachment 3. At first sight, a difference is notable between values of apparent specific resistances gained by direct-current (multi-electrode measurement) and measurement with GEM-2, which have only half values see the right scale of resistances. This difference is caused partially by influence of seasonal changes of specific resistances at the given environment (Novak 2003). In the Attachment 4, this difference is much lower, because measurement with GEM-2 is compared with average values of apparent resistances, obtained by monitoring resistance profiling in the corresponding time period of the month April. Values of resistances at lower frequencies 6325 and 16475 Hz are very similar to the values of the direct measurement, while resistances for higher frequency are acting differently they follow total increasing trend, under the approximately half values of resistances. Worth of attention is (in the detail) almost


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identical course of resistance curves with the spacing AB/2 = 1.75 m with resistances under the frequency 6325 Hz, presented in the Attachment 3. Disproportion between the direct-current and alternating-current measurements in the beginning parts of the profile up to the meter 5, is evidently caused by difference between the position of current position of the profile measurement by the GEM-2 and position of the profile from the previous time-period, because we did not succeed in renewal of exact position of the original profile. Locality Nizbor At this locality, in an area of the castle entrance gate, a detail geophysical measurement was performed for archeological survey (Dohnal et al. 2004), including the detail geo-electrical measurement. In its middle part, profile No. 36 intersects probably some buried ditch (increased resistances), what was well expressed at the depth resistance section derived from the multi-electrode tomography. Attachment 5 shows results of measurement at the 50 m long profile, where 3 spacings of resistance profiling, derived from multi-electrode measurement together with measurement apparatus GEM-2 and measurement DEMP apparatus CM-031 (Geofyzika Brno) are presented. Results of measurements by GEM-2 at frequency 16125 Hz may be well correlated with the resistance measurement; shift in the central part of the profile of 2-3 m is obviously caused by irregular move during walking through the distance of 50 m. During the measurement, markers were placed in the span of 10 m, but after the transfer of data into the computer, it was not possible to register these markers. On the basis of these negative experiences, at the consequent field measurements of reservoir dikes, a system of partial 10 m profile was used. Significant anomaly at yardage of 42.5 m is expression of green cover of water-supply service pipe. Noteworthy is oneness of values of apparent resistances in the extent of shown spacing of resistance profiling. Previously measured data at frequency 16125 Hz were strongly distorted by the noise; therefore the graph shows values, splined by 11-point running average. The own measurement was 5-times summed; therefore the individual points of measurement represent average of 55 values. At frequency 35625 Hz, significantly lower resistance values appear, again. Conductometric measurement apparatus CM-031 has a different character, which does not correspond with results of other methods. This comparison gives clear preference to the apparatus GEM-2.

Total evaluation

Performed experimental measurement at locality Albertov and comparative

measurement at locality Nizbor enable formulation of the following conclusions:

• Common level of interference in our conditions does not allow measurement at frequencies under 5 kHz;


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• Minimal number of summations of measured data is 10 15, in areas of strong interference, it is necessary to smooth these data consequently, e.g. by more-point running average;

• Optimal number of frequencies measured at the same time is 3 5; • The main problem of repeated measurements is precise localization of

measured points therefore, reliable activity of markers with possibility of sufficient correction of measured points position (function locate after the data transfer into the computer) is necessary;

• With regard to limited range of utilizable frequencies, processing of data via the frequency probing method is not realistic;

• Measured values of apparent resistances at lower frequencies are comparable with classical resistance measurement, under the spacings AB up to 10 m;

Precise localization of measured data during continual measurement could be (except the method of continual marking) assured by special carriage equipped with measuring wheel, similarly as it is at the geo-radar. For purposes of long-term monitoring, it would be suitable to get calibrating equipment from the producer (special calibration coil - Q coil calibration). Definitive decision on utilization of the apparatus GEM-2 for purposes of the fast diagnostics and monitoring of the state of protective dikes will be possible only after the analysis of results of testing measurement performed by the company GEO Group in the 2-nd stage of the monitoring of the Velký Bělčický pond. Eventual purchase of this apparatus should be conditioned by improved software with the fully functioning marking, equipment for calibration, and software for the 1D inversion. Quoted literature Dohnal J., Jáně Z., Kněz J., Valenta J., Vilhelm J., Zima L. (2004): Hrad Nibor poznatky komplexního geofyzikálního průzkumu v letech 2000 2002. Castellologica, 9, 2004 (v tisku) Novák P. (2003): Long-time monitoring of soil water content and apparent resistivity. Proceedings of 9th Meeting of Environmental and Engineering Geophysics, Prague, O-74.

List of pictures and attachments Picture 1 Situation plan of measured profiles

Picture 2a Comparison of electromagnetic resistivity measurement and direct-current four-electrode resistivity measurement. Basic profile P1.


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Picture 2b Comparison of electromagnetic resistivity measurement and direct-current four-electrode resistivity measurement. Perpendicular profiles.

Picture 3 Measurement by the method VLW. Searching for thin conductors (tectonic defects) in bedrock of embankment.

Picture 4 Method SP. Basic measurement before water filling-up of the pond.

Picture 5a Multielectrode method. Basic profile P1.

Picture 5b Multielectrode method. Perpendicular profiles K130 and K182.

Picture 6 Microgravimetry. Density model of embankment.

Picture 7 Shallow refraction seismics. Seismic cross section.

Attachement 1 GEM-2 measurements, the test site Albertov Attachement 2 GEM-2 measurements, the test site Albertov sliding of measured data by the five-points running average Attachement 3 Comparison of the measurements by GEM-2 and by ARS200, on the test site Albertov, the date: 6.3.2000 Attachement 4 Comparison of the measurements by GEM-2 with the measurements by the method of apparent resistances, obtained by monitoring resistance profiling in the corresponding time period of the month April Attachement 5 Comparison of the measurements by GEM-2 with the results by ARS200 and CM-031. Attachement 6 Fotos from the GEM-2 monitoring (Albertov)


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2-nd fase of the monitoring programme (on the Velký Bělčický pond):

In the range of terminal field stage (Spring 2004) of the project IMPACT, field proofs were realized at Velký Bělčický dam of an artificial lake. On all of the stated sites there has been realized geophysical measurement followed with a complex of methods: • geo-electric methods in several modifications (multi-electrode arrangement in

Anglo-Saxon literature standard way referred to as Resistivity Imaging, in the reports referred to as MEM, vertical electric sounding VES, symmetric resistivity profiling - SRP and measurement of filtration potentials - FP, resp. SP, georadar, shallow electric-magnetic profiling),

• gravimetry, • refractory seismics Selection of the complex of parameters and related geophysical methods is based on long-term experience with solution of this and similar problematics. Results obtained by separate methods can be summarized as follows: • MEM it was succeeded to diversify in detail the dam body as well as the

underlying units (identification of repaired sections, recess of the dam foundations, specification of gravel bodies or sublaying tertiary clays), good compliance with SRP as well as VES results,

• SRP principal division of the dam body and underlie,, • georadar definition of the dam body against underlie, identification of partial

local anomalies (boulders, engineering networks), definition of fine grained and sandy sections in the dam body (based on signal dump),

• shallow refractory seismology and seismic tomography tomography presented only partially, in the velocity crosscut defined the dam body, based on seismic velocity values evaluation on measure of material consolidation, in an underlie identified zones of decreased velocities (probably related to tectonic impairment of the clies),

• gravimetry identification of sections with increased porosity or decreased rate of compaction

• in total: defined problematic sections From the above it is clear, that the selected complex of methods is balanced and each of the methods contributes its own way to the general picture. Next to the above mentioned contribution of the separate methods, however there also has been found notable disproportion among separate methods from the point of costs, speed, operativeness and effectiveness. It is clear, that from the point of primary target of the project development of measurement methodics enabling very fast and with minimal costs to asses updated state, to find local inhomogeneities in these works and to asses occasional measure of hazard due to destruction of the objects investigated some of the stated methods are more suitable, and usage of


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other methods can be deemed at occasional more detailed investigation of the selected sections endangered. In the range of the stage Spring 2004 there has been realized measurement by the following complex of methods, on the above mentioned sites: • Method SP: Measurement has been realized in two-electrode potential variation

(voltage difference on the points measured against a firm basis) with a step of 2.5 m. Measurement has been performed with equipment GEOTER I and II. Method SP has been selected for the reason of high productivity and as a reliable indicator of water leakage and related features.

• Method MEM: Measurement has been realized with the apparatus ARS 200 at using 32 electrodes, configuration Wenner/Schlumberger was used, step of electrodes 2.5 m, alter. 5 m. Compared with Stage 2003 efforts have been orientated on newly repaired dam sections (approx. 70 - 90 m long), on which the most notable appearance of seasonal impacts can be expected. From the results of measurement of stage 2003 it is clear, that original already compacted dam sections are relatively monotone. In the places of keying (contact of the original unimpaired dam with the newly repaired part) there was performed detailed measurement of the contact with step of electrodes 1m. MEM method has been selected for the reason, that of the methods applied in Stage 2003, it provided the most detailed picture of internal construction of the dams including their near underlie enabling to deduce composition of the dams and their underlie.

• Shallow dipole electric-magnetic profiling: Notable advantage of this method is its non-contactness, high productivity and mobility, some disadvantage of this method is an integral character of the variable measured (i.e. on each measured point there is obtained an average data from the surroundings of the point measured approximately the size of half-sphere an average of the distance between the receiver and the transmitter). In the Stage 2003 in the range of complex of methods, there were tested equipments CM-031 and CM-132 at an effort to realize due to different transmitting frequencies of both instruments, measurement with different depth range and by this prompt and effective manner to obtain orientation impression on depth distribution apparent specific electric conductivity (alter. apparent specific resistance). During 2 stageS of measurement in the range of Stage 2003 it was concluded, that these instruments do not match expectations , especially from the point of view of (thermic) stability and repeatability of measurement.

Based on the study of a large amount of materials (web pages of machinery manufacturers, case studies, articles in impact journals, proceedings of international conferences) there has been selected GEM-2 equipment (producer GEOPHEX, U.S.A.). It is a broadband conductometer, which can measure parallel at more frequenciesand this way it can obtain at each point a set of data, corresponding to various depths.

With respect to the fact, that comparable technology has not been operated in the range of Czech Republic (and to information available even not in the range of Europe), routine operation of this equipment on the above mentioned sites has been


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preceded by confirmation of principal characteristics and abilities of this instrument by team of the Institute of Hydrogeology, Engineering Geology and Applied Geophysics of Faculty of Science, Charles University in Prague as an accredited branch educational workplace. At measurement on pilot sites, the measurements have been performed in 3 alignments, systematic manner:

• Horizontal coplanar orientation, longitudinal axis parallel to the dam, • Vertical coplanar orientation, longitudinal axis parallel to the dam, • Horizontal coplanar orientation, longitudinal axis perpendicular to the dam.

Dependence of the data measured from the elevation of the instrument above terrain (i.e. height of the concrete operator) has also been observed. Measurement with the apparatus GEM-2 have been realized parallel on 4 frequencies: 6325 Hz, 16475 Hz, 24475 Hz and 35625 Hz. These frequencies have been selected on one side according to expected depth range (with using experience with equipment CM series and information from literature dedicated to this class of conductometers), on the other side with respect to minimal local/regional noise at these bands (determined with special experimental procedure). Concept of the apparatus GEM-2, history of development of the instrument, orientation oof the company (manufacturer), measured parameters. The broadband digital, multifrequency, electric-magnetic (EM) apparatus working in the frequency range 330 Hz - 47970 Hz. It allows to measure parallel on up to 15 working frequencies (recommended 3 to 5). In difference from commonly used conductimeters (EM-31, EM-34, EM-38 etc. from the company Geonics and their derivations - CM-31 etc.), which work on 1 frequency (transmitter transmits monochromatic harmonic signal) and are tuned in for precisely determined combination of operational frequency versus distance of transmitting and receiving coils, this instrument is based on revolutionary concept using frequency synthesis of the signal transmitted (this allows to superimpose mutually selected number of frequencies) and separation of separate compounds off the signal measured (deconvolution). Principle of operation of the transmitter is described on pictures 1-3. Technical solution is enabled due to a modern particle basis (analog and digital electronics, microminiaturization), realization of this concept was unthinkable till the end of 90s. It is undoubtedly a hi-tech solution, declared dynamic range is 200 dB (!!!). The distance of transmitting and receiving coils is fixed, 1.66 m (at such a small distance remarkable demands arise on its dynamic range and sensitivity). Resonance frequencies of the coils must be very high, solved with small amount of turns (i.e. weak signal, necessary amplifiers of extraordinary quality, special coiling of the coils). Picture 8: Example of signal generated for 3 selected frequencies. Picture 9: Example of signal generated for 3 selected frequencies (detail). Picture 10: Signal spectrum.


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First mentioned such designed sensor has been in 80s. The concept is both used at the conductometer GEM-2 and at metal-detector GEM-3 as well as at airplane variants. Until the end of 90s activities of the producer have been linked to army contract, thus especially the apparatus GEM-3 is pushed forth. Sturdyness of construction and simplicity of tending correspond with the army demands. Measured parameters are real and imaginary compound of secondary magnetic field standardized on the size of primary field (measured in dimensionless units ppm, i.e. 10 -6). These values are transformed into apparent specific conductivities. Next to measurement of the secondary field generated by the primary field transmitted, the apparatus enables in special measurement modus, i.e. a) to measure throughout EM spectrum in its whole frequency range (this enables to specify frequencies impacted by noise) regretfully spectrum is represented in the form of dynamic parameters (dB), which does not allow to quantify noise from the point of the parameters measured (ppm), b) to further register also frequencies, which are not part of the signal transmitted, i.e. option of passive monitoring of electric-magnetic smog. The sensor has been developed in the middle of 90s, production license has been passed on to the company GSSI (recognized georadar producer). Apparatus produced has not been commercially successful nevertheless (measured data were significantly not reliable). Its production was (probably) stopped. The instrument is at the moment offered to rent only by one of the chief distributors GSSI. From the referees available, this apparatus has been used successfully especially at solving problems of permafrost nevertheless (i.e. somewhat specific case). Operation of the instrument The apparatus can be controlled in three ways: • Bracket, • Notebook by means of the program WinGEM, • Hand-held computer iPaq by means of the program winceGEM. At using the bracket (containing 8 row alphanumerical display) there are used 3 buttons, enabling standard operation (selection items from the menu, elementary set operation, starting/termination of the profile measurement, transition onto a new profile, saving markers). It is possible to select from 4 preset combination of frequencies, editing cannot be done in this regime. At using notebook, it is possible with WinGEM program to perform also editation of setting combination of frequencies , it is possible to realize transfer and alementary data processing, to depict the measured data, to transform the binary data set into CSV format accepted by table processors. This manner of operating the apparatus has not been confirmed systematically.Operation by means of the hand-held computer iPaq (working under Windows CE) by means of winceGEM program presents an interesting alternative graphic interface, full-value operation as by


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means of WinGEM (option for graphic depicting of measured data in real time, editation of setting etc.). Conclusions and recommendations We recommend torealize in another stage testing measurements at using multifrequency apparatus GEM-2 (producer GEOPHEX, U.S.A.). Based on the information available, this apparatus matches all demands imposed on the developed technology (especially productivity, effectiveness, undemanding operation, information gain and shallow depth focus) and of the technologies discussed in the range of IMPACT project matches these demands the best. In any case using multifrequency apparatus is much more effective than repeated measurements with several one-frequency instruments with varying positions of coils (next to other things there appears also the factor of reproduceability of the position of measurement). As a key issue however, we deem correlation of the results obtained with this technology with the results obtained by traditional technologies (these are parameters obtained by technology still not confirmed in this country, further in this relatively specific application). It can be summarized: • There were realized field measurements by means of the apparatus GEM-2, this

in April 2004. Despite very limited time space it was succeeded to manage very well usage of this technology at measurement, for routine implementation into practice there has to be finalized metodics however (eg. using markers etc.). It was also succeeded to manage elementary data processing and to present the data measured in form of graphs of apparent specific conductivities for separate frequencies.

• Selection of working frequencies has been performed on one side based on their expected depth range, on the other side based on noise analyses of separate bands.

• It can be concluded, that work with GEM-2 apparatus is very fast, operation undemanding and operative, work productivity is very high according to this criteria this technology matches all demands imposed on it from the point of its involvement into the complex of methods.

• Measured curves especially for lower frequencies are (not considering noise) substantially articulate, lateral distinguishing ability of GEM-2 apparatus (i.e. ability to detect even small lateral changes) is very high and in this sense it substantially exceeds CM line instruments, which because of larger distance transmitter receiver provide already markedly smoothened data. For the above reason, data measured by Cm instruments are also more relief dependant.

• It has to be stressed, that curves for the highest frequencies on use (over 24 Hz) are in practical monotonous, very little articulate and do have obviously very limited information value.

• The question of assigning specific depth range to specific frequencies is obviously very complex: in some cases amplitudes of anomalies for separate frequencies well corresponded with depth distribution according to resistivity crosscut to direct-current methods, in some cases however character/amplitude of curves of apparent specific resistances obviously did not correspond with model of environment. To obtain by means of data inversion on more frequencies, regular resistance cut, shall become subjected to consecutive stages of work.


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0 5 10 15

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GEM-2 Albertov 6.-19.4.2004[mS/m]

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Number of values 15Minimum 33.63Maximum 37.14Mean 35.484Standard deviation 0.97 (2,7%)

statistika mì øení v bodì 0


statistika mì øení v bodì 15

Attachement1: GEM-2 measurements, the test site Albertov.


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GEM-2 Albertov 6.-19.4.2004vyhlazené hodnoty - pì tibodový klouzavý prùmì r

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statistika mì øení v okolí bodu 0

statistika mì øení v okolí bodu 15

Attachement 2: GEM-2 measurements, the test site Albertov sliding of measured data by the five-points running average.


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0 5 10 15

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AB/2 = 0,75mAB/2 = 1,25mAB/2 = 1,75mAB/2 = 3,25modpor 6535 Hzodpor 16475 Hzodpor 24725 Hzodpor 35625 Hz

ρz [Ωm] ρz ∼ [Ωm]

Albertov

[m] Attachement 3: Comparison of the measurements by GEM-2 and by ARS200, on the test site Albertov, the date:

6.3.2000


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AB = 1.5 mAB = 3.0 mAB = 4.5 modpor 6325 Hzodpor 16475 Hzodpor 24725 Hzodpor 35625 Hz

ALBERTOV

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Attachement 4: Comparison of the measurements by GEM-2 with the measurements by the method of apparent resistances, obtained by monitoring resistance profiling in the corresponding time period of the month April.


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AB/2 = 1,5mAB/2 = 4,5mAB/2 = 6,5modpor 35625 Hzodpor 16125 HzCM-031

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Nibor P 36

Attachement 5: Comparison of the measurements by GEM-2 with the results by ARS200 and CM-031.


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Attachement 6 Fotos from the GEM-2 monitoring (Albertov)

Documents

IMPACT · Shallow refraction seismics utilizes refraction of the seismic wave on interface between the more slowly and faster environment (e.g. material of embankment against original