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5th EEGS-ES Meeting
- Conference date: 06 Sep 1999 - 09 Sep 1999
- Location: Budapest, Hungary
- ISBN: 978-94-6282-119-4
- Published: 06 September 1999
1 - 50 of 196 results
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Use of Euler deconvolution in recognising magnetic anomalies of archaeological objects
By M. El ErakiThis paper deals with the three dimensions magnetic interpretation of the buried archaeological bodies using Euler deconvolution. Euler's homogeneity relationship offers a quasi-automated way to derive plan location and depth estimates of buried archaeological objects, such as kilns, walls and tombs, from a gridded magnetic data set. Euler's homogeneity equation relates the magnetic field and its gradient components to the location of the source, with the degree of homogeneity expressed as a structural index. The structural index is a measure of the fall-off rate of the field with distance from the source and provides a way to discriminate between different source shapes. The method is also insensitive to field distortion caused by permanent magnetisation of source objects. Euler deconvolution method has been applied to data collected over Tell Basta historical site, Zagazig, Egypt. The analyses of the data sets have provided characteristic Euler deconvolution signatures and structural indices associated with typical buried archaeological bodies. The solutions obtained indicate the ability to quickly and accurately map the location and depth of buried archaeological objects from gridded magnetic survey data. The revealed depths to the different discontinuities agree fairly well with the archaeological evidences excavated later.
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Geophysical investigations of a possible impact structure near Raabs/Austria
Authors D. Kostial, F. Kohlbeck and C. KöberlA circular depression of about 400m diameter is located in the vicinity of Raabs/Austria (Fig.1). The question arose whether this structure could have been caused by an impact. The structure is very flat, about 5m of depth, whereas impact structures of this diameter should have a depth of at least 40m. However the circular shape of the flat central plane and the steep flanks resemble a crater that could have been filled by sediments at a later time. There are no rock exposures in the near vicinity, therefore shocked quartzes to prove the impact could not be found. The surrounding rock material is gneiss and amphibolite. Impact craters are associated with a low density zone of destroyed rock in the centre caused by the impact shock wave. This low density area can be recognised by different geophysical methods. At the Raabs site refraction-seismic, gravimetric, magnetic and geoelectric studies have been made. The refraction-seismic were carried out at two perpendicular profiles and indicated virgin rock with velocities of up to 6000m/s no deeper than 8m below the surface in the centre. Gravimetric and magnetic studies were carried out at stations on a regular grid of 50m mesh size covering the structure and extending to the outside region. In the centre measurements were taken on a more dense grid with 10m of mesh size. One line striking N-S was used for a comparison of all geophysical methods that had been applied. It was found that the gravity anomalies did not follow the shape of the surface. The Bouguer anomalies shown in fig. 2 are dipping about from SW to NE without any relation to the topography. 2-D modelling along the profiles was carried out. The models suggested that the amphibolite was dying out toward N beneath the structure. The depth to the solid rock found by the models fit the depth found by the seismic studies. Fig.3 shows the free air anomalies along the N-S profile passing the centre and the geological interpretation. A small micro-gravity high located near the centre is well defined and also shown. A correlation with a nearby magnetic high was suspected but could not be verified. EM34 measurements were taken along two profiles crossing the whole structure, whereas multielectrode measurements were made on a small profile in the centre only. The conductivities found with the EM34 showed an anomaly in the centre which could not be correlated with the seismics carried out on the same line. The Schlumberger measurements showed low resistivities (142 Ohmm) at the surface and high resistivities (1200 Ohmm) at a depth of 8m indicating solid rock. Very high magnetic anomalies have been found within a small region in the centre. This zone could not be identified by the other methods. The conclusion drawn from the measurement is, that the structure cannot have been caused by an impact. The comparison of the methods with each other shows a general agreement concerning the main structure. However, some differences and details could not be recognised by all methods.
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Integrated geophysical studies of St.Lawrence monastery's basements
Authors V. S. Safronov, D. K. Bolshakov, M. V. Kalisheva, I. N. Modin and A. Ju. PalenovIntegrated geophysical studies are carried out on St. Lawrence monastery's territory in Kaluga. The task of meals building basement's rests study in St. Lawrence monastery were considered. Magnetic, resistivity, GPR survey and metal detectors were used here. The main feature of the survey is a high amount of metal rubbish on territory (in particular in the uppermost layer). For example sheets of roofing iron at the total area about 20 sq. m were found here. Such strong source of noise could prevent not only magnetic prospecting, but also resistivity and georadar survey. Therefore geophysical studies of the area were conducted with preliminary searches of iron rubbish with metal detectors (TM-808, White's and ÑZ-7, Fisher) and its extraction. Magnetic prospecting Difficulties for magnetic survey were represented not only by metal rubbish, but also by high level of industrial noise, because the monastery is on Kaluga town territory. We used two proton magnetometers MMP-203, manufactured at St.-Petersburg enterprise «Geologorazvedka». To avoid the short period and high intensity variations (up to 50 nT) we used synchronous measurement's technology: the measurements on the area (13 x 20 m) and at base variational station were made simultaneously. The survey grid was 0.5 x 1 m. The survey accuracy consisted ± 5 nT (Fig. 1). The magnetic data interpretation was carried out with computer program DIPOLS-3 (K.M.Ermokhine), where field modeling is based on a dipole source method. Estimated anomalous magnetic field is presented at Fig.2. The model of the object producing such a field a field is presented at Fig. 3. Resistivity survey The survey territory was mapped by resistivity method with gradient array in two stages. At the first stage the current line was placed in S-N direction and measuring profiles were laid parallel to current line. Then the whole array was rotated on 90° (in E-W direction) and the survey was repeated. The area 18 x 25 m was mapped. The current line ÀÂ has length 50 - 60 ì, electric current value was 10 mA, frequency 4.88 Hz. Length of a measuring dipole was 0.5 m. The survey grid was 1 x 0.5 m. Resistivity instrument included the current generator (produced by ISR), and two measuring units ERA. The interpretation in accordance with the survey technology was carried out in two stages. At first stage the data for two different current line directions were analyzed separately, and then results for different polarizing fields' directions were united to receive a general map
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Geoenvironmental danger for Boyana church (Sofia, Bulgaria)
By M. MatovaIntroduction The Boyana church is a remarkable monument of the Middle Age history of the country. It is a subject of special protection from the side of UNESCO and the Bulgarian State. The small Boyana church was built in a quiet place, far away from the of the ancient Sofia city, near to the Vitosha mountain foots. The church was painted in the XIII century. The Christian icons of the church present one of the most important manifestations of the Bulgarian Renaissance.
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Multipurpose Geophysical Investigation on buda Castle Hill
Authors I. Holczinger, M. Pattanyús-Á., Zs. Prónay, L. Hermann and Zs. NyáriThe Buda Castle Hill in Budapest with its natural and artificial relics is an invaluable part of World’s Heritage. Cellars and caves, castle-walls and retaining walls, dwelling houses and palaces are the components which are in a tight relationship with each other and the urban environment too. The geological milieu is formed of marl covered by limestone and topsoil. The task of geophysics - on the basis of long-term city-planning - was to investigate the geological structure (especially the relief of the rock bed) and to detect archeological objects in three different areas.
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Application of Geomagnetic, Geoelectric and GPR Surveying to Archaeological studies
More LessThe ruins of the Roman-age town, Aquincum, can be found in the northern part of today's Budapest. Geophysical exploration for the Aquincum Museum started in October 1997 on a test area opposite to the museum. On one side, a main road and a railway with high voltage cable, on the other sides living houses border the area. Due to the artificial noises, the area was a good site to test how the different geophysical methods work in such environment. Magnetic method, direct current resistivity mapping and ground penetrating radar were tested. Aim of the work was mapping the ancient Roman town, which is presently covered by soil. An area of 3200 m2 was surveyed in a grid with spacing of 1 m, using proton precession magnetometer of GEOMETRICS G856 (sensitivity 1 nT). The magnetic field varies hundreds of nT in a few seconds period due to the electromagnetic noise of the city (Fig. 1). The amplitude of the magnetic anomalies caused by the archaeological objects is a few tens of nT. Due to the high frequency noise, interpolation of the data of a base station cannot be used to correct the measured data. This problem was solved by connecting the base station instrument and the measuring instrument by a cable and using an outer trigger to start the measurement. The measured data were corrected for the variation in time, the low-frequency, space-domain components were removed by a high-pass filter, the high-frequency, space-domain noises were smoothed by an optimum filter, the data were reduced to the pole, and finally, the magnetic field was downward continued. The synchronous measurement between the base and measuring instruments highly reduced the noise. To further reduce the noise, 5 measurements were made in each point in a smaller area of 300 m2. The mean of the measured values was taken. The repeated measurements resulted in a significant improvement of the quality and reliability of the magnetic map. The result of the repeated measurements is shown in the insert in Fig. 3. The resistivities of the limestone walls and the covering soil differ significantly. This initiated the idea of testing geoelectric methods in the area. DC measurements were made in an area of 50x40 meter using dipole-dipole electrode configuration. The length of the source and potential dipoles was 1 meter. The distance between the centres of dipoles was 2 meters. This electrode configuration gives information from a depth of about 1 meter. To speed up the profiling a "double-fork" electrode system has been constructed. The whole configuration was moved by one meter in the consecutive measurements. East-west as well as south-north profiles were measured. About 4500 data points were available for processing. An apparent resistivity map was computed from the combination of the two direction profiling (Fig. 3).
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The Application of Geophysical methods to resolve a fascinating archaeological query
By G. RanieriThe most interesting cultural characteristics in Sardinia are defined by the presence of more than seven thousand ancient buildings called ‘nuraghe.’ They are three thousand years old. Their impressiveness bears witness to the great capability of building of the ancient Sardinian population. In fact Sardinia is the only part of the world where ‘nuraghe’ occur. The nuragic civilization constructed varied building forms: towers with ‘tholos’ rooves, huts, sacred wells, multi-towered castles and so on. Well construction in very soft, fractured rocks and in coastal areas also demonstrated the great capability in searching for water, a rare resource on the island of Sardinia. The ‘Cuccuru su Nuraxi’ well in the village of Settimo S.Pietro near Cagliari the capital of Sardinia, is of particular interest. The well is more than thirty metres deep, two metres large and is covered by granite and sandstone blocks, fifty centimetres thick . The well head is eleven metres deep, in a room with a tholos roof. The well and the room are underground. A vertical stair leads to the summit of a hill where nuraghe artifacts are found (figure 1). From the exterior, the system appears as a regular conical hill formed by marl rocks. The context is formed by Miocene marl. Other irregular conical hills are found. Beyond archaeological interest, construction details along with the geological constitution of the hill pose some fascinating queries: Is the well an isolated sacred well, or is it inserted into a majestic castle and is it a water supply well? Was the well constructed upwards or downwards? Is the hill natural or artificial? What methods have they used to find water (there is still water in the well!) under marl formation? To answer these questions , crucial for the planning of archaeological excavations, geophysical surveys are carried out. Taking advantage of the conical form of the hill, we executed: three horizontal seismic tomographies at different levels; many gravimetric `Nettleton' profiles; a pole- pole electrical survey. The seismic tomographies show a relative high velocity zone only in the section around the well (figure 2) and constant velocity in the other sections. The tomographies were made with different shots-geophones array to verify the anomaly zones. They probably show the well in side a larger nuragic structure. Nettleton profiles have been utilized to determine the density of the hill (1.88 g/cm3), as well as to identify differences from right trend probably corresponding to voids. The well is very well indicated and a zone of 1.5 g/cm3 seems to indicate an old excavation (a trench ?). The pole-pole electric survey shows resistive anomalies which can be attributed to nuragic walls around the known well.
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Method of the equivalent dipole sources in the processing of results of the magnetic gradiometer surveys
Authors E. E. Nikitina and S. A. TikhotskyMeasurement of the vertical gradient (pseudo-gradient) of the magnetic field instead of total field measurement is one of the most popular methods in performing the high-accurate magnetic surveys especially when the noise level is very high. The process of transformation of the measured pseudo-gradient field into the total field is associated with the problem of the reconstruction of the long-wave field constituent. The common way in solving this problem is to add some small number of measurements of the total field T. Further processing of the survey results almost always includes the separation of the anomalous field into the constituents, associated with the distinct groups of sources. The traditional method in solving of formulated problems is the spectral operator application [e.g. C. Fechant, D. Orseau and N Florsch, 1998]. According to our experience spectral methods are not accurate enough and it could lead to the appearance of the false anomalies. These effects are due to the inaccurate Fourie series summation and edge effects (Gibbs effect). Following the ideas of V. Aronov (1963) and A. Bjerhammar (1964) we suggest an alternative method based on the simultaneous approximation of the measured values of the pseudo-gradient and the total field values by the model field of the equivalent dipole sources system. This algorithm allows us to calculate any magnetic field transformations, interpolation of the field values and to separate field into constituents. Let N be the number of the pseudogradient measurements (Gi) and M - the number of total field measurements Tj. N>M; i=1,..,N; j=N+1,..,N+M. We approximate these measurements by the system of K≤ N+M dipoles situated at some points (xk, yk, zk), k=1,..,K, in the lower half-space. In general magnetic moments of the dipoles mk can be determined from the following minimum condition:
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Enhancement and filtering of magnetic data in archeological sites
Authors F. Maurizio, F. Giovanni, P. Gaetano and R. Antonio.This work focuses on exploration of an archaeological site by magnetic survey. The aim of the survey is to evidence the presence of structures connected to an ancient Italic civilisation by means of new techniques for anomaly residuation and signal enhancement. The ancient settlement is located near S. Maria a Vico village, placed on a terrace of fluvial origin. The terrace is engraved in travertines covered by alluvial sediments and ash pyroclastic of Somma-Vesuvius and Flegrean Fields. In these sediments, altered by the weathering and about 3 meters thick, the remnants of proto-historical settlement to be investigated are buried. S. Maria a Vico is about 10 km far from Pontecagnano, site of a very important archeological deposit associated to the "Villanoviana" culture (Peninsular Italy, IX - VII century B. C.). Although the ancient site of S.Maria is associated to the "Tombe a fossa" culture, it was influenced by Villanoviana culture, being linked to the Etrurian-Campanian area and to the Greek colonies of the Ionian Sea. Excavation works, carried out in 1986 and 1991, did not allow to get clear elements about organization of the ancient settlement. Most of the remnants allow to estimate an age between VII and VI century B. C to the settlement.
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Monte Carlo simulation in geophysical site investigation
More LessGeophysical methods can substantially benefit the process of site investigation. It is equally true however that their full potential goes unrealised too often. A more rigorous and open approach to geophysical site assessment and forward modelling (i.e. geophysical `desk study') would go a long way to improving this situation. It is illustrated how the uncertainties involved in designing a geophysical investigation - for example uncertainties in the size and nature of the target, the accuracy and repeatability of measurements, the presence of cultural noise and the limitations of interpretation - can in many cases be understood and managed more clearly using a statistical method such as the Monte Carlo method. Examples of the application of such a method are described in the context of geophysical applications in site investigation. This type of approach can play a crucial role in defining the parameters of the investigation to an agreed or acceptable level of confidence. The approach can also aid the geophysicist in the design of the investigation and it improves clients' appreciation of the processes involved in designing, carrying out and interpreting the geophysical element of site investigation.
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Extremely fast IP used to delineate buried landfills
Authors N. R. Carlson, C. Mauldin Mayerle and K. L. ZongeDetermining the location of old, poorly documented buried landfills has become a significant concern in many places where development is hindered. Today new methodology provides a means for the efficient acquisition of induced polarization and resistivity data. Case studies of five landfills verified the accuracy of the technique and the acquisition speed of the system. At all sites IP anomalies (> 3 milliseconds) correspond to solid waste verified by drilling and trenching. Using a multiplexer capable of interfacing between a multichannel receiver, a transmitter and 30 electrodes allowed IP and resistivity data acquisition at extremely fast rates. Data acquisition in the dipole-dipole configuration consisted of a station spacing of 2.3 meters and a dipole size of 4.6 meters with twelve points collected along each diagonal (n=0.5, n=1.0, …n=6). The electric-field signal was sensed at the receiver site using tin-coated copper braid electrodes. A time domain, 0.5 Hz signal was used, stacking and averaging eight cycles to minimize random noise. Measuring all data points at least twice established repeatability of the data. A crew of three can acquire 3300 data points plus repeats in one day (approximately 800 line meters).
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Environmental assessment of municipal waste dump sites with electrical resistivity and induced polarization multielectrode methods
Authors V. Iliceto and G. MorelliA high resolution electrical resistivity and induced polarization survey was carried out inside and around a modern municipal waste dump site located in the Venice province, eastern Italy. The waste dump area is adjacent to two parallel rivers (Brenta and Bacchiglione), so that part of the measurement arrays had to be layed out directly on fluvial argins. The complex aquifer system was only partially known before the survey, although many boreholes were drilled in the past, before and after the construction of the dump site. However, the geology of the site is characterized by alternate thick clay and silt aquitards that should have guaranteed the protection of deeper aquifers. The most important things to assess with geophysical methods were the presence of higher permeability layers or lenses and the potential leakage from the waste area towards the above mentioned rivers or the adjacent farm land. The instrumentation used is a time domain resistivity/IP meter that allows for automatic SP compensation and digital stacking, with 1200 W maximum power and programmable IP windows. Special multi-core cables were constructed to ease the operations with up to 64 electrodes. The electrodes were made with standard copper-coated iron rods, that seemed to produce very low spurious polarization effects (although a cross-check with non polarizable porous pots is not easy to perform in multielectrode configuration). Different array configurations (Wenner, Dipole-Dipole) and different electrode spacings (2 to 5 meters) were tested, trying to optimize the results in terms of noise reduction and spatial resolution obtained. The data sets were interpreted using different 2D resistivity/IP inversion software, especially to test the influence of different IP modeling strategies on the final results. An example of a Resistivity / IP reconstructed section over the center part of the landfill is shown in the figure below: the solid waste mass appeared to be more resistive (25 -> 50 Ohm*m) than background soil (clay, pit and silty sand, ranging from 8 to 15 Ohm*m) and highly polarizable (over 20 mV/V). The IP results enabled to distinguish between different media having high conductivity, as clays or silty sands saturated with leachate originated by the waste. In fact, in the left part of the section a low resistivity layer ( 8 -> 15 meters depth, resistivity 4-5 Ohm*m) corresponds to a high chargeability zone, indicating a potential leakage from the bottom of the landfill (this older part was not protected with HDPE coating). From the center to the right of the section (the newer part of the landfill, with HDPE installed below 8 meters of waste) the chargeability decreases rapidly with depth to the low values encountered outside the area as well. In this case the conductive layer can be explained with the presence of a clay aquitard, indicated by the borings.
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Combined use of tem and RMT for the investigation of waste disposal sites
Authors S. Greinwald, A. Hördt, A. Hoheisl, G. Schaumann and B. TezkanAbout 25 km northwest of Hannover at Mellendorf a former sand pit was filled with industrial waste between about 1960 and 1980. The contents is mainly magnesium drosses. In 1987 the top of the site was sealed with a 1.5 m thick cover to avoid penetration of water into the waste. This was important because the bottom of the disposal at a depth of about 15 m below surface was not sealed. 1996 and 1997 TEM- (Transient Electromagnetic) and RMT-(Radio Magnetotelluric) measurements were carried out over the deposit. Goal of the combined use of RMT and TEM was a complete investigation of the vertical and horizontal resistivity distribution within the deposit and the directly surrounding area. The RMT measurements covered the deposit with a 10 m by 10 m grid, using four frequencies between 16 kHz and 200 kHz. For each frequency a pair of transmitters was chosen perpendicular to each other and located in the directions of the long and short axis of the deposit. For the TEM measurements the Protem 47 unit (Geonics) was used in an inloop configuration, the receiver being located at the centre of 50 m * 50 m transmitter loops. The interpretation of the data was performed in three steps. In a first step a one-dimensional inversion of the TEM data was used based on a three layer starting model. In the same step the interpretation of the RMT-data was based on a two-dimensional model. Secondly, for a further explanation of the RMT and the TEM data three-dimensional models were applied, based on the results of the preliminary interpretation of each method separately. Finally a three-dimensional model fitting both data sets was developed. The basic principle for determining the optimum model for both methods was to use the RMT-results for the top layers down to a depth of approximately 17 m and below that the results from the TEM data. This led to a final model combining the high resolution of the RMT method for the top layers and the better resolution at greater depth penetration of the TEM system. This model was further optimized, using trial and error, the only possible way at the moment. Figure 1a shows the combined model, which uses two different resistivity distributions for the western and eastern part of the deposit. The interesting result is that the high conductivity of the deposit extends to a greater depth than that of the former sand pit. The most likely explanation of the extension of the low resistivity below the known deposit depth is a leakage of waste water from the deposit into the ground below. To prove the validity of this result, modeling tests were undertaken with a reduced depth extend of the conducting layers for both sections of the deposit, the western (fig. 1b) and the eastern part (fig. 1c). Both models could not satisfactorily explain the measured data. This verifies with increased reliability the resistivity distribution of the optimized model shown in fig. 1a.
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Helicopter electromagnetic & magnetic surveying applied to a pipeline construction project in Canada
Authors A. Bouvier, P. Elkaim and G. HodgesFor construction Companies, the cost of a pipe laying project in a remote region depends on the risks taken when selecting a final pipe path according to the expected ground conditions within the first three to five metres of soil. Proper evaluation of the three engineering soil type distribution: unconsolidated, swampy deposits, loose ground (easily excavated), and hard rock (require drilling and blasting), is crucial to define the construction and environmental risks. It requires a costly and time consuming on-site investigation based on outcrop mapping and hand auger drilling. At best if access and logistics permit it, discontinuous soil sampling may be carried out along the proposed pipe path. For major pipeline projects, the selection of an appropriate path with respect to the lithological conditions is ensured through a helicopter-borne geophysical investigation combining electromagnetic and magnetic techniques. Electromagnetic data depict the surficial and near-surface apparent resistivity and discriminate between conductive layers (clay, marl, graphitic schist, salty formation..) and resistive ones ( dry sand, gravel, limestone, siltstone, sandstone, quartzite, gneiss, granite and volcanic rocks) . Magnetic data allow to distinguish between sedimentary and igneous rocks based on their magnetic susceptibility, which is several decades higher for igneous rocks. Methodology: a primary electromagnetic field is generated through transmitting coils using different frequencies in the 400 Hz – 100 000 Hz range. In a conductive body, eddy currents induce a secondary field, the component values of which (phase and amplitude) depend on the soil conductivity and are measured by a receiving coil. Transmitted frequency and depth of penetration share an inverse relationship whereby the higher the frequency, the shallower the depth, as expressed by the following formula: P = ½ Π √ 10 ρ/f where P is the depth of penetration (km), ρ is the ground resistivity (Ωm) and f is the frequency (Hz). This also means that the higher the soil conductivity (1/ρ), the shallower the penetration. The total intensity of the geomagnetic field is measured with a cesium vapor magnetometer simultaneously. Dighem method: the geophysical instruments are housed in a 8 metre long « bird » which is towed 30 m below a helicopter . Five pairs of coils (two coaxial and three coplanar) constitute the Dighem V electromagnetic system. A Geometrics 822A magnetometer is located between the transmitting and receiving coils. The helicopter flies lines at an altitude of 60 m and a speed of 100 km/h, maintening a ground clearance of 30 m for the bird. Along each line, an electromagnetic measurement is taken every tenth of a second, which represents a distance on the ground of about 3 m. The lines are positioned using an on board Ahstec GG24 GPS receiver, giving a lateral accuracy of about one meter.
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Locating burried waste using geomagnetic survey
Authors A. Kohler, S. Puszta and P. SteinbachDuring the operation of a refrigerator factory in the 50’s, 60’s and 70’s drummed hazardous wastes were dumped and covered with soil illegally in sand pits located near to the factory. About 20 years later the during an overall environmental remediation project these illegal waste disposal sites were excavated and restored. The locations of these dumping sites were determined by aerial photographs, and using local people recalls. As work progressed new reports were made to local government and factory management about waste dumping sites, all of which were examined by means of excavating, visual inspection, waste (if found), soil, and sometimes groundwater sampling. Originally 7 waste disposal sites were assigned for examination, the final number of the examined sites was fourteen. No. 14 site (WDS-14) was reported by an excavator operator of a sand mine, who actually was ordered to dig a trench, later in wich he witnessed dumped drums. Because of subsequent mining operations, and land use change the operator could not identify the location of a reportedly 40 x 2 meters trench, only approximately within a 100 x 100 meter area. The site was first excavated at several locations based upon the operator’s memory, but nothing was found, not even soil disturbance. As the operator insisted on the existence of the dump a geomagnetic survey was chosen to scan the area for geomagnetic anomalies, which could indicate burried metal objects. The site was prepared by creating a 20 x 20 m grid oriented to magnetic north, and handed over to the geomegnetic surveyors. Using fluxgate gradiometer one considerable anomaly and several disturbances were found within the area of interest. However, the proof of the existece of buried drums demanded systematic survey. A 120 x 120 meter area was systematically surveyed with a proton procession magnetometer. Measurements were taken at a 1 x 1 m grid, which means 12000 measurement points. The sensitivity of the instrument was 0,1 nT, and the probe was held at a 0,7 m level from ground surface. Data processing started with diurnal correction. This was followed by noise reduction as such reduction to the pole (resulted the elimination of anomalies that had permanent magnetic effect, since it could be presumed, that the burried objects have only induced magnetization). Geological effects (low frequency noises, e.g. facies changes, clay lenses etc.) were removed by band pass filtering and high frequency noises were compensated by optimum smoothing filtering. The picture of the burried objects was sharpened by downward continuation, and the result was visualized as a colour picture.
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Detection of old waste depostits and their contamination by using geophysical methods: Two case studies
Authors S. S. Seren and N. H. BlaumoserWaste deposits constitute a massive risk for the environment. Their re-development needs most exact information about their extent, content and their geological situation. This information can be improved cost-effectively and fast utilizing non-destructive geophysical methods, such as magnetic, electromagnetic and geoelectrical field measurements. To demonstrate the effectiveness and advantages resulting from combining these methods, two case studies are presented. 1) Waste deposit in Molln, Upper Austria: The waste of an aluminium-factory was deposed in a river. No precautions were made. The records showed no details about the quantity of the waste and the extent of the polluted area. Using geoelectrical-Wenner-array-measurements these informations could be retrieved. A 3m-array-distance was choosen for this purpose. The boundary of the waste deposit and even areas of high contaminations could be determined with high precision. The waste deposit showed lower apparant resistivity values than its surroundings, which consists of sand and gravel - both showing very high apparent resistivity values (> 500 Ohm-m). In addition, using two deep soundings after Schlumberger the specific resistivity of the waste deposit and of its surroundings could be determined. 2) Waste deposit in Karling, Upper Austria: The waste deposit, probably containing common house-garbage, was surveyed using magnetic, electromagnetic and geoelectrical methods. Based on five geoelectrical deep soundings after Schlumberger the hydrological and geological situation in the aerea of the waste deposit was determined. A 12 m thick ground-water-layer was endangered by this feature. At a depth of 18 m a layer showed up, which can play an important factor during a future re-development of the waste site. Electromagnetic and magnetic (total intensity and gradiometer) surveys were carried out with a measurement distance of 5x10m. The electromagnetic survey showed clearly the surrounding of the waste deposit and a strong contamination. The magnetic survey showed areas of strong anomalies, which are not typical for a normal household-waste. Hence, in the investigated area one should also expect to find steel barrels, car-wracks etc.
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Waste disposal site mapping using the CVES method
Authors M. Ezersky and A. Beckctions were constructed using Res2Dinv software, which is a computer program for rapid 2D resistivity inversion using the least-squares method by Loke and Barker (1996)1. The waste disposal site is 300x500m and consists of non-organic materials (mostly building waste), layered alternately with thin, sandy-loamy layers and underlain by a thick layer of dune sands. Visual inspection of the site revealed that, apart from bricks, wood and concrete fragments, there was also a substantial number of metallic objects randomly distributed throughout the waste volume. The sandy-loamy layers retain moisture whereas the sand is drained, thus there is a significant difference in electrical resistivity between the waste and the natural sandy base layer. At the northern and eastern sides of the site, the waste layer is higher than the sandy basement, while at the southern and western sides, both the waste and the basement are at the same level. It was, therefore, possible to correct the thickness of the waste layers using a forward calibration measured at the waste outcrops. Five CVES lines, totaling 1500 meters in length, were carried out in the waste disposal area. Fig. 1 shows a typical geoelectric cross-section of Line 2. Two layers are clearly observed: the upper layer (waste) is characterized by resistivities of 5-50 ohm-m, while the lower sandy layer has resistivities of 100-500 ohm-m. The corresponding interfaces are shown by dashed lines. The field between these interfaces may be interpreted as a “transition” zone, i.e. a sandy layer with different electrical properties. The results of the survey are shown in Fig. 2 as an lateral changes of the thickness of the waste layer. The map shows that the thickness of the waste layer varies from 0-30 m. The thickest part (more than 32 m) is located in the center of the map. An example of waste disposal site mapping using the CVES method with 2D inversion technique is given. This is one of the few geophysical methods that can be used successfully to map such heavily heterogeneous structures.
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Seismic investigations of waste disposals and biogas detection
Authors B. Elek, Zs. Prónay, L. Hermann and E. TörösThe investigation of waste disposals is not an easy task in geophysics. The general approximations, like homogeneous, near horizontal layers are not valid. That’s why besides conventional seismic methods new developments are necessary. On some recultivated old waste disposals the accumulation of organic originated gases (biogases) can endanger buildings, which is an other challenge for geophysics. ELGI has driven a project to try to solve those problems. The first step was to make test measurements on a typical location, and to compare and evaluate the results of different seismic methods. P-, S-wave refraction, reflection, diving wave tomography and surface-wave methods were tested. In deep seismics AVO (Amplitude-Versus-Offset) analysis seems to be the proper tool for direct gas detection, but in shallow cases the poor signal to noise ratio of reflected signals makes its application impossible. In highly inhomogeneous medium new methods needed. The first results are promising, on the test location the gas accumulation could have been detected and evaluated by drilling. In the recent years we want to continue the project to investigate more waste disposals for further testing of our method.
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Application of induced polarization and radiomagnetotellurics to a waste site in Nortii Cologne
Authors M. Hönig, S. Recher, B. Tezkan and F. M. NeubatuerThe application of geophysical methods to waste site exploration becomes increasingly important. On a waste site in Longerich measurements have been taken out using time domain dipole-dipole induced polarisation and frequency domain EM (RMT). The Time Domain IPequipment consists of a transient electromagnetic recording system and a portable transmitter whereas the RMT-method uses remote radio stations (10-300 kHz). The aim of the poster is to show first results of the survey and some details about the IP-equipment and measurement setup.
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Complez geophysical survey of an abandoned soviet military area polluted by the LNAPL in the creatceous of Bohemia
More LessIntroduction In the frame of the Czech-Italian joint research project "Groundwater movement and contamination risk of hydrogeological structures", a set of geophysical methods has been tested in an abandoned soviet military area with an airport for fighters near to the provincial town Mimon. Pollution history The selected test site has an extent 300 m2 and is situated in the area of the railway terminal, where the hydrocarbon products have been transferred from tank wagons to underground storage tanks. The leakage of tubes and bad insulated high volume tanks - getting rusty during the 30 years long period and finally being perforated - caused serious groundwater LNAPL pollution with first meters of the kerosene at the water table, with vadose zone pollution and traces of dissolved hydrocarbons in groundwater. The remedial activities (pumping fluids from clean-up wells situated in the dense network 10 x 10 m, bioventing, venting) are running continuously from 1993 under the supervision of the KAP Company. Under present conditions, the light heating oils and kerosene contaminate irregularly the vadose zone, in addition - after 3 year remedial activities - create thin layer above the groundwater level
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Geophysical monitoring for the Hungarian uranium remediation
Authors Zs. Berta, J. Csicsák, Zs. Koleszár and A. VárhegyiAfter the abandonment of uranium mining and ore processing activity in Mecsek mountains, a throughout close-out and remediation work has been started which is one of the biggest environmental protection project of Hungary. The sources of pollution are the objects created during the 40 years mining and milling operation, the open-air deposition of the waste material from the exploited cca. 18 million m3 rock. The most important polluting sources are the underground mines, the mining and milling facilities and areas, the transporting roads and pipelines, the waste rock piles, the alkaline heap leaching piles and first of all the two tailings pounds. In addition to the conventional mining remediation tasks the neighbourhood of a big city (Pécs), the contamination risk of its only underground drinking water reservoir and the radioactive pollution of the environment are also arise. The operated environmental monitoring system is unichronic with the Mecsek uranium industry. In that respect it preceded the usual practice of the age, and recently plays a particular role in the Hungarian environmental geophysics. Using the historical results of the monitoring system, completing with other surveys, a comprehensive environmental study has been created, which determine the most important tasks for the next 5 years. The study summarises the features all of the polluting objects (quantity, composition, radioactivity, horizontal and vertical delineation etc.) and risks, outlines the basic principles of environmental (first of all water) and radiation protection, and determines the main remediation tasks together with the related monitoring system.
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High resolution electrical monitoring of fluid flow through the unsaturated zone of a sandstone aquifer
Authors V. Hatzichristodulu, R. Barker and J. TellamThe unsaturated zone is very important because it influences recharge to the underlying aquifer and controls the transfer of pollutants to it. The typical methods by which the unsaturated zone has been studied are tensiometer and neutron probe field measurements. However these methods are invasive, laborious, time-consuming and do not provide spatially continuous information. Surface geophysics goes some way to eliminating these disadvantages. The aims of this work are to test the usefulness of surface high resolution electrical tomography in mapping the 3D movement of water in the unsaturated zone and to provide further understanding of the processes involved in the unsaturated zone flow.
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A wire net design for electrical monitoring of barriers
Authors C. Bernstone, T. Dahlin and P. UlriksenA wire net concept for monitoring of environmental barriers, aimed for permanent installations, is described. This presentation includes results from a laboratory study and from an installation constructed below a brine pond mineral liner at the Filborna landfill in Southern Sweden. A widespread leachate management strategy is ”containment and collection”, which implies that the detection of leakage at early stages from containers of hazardous liquids is a matter of great concern. Development work to construct effective and reliable systems for leakage detection based on geophysics has been underway since mid 1980s, commonly by using DC resistivity. The work on developing a new concept for electrical monitoring of impermeable barriers has been a part of research program at Lund University, with funds from the SEPA. The work started in late 1996 and runs as “Geophysical mapping and monitoring of waste deposits”. A Licentiate thesis has been produced [1] and this spring monitoring results from Filborna has been presented.
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Integrated geophysical survey for the detection of hydrocarbon pollution
Authors A. Godio and G. MorelliGeophysical methods provide useful tools to detect and map large areas potentially subjected to hydrocarbon pollution; many technical papers on the use of different geophysical techniques are reported in literature (Atenkwana, 1998, Godio and Morelli, 1998). Electromagnetic mapping, GPR investigation and electrical methods have proven to be effective for detecting polluted areas. An extensive survey has been undertaken on contaminated sites in Northern Italy over the last two years. The site assessment using the hydro-geological approach and destructive test (drillings) was completed with integrated geophysical investigations. The survey involved the characterisation of the subsoil assessment in some test sites, where oil recovering wells, tanks and pipelines were located. The main goal of the combined investigations was to find the presence of hydrocarbon pollution due to the leakage from the old tanks and pipelines. Because of the geological setting of the region, the main aim was to localise the free hydrocarbon phase above the shallow aquifer (2-3 meters in depth) and the presence of contamination in the vadose zone. Due to the long permanence of hydrocarbons in the soil (probably more than 20 years), chemical and biological reactions could have occurred in the polluted zones; these phenomena could complicate the interpretation of the geophysical response. A short description of the results of the geophysical tests is here reported: the selected examples relates to the survey performed on an area of about 50 x 100 m, previously employed to collect gas and oil. Most of the cultural features, such as tanks and pipelines, were removed from the site. The soil and water sampling confirmed the suspicion of the existence of a high contamination. The main goal of the geophysical survey was to verify the extension of the polluted zone, in order to plan a subsequent recovering of the area. Different methods were employed: a preliminary fast mapping was carried out using the low induction technique (LIN); the CM031 (Geophyzica) instrument in a dipole vertical mode was used. Ground probing radar (GPR) acquisition, which operates in the range between 100 MHz up to 500 MHz, was performed. Finally, electrical resistivity tomography (ERT) was carried out to confirm the response of the former geophysical investigations. Electrical measurements were performed employing the geo-electric intelligent nodes scanning system Syscal R2 (IRIS instrument). An electrode spacing of 2 meters was adopted in the dipole-dipole and Wenner array configuration for a total of 32 electrodes. The data processing involved the mapping of the in-phase and conductivity (quadrature component) of the electromagnetic response of the LIN acquisition; the GPR data were processed in order to obtain the time-slices of the reflection amplitude. ERT data were processed using a least square inversion (2D) with smoothness constrains, according to the procedure described by Morelli and Labrecque (1996).
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Contamination plume evaluation of two ditches using frequency domain electromagnetic method
Authors F. P. O. O. Figueiredo, F. A. Ferreira and M. J. Senos MatiasThe study site is located at 6 km NW of Coimbra, central Portugal, near two ditches: North ditch and the old Mondego river bed (Fig. 1). These ditches have been receiving sewage water from Coimbra municipality, causing severe environmental problems, such as ground-water contamination. This contamination is affecting agricultural and arboretum activities, which are the main use of the surroundings areas. In this area, the upper bedrock is of Cretaceous age and consist of siliciclastic sediments, limestones and marls, overlaid by recent sediments of the Mondego river. A geophysical survey, using electromagnetic method in frequency domain, was made near the two ditches with contaminated water. The purpose of this survey is to obtain information about the location of the conductive plume induced by organic and chemical pollution. This information will be used to locate fresh water aquifers, in order to provide the suitable location sites of water wells drilling.
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Comparison between geological models based on resistivity data and resistivity-IP data
More LessA geophysical study of a contaminated land area is presented. The main objective of this paper lies in the IP (induced polarization) technique — compared with EM and PVES (profile-vertical electrical soundings), what additional information on the geology and contamination is obtained when also the IP data is acquired? In glacial sediments lacking clay minerals, the induced polarization effect depends mainly on the grain-size distribution of the material and the resistivity of the pore-water. Vanhala (1997) measured large phase shifts to silts and fine-grained sands (Fig. 1), but small values to coarse sands and gravels, and also to clays. Very similar behavior has been reported also by Iliceto et al., (1982) and Dudas et al., 1991.
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Environmental examination of tailing ponds by geophysical methods
Authors I. Kaszás and J. StickelThe "Alkaloida" pharmaceutical factory located at the village of Tiszavasvári was established in 1927. Its main profile was to produce morphine from dry poppy-heads. During the decades, the activity of the factory and production of goods grew continuously, thus parallelly the amount of industrial wastes increased as well. The largest modification in the production circle took part in 1970 when the production of synthetic plant-protecting agents, fungicides and herbicides started, in addition to the traditional products of pharmaceutical industry. Until 1964, the communal and industrial waste water produced by the factory has been run into the Hortobágy Main Canal, after a. simple precipitation. Later the toxic waste waters which could not be cleaned biologically, were deposited in the abandoned clay pits of the neighbouring brickworks. The remediation program of the site started in 1997. The general task was to dewater the whole area, to separate it from the surroundings by slot walls, and later a complete recultivation. We carried out reconnaissance geophysical measurements in the site, the results of which were sufficient to start a detailed survey. The morphology of the site is quite plain, surface shaping by rivers can be detected, although these traces have been mixed with eolic sediments. The waste water tailing ponds are located on a plain ridge. The ponds are surrounded by earth dams. The River Tisza runs about 10 km far from the site, the most important irrigation canal of the country, the East Main Canal can be found at a distance of 1.8 km. Near the ponds to the south there are apple plantations, and about 200 m to the southwest fish pond are to be found. Thickness of the strongly stratified Pleistocene sediments is about 150 m. According to the different phases of fluvial sedimentation highly permeable sand and gravel layers alternate with impermeable layers of clay, silt and mud. A number of information on the near surface layers are available: archive drilling data and results of Engineering Geophysical Soundings, an advanced version of Cone Penetrating Tests. Moreover, aerial photos of the site taken in·1956 and 1994 are available in a scale of M = 1 : 5000. On the basis of investigation of the monitoring wells the groundwater table is between 1.0 and 2.0 m in the average, the direction of the groundwater flow is W. The task of geophysics was to delineate the deposited contaminating material both in horizontal and vertical directions, and to detect the pollution occurred. Moreover, the depth, thickness and hydraulic conductivity of the impermeable clay layer should also be determined. An important requirement was that the shallow impermeable clay layer be unsettled in the possible slightest degree. Ground geophysical methods meet these demands; four of them were chosen:
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Wavelet technique applied to localisation and interpretation of electrical signal distortions produced by pollutants in soils
More LessOn the bases of previous laboratory results, we performed and tested a Geoelectrical Integrated Method (GIM) to investigate and monitor the presence of chemical and organic pollutants near riverbed, water tables, and urban waste deposits. The GIM is based on the electrical parameters obtained both at frequency and DC conditions in laboratory and in `in situ'. So doing no destructive and low cost investigations allow find the presence of pollutants and evaluate the volume of polluted terrain. In particular it will be possible discriminate macromolecules of organic pollutants from metallic ions. To find the class and the quantity of pollutants in soils and rocks we can use the different electrical behaviour of organic compounds and metallic ions at the solid-liquid interface as function of electrical frequency in the DC÷200kHz range. In particular the electrical behaviour of polluted soil samples resulted strongly no-linear and consequently the output waveform are distorted. We used the classical THD% (% Total Harmonic Distortion) and the HD%(ω) parameters to quantify the deformation level. This method allows to identify the frequency interval of nonlinear electrical behaviour, that is strictly dependent of the pollutant molecular size and give an "electrical signature" of the pollutant-mineral interaction.
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Electrokinetic treatment of cadmium spiked clays. influence of the heterogeneities.
Authors P. Marceau, P. Broquet and P. BaticleRecently, attention has focused on electrokinetic techniques that make it possible to move and extract contaminants from fine-grained soil under an electric field (Figure 1). The application of a constant electric current has several effects: (1) electrolysis of water, plating reactions and gas formation occur at the electrodes. H+ is produced at the anode, and OH- at the cathode (2) the electric potential difference leads to electroosmosis , the pore water flow is toward the cathode, since most soils have a negative surface charge (3) the electric field initiates electromigration of species available in the pore fluid and of those introduced at the electrolytes. These phenomena change the chemical pore fluid composition and induce sorption reactions in the soil. Bench scale studies showed that inorganic species and heavy metals such as arsenic, cadmium, chrome, copper, iron, lead, mercury, nickel and zinc can be efficiently extracted from polluted or spiked soils (Pamucku and Wittle, 1992; Acar and al., 1995; Cox and al., 1996; Reed and al., 1995; Li and al., 1996; Acar and Alshawabkeh, 1996; Marceau and al., 1999). Extraction rates of over 90 % are reported. But species seem to precipitate with OH- near the catholyte. Organic species such as benzene, phenol and acetic acid can also be removed (de Marsily and al., 1992; Bruell and al., 1992; Acar and al., 1992).
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Time-Lapse resistivity imaging inversion
By M. Heng LokeIn studying the changes of the subsurface resistivity with time, two-dimensional resistivity imaging surveys are often repeated over the same line at different times. Such studies include the flow of water through the vadose zone, changes in the water table due to water extraction, flow of chemical pollutants and leakage from dams. Normally, the data from the surveys conducted at different times are inverted independently, frequently with a smoothness-constrained least-squares inversion method (deGroot-Hedlin and Constable 1990). The changes in the subsurface resistivity values are then determined by comparing the model resistivity values obtained from the inversions of an initial data set and the later data sets. In many cases, such an approach has given satisfactory results. However, in theory, since the inversion of each data set is carried out independently, there is no guarantee that the differences in the resistivity values are only due to actual changes in the subsurface resistivity with time. Each inversion attempts to minimise the difference between the observed and calculated apparent resistivity values for an individual data set without taking into account the model obtained from the initial data set. The use of joint inversion techniques using three different types of cross-model constrains is investigated in this paper. The model obtained from the inversion of the initial data set is used as a reference model to constrain the inversion of the later time-lapse data sets. Firstly, a simple damped least-squares constrain to minimise the differences in the model resistivity values between the initial model and the time-lapse model was used. The second method uses a least-squares smoothness constrain to ensure that the differences in the model resistivity values vary in a smooth manner. Thirdly, a robust (Claerbout and Muir 1973) smoothness constrain which minimises the absolute changes in the model resistivity values was used. Figure 1a shows a test model where the base model has a 50 ohm.m faulted block at the bottom-left side with a surrounding medium of 10 ohm.m. In the time-lapse model, a small rectangular block with a resistivity of 20 ohm.m was added. This is a relatively difficult test model as the apparent resistivity anomaly caused by the faulted block is much large than that caused by the small block (Figures 1b and 1c). Figure 2 shows the differences in the resistivity of the models obtained from the inversion of the initial and the time-lapse data sets. Ideally, the difference section should show a value of 100% where the small block is located and 0% elsewhere. The difference sections for all the inversion methods do show large positive values of over 70% at the location of the small block. However, the section obtained with independent inversions with no cross-model constrains show significant changes in other areas, particularly near the upper-right corner of the faulted block (Figure 2a). The distortions are smaller with the damped least-squares (Figure 2b) and the least-squares smoothness crossmodel constrain (Figure 2c) methods. However, the best result is obtained with the robust smoothness cross-model constrain method (Figure 2d).
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Effects pf dispersive conductivity on shallow em response
Authors W. Frangos, C. Scott and H. F. MorrisonElectromagnetic (EM) geophysical exploration methods, both time and frequency domain, have historically operated in two disparate frequency bands. Methods such as magnetotellurics (MT), CSAMT, loop-loop, and TEM work at frequencies of a millihertz to tens of kilohertz, investigating depths from several tens of meters to tens of kilometers. Wave propagation methods such as Ground Penetrating Radar (GPR) operate at frequencies above a few tens of megahertz, with exploration depths of a few meters. The intervening gap in exploration frequencies is critical to much near-surface geophysics as practiced by the environmental and hydrologic geophysics communities. The transition between the diffusion and wave modes of electromagnetic propagation occurs in a small spectral interval of about one decade of frequency. Below the transition, displacement currents are negligible and conventional diffusive electromagnetic theory provides an adequate description, while above the transition, conduction currents are small and wave propagation theory applies. The rock properties conductivity and dielectric permittivity are frequency dependent, i.e., best described as complex quantities over portions of the spectrum with conductivity increasing with frequency while permittivity decreases. At issue for any work in the transition portion of the spectrum is how the dispersive behavior of the properties may influence the observed data in electromagnetic surveys for delineation of targets associated with the shallow subsurface.
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Enviromt. A new radio/controlled source system
Authors L. B. Pedersen, L. Dynesius, M. Bastani, M. Gharibi and U. MatzanderNovel features of the EnviroMT system Broad band radio-signal reception For the estimation of transfer functions we only make use of the signal from distant manmade sources in the form of VLF and Radio transmitters in the LW and MW band in contrast to standard MT where the whole frequency band is considered as plane wave sources perhaps mixed with non-planar events. In Northern Europe there are generally more than two independent transmitters available in each half-octave/octave band that transfer functions can be estimated starting at 14 kHz. Controlled source with synchronous detection Whereas the standard estimation of transfer functions in the RMT mode is based upon the assumption that they are constant in each sub-band no such assumption is necessary in the controlled source mode. Here, unique transfer functions can be found right at the transmitter frequencies chosen because the transmitter consists of two independent horizontal dipole coils. Even though these transfer functions are most easily interpreted in the plane wave (or far field) case they can still be uniquely defined in the intermediate and near field range. Built in database handling In addition to showing pseudo-sections the system also provides a facility for automatic 1D inversion of selected impedance combinations (off-diagonal elements, determinant, etc.) along selected profiles. In the future even a fast 2-D inversion code will be added to the system to make it more flexible and self-contained. All the above mentioned manipulations can be done in-field. The built-in colour display unit is of excellent quality and with the fast built-in computer, in a few minutes the user can get a complete overview of the data collected along various profiles.
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Continuously recording radio-frequency electromagnetic (RF-EM) method (15–300 KHZ) without ground contact: A powerful tool for groundwater vulnerability mapping in fissured rocks
Authors F. P. Bosch, P. Turberg and I. MüllerIntroduction Density and size of open fractures, which connect the water reservoir with the surface, are main features for characterizing the vulnerability of fractured aquifers. The Hydrogeology Centre, University of Neuchâtel has developed a very fast and precise geophysical survey technique to locate and extensively map these types of high-permeability fracture zones: a Radio Frequency Electromagnetic (RF-EM) system. Theoretical background The RF-EM method measures the earth’s electromagnetic response (secondary field) induced by radio signals emitted from terrestrial transmitters (horizontal, primary field). These transmitters are used for navigation and submarine communication in the frequency range of 15-300 kHz. The induced secondary field has components that are oscillating both ‘in phase’ and ‘out of phase’ with the primary field. The intensity of the vertical component of the secondary field depends on the size of the conductive body. Therefore electrically conductive features such as geologic faults and fractures are detectable. In particular using the higher frequencies (up to 300 kHz) enables fracture detection in the epikarstic zone. Our prototype instrumentation continuously measures both fields with two orthogonal induction coils and
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Electromagnetic measurements on the surface of a lake
More LessThe contamination of lakes and rivers by industrial activities constitutes a serious problem for human health, especially when lakes are used for swimming an fishing. The radiomagnetotelluric technique (RMT) has been successfully applied for the exploration on waste sites, the detection of contaminative plumes and for archaeological prospection. (Tezkan et al., 1996, Zacher et al., 1996, Recher et al., 1998) In order to test the applicability of the RMT technique for the geophysical investigation of lakes and rivers, RMT measurements were carried out on a profile crossing from one shore to the other shore of a lake close to Cologne in Germany. Fig. 1 demonstrates the field setup of the RMT survey on a lake schematically. A small boat was used for the survey and civilian and military radio stations were used as transmitters broadcasting in frequency range between 10 kHz and 1 MHz. The magnetic field was measured by a coil in the boat and the electric field by an electric antenna placed on the lake’s surface. Apparent resistivities and phase values were derived from the measured magnetic and electric fields so that a sounding information was available for each station. The data were interpreted by a 2D inversion algorithm (Mackie et al., 1997). Thus a 2D conductivity model was derived for the investigated lake and its surrounding host which is demonstrated in Fig. 2. The lake can be seen as a strong conductive anomaly in the 2D inversion model.
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The TEM arbitrary impulse equipment
Authors M. I. Epov, E. Y. Antonov and I. O. IssaevIntroduction Transient electromagnetic (TEM) sounding is an important method for environmental investigation and engineering geophysics. The similar existing TEM techniques proved to have, along with many advantages, numerous essential limitations, such as: -difficulties in studying the upper part of the section close to the surface; this is because only the late stage of transient process is interpretable; -inapplicability of many statistical methods for noisy data processing because of logarithmic time-step used and only accumulated data registered; -poor spectrum of common used signal (turning on/off of current). In this paper we describe an equipment for measuring and a method for interpreting the nearsurface TEM data. This method is based on using the arbitrary fully controlled current impulse. All soundings are registered separately with arithmetic time step. It is shown, that with technique suggested, one can overcome the above pointed difficulties.
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3-D Electromagnetic research of the structure and state of the rock massive
Authors O. A. Hachay, E. N. Novgorodova and A. Yu. HachayIt is offered a new approach to the research of high complicated geological media, which are rock massives [1-2]. That approach is based on the theory of 3-d interpretation of alternating electromagnetic fields. The effect of it's use for the research of rock massives in mines and the possibility of structure identification and state of it is shown , using natural observations. It is offered a new effective method of the electromagnetic monitoring of inhomogeneous rock massives, which are influenced by intensive technogenic forces. There had been analysed and outlined parameters, which characterise the massive as a whole, and that, which are sensitive to local changes into the massive, which can be a potential dangerous zones and must to be systematically observed by additional geophysical and geomechanical methods. That parameters are: the geometrical distribution of apparent resistivity (fig1.c points - before rock outworking and squares - after rock outworking) and profile distribution of the average parameter of geoelectrical inhomogeneities (fig 1. a – before rock outworking and b – after rock outworking).
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3-D Resistivity and IP probability tomography in near-surface geophysics
Authors P. Mauriello and D. PatellaIn a previous paper (Patella and Mauriello, 1998) we extended to resistivity and induced polarization (IP) measurements the principles of probability tomography (Patella, 1997). In this work, we deal with the 3-D case for both types of data and propose a method for a combined tomography, always based on a probabilistic point of view. In the present formulation we start from the tensorial analysis of geoelectric data (Bibby, 1986), now extended to the IP case. In fact, from the apparent resistivity or chargeability tensor it is possible to obtain tensor invariants, which are independent of the direction of the source field and well confined in the neighbourhood of the causative bodies. In particular we focus our attention on the trace tr(A) of the data apparent anomaly tensor. Implementing the mathematical backgrounds reported in the previous paper (Patella and Mauriello, 1998), we write the departure of the measured tensor trace from that calculated on the basis of a reference model as:
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Electrical anisotropy parameters and resistivity sounding interpretation
More LessIn general, resistivity sounding data interpretation assumes a homogeneous, isotropic and horizontally layered earth model. However, in many field situations these conditions are not valid as the complexity of local geology can originate strong orientation effects caused by lateral inhomogeneities and anisotropy. Under these circumstances it is good practice to keep constant the orientation of the electrode array in use and therefore orientational variation of resistivity data is often ignored. If the strike of concealed formations is known it is possible to choose an array orientation perpendicular to the strike and to obtain further measurements perpendicular to that orientation. This procedure allows a better description of the area but in regions where moderate to strong anisotropic effects exist can lead to results such as the ones shown on Fig.1.
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Two-dimensional resistivity inversion using a new topographical correction method
Authors M. Varga, A. Kovács, C. Gáspár and S. GisbertIn shallow geoelectric exploration, e.g. in problems occurring in environmental and engineering geophysics, the topographical effects can be larger in measured values than changes caused by resistivity inhomogeneities to be found. In the simplest topographical correction method the forward problem is solved for homogeneous halfspace having actual topographical relief and the measured apparent resistivities are corrected before interpretation with coefficients calculated for homogeneous halfspace. This method is inaccurate if large resistivity variations occur near to the surface and it is problematic to represent the results of two-dimensional inversion (section prepared from cell-resistivities) taking into account the topographical data, too. Therefore it is very important to use real topography both in the forward and inverse modeling but using conventional 2-D methods it is very time-consuming. To solve the reconstruction (inverse) problem by e.g. a Newton-type method a number of direct problems have to be solved. Therefore the choice of the applied solution method of the direct problem is of great importance. Here we used a numerical method adopted from the computational flow modeling which makes it possible to reduce the computational cost by a remarkable amount. The method is based on a non-equidistant, non-uniform but Cartesian cell-system (computational grid) generated by the so-called quadtree algorithm. This algorithm came from the computational graphics and is a heart of the “unconstructed grid generation”. Special finite-difference schemes have been defined on the above computational grid. To speed up the computations a simple but efficient multigrid technique has been developed in the quadtree context. It is shown that to solve the direct problem (2-D Poisson equation) the number of the necessary arithmetic operations is proportional to the first power of the number of the surface points only which results in a much more economic method compared with the traditional solvers. Grid shown in Figure 1 is generated automatically using only some control parameters for a halfspace having rough enough topography and two buried inhomogeneities. This is only an initial grid because by the help of multigrid technique using e.g. 10 levels every cell will be split up into 1024 small cells. On the earth-air interface and near the boundaries of inhomogeneities the size of cells is small already in the starting grid and going away from this regions, which are the most interesting in the description of the resistivity distribution, the size of cells increases gradually. Of course the presented grid will be expanded both on the left and right to fulfill by solution the prescribed boundary conditions. On the upper figure that grid can be seen which was generated automatically on the prescribed rectangle. However during the solution of forward problem those cells are not taken into account which are in full extent in the air so only the grid shown in the lower figure is used in the forward ( and inverse ) modeling. During inversion conductivities of the cells of the initial grid are determined from the apparent resistivities measured by a multi-electrode system using electrodes laid down equidistantly along real topography. It is important that the effect of topography is taken into account not by applying a topographical correction on measured data before inversion using a homogeneous halfspace having the actual topography but the real topography is used in every step of inversion.
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Problems in interpretation of resistivity sounding data, distorted by geological noise
Authors V. A. Shevnin, A. A. Bobachev, I. N. Modin and E. V. PervagoThe traditional resistivity sounding method deals with isolated sounding sites located casually on profile or area of investigation and executed with logarithmic step in distance growth. This technology has very restricted possibilities. Therefore it has begin to exchange practically simultaneously in different countries into new sounding technology, called multielectrodic sounding, or resistivity tomography, electrical imaging, etc. Its main advantage - more dense survey along profile - is clear visible, while the other - overlapping system of measurements is not so evident. Our group in MSU began to apply multi-electrodic resistivity sounding to overcome distorting influence of geological noise. From the end of the 80-th we have been working in areas with high resistivity contrast along both vertical, and horizontal direction. Such sounding data have different features of distortions or differences from response for horizontally layered models. At that period we developed theory of distortions of DC electric field by deep and near-surface inhomogeneities (Electrical..., 1994) and software to establish VES data distortion with the help of various indicators, noticeable on VES curves. We have found, that more than 70 % of all VES curves are distorted. That means, that the distortions are practically constant features of resistivity sounding. Carrying out the large volumes of VES field data and their interpretation (up to 1000 VES sites a year) we analyzed misfit errors of VES curves interpretation. The average value of misfit error (average RMS or D , where D is dispersion) consists 8-12%, that seems rather high value and needs to be explained.
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The resistivity grid to optimize tomographic 3D imaging
Authors P. L. Cosentino, R. Martorana and L. M. TerranovaIn this paper a modified version of the old resistivity "AB rectangle" is presented. Here the potential electrodes are located along a regular grid of suitable dimensions, having a triangular or square unitary cell. The current electrodes, instead, are generally located outside of the grid (see figure below), and various directions of the transmitting dipoles may be selected so to optimize the lighting of the subsoil under investigation (as expected the results improve by increasing the number of the directions).
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Field meauserements by using geoelectrical nullarrays on fissured limestone in the Swiss Jura
Authors S. Szalai, F. Bosch, P. Turberg, I. Müller, E. Prácser and L. SzarkaUntil now no systematical field studies have been carried out by using geoelectrical arrays, having zero potential difference over a homogeneous halfspace. We call hereafter such configurations as null-arrays. In this paper we present field results obtained over near-surface fissures in limestone by using three different null-arrays. The null-arrays were selected according to three possible types of the current distribution around the current electrodes. In the first type of the current distribution the current is propagating radially. In the second one the current lines are more or less parallel with each other. In the last one the current flow is considered as it would be of a dipole origin. The first type of null-arrays was realized by a three-electrode geometry. The second type was realized by the so called Schlumberger null-array and in the last case both dipole equatorial and dipole axial configurations were transformed into null-array geometries. The connecting line of the potential electrodes in the null-arrays are always perpendicular to that in the classical arrays. This feature made it possible to obtain a very quick joint application of these two array types and also to compare the results between the traditionally used arrays and the null-arrays. All geoelectric profiles were evaluated in a close vicinity of a quarry, where the fissures were directly seen. As we found, the null-arrays located the fissures with a higher accuracy and provided a more significant anomaly than the classical configurations did, especially in case of a thicker overburden. Since the null-arrays and classical arrays give complementary information we recommend their combined use in such near-surface studies. Besides the theoretical aspects, these field test measurements have been found to be very useful in hydrogeological applications, since fractures and fissures are the most important geological features for determining groundwater vulnerability.
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Resistivity surveying as pre-investigation method for the hallandsas tunnel project
By T. DahlinResistivity surveying with combined profiling and sounding, also called continuous vertical electrical sounding, has been carried out along large parts of the planned railway tunnel through Hallandsås using multi-electrode data acquisition equipment. The measured data was processed by means of inverse numerical modelling. (Dahlin 1996) The first surveying was carried out over the Southern Marginal Zone in 1995 as part of complementary investigations of the rock quality (Dahlin et al. 1999). Due to the good results achieved, the technique has since been applied along most of the 8.6 km long tunnel line. In the surveys carried out in 1998 larger electrode separations, with 800 metre cable layouts, were used for increased depth penetration. Expanded measurement protocols were also employed for enhanced resolution. The results (see figure 1) display a strong variation in modelled resistivities along the line, where in particular the Southern Marginal Zone and the Möllebäcken Zone stand out as low resistive zones. Apart from these, several low resistive zones of smaller extent are evident, which can be interpreted as fractured or weathered zones. Limited parts of the tunnel stretch exhibit the high resistivities that are normally expected for crystalline rock of low fracture density below the groundwater level. The resistivity models were used, along with other available geophysical data, as a basis for creating conceptual models of the structural geology and the variation in quality of the rock. These models were used as a basis for siting test drillings, to verify and refine the conceptual models. A comparison with core drilling data shows that the resistivity results give a good overview of the structural geology and variation in relative rock quality. It was, however, not possible to correlate e.g. Q-values with resistivity on a detail level. This is partly dependent on the differences in scale and resolution of the two types of data, but there may also be a lack of a simple connection between Q-value and resistivity. It is quite possible that a better correlation could be achieved with a different parameter as measure of the rock quality. Furthermore, the resistivity models as well as drilling data and documentation from the tunnelling (Dahlin and Sturk 1998) shows a strong inhomogeneity of the rock in Hallandsås, which means that the individual boreholes are probably only representative for a very small volume in their vicinity.
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SNMR test measurements in Austria
More LessIn December 1997 the NMR- method (NUMIS-equipment from IRIS-instruments) was tested the first time in Austria for groundwater research. NMR soundings were carried out in two different areas of Austria. The locations were chosen at sites where other geophysical methods, especially geoelectric multielectrode profiles, had been used before, to correlate the results with the findings of these methods. Unfortunately at location Pulkau the proton resonance frequency, calculated from the measured local magnetic field, was a multiple of the 50 Hz frequency. Therefore no reliable measurement was possible in that area due to the high background noise. In the area of Marz a total of 9 NMR soundings were carried out using both square loop and eight-shaped-loop. At two locations also no measurements were possible because of the high background noise of 6000 to 60000 nV. Only at the location “Marzer Kogel” reliable measurements were possible at some points, e.g the Paulinerkloster. In this area an intensive geoelectric research program was carried out for groundwater research purposes the year before. The results of the NMR sounding at the location Paulinerkloster are shown in fig.1. Here only the eight-shaped-loop gave good results. The inversion results show a constant water content of 20% between 12 and 50m. Two months before a multielectrode geoelectric profile was measured crossing the location of the NMR-sounding. The results are shown in fig.2. At the location of the NMR-sounding a high resistive area down to about 80 m can be seen. The results of both methods were so promising that in the summer of 1998 a borehole was made there. The results of the drilling fully confirmed the findings of the geophysical research. As the quantity and quality of water is sufficient, a local water supply station will be planned for the near future.
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Detection and estimation of problem zones in salt rock with the aid of underground geophysical exploration methods
Authors J. Kulenkampff, L. Aschmann, E. Moïse, F. Jacobs, G. Kurz and B. FlucheHaving a very low permeability, a high plasticity, and a high heat conductivity, salt rocks are principally suited for deposits of toxic and radioactive waste. In Germany exist a big number of salt domes and bedded layers which could serve for underground storage of hazardous waste. However, recent tectononic fault zones and areas of high water content may bear severe risks for mining and underground deposits, which are mainly due to the high solubility of salt rocks. Therefore reliable detection of fault zones and humid areas with non destructive methods is crucial for the appraisal of underground nuclear and toxic waste deposition sites in salt rocks. A qualitative detection of such problem zones with non destructive geophysical methods is possible. But it is not yet reliable enough for legal purposes. The security appraisal of hazardous waste deposits based only on non destructive geophysical methods not seems possible yet. In the past, these methods were usually applied as stand alone methods. In the frame of a research program of the German Research Ministery (BMBF) a complex interpretation system of geophysical methods shall be developed, including seismics and ultrasonic acoustics, high and low frequency electrical methods, in order to reduce the uncertainty of the geophysical results and to approve a more quantitative evaluation.
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An other Gis: The national geoelectric information system
Authors L. Sőrés and E. PrácserThe extensive use of electromagnetic geophysical methods in Hungary was started in the 50- ies by the Eötvös Loránd Geophysical Institute (ELGI). Tens of thousands of DC soundings have been measured, reported and closed to files since the first systems started to collect geophysical data. Long before the PC age, most of them were processed by hand, using precalculated curve albums. In the store-rooms of ELGI 40 meters of geophysical documentation was sleeping it's Rosen-bud dream and waiting for the kiss of modern information technology. In 1998 a long term data archiving project has been started in the ELGI. The aim of this project is to save the tremendous amount of geophysical information that has been collected in Hungary during the last decades, to build it into a GIS - database system and bring it back to life. The database is proposed to include all available DC and TDEM sounding information and to assure access to gravity, to ground and airborne magnetic data as well. Data archiving started with the engineering geophysical measurements carried out on and around the lake Balaton from 1974. Sorting out and reorganising the old documentation, to prepare it to uploading is a very long process with high man-power requirements, but definitely pays off, if we look at the results. In 1998 about 3500 DC soundings from that area were entered, in 1999 an other 3000 is going to be entered into the system. All these data will be available again for geophysical reprocessing and environmental analysis.
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Stream-function used for current-lines' construction in 2-dimensional DC modeling
Authors A. A. Bobachev, I. N. Modin, E. V. Pervago and V. A. ShevninThe stream function described is employed for the presentation of 2D DC modeling results. The 2D model is understood as a 2D medium with linear current electrodes, oriented along the inhomogeneities' strike direction. In this case both the medium and the electric field depend on two space coordinates only. Modelling becomes much easier than considering point current electrodes, where the electrical field always is three-dimensional. Meanwhile the actual results of such modelling are qualitatively equivalent to 3D modelling with point electrodes, as long as the measurements are conducted across the objects. The classical modelling presentation is in apparent resistivity which reflects an electric field distribution on the earth's surface. Quite often the connection of measured anomalies with a geoelectrical model is rather complex (fig. 1, A and C). The visualization of DC current lines simplifies understanding of the electric field's structure. Current lines are used in almost each textbook, but a practical techniques for their construction is usually not included. The evident way for drawing current-lines is the step by step continuation of a line from some point along the electric field direction. The practical realization of such approach is not trivial. For a 2D field it is possible to make use of the stream-function. This function is often used in EM field modeling [flux function, Berdichevsky, 1984]. A contour map of the stream-function corresponds to the stream-line distribution. Thus the problem of current streamlines' construction is reduced to the calculation of the stream-function in the research area. This can be achieved by calculating secondary surface charges, which are determined by 2D modeling, using Fredholm's integral equation of the second type relatively of electric field [Escola, 1979]. The stream-function’s (ψ) physical definition is the difference between stream-function's values in two points in space is equal to the electric current intersecting a curve connecting them:
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Electrical imaging over simple sediment structures
More LessElectrical imaging or tomography is a survey technique employed to build up both a laterally and vertically varying picture of the electrical properties of the subsurface. The image can therefore be related to lithology and fluid variations of the underlying sediments. The technique is now widely used in geological and resource studies, archaeological surveys and in various types of hydrogeological investigations. Electrical imaging appears to be a versatile technique and has been used here to try to determine and identify simple structures and trends within shallow Quaternary sediments.
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Recent advances in engineering geophysics Spectral IP and Multifrequency EM
Authors E. Niederleithinger and G. PlaumannThere are two main needs to improve the usability of geophysical methods for geotechnical and environmental investigations: the speed has to be increased to optimize costs and better correlation is needed between geophysical results and true subsurface parameters. Two methods - multifrequency EM and Spectral IP - are presented. None of them is entirely new, but they are going to be used in practise now. Measurements from different sites are shown.
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A new wide-band radiomagnetotelluric measurement system with HTC-Squid
Authors T. Radic and H. BurkhardtThe radiomagetotelluric (RMT) method estimates the resistivity distribution of soil by measuring and evaluating the em-fields of radiotransmitters in the very low-, long- and medium wave frequency range. The method potentialy does not require many personnel for field measurements. It has all the features for fast recording of complex geological structures. Presently available RMT instruments have serious limitations. Therefore, we have designed and tested the following improvements: Estimating full impedance tensor To explore complex shaped subsoils knowledge of full electrical impedance tensor is indispensible. We introduce a method to estimate all four tensor elements in the radio frequency range. This makes it possible to evaluate RMT-data by using conventional and powerful MT inversions programs. Fast speed in measurement Progress in measurement is determined by the time to move the instrument from one point to another, registration time and time necessary to process the time series of typically some ten million samples per channel. To minimize the registration and the processing time we build a very fast digital time series preprocessing electronics. It calculates impedance tensor for 16 frequencies in RMT range in parallel from one registration within a few seconds. Using superconducting quantum interference device (SQUID) magnetometer A liquid nitrogen cooled SQUID Magnetometer was tested. SQUIDs are invaluable for the near future especially if wideband registration with mobile instruments, as presented here, is required. Extended frequency range The RMT technique is often unable to penetrate highly conductive overburden like dump sites, clay layers or contaminated ground water. The reason for this is that the lowest available frequency is too high. Therefore, we have combined the RMT frequency range (10 kHz to 1600 kHz) with the CSAMT (frequency: 1 kHz to 10 kHz). This permits a threefold increase in the depth of penetration. At low frequencies we use a light battery-driven horizontaly orienteted magnetic transmitter to generate the em-field. This is necessary because of absence of radio transmitters below 10 kHz.
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