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Ž .Journal of Applied Geophysics 44 2000 237–256www.elsevier.nlrlocaterjappgeo

Two-dimensional radiomagnetotelluric investigation of industrialand domestic waste sites in Germany

B. Tezkan a,), A. Hordt a, M. Gobashy b¨a Institut fur Geophysik und Meteorologie, UniÕersitat zu Koln, Albertus-Magnus-Platz, 50923 Cologne, Germany¨ ¨ ¨

b Cairo UniÕersity, Faculty of Sciences, Department of Geophysics, Cairo, Egypt

Received 25 March 1998; accepted 10 March 1999

Abstract

Radiomagnetotelluric surveys were carried out on two different waste sites in order to map them and to detect thesurrounding hosts. One was filled with industrial waste and the other with household refuse. Both had been recultivated.

Ž .Powerful military and civilian radiostations 10–300 kHz located parallel and perpendicular to the strike direction of bothwaste sites served as transmitters. Hence, apparent resistivity and phase data were observed for several selected frequencies.Using this field setup, the data were associated to the E- and B-polarisation directions of the magnetotelluric field. Thelateral borders of both waste sites are located accurately in the field. They are characterized by strongly decreasing apparentresistivities observed at nearly all frequencies at the border between the waste site and the undisturbed geology. The datawere quantitatively interpreted by conductivity models using a 2D inversion algorithm. The derived 2D conductivity modelsgive information about the vertical extent of the studied waste sites and about the structure of the surrounding hosts. q 2000Elsevier Science B.V. All rights reserved.

Keywords: Waste sites; Radiomagnetotellurics; 2D inversion; Conductivity models

1. Introduction

In Europe, especially in Germany, the appli-cation of geophysical methods to waste siteexploration becomes increasingly importantsince their contribution to the pollution riskestimation has been recognized. In Germany,for example, the number of registered waste

) Corresponding author. E-mail:[email protected]

sites amounted to 93,723 in December 1993Ž .Franzius, 1994 ; among them 350 in the city ofCologne. These numerous waste sites are tradi-

Žtionally explored by geotechnical methods e.g.,.drillings . Applied geophysics can offer many

non-invasive techniques for mapping and shouldbe considered as a main source of informationfor selecting borehole locations. In general,waste sites are characterized by their low resis-tivities compared to the surrounding hosts, be-cause industrial waste and household refusecause an increase of electrical conductivity due

0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0926-9851 99 00014-2

( )B. Tezkan et al.rJournal of Applied Geophysics 44 2000 237–256238

to biological decomposition of the organic ma-terials. Since the mid-1990s, the radiomagne-

Ž .totelluric method RMT is extensively used inŽconnection with waste site exploration Dautel,

1996; Tezkan et al., 1996; Zacher et al., 1996a;.Tezkan et al., 1997 . The RMT method allows

both sounding and mapping by using severalfrequencies. The data collection is relatively fastand is interpreted quantitatively by using well-known magnetotelluric modeling programs.Several RMT surveys have also been conductedsuccessfully for hydrogeological investigations,for engineering and for archeological explo-

Žrations Turberg, 1991; Hollier-Larousse et al.,1994; Dupis and Choquier, 1996; Zacher et al.,

.1996b .Two selected RMT field surveys are pre-

sented in this paper to demonstrate the effi-ciency and suitability of the method for wastesite exploration of different types. The 2D in-version of the RMT data will be discussed indetail.

The waste site of Volkswagen in Mellendorfwas chosen as a representative example for anindustrial deposit for the RMT evaluation. Asmall number of boreholes give informationabout the vertical extent of this waste site. Nogeophysical techniques were applied for the ex-ploration of this site previous to the detailedRMT measurements. For comparison, the wastedeposit Hermsdorf filled with household refusewas studied exclusively by geophysical methodswithout any boreholes. Magnetic, DC, EM 31measurements were carried out to map the de-posit. The RMT measurements should enable adetailed conductivity depth information of thisarea by applying 2D inversion algorithm to theobserved data.

No geographic and topographic maps of bothwaste sites will be shown for confidential rea-

sons. Instead, the efficiency of the RMT tech-nique for the exploration of these different typesof waste sites and the resolutions of the 2Dinversion models are emphasized.

2. Basic concepts of the RMT technique

The RMT technique uses as transmitters mili-tary and civilian radiostations broadcasting in afrequency range between 10 kHz and 1 MHz. InFig. 1a, the principle of this method is demon-strated schematically. The electromagneticwaves radiated from these transmitters diffuseinto the conductive earth and induce electricalcurrent systems which are connected with elec-trical and magnetic alternating fields. The mag-netic field can be measured for selected fre-quencies by a coil and the electric field by twogrounded electrodes. Fig. 1b shows the RMT-instrument, a prototype developed in Switzer-

Ž .land Muller, 1983 . The magnetic coil has a¨diameter of 0.4 m and the distance between theelectrodes can be chosen to be 1 or 5 m. Thefrequency range of this instrument is limited to10–300 kHz but can be extended to 1 MHzafter hardware modifications in the future. Notransmitters are available below 10 kHz. De-pending on the conductivity of the subsurfacethis limitation might cause the near surface andthe deep layers to be unresolved by this method.This will be discussed in Section 4 by the 2Dmodeling results of the observed data.

The measuring device only weighs 7 kg andabout 3 min are required for measuring fourfrequencies at one station. Thus, large areas canbe mapped quickly.

For the selected frequencies apparent resistiv-ity and phase data are derived from the mea-

Ž .Fig. 1. a Schematic diagram for illustrating a RMT field set up over a hazardous waste site. A sounding information can beŽ .obtained by using transmitters with different frequencies from the same direction. b RMT instrument used in the field: a

Ž .prototype developed by the Hydrogeological Institute of the University of Neuchatel, Switzerland Muller, 1983 .ˆ ¨

( )B. Tezkan et al.rJournal of Applied Geophysics 44 2000 237–256 239


sured electric and magnetic field using thewell-known formulae of magnetotelluricsŽ .Cagniard, 1953

21 Ex

r s 1Ž .ax y 2pm f H0 y

ExI ž /Hyy1f s tan 2Ž .x y ExR ž /Hy

where E and H denote the electric and mag-x y

netic field, m the permeability of the free space0

and f the selected frequency.Displacement currents can be neglected for

the RMT frequencies under normal conditionsin Europe. The plane wave assumption is validwhen receiver locations are located at least sevenskin depths away from the radio transmittersŽ .Schroder, 1994 . In this case, the well devel-¨oped and tested magnetotelluric interpretationsoftware can be directly applied to the RMTdata. The main difference between the RMTand the VLF-R technique lies in the extension

Ž .of VLF frequencies 10–30 kHz to higher fre-Ž .quencies Tezkan et al., 1996 . Due to the very

limited frequency range, VLF-R is only used forprofiling and the interpretation of this data is

Ž .carried out visually Berktold et al., 1992 . Insome cases, however, VLF data can be inter-preted in terms of 1D and 2D conductivity

Ž .models Beamish, 1998 . The RMT techniqueenables a sounding which can be interpretedquantitatively. One important aspect is the se-lection of transmitters by considering the strikedirection of the waste site. Radio transmitterslocated parallel and perpendicular to the generalstrike direction given by geological or anthropo-logical structures can be selected for the givenfrequency range. Assuming a two-dimensionalconductivity structure, this data is associated to

Žthe E- and B-polarisation directions e.g., Zx y

and Z elements of the magnetotelluricy x.impedance tensors and then interpreted by us-

Žing 2D inversion techniques Smith and Booker,.1991; Mackie et al., 1997 . Scalar measure-

ments can only be carried out by the RMTsystem used in this study. The apparent resistiv-ity and phase values associated to Z and Zx x y y

elements of the magnetotelluric impedance ten-sor are not measured in the field. If no informa-tion about the strike direction is available thesame field procedure is used by choosing trans-mitters with similar frequencies from NS andEW directions and the obtained data is alsointerpreted by 2D inversion procedures whereas3D effects are not considered.

3. Selected RMT surveys for the explorationof different waste site types

In the following, two selected RMT surveysfor waste site exploration in Germany will bediscussed. An industrial waste site and a very

Žcommon type of a waste site embedded with.household refuse and building debris are cho-

sen as an example.

3.1. The Mellendorf industrial waste site

The industrial waste site of Volkswagen atMellendorf near Hanover was used from 1960to 1984. Magnesium drosses and other indus-trial wastes were stored in a sand pit. Thebottom of this sand pit was not made watertight. Magnesium drosses are a waste productfrom motor fabrications containing magnesium,magnesium salts and magnesium oxides. Theclay layer beneath the area of the waste site is ata depth of 60 m, i.e., relatively deep. Betweenthe surface and the clay layer a sand layer ofQuaternary age was geologically mapped con-taining thin clay layers in some localities. Thewaste site was recultivated in 1984. Fig. 2shows a lithological stratification beneath thesurvey area derived from boreholes on the waste


Fig. 2. The structure at the industrial waste site in Mellen-dorf derived from borehole results.

Ž .site Renno and Christofzik, 1987 . The top andthe bottom of the waste site is expected at 1.6and 16 m depth, respectively. During the recul-tivation the waste site was covered by a layercontaining soil, sand with gravel, clay and grav-

Ž .els Fig. 2 .For the RMT survey four frequency pairs

between 16 and 200 kHz from radiostations inNorth–South and East–West directions werechosen. The profile direction was East–Westwhich was perpendicular to the assumed strikedirection of the waste site. The distances be-

tween the profiles and the RMT stations were12.5 m, except at the border between the wastesite and the undisturbed geology, where thedistance was 5 m.

Figs. 3 and 4 give an overview of the lateraldistribution of the apparent resistivities andphases for 207 and 177 kHz. The triangles onthe figures denote the locations of the RMTstations. The 207 kHz data belong to the trans-mitter located in North–South direction. In thiscase, the measured electric field associated withthe E-polarization is oriented in NS direction.The electric field of the 177 kHz data is ori-ented in the direction of the profile and repre-sents the B-polarization. The waste site as awell conducting anomaly in comparison to thesurrounding host is characterized by apparentresistivity values -30 Vm for both polariza-

Žtion directions e.g., 207 and 177 kHz apparent.resistivity data and all lateral boundaries of the

waste site are clearly detected. A clear phaseŽ . Ždifference outside Ff508 and inside Ff

.358 the waste site is also observed.Figs. 5 and 6 show the apparent resistivity

and phase data for all observed frequenciesfrom NS and EW transmitters for the profile

Ž .ys0 Figs. 3 and 4 as a representative exam-ple. The boundaries of the waste site are charac-terized for both transmitter locations by stronglydecreasing apparent resistivity values at profileposition 5 m and by increasing apparent resistiv-ity at profile position 190 m for all frequencies.

ŽThe difference between E-polarization NS-. Žtransmitters and B-polarization EW-trans-

.mitters data can also easily be seen in thesefigures. Due to the discontinuity of the electricfield at vertical boundaries, a very sharp transi-tion from the undisturbed geology to the wastesite is observed for the B-polarization case inFig. 6, whereas this transition is, as expected,relatively smooth for the E-polarization case inFig. 5. There is only a small difference betweenthe observed apparent resistivity values for allfrequencies on the waste site in Figs. 5 and 6.On the other hand, the phases vary for everystation on the waste site from 608 for the lowest


Fig. 3. Spatial distribution of apparent resistivity and phase for the frequency 207 kHz in Mellendorf as derived from thetransmitter in NS direction. Triangles denote the locations of the RMT stations.


Fig. 4. Spatial distribution of apparent resistivity and phase for the frequency 177 kHz in Mellendorf as derived from thetransmitter in EW direction. Triangles denote the locations of the RMT stations.


Ž .Fig. 5. Observed apparent resistivities and phases in Mellendorf on the profile ys0 NS transmitters for four frequencies.

frequency to 358 for the highest observed fre-Ž .quency Fig. 5 . Information about the structure

beneath the waste site can mostly be derivedfrom the phase information. This observation


Ž .Fig. 6. Observed apparent resistivities and phases in Mellendorf on the profile ys0 EW transmitters for four frequencies.

has also been studied and confirmed by ZiebellŽ .1997 in his 2D model calculations for the

resolution of the bottom of waste sites usingsynthetic data.


The quantitative interpretation of the nineprofiles crossing the industrial waste site in theEW direction by means of 2D model calcula-tions will be shown in Section 4.

3.2. The Hermsdorf waste site

A sand pit of approximately 10 m depth wasŽused as a waste site in Hermsdorf Thuringenr¨

Fig. 7. Spatial distribution of apparent resistivities in Hermsdorf as derived from the transmitters in NWrSE directionŽ .E-polarization for four frequencies.


.Germany until 1979. Household refuse and in-dustrial wastes from a ceramic factory weredeposited in this waste site. It was recultivated

and approximately a 1 m thick soil layer wasplaced above the waste. The surrounding hostconsists of bunter sandstone. No boreholes exist

Ž .Fig. 8. Spatial distribution of phases in Hermsdorf as derived from the transmitters in NWrSE direction E-polarization forfour frequencies.


to derive the lithology and the dimension of thewaste site. Previous geophysical studies on thewaste disposal site at Hermsdorf which includedtotal and vertical magnetic gradient measure-

Žments, geoelectric mapping and soundings Lin-. Ždner et al., 1993 , self potential Pretzschner et

. Žal., 1993 and refraction seismics Dresen and.Schneider, 1994 identified the waste location

with an approximate thickness of about 8–12 m.Another aim of these measurements was thedetection of possible contamination plumes fromthe waste site.

The objective of the RMT measurements onthis area was to test their applicability andefficiency with regard to the exploration of ageophysically well studied waste. Especially avery detailed information about the lateralboundaries and vertical extension of a waste siteis expected from the RMT measurements.

Four pairs of frequencies from radio transmit-ters perpendicular to each other 23.8, 68, 128.5and 270 kHz for NWrSE direction and 18.3,75, 153 and 261 kHz for NErSW directionhave been used. Previous geophysical measure-ments indicate that the strike of the waste site isin the NWrSE direction, which is associatedwith the E-polarization case. Apparent resistiv-ity and phase data were collected on 14 profilesusing a station interval varying between 12.5and 50 m. The distance between the profilesvaries from 20 m in the waste to 50 m outsideof it. The expected boundaries of the waste siteare densely sampled.

The observed apparent resistivity and phasedata are shown in Figs. 7 and 8 as derived fromtransmitters located in NWrSE direction. Theyclearly show the approximate location of thewaste site, which is bounded by the profiles 250

Ž .and 450 y-direction and the stations 150 andŽ .360 x-direction . The apparent resistivity val-

ues in Fig. 7 show a relatively heterogeneouscomposition of the waste site with minimumresistivities of 10 Vm in some locations. Thecorresponding phase values in Fig. 8 generallyreflect the same variability over the waste dis-posal. The average value of the phase is about

UŽ U.388. According to the r z transformationŽ .Schmucker, 1987 , this indicates a more resis-tive unit below the well-conducting waste mate-rial. The apparent resistivities and phasesobserved by using transmitters from NErSWdirection show the same qualitative results.

The observed data of the 19 profiles areinterpreted by means of 2D inversion algorithmsand will be shown together with the inversionresults of the industrial waste site Mellendorf inthe following.

4. Two-dimensional interpretation of theRMT data

In general, more than 300 RMT stations areused for a waste site exploration. More than 30stations on a profile are very common. Thenumber of RMT stations to be interpreted by aconductivity model is much larger than in thecase of a classic magnetotelluric survey. Trialand error fitting using 2D forward modelingproves to be very time consuming for such alarge number of stations. A 2D inversion algo-rithm is used for the interpretation of RMT data.

ŽThe field setup described in Section 2 e.g.,transmitters with similar frequencies parallel andperpendicular to the assumed strike direction of

.a waste site enables the application of 2Dinversion algorithms on the data. Compared toclassic MT measurements only informationabout Z and Z elements of the impedancex y y x

tensor is available. The Z and Z elementsx x y y

are not estimated and assumed to be zero asthey should be in ideal case of a 2D conductiv-ity anomaly. However, Z and Z may devi-x y y x

ate from the estimated ones if 3D effects arepresent.

Ž .Ziebell 1997 carried out model calculationswith different types of modeling algorithms us-ing synthetic and field data. He shows that the

Ž .2D inversion algorithm of Mackie et al. 1997is very suitable for the interpretation of theRMT data.


An inversion of the complete 2D Helmholtz-equation is carried out in the algorithm of

Ž .Mackie et al. 1997 . First, the subsurface struc-ture is parameterized by using the finite differ-ence technique. The components of the sensitiv-

Žity matrix e.g., the partial derivative of the field.values with respect to the model parameters are

calculated by using the technique of adjugateGreen’s functions. For N stations and F fre-quencies per E- and B-polarization mode, N=Fforward modeling calculations should be carriedout to calculate the elements of the sensitivitymatrix. The inversion procedure can be startedafter the calculation of the sensitivity matrix. Ina two-dimensional case, the problem is ill-posedand a regularization procedure must be imple-

Ž .mented. Mackie et al. 1997 use Tikhonov’smethod which defines a regularized solution ofthe inverse problem as a model m that mini-mizes the objective function.

T y1S m s dyF m R dyF mŽ . Ž Ž . Ž .Ž .dd

5 5 2qt L mym 3Ž . Ž .0

Žwhere dsobserved data vector e.g., apparent.resistivities and phases , Fs forward modeling

Žoperator e.g., the differential equations for thedifferent field components of the E- and B-

.polarization case , msunknown model vector,R serror covariance matrix, m sa prioridd 0

model, ts regularization parameter and L is alinear operator for the regularization procedure.

Ž .Mackie et al. 1997 use a Laplacian operatorŽ .LsD , yielding

225 5L mym s D m x ym x d x .Ž . Ž . Ž .Ž .Ž .H0 0

4Ž .

A very important parameter which influencesthe result of the inversion is the regularisation

Ž .parameter t . According to Mackie et al. 1997 ,the value of t should be chosen in such a waythat a RMS error for the inversion is between1.0 and 1.5%. Typical values for t vary be-tween 3 and 300. Large t values cause a

Ž .smoother model. Ziebell 1997 examined with

Žsynthetic data typical waste site locations e.g.,good conductive anomaly beneath a 1–3 m top

.layer embedded in a poorly conductive hostusing the 2D inversion algorithm of Mackie et

Ž .al. 1997 . He found that the top layer and thewaste are well resolved by using the RMT

Ž .frequencies 10–300 kHz if t is chosen to be100. In the following, this value was chosenduring the inversion of the RMT field data. AGauss–Newton algorithm with matrix inversionwas used for numerical minimization of theobjective function S. Then a minimum differ-ence in the structure between start model andfinal model was looked for. The resulting final2D conductivity model minimizes also the dif-ference between observed and calculated data.

4.1. Two-dimensional conductiÕity models forthe industrial waste site Mellendorf

The RMT data of nine profiles in EW direc-tion associated with the E- and B-polarization ofthe magnetotelluric field have been interpretedby the 2D inversion technique of Madden and

Ž .Mackie 1989 . Fig. 9 shows the result of the2D inversion as a resistivity cross-section forthe profiles ys0 m and ys25 m. The wastesite is clearly emphasized by low resistivitiesŽ .-35 Vm . Its lateral borders at profile loca-tions 10 and 180 m are clearly detected by themodel. The local good conductive zone outsideof the waste site at 3 m can be interpreted as asmall clay body in the sandy layer. The sameanomaly was detected in the same area by TEMmeasurements which were carried out just be-

Ž .fore the RMT survey Schaumann, 1997 . Thebottom of the waste site can be estimated to be

Ž15 m. However, the lowest frequency e.g., 16.kHz is not sufficiently low for clearly detecting

it. The central depth of the in-phase inducedU Ž .currents z Schmucker, 1987 which is defined

as

raUz s sinf 5Ž .(vm0


Fig. 9. Two-dimensional inversion results for the profiles ys0 and ys25 in Mellendorf. The dashed lines indicate thelateral and vertical boundaries derived from the 2D conductivity models.

has been used as a guide for the maximumdepth from which information about the conduc-tivity structure can be obtained. The 1D and 2D

Ž .model calculations of Ziebell 1997 have shownthat 2 zU can be used for RMT frequencies for

maximum interpretation depth in the case ofŽwaste site surveys e.g., a well-conductive body

.embedded in a poorly conductive host . VLFfrequencies have been used for the determina-

Ž Ution of the maximum depth 2 z values on the


.waste site which is about 20 m for 16 kHzŽ . ŽE-polarization and for 20.3 kHz B-polariza-

.tion . Similar inversion results have been ob-tained for the other profiles on the waste site inMellendorf. A quasi-3D resistivity distributionat 3 m depth derived from the 2D modeling ofall RMT profiles is shown in Fig. 10 as anexample. The lateral borders of the waste de-posit have been determined accurately. There isa clear difference between undisturbed geologyŽ . Ž .r)100 Vm and the waste site r-35 Vmat this depth. The calculated 2D inversion re-sults also enable resistivity slices to be derivedfrom the modeled data set down to the maxi-

Ž U.mum interpretation depth e.g., 2 z . Such rep-resentations can be seen as a final interpretation

Ž .step for a RMT survey see also Section 4.2 .

4.2. Two-dimensional conductiÕity models forthe Hermsdorf waste site

As in the case of the waste site in Mellen-dorf, the RMT data from 19 profiles in NErSW

Ž .direction Fig. 8 were interpreted using the 2D

Ž .inversion algorithm of Mackie et al. 1997 .Apparent resistivities and phases for four fre-

Ž .quencies in NWrSE E-polarization andŽ .NErSW B-polarization direction were used as

input data. Fig. 11 shows the inversion resultsof profiles ys350 m and ys375 m crossingthe waste site. Again, the waste site can clearlybe seen as a good conductive body with resistiv-ities less than 35 Vm. The lateral borders arealso well defined. The vertical extension of thewaste site cannot be resolved accurately. It canquantitatively be estimated to be at 15 m fromthe 2D inversion models. The 2 zU values onthe waste site vary for the VLF frequenciesbetween 14 and 16 m. A well-conducting struc-ture outside of the waste site at a depth of10–17 m can also be seen on the 2D modelsand could be interpreted as a contaminationplume. The previous geophysical measurementsŽ .e.g., DC soundings also confirm these anoma-

Ž .lies Lindner et al., 1993 . The other possibleinterpretation for this good conductive anomalyis the increase of loose packed materials in thebunter sandstone which can produce the sameanomaly. Without borehole information no

Fig. 10. Resistivity distribution beneath the industrial waste site Mellendorf in 3 m depth, derived from the 2D inversionresults.


Fig. 11. Two-dimensional inversion results for the profiles ys350 and ys375 in Hermsdorf. The dashed lines indicate thelateral and vertical boundaries derived from the 2D conductivity models.

definitive interpretation about this anomaly ispossible from surface measurements. The 2Dconductivity models in Figs. 9 and 11 are the

optimum models which provide a minimummisfit between the observed and modeled data.Fig. 12 shows a comparison of the observed and


Fig. 12. Observed and calculated apparent resistivity and phases for the highest frequency 270 kHz on profile ys350 inHermsdorf. The 2D model in Fig. 11 was used for calculating theoretical apparent resistivity and phase data.

calculated data for the highest frequency of 270kHz on the profile ys300 m. This comparison

serves as a representative example of a generalfitting for all calculated 2D models. In general,


Fig. 13. Resistivity distribution beneath the waste site Hermsdorf in 3 m depth, derived from the 2D inversion results.

the fit is good for such an inhomogeneous area.The main features of the data could be ex-plained by the 2D conductivity models. How-ever, there are some large misfits between ob-served and modeled data. Especially the steepabrupt change of data from station to stationcannot be explained by this type of model dueto smoothing criteria in the inversion algorithm.The observed data can be influenced by localinhomogeneities. The complete impedance ten-sor cannot be measured by the RMT instrumentused for these surveys. There are also no stable3D inversion algorithm available. These are themain reason why the interpretation of the RMTdata were carried out by 2D inversion algorithmassuming a 2D conductivity structure. Similar tothe case in Mellendorf, the 2D inverted profilescan be presented as slices through certain depthsin order to obtain a quasi-3D resistivity distribu-tion of the survey area. Fig. 13 shows theresistivity distribution at 3 m depth derivedfrom all 2D inversion results. About 20 min.CPU time is needed for each inversion. The

lateral borders of the waste site can again bedefined accurately. There is a significant resis-tivity difference between the waste site -35Vm and the surrounding host.

5. Conclusions

The radiomagnetotelluric technique is a pow-erful tool for waste site exploration. The casehistories shown in this paper and the previousRMT surveys proved the efficiency of thismethod.

The great advantage of RMT lies in the factthat it combines both profiling and sounding. A

Žspecially designed field setup e.g., the use ofradio transmitters parallel and perpendicular to

.the assumed strike direction of a waste siteenables the interpretation of the data by 2Dconductivity models giving depth information ofthe survey area.

Only a few boreholes exist on the investi-gated waste sites in Hermsdorf and Mellendorf,


hence just a rough information about their lat-eral borders could be deduced. On the otherhand, the 2D inversion algorithm of Mackie et

Ž .al. 1997 has been used to interpret the RMTdata and enabled an excellent resolution of thelateral borders of the waste sites in Hermsdorfand Mellendorf which are also confirmed byhistorical records about the waste deposits. Thebottom of these waste sites could also be esti-mated.

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