Geophys. J. Int. (2000) 140, 577–586

Interpretation of long-offset transient electromagnetic data from theOdenwald area, Germany, using two-dimensional modelling

A. Hordt, S. Dautel, B. Tezkan and H. Thern*Universitat zu Koln, Institut fur Geophysik und Meteorologie, Albertus Magnus Platz, 50923 Koln, Germany. E-mail: hoerdt@ageo.uni-koeln.de

Accepted 1999 October 12. Received 1999 October 11; in original form 1999 September 1

SUMMARYThe standard 1-D inversion approach for the interpretation of transient electromagnetic(TEM) data usually fails in the presence of near-surface conductivity anomalies. Sincemultidimensional inversion codes are not routinely available, the only alternative todiscarding the data may be trial-and-error forward modelling. We interpret data froma long-offset transient electromagnetic (LOTEM) survey which was carried out in 1995in the Odenwald area, using 2-D finite-difference modelling. We focus on a subsegmentof the LOTEM profile, which was shot with two different electric dipole transmitters.A model is found which consistently explains the electric and magnetic field data ateight locations for both transmitters. First, we introduce a conductive dyke under thereceiver spread to explain sign reversals in the magnetic field transients. A conductiveslab under one of the transmitters is required to obtain a reasonable quantitative fitfor that transmitter. Consideration of the electric field data then requires a modificationof the layered earth background. Finally, we study the response of a crustal conductor,which was the original target of the survey. The data are sensitive to the conductor, andfor the investigated subset of the data the fits are slightly better without the conductivelayer.

Key words: crustal structure, electrical conductivity, electromagnetic modelling,electromagnetic survey.

Here, we present a 2-D interpretation case history of a1 INTRODUCTION

densely covered LOTEM profile, including both magnetic andThe routine interpretation of transient electromagnetic (TEM) electric field data. For this data set, a 1-D joint inversion doesdata is still based on one-dimensional (1-D) inversion. A few not give reasonable results, or is not possible at all becausetechniques have been developed for multidimensional inversion the magnetic field data exhibit sign reversals. The data areor approximate imaging (Pellerin & Hohmann 1993; Oristaglio corrupted mainly by near-surface conductivity anomalies. Usinget al. 1993; Zhdanov & Portniaguine 1997), but case histories a trial-and-error forward modelling strategy, and previouswith field data are rare. Another way to deal with the problem experience with near-surface distortions (Hordt et al. 1992),is to study the validity of 1-D inversion in the presence of 2-D we find a 2-D model that explains data from eight receiveror 3-D structures (Goldman et al. 1994; Newman et al. 1987; locations, shot with two different transmitters.Rabinovich 1995). In cases where the data are so strongly The data comprise a subset of a survey that was carrieddistorted that 1-D inversion does not give reasonable results, out together with magnetotelluric (MT) measurements in themultidimensional trial-and-error forward modelling may be the Odenwald area, Germany. The Odenwald is part of the Mid-only practical alternative. Examples have been given by Helwig German Crystalline High, which is located at the suture zoneet al. (1994) for loop–loop TEM data and Hordt et al. (1992) between the Rhenoherzynian and Saxothuringian units. Thefor long-offset transient electromagnetic (LOTEM) data. These original aim of the survey was to investigate the possible south-studies focused on the time derivative of the vertical magnetic west extension of an extremely conductive layer. The conductorfield (referred to as magnetic field data throughout this paper), had been deduced previously by Hofer (1990) from MT andand the data were recorded with one single transmitter for geomagnetic depth-sounding data along a profile crossingeach receiver. the same suture zone approximately 50 km northeast of the

Odenwald. There is a considerable increase in the conductance,

up to a maximum of about 9000 S, below the Mid-German*Now at: Baker Atlas Logging Services, PO Box 1407, Houston,

Texas 77251-1407, USA. Crystalline High. If the conductor is related to the subduction

577© 2000 RAS

578 A. Hordt et al.

of the Rhenoherzynian under the Saxothuringian plate, it different shadings of the Odenwald crystalline rock. The borders

should be a regional phenomenon extending in both directions to the Rhenoherzynian and the Saxothuringian are north andalong the SW–NE strike. south of this part of the map.

Although all information, including the MT data, will have The data acquisition was performed using the TEAMEXto be considered in the final geological conclusions, we assess multichannel system (Ruter & Strack 1991). The remote unitsat this stage whether the corrupted LOTEM data still contain with two channels each were digitally connected and controlledinformation about the deeper structure. In a modelling study we by a PC. This required the receivers to be laid out in sub-find that a crustal conductor above 15 km depth causes signifi- segments called spreads. The typical length of such a spreadcant variations in the responses. This means that the effort to was between 1 and 1.5 km, using eight remote units with aexplain quantitatively the near-surface effects is worthwhile, distance of about 150 m. One of the two channels was used toand there is no need to discard any of the data. In a combined record the time derivative of the vertical magnetic field, andLOTEM–MT interpretation, the LOTEM data will contribute the other recorded the horizontal electric field componentsignificantly to conclusions about the existence of the conductor. parallel to the transmitter bipole, which gives the maximum

signal strength.

Each spread was shot with typically 2–3 different trans-2 THE SURVEYmitters, if possible on either side of the spread, giving a total

The survey was carried out during three weeks in the summer of 17 different transmitter–spread combinations. The trans-of 1995, with a permanent crew of eight people in the field. mitter bipoles were 1–2 km long, and the distances betweenThe profile was laid out perpendicular to the regional geo- transmitters and receiver spreads varied between 6 and 14 km.logical strike, which is SW–NE, as indicated in Fig. 1 by The transmitter electrodes were strong aluminium foils and/or

metal sheets in ditches of a few square metres, filled with water

and bentonite. A Zonge GGT30 transmitter with 30 kVA power

generator supplied the input current, which varied between 6

and 23 A.

Man-made noise was large in the area compared with the

locations of previous LOTEM surveys (e.g. Strack et al. 1989;

Strack 1992), due to the vicinity of Darmstadt and the high

industrialization of the area in general. Fig. 2 illustrates the

noise level compared with the signal. The single raw transient

is dominated by 50 Hz power-line noise and 16 2/3 Hz railroad

noise, and the signal is hardly visible. Typically, 1000–2000

raw transients were collected at each station, with a sampling

interval of 1 ms. They were digitally filtered and selectively

stacked to obtain the earth response. In this particular case,

the maximum signal level is one order of magnitude smaller

than the noise in the raw data. This is an example for a

relatively high signal-to-noise ratio. For many other receivers,

the signal would not even be visible if plotted on the same

Figure 1. Locations of transmitters and receivers along the profile.

Receiver spread 6, shot with transmitter 3 in the north and transmitter

12 in the south, will be treated in detail. The lengths of the transmitter

bipoles and of the spreads are exaggerated for the sake of visibility.

Figure 2. Example of raw data compared with the extracted signalThe shading indicates the surface geology. The dark shadings denote

the Odenwald crystalline rock, lighter and darker variations are mainly (processed data). The transmitter switch is at t=0, when the signal

starts. The peak at −400 ms in the signal is a ringing effect of thegranites and diorites. Sediments are white, and the large white area in

the west is the Upper Rhine Graben. digital recursive filter.

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2-D L OT EM interpretation 579

linear scale as the raw data. Even with analogue notch filters,3 ONE-DIMENSIONAL INVERSION

which were used in most cases, the noise was orders ofmagnitude larger than the signal. First, we attempted to fit the data with 1-D inversion. This

worked well for some of the stations, but there were manyTo overcome this problem, the existing digital recursive

notch filtering techniques (Strack et al. 1989) were modified along the profile where it was not possible to obtain areasonable data fit, mainly due to sign reversals in the magneticand new filters were developed (Hanstein 1996). The basic

idea is to use knowledge about the behaviour of the signal, field transients. Even if the data could be explained with 1-D

models, the results were inconsistent along the profile, orwhich is zero before t=0, strongly varying at early times, andsmoothly varying at late times. Using time-dependent filters, a inconsistent for the different field components. Fig. 3 shows an

example where the 1-D inversion gives reasonable results forgood compromise between minimum signal distortion by the

filters and maximum noise reduction can be obtained. The the magnetic and electric fields separately, but where no 1-Dmodel could be found to explain them simultaneously.signal shown in Fig. 2 is a result of this processing. The high

amplitude at –400 ms is a typical ringing effect of the recursive The top panel shows the early-time apparent resistivity

calculated from vertical magnetic field time derivatives. Thefilter. The large bandwidth in this time range keeps the ringingshort and prevents it from distorting the signal. Details of data are fitted well by the layered model, which consists of a

homogeneous half-space with a conductive layer embeddedthe application of the techniques to the Odenwald data are

described in Thern et al. (1996) and Thern (1996). between 2 and 3 km (top right panel ). Note that the apparentresistivity curve at early times approaches zero instead ofMagnetotelluric data were recorded near every LOTEM

receiver spread (Fig. 1). In addition, the profile was extended 1000 V m, as it should for this particular model. This is mainly

the result of the presence of the analogue low-pass filter. Thefurther to the south. The noise was also a problem for the MTprocessing, and special measures had to be taken to obtain an impulse response of this filter is known, but since deconvolution

is numerically unstable, the theoretical curves are convolvedacceptable data quality. Details are given in Michel & Tezkan

(1996). The joint interpretation of the MT and LOTEM data with the filter impulse response instead. In fact, the filterresponse is part of the so-called system response, which alsois still the subject of current research.

Figure 3. 1-D inversion results for the magnetic field time derivative receiver 81 (top) and electric field receiver 82 (bottom) on spread 6, shot with

transmitter 3. The left column shows the measured data with error bars, and the calculated curve (solid line) as a function of time after the

transmitter current switch. The right column depicts the 1-D model. The dashed line in the bottom left panel is the calculated curve for the model

which fits the magnetic field transients (top right panel ).

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580 A. Hordt et al.

includes 50 Hz and 16 2/3 Hz notch filters, as well as deviations reversal in the magnetic field. This will mainly involve near-

surface structures. Next, we include the electric field and findof the transmitter input waveform from an ideal step, andsensor characteristics. It is measured in the field by placing the model that best explains all the features in the magnetic

and electric field data. Finally, we carry out a modellingthe transmitter so close to the receiver that the earth response

can be considered as an impulse (Strack 1992). exercise to assess the resolution with respect to a possiblecrustal conductor. We restrict ourselves to the data of spreadThe bottom panel shows the electric field transient and fit

by a different 1-D model. The model is a simple three-layer 6, shot with transmitters 3 and 12, because the near-surface

effects are consistent and it seems realistic to find a quantitativemodel, but the conductive layer is significantly shallower thanfor the magnetic field transient. The dashed line in the left explanation.panel was calculated for the model which gives the best fit

for the magnetic field data. The curve shapes are completely 4 MODELLING THE SIGN REVERSALdifferent at early times. Attempts to find a 1-D model which

Structures that may cause a sign reversal in the verticalexplains both data sets using 1-D joint inversion (Jupp &magnetic field time derivatives are known from previous work.Vozoff 1975) fail. They end up in a compromise which doesNewman (1989) modelled sign reversals in LOTEM datanot give satisfactory fits for either of the two transients.with a near-surface conductive body, and Hordt et al. (1992)The fact that magnetic and electric field data cannot beexplained a measured LOTEM transient with a sign reversalconsistently explained by 1-D inversions is a clear indicationbased on conductive dykes. By making use of these experiences,that multidimensional effects corrupt the data. This is con-and analysing the locations of the stations that have a signfirmed by other stations on the spread, where most of thereversal and those that do not, it was possible to find a modelmagnetic field transients exhibit a sign reversal. One examplethat qualitatively gives the desired effect. Fig. 4 shows theis shown in Fig. 5. Station 87 is only 400 m from the locationsection of a model which includes a near-surface conductiveof stations 81 and 82. The amplitude is negative at early times,dyke running perpendicular through the spread. The dyke iswhich cannot be explained by a layered model.infinitely extended in the direction perpendicular to the section,During the following trial-and-error modelling procedure,and is embedded in a homogeneous half-space. The nature ofwe first find a model that quantitatively explains the signthe dyke, and whether or not it is geological, will be discussed

further below.The modelling was done using the 3-D finite-difference code

of Druskin & Knizhnerman (1988), where an infinite dyke is

realized by extending it to the boundaries of the discretizationvolume. A code that allows a 3-D source over a 2-D conductivitystructure [sometimes called a 2.5-D problem (Goldman et al.1994)] could have been used instead for the models shownhere. However, the gain in computing time is much less thanfor MT modelling, where the source can be considered as a

plane wave. Moreover, additional simulations were run whichrequire a full 3-D model, to investigate the influence of the

Figure 4. 2-D model to explain the sign reversal in the magnetic field Upper Rhine Graben sediments and to study the effect of atransients. The dyke has a resistivity of 10 V m, and infinite extension finite length of the conductive dyke.perpendicular to the section. The 500 V m background extends in all

Fig. 5 shows the model curve and measured data for twodirections below the transmitters. The receivers are all on spread 6,

stations on the spread that have a sign reversal, shot withand the spread length is approximately 1 km. Receiver locations were

transmitter 3. The conductor is located between the stationsprojected onto the section, and the real locations were considered inand the transmitter. Station 87 is quite close to the dyke, andthe modelling. Odd receiver numbers were used for magnetic receivers,station 95 is farther away. All the receivers between these twoeven numbers for electric receivers, so, for example, 81 and 82 are at

the same location. show the same behaviour, with negative early-time apparent

Figure 5. Early-time apparent resistivities calculated from measured vertical magnetic field time derivatives for receivers 87 and 95, shot with

transmitter 3, together with calculated curves for the model shown in Fig. 4. The plus and minus signs denote the polarity of the curve segments

for both measured and calculated data.

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2-D L OT EM interpretation 581

resistivities (Fig. 14). The model curves explain the data at station 81 even in the mid-time range where the sign

changes, and how well the mid-time range around the maxi-qualitatively. The sign reversal is approximately at the correcttime, and the amplitudes of positive and negative values are mum at station 87 is fit. Only at late times is the decay of the

model curve faster than that of the data for both receivers.roughly correct. In the measured data, however, the amplitude

of the negative part is significantly smaller for receiver 95 than The nature of the dyke, whether it is geological or whetherit is a smaller man-made feature with an equivalent transversefor receiver 87, because 95 is farther away from the dyke. In

the modelled data, this amplitude difference is less pronounced. conductance, cannot be resolved from the data alone. However,

there are some indications in favour of a geological structure.This discrepancy can be improved if the dyke is modelled witha finite length, but during the model finding procedure we Owing to the high station density, the location can be deter-

mined with an error of less than 100 m. There is no railroadrestricted ourselves to a 2-D model in order to keep the

number of parameters small. or gas pipeline in this area. Moreover, the strike direction ofthe dyke is parallel to the general geological strike, and theA sign reversal does not occur for receivers between the

transmitter and the dyke, for example at station 81 (Fig. 6). resistivity (10 V m) is not unnaturally low. Therefore, an

accumulation of fluids along a former shear zone is a possibleThis behaviour is typical for near-surface conductors. Asexplained by Newman (1989) and Hordt et al. (1992), currents explanation. A close look at the geological map (Fig. 1) shows

that there is indeed an elongated structure going throughare channelled in the body, and have an antisymmetric effect

on the receivers with respect to the body location. the spread.For future surveys, it might be useful to supplement theThe model also explains the transients of the same spread

shot with transmitter 12, which is on the opposite side of the LOTEM measurements with TEM measurements with shorter

offsets, an earlier time range, and a smaller station distance.dyke. Fig. 7 shows two receivers as an example. Now station81 has a sign reversal in both model and field data, because This would provide detailed information on near-surface

structures, which could be used as input for the LOTEMthe dyke is between the receiver and the transmitter. Stations

87 and 95 are between the transmitter and the dyke, and do not interpretation. In this particular case, it would have allowed abetter understanding of the nature of the dyke. The additionalhave a sign reversal, which is consistent with the explanation

given above. Again, all field data at receivers that are not time required to do these measurements might be compensatedfor by using a larger spacing between the LOTEM stations.shown here are qualitatively consistent. The electric fields will

be discussed in the next section, and we will see that the At present, the maximum length of a LOTEM spread with the

TEAMEX system is limited by technological constraints tohigher-dimensional effects which prevent a successful 1-D jointinversion are also explained by the same dyke. about 2 km, but this may be different for the next generation

of data acquisition systems.The fit for this transmitter is also good in a quantitative

sense. Note how well the amplitude and shape are explained In order to obtain a better quantitative fit for transmitter 3,we modified the parameters of the dyke, and included otherlocal structure in the vicinity of the spread. These attempts

failed because if they improved the fit for transmitter 3, the fitfor transmitter 12 deteriorated. The solution was to include aconductive slab below transmitter 3, as shown in Fig. 8.

Geologically, this can be justified, because transmitter 3 is inthe north, where the crystalline base rock is thin and thesediments dominate. The model parameters, such as thickness

and resistivity of the slab should not be taken to reflect thetrue geological situation. The slab is used as a means to explainthe bulk effect caused by structure around the transmitter,

without attempting to resolve details.The data fit for the two receivers on spread 6 is shown in

Fig. 9. The left panel shows the transient, which does not havea sign reversal. The shape is now well matched over the entireFigure 6. Measured data for receiver 81, shot with transmitter 3

(same as in Fig. 2), and calculated curve for the model shown in Fig. 4. time range except for late times, and the fit is considerably

Figure 7. Measured data and calculated curves for receivers 81 and 87, shot with transmitter 12, for the model shown in Fig. 4.

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582 A. Hordt et al.

Figure 8. 2-D model with a conductive slab under transmitter 3. The slab thickness is 2 km, and it is infinitely extended in the direction

perpendicular to the section and to the left. The dyke under the receiver spread is the same as in Fig. 4.

Figure 9. Measured data and calculated curves for receivers 81 and 87, shot with transmitter 3, for the model shown in Fig. 8.

improved compared with Fig. 6. The same applies to the right The corresponding data fits for the two magnetic fieldtransients of transmitter 12 are shown in the top panels ofpanel, which has to be compared with the left panel of Fig. 5.

The amplitude of the negative part as well as the mid-time Fig. 11. Unlike in Fig. 7, where the background was a 500 V m

half-space, the late-time branches are now well explained; inrange around the sign reversal are now correctly simulated.To find a model that explains the late-time branch of the particular, the concave shape of the curves is well matched.

magnetic field data for transmitter 12, we replace the homo-

geneous half-space by horizontal layers. Models that are more5 ELECTRIC FIELDS

complicated are not considered at this stage in order to keep

the number of parameters small. Another advantage of using At this stage, we take the electric fields into consideration(Fig. 11, bottom panels). The shape of the ascending branchhorizontal layers is that model studies with a much faster 1-D

program could be used to support the model-finding procedure. (times between 0.005 and 0.05 s), which is influenced by the

conductive dyke, is well explained by the model data. ToA direct transformation of the 1-D studies to the 2-D modelwas not possible, because the response of the layers is modified understand the influence of the dyke on the electric field, it is

useful to look at the right panel of Fig. 3 again. In the 1-Dby the near-surface structure; however, the 1-D modelling

still helped in understanding the behaviour of the curves. inversion, the early-time shape requires a conductive layer ata depth that is inconsistent with the depth obtained from theThe resulting model (Fig. 10) consists of the conductive dyke,

embedded in a 400 V m top layer, and a 100 V m basement at magnetic field data inversion. By including the conductive

dyke, all these features are consistently explained for magnetic4 km depth.

Figure 10. Model that gives the best fit for magnetic field data, shot with transmitter 12. The slab and the dyke are the same as in Figs 4 and 8.

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2-D L OT EM interpretation 583

Figure 11. Measured and calculated data for the model of Fig. 10, transmitter 12. Top row: magnetic field data for receivers 81 and 87. Bottom

row: electric field data for receivers 82 and 88. Receivers 81 and 82 are at the same location, and receivers 87 and 88 are at the same location.

and electric fields. These considerations are valid even though 6 km thickness. The results for transmitter 12 are shown inFig. 13. The fit for the magnetic field late-time branches isFig. 3 is for transmitter 3, while Fig. 11 is for transmitter 12.

The dyke also explains the electric field data for transmitter 3 slightly deteriorated (top panel, solid line) compared with

Fig. 11. The fit of the corresponding electric field data (bottom(Fig. 14). The physics behind this is that, unlike for magneticfields, the effect of the dyke on the electric field is symmetric. panel, solid line) is now much better compared with Fig. 11.

The more resistive basement keeps the electric field at a fairlyThe dyke always causes a drop of the electric field at early

times, independent of the receiver location. constant level.The odd-looking behaviour of the model curve for stationThe late times of the electric fields are not fit at all. In the

model curves, there is a significant drop at late times, whereas 81 is the result of the convolution with the system response,

which has a minimum at 25 ms. When the sign reversal in thethe data are nearly constant at times later than 0.1 s. The dropin the model data is caused by the relatively conductive layer theoretical data is in the same time range as the minimum in

the system response, the convolution can give several valuesat 4 km, which is required to produce the concave late-time

branch in the magnetic field data. close to zero. In a log–log display this can look like noise, asis the case for station 81.The model that gives the best compromise between the

electric and magnetic fields is shown in Fig. 12. The two-layer Fig. 14 shows the fit for all receivers along the spread for

transmitter 3. The consistency of the data along the spreadmodel has a 1000 V m basement below a 300 V m layer of

Figure 12. Best compromise model for the electric and magnetic field data of both transmitters.

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584 A. Hordt et al.

Figure 13. Measured and calculated data for the model of Fig. 12, transmitter 12. Top row: magnetic field data for receivers 81 and 87. Bottom

row: electric field data for receivers 82 and 88. The dashed lines were calculated with a 10 V m layer, 1000 m thick, at two different depths.

Figure 14. Measured and calculated data for the model of Fig. 12, transmitter 3. Left panel: magnetic field data. Right panel: electric field data.

The scales refer to the top transient only.

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2-D L OT EM interpretation 585

is remarkable. There is little variation in the shape of the

transients, except between stations 83 and 85, where the dykeis located. Overall, considering that the model includes a near-surface dyke and needs to satisfy electric and magnetic field

data for eight locations shot with two different transmitters, themodel can be regarded as a reasonable explanation of the data.

6 RESOLUTION STUDY

The original aim of the survey was to investigate the possibilityof the existence of a conductive layer in the crust. Geological

conclusions have to include the remaining LOTEM and MT

data, but it is interesting to investigate whether the LOTEM

data are sensitive to deeper structure. The concern is that thenear-surface effects might dominate the response and decrease

the resolution. Therefore, additional simulations were run with

a conductive layer included at a variety of depths. The models

chosen for illustration have a layer of 1 km thickness and10 V m resistivity embedded at 9 and 15 km. The results for

transmitter 12 are shown in Fig. 13 (dashed lines). For the

conductor at 9 km, the differences between the dashed and

solid lines are quite significant. For magnetic fields (top row)the conductor first accelerates the decay, and then delays it,

producing a concave curve shape at late times. For the electric

fields, the conductor causes a late-time amplitude drop. Forthe conductor at 15 km, the effects are qualitatively the same,

Figure 15. Measured and calculated data for the model of Fig. 12,but the response is quite weak, so this can be considered as

transmitter 3. Top panel: magnetic field data at station 81. Bottomroughly the resolution limit. panel: electric field data at station 82.

The conductor slightly deteriorates the fit for both magnetic

and electric fields. However, some ambiguity remains in theinterpretation, and the results can at most be considered as

weak evidence that there might be no extremely conductive7 CONCLUSIONSlayer above 9 km.

Fig. 15 shows the simulation results for a magnetic and It is possible to interpret LOTEM data even if the data arean electric field transient for transmitter 3. The magnetic corrupted by near-surface effects. This was shown here forfield resolution is slightly poorer than for transmitter 12. The data acquired in the Odenwald area, where a conductive slabdifferences between the curves with and without the conductor under a transmitter and a near-surface dyke were required toare smaller. This is because the transmitter is on the conductive fit the data. The data are still sensitive to deeper structure,slab, and most of the currents flow in the conductive slab and are suitable for interpretation in terms of geologicallyinstead of the resistive background. Therefore, the resolution relevant structure. The case history is unique in the sense thatwith respect to deeper structure decreases. This may have the final model jointly explains both magnetic and electricimportant implications for survey design. It seems natural to field transients, shot with two different transmitters. The useput the transmitter on more conductive material, because this of a multichannel data acquisition system allowed a highallows stronger transmitter currents and increased signal strength. spatial redundancy, which helped to provide an accurateApparently, this has to be traded off against resolution, and location for the dyke under the receiver spread.the advantage may turn into a disadvantage. For the particular subset of the data investigated here, the

The second observation is that the effect of the deep con- fits are best without a crustal conductor. A conductor causesductor on the magnetic field response is different from that for a late-time drop in the electric field transients, which is nottransmitter 12. In Fig. 13 (top row), the main effect of the deep present in the measured data. However, there is some ambiguityconductor is an acceleration of the decay, resulting in a late- left in the interpretation, and it is not clear whether large-scaletime amplitude decrease. In Fig. 15, the effect of the layer is a conductivity variations may alter the response of a crustalslower decay at late times. This must be due to the slab, which conductive layer. Therefore, final conclusions will have tois the main difference between the models for transmitters 12 include the MT data, which are still the subject of research.and 3. In contrast, the slab does not modify the conductor The trial-and-error forward modelling procedure wasresponse in the electric fields. For both transmitters, the cumbersome. It took about 200 forward modelling runs withconductive layer causes an amplitude drop at late times. a 3-D finite difference code, and is far from being a routine

Additional model studies were carried out to assess whether process. The ultimate goal is to develop a 2-D or 3-D inversion.the Upper Rhine Graben sediments influence the curves and Modelling studies with field data like the one presented herechange the conclusions. It is concluded that the effects of will be extremely useful in assessing the performance of thelateral structures are not significant, and in particular are inversion code and in supporting the development of efficient

algorithms.smaller than those of a potential crustal conductor.

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586 A. Hordt et al.

Jupp, D.L.B. & Vozoff, K., 1975. Stable iterative methods forACKNOWLEDGMENTS the inversion of geophysical data, Geophys. J. R. astr. Soc., 42,

957–976.We thank Tilman Hanstein for his support during the dataMichel, J. & Tezkan, B., 1996. Prozessing von Magnetotellurik-Datenprocessing. The field work was undertaken with the help of 20

aus dem Odenwald, in Elektromagnetische T iefenforschung, 16.students from the University of Cologne. Vladimir Druskin and

Kolloquium, pp. 162–166, eds Bahr, K. & Junge, A., DGG, Potsdam.Leonid Knizhnerman allowed us to use their code. We would

Newman, G.A., 1989. Deep transient electromagnetic soundings withlike to thank Markus Eisel and Mark Goldman for helpful a grounded source over near-surface conductors, Geophys. J. Int.,reviews. The company Alu Feron generously provided high- 98, 587–601.quality aluminium foil for the transmitter electrode coupling. The Newman, G.A., Anderson, W.L. & Hohmann, G.W., 1987.

Interpretation of transient electromagnetic soundings over three-project was sponsored by the Deutsche Forschungsgemeinschaftdimensional structure for the central-loop configuration, Geophys.(project numbers Ne220/14-1 and Ho1506/6-2).J. R. astr. Soc., 89, 889–914.

Oristaglio, M., Wang, T., Hohmann, G.W. & Tripp, A., 1993.

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