Directory UMM :Data Elmu:jurnal:J-a:Journal Of Applied Geophysics:Vol44.Issue2-3.2000:
Journal of Applied Geophysics 44 Ž2000. 237–256
www.elsevier.nlrlocaterjappgeo
Two-dimensional radiomagnetotelluric investigation of industrial
and domestic waste sites in Germany
B. Tezkan
a
a,)
, A. Hordt
¨ a, M. Gobashy
b
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 the
surrounding hosts. One was filled with industrial waste and the other with household refuse. Both had been recultivated.
Powerful military and civilian radiostations Ž10–300 kHz. located parallel and perpendicular to the strike direction of both
waste 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. The
lateral borders of both waste sites are located accurately in the field. They are characterized by strongly decreasing apparent
resistivities observed at nearly all frequencies at the border between the waste site and the undisturbed geology. The data
were quantitatively interpreted by conductivity models using a 2D inversion algorithm. The derived 2D conductivity models
give information about the vertical extent of the studied waste sites and about the structure of the surrounding hosts. q 2000
Elsevier Science B.V. All rights reserved.
Keywords: Waste sites; Radiomagnetotellurics; 2D inversion; Conductivity models
1. Introduction
In Europe, especially in Germany, the application of geophysical methods to waste site
exploration becomes increasingly important
since their contribution to the pollution risk
estimation has been recognized. In Germany,
for example, the number of registered waste
)
Corresponding author. E-mail:
tezkan@geo.uni-koeln.de
sites amounted to 93,723 in December 1993
ŽFranzius, 1994.; among them 350 in the city of
Cologne. These numerous waste sites are traditionally explored by geotechnical methods Že.g.,
drillings.. Applied geophysics can offer many
non-invasive techniques for mapping and should
be considered as a main source of information
for selecting borehole locations. In general,
waste sites are characterized by their low resistivities compared to the surrounding hosts, because industrial waste and household refuse
cause an increase of electrical conductivity due
0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 9 8 5 1 Ž 9 9 . 0 0 0 1 4 - 2
238
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
to biological decomposition of the organic materials. Since the mid-1990s, the radiomagnetotelluric method ŽRMT. is extensively used in
connection with waste site exploration Ž Dautel,
1996; Tezkan et al., 1996; Zacher et al., 1996a;
Tezkan et al., 1997. . The RMT method allows
both sounding and mapping by using several
frequencies. The data collection is relatively fast
and is interpreted quantitatively by using wellknown magnetotelluric modeling programs.
Several RMT surveys have also been conducted
successfully for hydrogeological investigations,
for engineering and for archeological explorations ŽTurberg, 1991; Hollier-Larousse et al.,
1994; Dupis and Choquier, 1996; Zacher et al.,
1996b..
Two selected RMT field surveys are presented in this paper to demonstrate the efficiency and suitability of the method for waste
site exploration of different types. The 2D inversion of the RMT data will be discussed in
detail.
The waste site of Volkswagen in Mellendorf
was chosen as a representative example for an
industrial deposit for the RMT evaluation. A
small number of boreholes give information
about the vertical extent of this waste site. No
geophysical techniques were applied for the exploration of this site previous to the detailed
RMT measurements. For comparison, the waste
deposit Hermsdorf filled with household refuse
was studied exclusively by geophysical methods
without any boreholes. Magnetic, DC, EM 31
measurements were carried out to map the deposit. The RMT measurements should enable a
detailed conductivity depth information of this
area by applying 2D inversion algorithm to the
observed data.
No geographic and topographic maps of both
waste sites will be shown for confidential rea-
sons. Instead, the efficiency of the RMT technique for the exploration of these different types
of waste sites and the resolutions of the 2D
inversion models are emphasized.
2. Basic concepts of the RMT technique
The RMT technique uses as transmitters military and civilian radiostations broadcasting in a
frequency range between 10 kHz and 1 MHz. In
Fig. 1a, the principle of this method is demonstrated schematically. The electromagnetic
waves radiated from these transmitters diffuse
into the conductive earth and induce electrical
current systems which are connected with electrical and magnetic alternating fields. The magnetic field can be measured for selected frequencies by a coil and the electric field by two
grounded electrodes. Fig. 1b shows the RMTinstrument, a prototype developed in Switzerland ŽMuller,
1983.. The magnetic coil has a
¨
diameter of 0.4 m and the distance between the
electrodes can be chosen to be 1 or 5 m. The
frequency range of this instrument is limited to
10–300 kHz but can be extended to 1 MHz
after hardware modifications in the future. No
transmitters are available below 10 kHz. Depending on the conductivity of the subsurface
this limitation might cause the near surface and
the deep layers to be unresolved by this method.
This will be discussed in Section 4 by the 2D
modeling results of the observed data.
The measuring device only weighs 7 kg and
about 3 min are required for measuring four
frequencies at one station. Thus, large areas can
be mapped quickly.
For the selected frequencies apparent resistivity and phase data are derived from the mea-
Fig. 1. Ža. Schematic diagram for illustrating a RMT field set up over a hazardous waste site. A sounding information can be
obtained by using transmitters with different frequencies from the same direction. Žb. RMT instrument used in the field: a
prototype developed by the Hydrogeological Institute of the University of Neuchatel,
1983..
ˆ Switzerland ŽMuller,
¨
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
239
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
240
sured electric and magnetic field using the
well-known formulae of magnetotellurics
ŽCagniard, 1953.
ra x y s
1
Ex
2
2pm 0 f H y
I
ž /
ž /
Ex
Hy
f x y s tany1
R
Ž1.
Ex
Ž2.
Hy
where E x and H y denote the electric and magnetic field, m 0 the permeability of the free space
and f the selected frequency.
Displacement currents can be neglected for
the RMT frequencies under normal conditions
in Europe. The plane wave assumption is valid
when receiver locations are located at least seven
skin depths away from the radio transmitters
ŽSchroder,
1994.. In this case, the well devel¨
oped and tested magnetotelluric interpretation
software can be directly applied to the RMT
data. The main difference between the RMT
and the VLF-R technique lies in the extension
of VLF frequencies Ž10–30 kHz. to higher frequencies Ž Tezkan et al., 1996.. Due to the very
limited frequency range, VLF-R is only used for
profiling and the interpretation of this data is
carried out visually ŽBerktold et al., 1992. . In
some cases, however, VLF data can be interpreted in terms of 1D and 2D conductivity
models ŽBeamish, 1998.. The RMT technique
enables a sounding which can be interpreted
quantitatively. One important aspect is the selection of transmitters by considering the strike
direction of the waste site. Radio transmitters
located parallel and perpendicular to the general
strike direction given by geological or anthropological structures can be selected for the given
frequency range. Assuming a two-dimensional
conductivity structure, this data is associated to
the E- and B-polarisation directions Že.g., Z x y
and Z y x elements of the magnetotelluric
impedance tensors. and then interpreted by using 2D inversion techniques ŽSmith and Booker,
1991; Mackie et al., 1997. . Scalar measurements can only be carried out by the RMT
system used in this study. The apparent resistivity and phase values associated to Z x x and Z y y
elements of the magnetotelluric impedance tensor are not measured in the field. If no information about the strike direction is available the
same field procedure is used by choosing transmitters with similar frequencies from NS and
EW directions and the obtained data is also
interpreted by 2D inversion procedures whereas
3D effects are not considered.
3. Selected RMT surveys for the exploration
of different waste site types
In the following, two selected RMT surveys
for waste site exploration in Germany will be
discussed. An industrial waste site and a very
common type of a waste site Žembedded with
household refuse and building debris. are chosen as an example.
3.1. The Mellendorf industrial waste site
The industrial waste site of Volkswagen at
Mellendorf near Hanover was used from 1960
to 1984. Magnesium drosses and other industrial wastes were stored in a sand pit. The
bottom of this sand pit was not made water
tight. Magnesium drosses are a waste product
from motor fabrications containing magnesium,
magnesium salts and magnesium oxides. The
clay layer beneath the area of the waste site is at
a depth of 60 m, i.e., relatively deep. Between
the surface and the clay layer a sand layer of
Quaternary age was geologically mapped containing thin clay layers in some localities. The
waste site was recultivated in 1984. Fig. 2
shows a lithological stratification beneath the
survey area derived from boreholes on the waste
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 2. The structure at the industrial waste site in Mellendorf derived from borehole results.
site ŽRenno and Christofzik, 1987. . The top and
the bottom of the waste site is expected at 1.6
and 16 m depth, respectively. During the recultivation the waste site was covered by a layer
containing soil, sand with gravel, clay and gravels ŽFig. 2..
For the RMT survey four frequency pairs
between 16 and 200 kHz from radiostations in
North–South and East–West directions were
chosen. The profile direction was East–West
which was perpendicular to the assumed strike
direction of the waste site. The distances be-
241
tween the profiles and the RMT stations were
12.5 m, except at the border between the waste
site and the undisturbed geology, where the
distance was 5 m.
Figs. 3 and 4 give an overview of the lateral
distribution of the apparent resistivities and
phases for 207 and 177 kHz. The triangles on
the figures denote the locations of the RMT
stations. The 207 kHz data belong to the transmitter located in North–South direction. In this
case, the measured electric field associated with
the E-polarization is oriented in NS direction.
The electric field of the 177 kHz data is oriented in the direction of the profile and represents the B-polarization. The waste site as a
well conducting anomaly in comparison to the
surrounding host is characterized by apparent
resistivity values - 30 V m for both polarization directions Že.g., 207 and 177 kHz apparent
resistivity data. and all lateral boundaries of the
waste site are clearly detected. A clear phase
difference outside ŽF f 508. and inside ŽF f
358. the waste site is also observed.
Figs. 5 and 6 show the apparent resistivity
and phase data for all observed frequencies
from NS and EW transmitters for the profile
y s 0 Ž Figs. 3 and 4. as a representative example. The boundaries of the waste site are characterized for both transmitter locations by strongly
decreasing apparent resistivity values at profile
position 5 m and by increasing apparent resistivity at profile position 190 m for all frequencies.
The difference between E-polarization Ž NStransmitters. and B-polarization Ž EW-transmitters. data can also easily be seen in these
figures. Due to the discontinuity of the electric
field at vertical boundaries, a very sharp transition from the undisturbed geology to the waste
site is observed for the B-polarization case in
Fig. 6, whereas this transition is, as expected,
relatively smooth for the E-polarization case in
Fig. 5. There is only a small difference between
the observed apparent resistivity values for all
frequencies on the waste site in Figs. 5 and 6.
On the other hand, the phases vary for every
station on the waste site from 608 for the lowest
242
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 3. Spatial distribution of apparent resistivity and phase for the frequency 207 kHz in Mellendorf as derived from the
transmitter in NS direction. Triangles denote the locations of the RMT stations.
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
243
Fig. 4. Spatial distribution of apparent resistivity and phase for the frequency 177 kHz in Mellendorf as derived from the
transmitter in EW direction. Triangles denote the locations of the RMT stations.
244
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 5. Observed apparent resistivities and phases in Mellendorf on the profile y s 0 ŽNS transmitters. for four frequencies.
frequency to 358 for the highest observed frequency ŽFig. 5.. Information about the structure
beneath the waste site can mostly be derived
from the phase information. This observation
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
245
Fig. 6. Observed apparent resistivities and phases in Mellendorf on the profile y s 0 ŽEW transmitters. for four frequencies.
has also been studied and confirmed by Ziebell
Ž1997. in his 2D model calculations for the
resolution of the bottom of waste sites using
synthetic data.
246
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
The quantitative interpretation of the nine
profiles crossing the industrial waste site in the
EW direction by means of 2D model calculations will be shown in Section 4.
3.2. The Hermsdorf waste site
A sand pit of approximately 10 m depth was
used as a waste site in Hermsdorf ŽThuringenr
¨
Fig. 7. Spatial distribution of apparent resistivities in Hermsdorf as derived from the transmitters in NWrSE direction
ŽE-polarization. for four frequencies.
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Germany. until 1979. Household refuse and industrial wastes from a ceramic factory were
deposited in this waste site. It was recultivated
247
and approximately a 1 m thick soil layer was
placed above the waste. The surrounding host
consists of bunter sandstone. No boreholes exist
Fig. 8. Spatial distribution of phases in Hermsdorf as derived from the transmitters in NWrSE direction ŽE-polarization. for
four frequencies.
248
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
to derive the lithology and the dimension of the
waste site. Previous geophysical studies on the
waste disposal site at Hermsdorf which included
total and vertical magnetic gradient measurements, geoelectric mapping and soundings Ž Lindner et al., 1993. , self potential Ž Pretzschner et
al., 1993. and refraction seismics Ž Dresen and
Schneider, 1994. identified the waste location
with an approximate thickness of about 8–12 m.
Another aim of these measurements was the
detection of possible contamination plumes from
the waste site.
The objective of the RMT measurements on
this area was to test their applicability and
efficiency with regard to the exploration of a
geophysically well studied waste. Especially a
very detailed information about the lateral
boundaries and vertical extension of a waste site
is expected from the RMT measurements.
Four pairs of frequencies from radio transmitters perpendicular to each other 23.8, 68, 128.5
and 270 kHz for NWrSE direction and 18.3,
75, 153 and 261 kHz for NErSW direction
have been used. Previous geophysical measurements indicate that the strike of the waste site is
in the NWrSE direction, which is associated
with the E-polarization case. Apparent resistivity and phase data were collected on 14 profiles
using a station interval varying between 12.5
and 50 m. The distance between the profiles
varies from 20 m in the waste to 50 m outside
of it. The expected boundaries of the waste site
are densely sampled.
The observed apparent resistivity and phase
data are shown in Figs. 7 and 8 as derived from
transmitters located in NWrSE direction. They
clearly show the approximate location of the
waste site, which is bounded by the profiles 250
and 450 Ž y-direction. and the stations 150 and
360 Ž x-direction.. The apparent resistivity values in Fig. 7 show a relatively heterogeneous
composition of the waste site with minimum
resistivities of 10 V m in some locations. The
corresponding phase values in Fig. 8 generally
reflect the same variability over the waste disposal. The average value of the phase is about
388. According to the r U Ž zU . transformation
ŽSchmucker, 1987., this indicates a more resistive unit below the well-conducting waste material. The apparent resistivities and phases
observed by using transmitters from NErSW
direction show the same qualitative results.
The observed data of the 19 profiles are
interpreted by means of 2D inversion algorithms
and will be shown together with the inversion
results of the industrial waste site Mellendorf in
the following.
4. Two-dimensional interpretation of the
RMT data
In general, more than 300 RMT stations are
used for a waste site exploration. More than 30
stations on a profile are very common. The
number of RMT stations to be interpreted by a
conductivity model is much larger than in the
case of a classic magnetotelluric survey. Trial
and error fitting using 2D forward modeling
proves to be very time consuming for such a
large number of stations. A 2D inversion algorithm is used for the interpretation of RMT data.
The field setup described in Section 2 Že.g.,
transmitters with similar frequencies parallel and
perpendicular to the assumed strike direction of
a waste site. enables the application of 2D
inversion algorithms on the data. Compared to
classic MT measurements only information
about Z x y and Z y x elements of the impedance
tensor is available. The Z x x and Z y y elements
are not estimated and assumed to be zero as
they should be in ideal case of a 2D conductivity anomaly. However, Z x y and Z y x may deviate from the estimated ones if 3D effects are
present.
Ziebell Ž1997. carried out model calculations
with different types of modeling algorithms using synthetic and field data. He shows that the
2D inversion algorithm of Mackie et al. Ž1997.
is very suitable for the interpretation of the
RMT data.
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
An inversion of the complete 2D Helmholtzequation is carried out in the algorithm of
Mackie et al. Ž 1997. . First, the subsurface structure is parameterized by using the finite difference technique. The components of the sensitivity matrix Že.g., the partial derivative of the field
values with respect to the model parameters. are
calculated by using the technique of adjugate
Green’s functions. For N stations and F frequencies per E- and B-polarization mode, N = F
forward modeling calculations should be carried
out to calculate the elements of the sensitivity
matrix. The inversion procedure can be started
after the calculation of the sensitivity matrix. In
a two-dimensional case, the problem is ill-posed
and a regularization procedure must be implemented. Mackie et al. Ž1997. use Tikhonov’s
method which defines a regularized solution of
the inverse problem as a model m that minimizes the objective function.
T
S Ž m . s Ž d y F Ž m . Ry1
dd Ž d y F Ž m . .
q t 5 LŽ m y m0 . 5
2
Ž3.
where d s observed data vector Ž e.g., apparent
resistivities and phases. , F s forward modeling
operator Že.g., the differential equations for the
different field components of the E- and Bpolarization case., m s unknown model vector,
R dd s error covariance matrix, m 0 s a priori
model, t s regularization parameter and L is a
linear operator for the regularization procedure.
Mackie et al. Ž1997. use a Laplacian operator
Ž L s D . , yielding
5 LŽ m y m0 . 5 2 s
H Ž D Ž m Ž x . y m Ž x . . . d x.
2
0
Ž4.
A very important parameter which influences
the result of the inversion is the regularisation
parameter t . According to Mackie et al. Ž 1997. ,
the value of t should be chosen in such a way
that a RMS error for the inversion is between
1.0 and 1.5%. Typical values for t vary between 3 and 300. Large t values cause a
smoother model. Ziebell Ž 1997. examined with
249
synthetic data typical waste site locations Že.g.,
good conductive anomaly beneath a 1–3 m top
layer embedded in a poorly conductive host.
using the 2D inversion algorithm of Mackie et
al. Ž 1997. . He found that the top layer and the
waste are well resolved by using the RMT
frequencies Ž10–300 kHz. if t is chosen to be
100. In the following, this value was chosen
during the inversion of the RMT field data. A
Gauss–Newton algorithm with matrix inversion
was used for numerical minimization of the
objective function S. Then a minimum difference in the structure between start model and
final model was looked for. The resulting final
2D conductivity model minimizes also the difference between observed and calculated data.
4.1. Two-dimensional conductiÕity models for
the industrial waste site Mellendorf
The RMT data of nine profiles in EW direction associated with the E- and B-polarization of
the magnetotelluric field have been interpreted
by the 2D inversion technique of Madden and
Mackie Ž 1989. . Fig. 9 shows the result of the
2D inversion as a resistivity cross-section for
the profiles y s 0 m and y s 25 m. The waste
site is clearly emphasized by low resistivities
Ž- 35 V m.. Its lateral borders at profile locations 10 and 180 m are clearly detected by the
model. The local good conductive zone outside
of the waste site at 3 m can be interpreted as a
small clay body in the sandy layer. The same
anomaly was detected in the same area by TEM
measurements which were carried out just before the RMT survey Ž Schaumann, 1997. . The
bottom of the waste site can be estimated to be
15 m. However, the lowest frequency Ž e.g., 16
kHz. is not sufficiently low for clearly detecting
it. The central depth of the in-phase induced
currents zU ŽSchmucker, 1987. which is defined
as
zU s
(
ra
vm 0
sin f
Ž5.
250
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 9. Two-dimensional inversion results for the profiles y s 0 and y s 25 in Mellendorf. The dashed lines indicate the
lateral and vertical boundaries derived from the 2D conductivity models.
has been used as a guide for the maximum
depth from which information about the conductivity structure can be obtained. The 1D and 2D
model calculations of Ziebell Ž1997. have shown
that 2 zU can be used for RMT frequencies for
maximum interpretation depth in the case of
waste site surveys Ž e.g., a well-conductive body
embedded in a poorly conductive host. . VLF
frequencies have been used for the determination of the maximum depth Ž2 zU values on the
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
waste site. which is about 20 m for 16 kHz
ŽE-polarization. and for 20.3 kHz Ž B-polarization.. Similar inversion results have been obtained for the other profiles on the waste site in
Mellendorf. A quasi-3D resistivity distribution
at 3 m depth derived from the 2D modeling of
all RMT profiles is shown in Fig. 10 as an
example. The lateral borders of the waste deposit have been determined accurately. There is
a clear difference between undisturbed geology
Ž r ) 100 V m. and the waste site Ž r - 35 V m.
at this depth. The calculated 2D inversion results also enable resistivity slices to be derived
from the modeled data set down to the maximum interpretation depth Že.g., 2 zU .. Such representations can be seen as a final interpretation
step for a RMT survey Žsee also Section 4.2..
4.2. Two-dimensional conductiÕity models for
the Hermsdorf waste site
As in the case of the waste site in Mellendorf, the RMT data from 19 profiles in NErSW
direction Ž Fig. 8. were interpreted using the 2D
251
inversion algorithm of Mackie et al. Ž 1997. .
Apparent resistivities and phases for four frequencies in NWrSE Ž E-polarization. and
NErSW ŽB-polarization. direction were used as
input data. Fig. 11 shows the inversion results
of profiles y s 350 m and y s 375 m crossing
the waste site. Again, the waste site can clearly
be seen as a good conductive body with resistivities less than 35 V m. The lateral borders are
also well defined. The vertical extension of the
waste site cannot be resolved accurately. It can
quantitatively be estimated to be at 15 m from
the 2D inversion models. The 2 zU values on
the waste site vary for the VLF frequencies
between 14 and 16 m. A well-conducting structure outside of the waste site at a depth of
10–17 m can also be seen on the 2D models
and could be interpreted as a contamination
plume. The previous geophysical measurements
Že.g., DC soundings. also confirm these anomalies ŽLindner et al., 1993.. The other possible
interpretation for this good conductive anomaly
is the increase of loose packed materials in the
bunter sandstone which can produce the same
anomaly. Without borehole information no
Fig. 10. Resistivity distribution beneath the industrial waste site Mellendorf in 3 m depth, derived from the 2D inversion
results.
252
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 11. Two-dimensional inversion results for the profiles y s 350 and y s 375 in Hermsdorf. The dashed lines indicate the
lateral and vertical boundaries derived from the 2D conductivity models.
definitive interpretation about this anomaly is
possible from surface measurements. The 2D
conductivity models in Figs. 9 and 11 are the
optimum models which provide a minimum
misfit between the observed and modeled data.
Fig. 12 shows a comparison of the observed and
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
253
Fig. 12. Observed and calculated apparent resistivity and phases for the highest frequency 270 kHz on profile y s 350 in
Hermsdorf. The 2D model in Fig. 11 was used for calculating theoretical apparent resistivity and phase data.
calculated data for the highest frequency of 270
kHz on the profile y s 300 m. This comparison
serves as a representative example of a general
fitting for all calculated 2D models. In general,
254
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
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 explained by the 2D conductivity models. However, there are some large misfits between observed and modeled data. Especially the steep
abrupt change of data from station to station
cannot be explained by this type of model due
to smoothing criteria in the inversion algorithm.
The observed data can be influenced by local
inhomogeneities. The complete impedance tensor cannot be measured by the RMT instrument
used for these surveys. There are also no stable
3D inversion algorithm available. These are the
main reason why the interpretation of the RMT
data were carried out by 2D inversion algorithm
assuming a 2D conductivity structure. Similar to
the case in Mellendorf, the 2D inverted profiles
can be presented as slices through certain depths
in order to obtain a quasi-3D resistivity distribution of the survey area. Fig. 13 shows the
resistivity distribution at 3 m depth derived
from all 2D inversion results. About 20 min.
CPU time is needed for each inversion. The
lateral borders of the waste site can again be
defined accurately. There is a significant resistivity difference between the waste site - 35
V m and the surrounding host.
5. Conclusions
The radiomagnetotelluric technique is a powerful tool for waste site exploration. The case
histories shown in this paper and the previous
RMT surveys proved the efficiency of this
method.
The great advantage of RMT lies in the fact
that it combines both profiling and sounding. A
specially designed field setup Ž e.g., the use of
radio transmitters parallel and perpendicular to
the assumed strike direction of a waste site.
enables the interpretation of the data by 2D
conductivity models giving depth information of
the survey area.
Only a few boreholes exist on the investigated waste sites in Hermsdorf and Mellendorf,
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
hence just a rough information about their lateral borders could be deduced. On the other
hand, the 2D inversion algorithm of Mackie et
al. Ž1997. has been used to interpret the RMT
data and enabled an excellent resolution of the
lateral borders of the waste sites in Hermsdorf
and Mellendorf which are also confirmed by
historical records about the waste deposits. The
bottom of these waste sites could also be estimated.
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¨
www.elsevier.nlrlocaterjappgeo
Two-dimensional radiomagnetotelluric investigation of industrial
and domestic waste sites in Germany
B. Tezkan
a
a,)
, A. Hordt
¨ a, M. Gobashy
b
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 the
surrounding hosts. One was filled with industrial waste and the other with household refuse. Both had been recultivated.
Powerful military and civilian radiostations Ž10–300 kHz. located parallel and perpendicular to the strike direction of both
waste 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. The
lateral borders of both waste sites are located accurately in the field. They are characterized by strongly decreasing apparent
resistivities observed at nearly all frequencies at the border between the waste site and the undisturbed geology. The data
were quantitatively interpreted by conductivity models using a 2D inversion algorithm. The derived 2D conductivity models
give information about the vertical extent of the studied waste sites and about the structure of the surrounding hosts. q 2000
Elsevier Science B.V. All rights reserved.
Keywords: Waste sites; Radiomagnetotellurics; 2D inversion; Conductivity models
1. Introduction
In Europe, especially in Germany, the application of geophysical methods to waste site
exploration becomes increasingly important
since their contribution to the pollution risk
estimation has been recognized. In Germany,
for example, the number of registered waste
)
Corresponding author. E-mail:
tezkan@geo.uni-koeln.de
sites amounted to 93,723 in December 1993
ŽFranzius, 1994.; among them 350 in the city of
Cologne. These numerous waste sites are traditionally explored by geotechnical methods Že.g.,
drillings.. Applied geophysics can offer many
non-invasive techniques for mapping and should
be considered as a main source of information
for selecting borehole locations. In general,
waste sites are characterized by their low resistivities compared to the surrounding hosts, because industrial waste and household refuse
cause an increase of electrical conductivity due
0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 9 8 5 1 Ž 9 9 . 0 0 0 1 4 - 2
238
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
to biological decomposition of the organic materials. Since the mid-1990s, the radiomagnetotelluric method ŽRMT. is extensively used in
connection with waste site exploration Ž Dautel,
1996; Tezkan et al., 1996; Zacher et al., 1996a;
Tezkan et al., 1997. . The RMT method allows
both sounding and mapping by using several
frequencies. The data collection is relatively fast
and is interpreted quantitatively by using wellknown magnetotelluric modeling programs.
Several RMT surveys have also been conducted
successfully for hydrogeological investigations,
for engineering and for archeological explorations ŽTurberg, 1991; Hollier-Larousse et al.,
1994; Dupis and Choquier, 1996; Zacher et al.,
1996b..
Two selected RMT field surveys are presented in this paper to demonstrate the efficiency and suitability of the method for waste
site exploration of different types. The 2D inversion of the RMT data will be discussed in
detail.
The waste site of Volkswagen in Mellendorf
was chosen as a representative example for an
industrial deposit for the RMT evaluation. A
small number of boreholes give information
about the vertical extent of this waste site. No
geophysical techniques were applied for the exploration of this site previous to the detailed
RMT measurements. For comparison, the waste
deposit Hermsdorf filled with household refuse
was studied exclusively by geophysical methods
without any boreholes. Magnetic, DC, EM 31
measurements were carried out to map the deposit. The RMT measurements should enable a
detailed conductivity depth information of this
area by applying 2D inversion algorithm to the
observed data.
No geographic and topographic maps of both
waste sites will be shown for confidential rea-
sons. Instead, the efficiency of the RMT technique for the exploration of these different types
of waste sites and the resolutions of the 2D
inversion models are emphasized.
2. Basic concepts of the RMT technique
The RMT technique uses as transmitters military and civilian radiostations broadcasting in a
frequency range between 10 kHz and 1 MHz. In
Fig. 1a, the principle of this method is demonstrated schematically. The electromagnetic
waves radiated from these transmitters diffuse
into the conductive earth and induce electrical
current systems which are connected with electrical and magnetic alternating fields. The magnetic field can be measured for selected frequencies by a coil and the electric field by two
grounded electrodes. Fig. 1b shows the RMTinstrument, a prototype developed in Switzerland ŽMuller,
1983.. The magnetic coil has a
¨
diameter of 0.4 m and the distance between the
electrodes can be chosen to be 1 or 5 m. The
frequency range of this instrument is limited to
10–300 kHz but can be extended to 1 MHz
after hardware modifications in the future. No
transmitters are available below 10 kHz. Depending on the conductivity of the subsurface
this limitation might cause the near surface and
the deep layers to be unresolved by this method.
This will be discussed in Section 4 by the 2D
modeling results of the observed data.
The measuring device only weighs 7 kg and
about 3 min are required for measuring four
frequencies at one station. Thus, large areas can
be mapped quickly.
For the selected frequencies apparent resistivity and phase data are derived from the mea-
Fig. 1. Ža. Schematic diagram for illustrating a RMT field set up over a hazardous waste site. A sounding information can be
obtained by using transmitters with different frequencies from the same direction. Žb. RMT instrument used in the field: a
prototype developed by the Hydrogeological Institute of the University of Neuchatel,
1983..
ˆ Switzerland ŽMuller,
¨
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
239
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
240
sured electric and magnetic field using the
well-known formulae of magnetotellurics
ŽCagniard, 1953.
ra x y s
1
Ex
2
2pm 0 f H y
I
ž /
ž /
Ex
Hy
f x y s tany1
R
Ž1.
Ex
Ž2.
Hy
where E x and H y denote the electric and magnetic field, m 0 the permeability of the free space
and f the selected frequency.
Displacement currents can be neglected for
the RMT frequencies under normal conditions
in Europe. The plane wave assumption is valid
when receiver locations are located at least seven
skin depths away from the radio transmitters
ŽSchroder,
1994.. In this case, the well devel¨
oped and tested magnetotelluric interpretation
software can be directly applied to the RMT
data. The main difference between the RMT
and the VLF-R technique lies in the extension
of VLF frequencies Ž10–30 kHz. to higher frequencies Ž Tezkan et al., 1996.. Due to the very
limited frequency range, VLF-R is only used for
profiling and the interpretation of this data is
carried out visually ŽBerktold et al., 1992. . In
some cases, however, VLF data can be interpreted in terms of 1D and 2D conductivity
models ŽBeamish, 1998.. The RMT technique
enables a sounding which can be interpreted
quantitatively. One important aspect is the selection of transmitters by considering the strike
direction of the waste site. Radio transmitters
located parallel and perpendicular to the general
strike direction given by geological or anthropological structures can be selected for the given
frequency range. Assuming a two-dimensional
conductivity structure, this data is associated to
the E- and B-polarisation directions Že.g., Z x y
and Z y x elements of the magnetotelluric
impedance tensors. and then interpreted by using 2D inversion techniques ŽSmith and Booker,
1991; Mackie et al., 1997. . Scalar measurements can only be carried out by the RMT
system used in this study. The apparent resistivity and phase values associated to Z x x and Z y y
elements of the magnetotelluric impedance tensor are not measured in the field. If no information about the strike direction is available the
same field procedure is used by choosing transmitters with similar frequencies from NS and
EW directions and the obtained data is also
interpreted by 2D inversion procedures whereas
3D effects are not considered.
3. Selected RMT surveys for the exploration
of different waste site types
In the following, two selected RMT surveys
for waste site exploration in Germany will be
discussed. An industrial waste site and a very
common type of a waste site Žembedded with
household refuse and building debris. are chosen as an example.
3.1. The Mellendorf industrial waste site
The industrial waste site of Volkswagen at
Mellendorf near Hanover was used from 1960
to 1984. Magnesium drosses and other industrial wastes were stored in a sand pit. The
bottom of this sand pit was not made water
tight. Magnesium drosses are a waste product
from motor fabrications containing magnesium,
magnesium salts and magnesium oxides. The
clay layer beneath the area of the waste site is at
a depth of 60 m, i.e., relatively deep. Between
the surface and the clay layer a sand layer of
Quaternary age was geologically mapped containing thin clay layers in some localities. The
waste site was recultivated in 1984. Fig. 2
shows a lithological stratification beneath the
survey area derived from boreholes on the waste
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 2. The structure at the industrial waste site in Mellendorf derived from borehole results.
site ŽRenno and Christofzik, 1987. . The top and
the bottom of the waste site is expected at 1.6
and 16 m depth, respectively. During the recultivation the waste site was covered by a layer
containing soil, sand with gravel, clay and gravels ŽFig. 2..
For the RMT survey four frequency pairs
between 16 and 200 kHz from radiostations in
North–South and East–West directions were
chosen. The profile direction was East–West
which was perpendicular to the assumed strike
direction of the waste site. The distances be-
241
tween the profiles and the RMT stations were
12.5 m, except at the border between the waste
site and the undisturbed geology, where the
distance was 5 m.
Figs. 3 and 4 give an overview of the lateral
distribution of the apparent resistivities and
phases for 207 and 177 kHz. The triangles on
the figures denote the locations of the RMT
stations. The 207 kHz data belong to the transmitter located in North–South direction. In this
case, the measured electric field associated with
the E-polarization is oriented in NS direction.
The electric field of the 177 kHz data is oriented in the direction of the profile and represents the B-polarization. The waste site as a
well conducting anomaly in comparison to the
surrounding host is characterized by apparent
resistivity values - 30 V m for both polarization directions Že.g., 207 and 177 kHz apparent
resistivity data. and all lateral boundaries of the
waste site are clearly detected. A clear phase
difference outside ŽF f 508. and inside ŽF f
358. the waste site is also observed.
Figs. 5 and 6 show the apparent resistivity
and phase data for all observed frequencies
from NS and EW transmitters for the profile
y s 0 Ž Figs. 3 and 4. as a representative example. The boundaries of the waste site are characterized for both transmitter locations by strongly
decreasing apparent resistivity values at profile
position 5 m and by increasing apparent resistivity at profile position 190 m for all frequencies.
The difference between E-polarization Ž NStransmitters. and B-polarization Ž EW-transmitters. data can also easily be seen in these
figures. Due to the discontinuity of the electric
field at vertical boundaries, a very sharp transition from the undisturbed geology to the waste
site is observed for the B-polarization case in
Fig. 6, whereas this transition is, as expected,
relatively smooth for the E-polarization case in
Fig. 5. There is only a small difference between
the observed apparent resistivity values for all
frequencies on the waste site in Figs. 5 and 6.
On the other hand, the phases vary for every
station on the waste site from 608 for the lowest
242
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 3. Spatial distribution of apparent resistivity and phase for the frequency 207 kHz in Mellendorf as derived from the
transmitter in NS direction. Triangles denote the locations of the RMT stations.
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
243
Fig. 4. Spatial distribution of apparent resistivity and phase for the frequency 177 kHz in Mellendorf as derived from the
transmitter in EW direction. Triangles denote the locations of the RMT stations.
244
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 5. Observed apparent resistivities and phases in Mellendorf on the profile y s 0 ŽNS transmitters. for four frequencies.
frequency to 358 for the highest observed frequency ŽFig. 5.. Information about the structure
beneath the waste site can mostly be derived
from the phase information. This observation
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
245
Fig. 6. Observed apparent resistivities and phases in Mellendorf on the profile y s 0 ŽEW transmitters. for four frequencies.
has also been studied and confirmed by Ziebell
Ž1997. in his 2D model calculations for the
resolution of the bottom of waste sites using
synthetic data.
246
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
The quantitative interpretation of the nine
profiles crossing the industrial waste site in the
EW direction by means of 2D model calculations will be shown in Section 4.
3.2. The Hermsdorf waste site
A sand pit of approximately 10 m depth was
used as a waste site in Hermsdorf ŽThuringenr
¨
Fig. 7. Spatial distribution of apparent resistivities in Hermsdorf as derived from the transmitters in NWrSE direction
ŽE-polarization. for four frequencies.
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Germany. until 1979. Household refuse and industrial wastes from a ceramic factory were
deposited in this waste site. It was recultivated
247
and approximately a 1 m thick soil layer was
placed above the waste. The surrounding host
consists of bunter sandstone. No boreholes exist
Fig. 8. Spatial distribution of phases in Hermsdorf as derived from the transmitters in NWrSE direction ŽE-polarization. for
four frequencies.
248
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
to derive the lithology and the dimension of the
waste site. Previous geophysical studies on the
waste disposal site at Hermsdorf which included
total and vertical magnetic gradient measurements, geoelectric mapping and soundings Ž Lindner et al., 1993. , self potential Ž Pretzschner et
al., 1993. and refraction seismics Ž Dresen and
Schneider, 1994. identified the waste location
with an approximate thickness of about 8–12 m.
Another aim of these measurements was the
detection of possible contamination plumes from
the waste site.
The objective of the RMT measurements on
this area was to test their applicability and
efficiency with regard to the exploration of a
geophysically well studied waste. Especially a
very detailed information about the lateral
boundaries and vertical extension of a waste site
is expected from the RMT measurements.
Four pairs of frequencies from radio transmitters perpendicular to each other 23.8, 68, 128.5
and 270 kHz for NWrSE direction and 18.3,
75, 153 and 261 kHz for NErSW direction
have been used. Previous geophysical measurements indicate that the strike of the waste site is
in the NWrSE direction, which is associated
with the E-polarization case. Apparent resistivity and phase data were collected on 14 profiles
using a station interval varying between 12.5
and 50 m. The distance between the profiles
varies from 20 m in the waste to 50 m outside
of it. The expected boundaries of the waste site
are densely sampled.
The observed apparent resistivity and phase
data are shown in Figs. 7 and 8 as derived from
transmitters located in NWrSE direction. They
clearly show the approximate location of the
waste site, which is bounded by the profiles 250
and 450 Ž y-direction. and the stations 150 and
360 Ž x-direction.. The apparent resistivity values in Fig. 7 show a relatively heterogeneous
composition of the waste site with minimum
resistivities of 10 V m in some locations. The
corresponding phase values in Fig. 8 generally
reflect the same variability over the waste disposal. The average value of the phase is about
388. According to the r U Ž zU . transformation
ŽSchmucker, 1987., this indicates a more resistive unit below the well-conducting waste material. The apparent resistivities and phases
observed by using transmitters from NErSW
direction show the same qualitative results.
The observed data of the 19 profiles are
interpreted by means of 2D inversion algorithms
and will be shown together with the inversion
results of the industrial waste site Mellendorf in
the following.
4. Two-dimensional interpretation of the
RMT data
In general, more than 300 RMT stations are
used for a waste site exploration. More than 30
stations on a profile are very common. The
number of RMT stations to be interpreted by a
conductivity model is much larger than in the
case of a classic magnetotelluric survey. Trial
and error fitting using 2D forward modeling
proves to be very time consuming for such a
large number of stations. A 2D inversion algorithm is used for the interpretation of RMT data.
The field setup described in Section 2 Že.g.,
transmitters with similar frequencies parallel and
perpendicular to the assumed strike direction of
a waste site. enables the application of 2D
inversion algorithms on the data. Compared to
classic MT measurements only information
about Z x y and Z y x elements of the impedance
tensor is available. The Z x x and Z y y elements
are not estimated and assumed to be zero as
they should be in ideal case of a 2D conductivity anomaly. However, Z x y and Z y x may deviate from the estimated ones if 3D effects are
present.
Ziebell Ž1997. carried out model calculations
with different types of modeling algorithms using synthetic and field data. He shows that the
2D inversion algorithm of Mackie et al. Ž1997.
is very suitable for the interpretation of the
RMT data.
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
An inversion of the complete 2D Helmholtzequation is carried out in the algorithm of
Mackie et al. Ž 1997. . First, the subsurface structure is parameterized by using the finite difference technique. The components of the sensitivity matrix Že.g., the partial derivative of the field
values with respect to the model parameters. are
calculated by using the technique of adjugate
Green’s functions. For N stations and F frequencies per E- and B-polarization mode, N = F
forward modeling calculations should be carried
out to calculate the elements of the sensitivity
matrix. The inversion procedure can be started
after the calculation of the sensitivity matrix. In
a two-dimensional case, the problem is ill-posed
and a regularization procedure must be implemented. Mackie et al. Ž1997. use Tikhonov’s
method which defines a regularized solution of
the inverse problem as a model m that minimizes the objective function.
T
S Ž m . s Ž d y F Ž m . Ry1
dd Ž d y F Ž m . .
q t 5 LŽ m y m0 . 5
2
Ž3.
where d s observed data vector Ž e.g., apparent
resistivities and phases. , F s forward modeling
operator Že.g., the differential equations for the
different field components of the E- and Bpolarization case., m s unknown model vector,
R dd s error covariance matrix, m 0 s a priori
model, t s regularization parameter and L is a
linear operator for the regularization procedure.
Mackie et al. Ž1997. use a Laplacian operator
Ž L s D . , yielding
5 LŽ m y m0 . 5 2 s
H Ž D Ž m Ž x . y m Ž x . . . d x.
2
0
Ž4.
A very important parameter which influences
the result of the inversion is the regularisation
parameter t . According to Mackie et al. Ž 1997. ,
the value of t should be chosen in such a way
that a RMS error for the inversion is between
1.0 and 1.5%. Typical values for t vary between 3 and 300. Large t values cause a
smoother model. Ziebell Ž 1997. examined with
249
synthetic data typical waste site locations Že.g.,
good conductive anomaly beneath a 1–3 m top
layer embedded in a poorly conductive host.
using the 2D inversion algorithm of Mackie et
al. Ž 1997. . He found that the top layer and the
waste are well resolved by using the RMT
frequencies Ž10–300 kHz. if t is chosen to be
100. In the following, this value was chosen
during the inversion of the RMT field data. A
Gauss–Newton algorithm with matrix inversion
was used for numerical minimization of the
objective function S. Then a minimum difference in the structure between start model and
final model was looked for. The resulting final
2D conductivity model minimizes also the difference between observed and calculated data.
4.1. Two-dimensional conductiÕity models for
the industrial waste site Mellendorf
The RMT data of nine profiles in EW direction associated with the E- and B-polarization of
the magnetotelluric field have been interpreted
by the 2D inversion technique of Madden and
Mackie Ž 1989. . Fig. 9 shows the result of the
2D inversion as a resistivity cross-section for
the profiles y s 0 m and y s 25 m. The waste
site is clearly emphasized by low resistivities
Ž- 35 V m.. Its lateral borders at profile locations 10 and 180 m are clearly detected by the
model. The local good conductive zone outside
of the waste site at 3 m can be interpreted as a
small clay body in the sandy layer. The same
anomaly was detected in the same area by TEM
measurements which were carried out just before the RMT survey Ž Schaumann, 1997. . The
bottom of the waste site can be estimated to be
15 m. However, the lowest frequency Ž e.g., 16
kHz. is not sufficiently low for clearly detecting
it. The central depth of the in-phase induced
currents zU ŽSchmucker, 1987. which is defined
as
zU s
(
ra
vm 0
sin f
Ž5.
250
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 9. Two-dimensional inversion results for the profiles y s 0 and y s 25 in Mellendorf. The dashed lines indicate the
lateral and vertical boundaries derived from the 2D conductivity models.
has been used as a guide for the maximum
depth from which information about the conductivity structure can be obtained. The 1D and 2D
model calculations of Ziebell Ž1997. have shown
that 2 zU can be used for RMT frequencies for
maximum interpretation depth in the case of
waste site surveys Ž e.g., a well-conductive body
embedded in a poorly conductive host. . VLF
frequencies have been used for the determination of the maximum depth Ž2 zU values on the
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
waste site. which is about 20 m for 16 kHz
ŽE-polarization. and for 20.3 kHz Ž B-polarization.. Similar inversion results have been obtained for the other profiles on the waste site in
Mellendorf. A quasi-3D resistivity distribution
at 3 m depth derived from the 2D modeling of
all RMT profiles is shown in Fig. 10 as an
example. The lateral borders of the waste deposit have been determined accurately. There is
a clear difference between undisturbed geology
Ž r ) 100 V m. and the waste site Ž r - 35 V m.
at this depth. The calculated 2D inversion results also enable resistivity slices to be derived
from the modeled data set down to the maximum interpretation depth Že.g., 2 zU .. Such representations can be seen as a final interpretation
step for a RMT survey Žsee also Section 4.2..
4.2. Two-dimensional conductiÕity models for
the Hermsdorf waste site
As in the case of the waste site in Mellendorf, the RMT data from 19 profiles in NErSW
direction Ž Fig. 8. were interpreted using the 2D
251
inversion algorithm of Mackie et al. Ž 1997. .
Apparent resistivities and phases for four frequencies in NWrSE Ž E-polarization. and
NErSW ŽB-polarization. direction were used as
input data. Fig. 11 shows the inversion results
of profiles y s 350 m and y s 375 m crossing
the waste site. Again, the waste site can clearly
be seen as a good conductive body with resistivities less than 35 V m. The lateral borders are
also well defined. The vertical extension of the
waste site cannot be resolved accurately. It can
quantitatively be estimated to be at 15 m from
the 2D inversion models. The 2 zU values on
the waste site vary for the VLF frequencies
between 14 and 16 m. A well-conducting structure outside of the waste site at a depth of
10–17 m can also be seen on the 2D models
and could be interpreted as a contamination
plume. The previous geophysical measurements
Že.g., DC soundings. also confirm these anomalies ŽLindner et al., 1993.. The other possible
interpretation for this good conductive anomaly
is the increase of loose packed materials in the
bunter sandstone which can produce the same
anomaly. Without borehole information no
Fig. 10. Resistivity distribution beneath the industrial waste site Mellendorf in 3 m depth, derived from the 2D inversion
results.
252
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
Fig. 11. Two-dimensional inversion results for the profiles y s 350 and y s 375 in Hermsdorf. The dashed lines indicate the
lateral and vertical boundaries derived from the 2D conductivity models.
definitive interpretation about this anomaly is
possible from surface measurements. The 2D
conductivity models in Figs. 9 and 11 are the
optimum models which provide a minimum
misfit between the observed and modeled data.
Fig. 12 shows a comparison of the observed and
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
253
Fig. 12. Observed and calculated apparent resistivity and phases for the highest frequency 270 kHz on profile y s 350 in
Hermsdorf. The 2D model in Fig. 11 was used for calculating theoretical apparent resistivity and phase data.
calculated data for the highest frequency of 270
kHz on the profile y s 300 m. This comparison
serves as a representative example of a general
fitting for all calculated 2D models. In general,
254
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
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 explained by the 2D conductivity models. However, there are some large misfits between observed and modeled data. Especially the steep
abrupt change of data from station to station
cannot be explained by this type of model due
to smoothing criteria in the inversion algorithm.
The observed data can be influenced by local
inhomogeneities. The complete impedance tensor cannot be measured by the RMT instrument
used for these surveys. There are also no stable
3D inversion algorithm available. These are the
main reason why the interpretation of the RMT
data were carried out by 2D inversion algorithm
assuming a 2D conductivity structure. Similar to
the case in Mellendorf, the 2D inverted profiles
can be presented as slices through certain depths
in order to obtain a quasi-3D resistivity distribution of the survey area. Fig. 13 shows the
resistivity distribution at 3 m depth derived
from all 2D inversion results. About 20 min.
CPU time is needed for each inversion. The
lateral borders of the waste site can again be
defined accurately. There is a significant resistivity difference between the waste site - 35
V m and the surrounding host.
5. Conclusions
The radiomagnetotelluric technique is a powerful tool for waste site exploration. The case
histories shown in this paper and the previous
RMT surveys proved the efficiency of this
method.
The great advantage of RMT lies in the fact
that it combines both profiling and sounding. A
specially designed field setup Ž e.g., the use of
radio transmitters parallel and perpendicular to
the assumed strike direction of a waste site.
enables the interpretation of the data by 2D
conductivity models giving depth information of
the survey area.
Only a few boreholes exist on the investigated waste sites in Hermsdorf and Mellendorf,
B. Tezkan et al.r Journal of Applied Geophysics 44 (2000) 237–256
hence just a rough information about their lateral borders could be deduced. On the other
hand, the 2D inversion algorithm of Mackie et
al. Ž1997. has been used to interpret the RMT
data and enabled an excellent resolution of the
lateral borders of the waste sites in Hermsdorf
and Mellendorf which are also confirmed by
historical records about the waste deposits. The
bottom of these waste sites could also be estimated.
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