Journal of Applied Geophysics: Franjo Šumanovac, Jasna Ore Šković
Journal of Applied Geophysics: Franjo Šumanovac, Jasna Ore Šković
Journal of Applied Geophysics: Franjo Šumanovac, Jasna Ore Šković
a r t i c l e i n f o a b s t r a c t
Article history: On the selected cases, Gotalovec in the area of Pannonian basin and Baška in the Dinaridic karst area, that are
Received 19 October 2017 representing a common hydrogeological model in both regions of Croatia, CSAMT data together with data of
Received in revised form 28 March 2018 other geophysical methods (electrical resistivity tomography, electrical sounding and seismic reflection) enabled
Accepted 7 April 2018
the definition of a reliable prognostic geological model. The model consists of carbonate aquifer which underlies
Available online 11 April 2018
an impermeable thick package of clastic deposits. There are great variations of the dolomitic aquifer depths in the
Keywords:
Gotalovec area due to strong tectonic activity, while in the Baška area depth changes are caused by the layer
CSAMT folding.
Groundwater The CSAMT method provides the most complete data on lithological and structural relationships in cases of
Carbonate aquifer hydrogeological targets deeper than 100 m. Based on the presented models we can conclude that the CSAMT
Forward modelling method can provide greater exploration depth than electrical resistivity tomography (ERT) and can be consid-
Inversion ered as a fundamental geophysical method for exploration of buried carbonate aquifers, deeper than 100 m.
But, the CSAMT research may demonstrate its advantages only in the case of very dense layout of CSAMT stations
(25–50 m), due to the greater sensitivity to noise in relation to resistivity methods. Interpretation of CSAMT data
is more complex in relation to resistivity methods, and a forward modelling method sometimes gives better re-
sults than an inversion due to possibility of the use of additional data acquired by other geophysical methods
(ERT, electrical sounding and seismic reflection). At greater depths, the resolution of all electrical methods includ-
ing the CSAMT method is significantly reduced, and seismic reflection can be very useful to resolve deeper lith-
ological interfaces.
© 2018 Elsevier B.V. All rights reserved.
https://doi.org/10.1016/j.jappgeo.2018.04.007
0926-9851/© 2018 Elsevier B.V. All rights reserved.
48 F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63
rock; based on the measured rock resistivity, and data about rock type comparison and verification. The main disadvantage of the CSAMT
and rock condition (whether it is compact, fractured or porous) can be method compared to the direct current methods is greater sensitivity
obtained. to urban and surface noises, which is especially prominent in urban en-
Electrical tomography is also applied to solve geological models in vironments and in the vicinity of electrical infrastructural facilities.
sedimentary basin. However, in the case of aquifers composed of thin
sandy or gravelly layers, the other geophysical methods should be ap- 2. Geological setting
plied, especially seismic reflection, to determine a reliable predicted
geological model (Šumanovac, 2006). The targets of geophysical explorations in Croatia are often carbon-
Standard equipment for ERT is efficient up to depths of about 100 m, ate rocks that underlie clastic deposits. Carbonate rocks, Triassic dolo-
but in the case of deeper targets in hydrogeological and geothermal in- mites and Jurassic and Cretaceous limestones and dolomites, may be
vestigations, ERT is not usually carried out because the field equipment tectonically fractured and weathered and are therefore permeable and
would be too expensive and impractical. The tomographic profiles as such act as aquifer. Clastic sediments above them consist mainly of
should be very long to reach deep targets, and there is often not enough impermeable clay, shale and marl, which are insulating rocks. This
space to set such long profiles. Length of the profiles could be often 800 model is found in both geological regions in Croatia, in the Pannonian
m only, which means the depth penetration of 130 m in the middle of Basin and the Dinaridic karst area (Fig. 1). In the Dinarides clastic de-
the profile. posits are usually Eocene flysch sediments. Although they may contain
Deeper hydrogeological investigations seek depth penetration of permeable rocks such as sands, sandstones and carbonates, they are in
several hundred meters, and geothermal investigations even more a package of impermeable rocks, clay and marl, so the entire package
than 1 km. In such cases, standard ERT cannot be applied in an econom- acts as an impermeable barrier. Two representative locations are se-
ical and routine way, so other geophysical methods should be consid- lected in order to analyse the possibilities of CSAMT method in ground-
ered. The use of CSAMT can provide greater depths of research, as well water research: Gotalovec in the Pannonian Basin and Baška on island
as good space sampling and relatively dense measurements. A common Krk that belongs to the External Dinarides.
hydrogeological model, which can be found in both main regions in
Croatia, Pannonian basin and Dinarides (Fig. 1), can be solved by the 2.1. Gotalovec area
CSAMT method. The carbonate aquifer is underlying an impermeable
package of clastic deposits. The main goal is to define as precisely as pos- The first study area - Gotalovec, is located in the north-western
sible the carbonate-clastic interface which will enable determination of Croatia, at the foot of Mount Ivančica (Fig. 1). The geological model in
optimal exploratory borehole positions. Two characteristic examples of this part of Croatia consists of Quaternary and Neogene sediments
groundwater investigations carried out in the Pannonian basin and (Miocene layers), and Cretaceous volcanoclastic rocks (K1,2) which dis-
Dinarides in Croatia are shown. The results achieved by the CSAMT cordantly lay on a surface made of Upper and Middle Triassic dolomites
method were correlated with data obtained by other geophysical (T3, T2), Šimunić et al. (1982; Fig. 2). These dolomites were exposed to
methods, seismic reflection and electrical resistivity tomography, for intense tectonic activities in several stages during geological history.
Fig. 1. Positional map of the surveys. Locations 1 and 2 belong to Pannonian basin, while location 3 belongs to the Dinarides. 1 – position of the profile P-1 at Gotalovec, situated in a hilly
area; 2 – position of the profile P-2 at Gotalovec, situated in a valley; 3 – position of the profile P-3 on karstic island Krk.
F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63 49
For that reason the secondary, fracture porosity was created and the found between the surface and a depth of 266.5 m, followed by Mio-
ground rock massif of the dolomites have become the main aquifer in cene, Upper Badenian Lithothamnium limestone to a depth of 339 m,
this part of Croatia with a significant water supply potential. Neogene and Triassic dolomites (Pickel, 2004). Lithothamnium limestone ap-
rocks (Miocene and Pliocene) are mostly built of marls, clays, shales pears in the form of reef, and thus large lateral changes in its thickness
and fine-grained sandstones are impermeable as such. Occasionally can be expected.
Lithothamnium limestone from Badenian can be found in the middle
part of Miocene deposits, at the contact with Triassic dolomites. They 2.2. Baška area
come as irregularly-shaped bodies, in reef development, and are also
aquifers when in contact with dolomites. The Lower Triassic clastic sed- The second micro-location is in the south-eastern part of Adriatic is-
iments are placed beneath the Triassic carbonate aquifer, which are con- land Krk, which belongs to the External Dinarides and can be character-
sidered as impermeable or low permeable layers. In view of the fact that ized as a karstic area (Fig. 1). According to the previous geological
impermeable clastics and permeable carbonates are clearly distin- investigations Eocene flysch layers (E2,3), Eocene limestones (E1) and
guished by their resistivities, this model can be very efficiently investi- Upper Cretaceous carbonates (K22, 2K1,2 2 ) are found at the survey area
gated using electrical and electromagnetic methods. (Mamužić et al., 1969; Fig. 3). Eocene flysch layers are mostly composed
Low mineral and thermal water (temperature of 25.6 °C) from the of a package of clastic deposits (marls and sandstones) with limestone
spring Topličica, close to the study area (Fig. 2), according to the ionic inserts. Although the package can include some permeable layers, it
composition belongs to the CaMg-HCO–3 (calcium magnesium hydrogen usually acts as an impermeable barrier. Foraminiferal limestones (E1)
carbonate) mixed type (Borović et al., 2016). This water type also points are present in the Lower Eocene rocks which discordantly lay on the
to the aquifer which is built of Badenian limestone and Triassic dolo- Cretaceous rudist limestones (K22). Foraminiferal limestones can be
mites as a unique aquifer. The water has been recently utilized for often poorly or sometimes well permeable depending on the degree
water bottling, as well. The Triassic dolomites appear at the surface on of fracturing, while the Cretaceous limestones are usually fractured
Mount Ivančica, which are also connected to the dolomite basement and weathered which means they are permeable, and they can be con-
in the Mountain foothills enabling water recovery in the aquifer. sidered as the main aquifer in the survey area.
Dragičević et al. (1998) evaluated secondary porosity in the range 5– Krk is one of the biggest Adriatic islands where are several locations
25% in the mountain aquifers based on observing outcrops on larger with significant water resources. To the south of the study area there are
quarries. The aquifer thicknesses vary considerably ranging from 200 wells which exploit fresh water from sandstone layers within flysch
m to 1800 m. They also concluded on the basis of established long pe- layers, as an exception in that region of Dinarides. The flysch layers in
riod balance between recharge and storage that renewable resources the survey area are mainly impermeable, and the main goal is karstified
for these aquifers are similar to the annual discharge at aquifer springs. and fractured Cretaceous limestone in the basement. On the basis of test
According to previous investigations conducted by an exploratory pumping in the exploited wells on the Adriatic karstified islands, Terzić
borehole located at one of the deep electrical sounding site in the (2006) estimated average hydraulic conductivity in the range 10−5–
wider area (Fig. 2), Pliocene and Miocene clastics, mostly marls, are 10−6 m/s for rock masses.
Fig. 2. Geological map (Šimunić et al., 1982) and positions of the profiles at the Gotalovec area placed in the Pannonian basin.
50 F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63
Fig. 3. Geological map (Mamužić et al., 1969) and positions of the profiles at the Baška area (island Krk) placed in the Dinarides.
The survey area is placed in the syncline structure (Fig. 3). Based on At the second micro-location, deep vertical electrical soundings
previous investigations in the wider area we expect great variations in using Schlumberger array (electrode spacing AB/2 = 1500 m) were
the thicknesses of the flysch impermeable layers above the carbonate performed, covering a wider area in order to determine the general lith-
basement as steeply sloping carbonate-clastic interface are suggested. ological relationships, that is the common depth of Triassic dolomites.
Thus, the predicted depth of the limestone aquifer ranges from 100 to Since the resolution of electrical methods is strongly reduced with
300 m. depth, seismic reflection measurements were performed. It enables
very high resolution even at depths of several hundred meters. The
3. Data acquisition aim of this investigation was to define more precisely discontinuities,
lithologic boundaries and potential faults. In particular, it was expected
In the area of Gotalovec, investigations were carried out at two micro- to define more precisely the contact of clastic rocks and carbonates,
locations. The first one is located in a hilly area, and the expected depth of which is the relief of Triassic dolomites underlying clastic rocks. There-
Triassic dolomites is relatively small (tens to hundreds of meters) since in fore, two reflection profiles, RF-1 and RF-2 were set up. Seismic reflec-
the surroundings can be found outcrops of Triassic dolomites. The second tion measurements were carried out using 24-channel ABEM
micro-location is in the plain area and expected depth of Triassic dolo- Terralock MK6 and standard CDP-method was applied. The data were
mites is few hundred meters because the borehole in the vicinity of re- recorded using the in-line spread (source-receiver geometry) with an
search area revealed dolomites at 266 m (Pickel, 2004). offset of 20 m and 5 m geophone spacing. This resulted in a 12-fold cov-
At the first micro-location ERT and CSAMT measurements were car- erage with a CDP distance of 2.5 m. The source was small explosive
ried out on the same profile. Electrical tomography profile was 700 m charge (100–200 g of explosive) placed in a shallow borehole about 1
long. The measurement was performed using a Wenner electrode m deep. The CSAMT measurements were carried out along seismic re-
array with a unit electrode spacing of 10 m, so the depth of investigation flection profile RF-1, on 15 CSAMT stations at a distance of 25 m. The
was 135 m. The inversion was done using the software based on the electrical sounding results and reflection data allow detailed control
Loke and Barker (1995) algorithm. Since expected depths are rather and analysis of CSAMT results.
small, and resolution of ERT to those depths is good, it can be used for In the area of Baška, measurements were performed at several
control and evaluation of CSAMT measurements. CSAMT measurements micro-locations and one has been selected as characteristic one
were carried out at 16 CSAMT sounding sites distributed along the elec- (Figs. 1 and 3). CSAMT measurements were completed on the profile,
trical tomography profile. The distance between first three sites is 25 m, and 13 CSAMT stations were measured with spacing about 50 m. ERT
and the others are at a distance of 50 m, so the total length of the profile measurements were done on the profile that stretches parallel to the
is 700 m. CSAMT-profile, using a Wenner electrode array with unit spacing of
F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63 51
Fig. 4. Examples of measured data: apparent resistivity and phase in the xy and yx direction plotted as a function of frequency, represented with two characteristic stations at each location:
(a) for the first location at Gotalovec, (b) for the second location at Gotalovec and (c) Baška on island Krk.
inversion is usually carried out by an iterative procedure where forward interpreter. During forward modelling it is possible to test several op-
modelling is included to calculate resistivity responses for initial tional models because sometimes different boundary depths and resis-
models. For the first initial model a half-space of constant resistivity is tivity contrasts are possible. Due to the fact that the final model needs to
assumed. The only influence of the interpreter is to define inversion al- fit both, a priori geologic knowledge and observed CSAMT data, the final
gorithm and several inversion parameters, such as resistivity limits, interpreted model will be often more reliable than the model acquired
smoothing factor and smoothness ratio. Depending on the data quality by an automatic inversion.
and noises the inversion model can sometimes give unreliable results. The inversion could be done using just apparent resistivity data, just
Also, we have to keep in mind that inversion is non-unique and many phase or both. In the present analysis we have employed an Occam's in-
resistivity models can fit the observed data. However, the inversion version (deGroot-Hedlin and Constable, 1990), with use of smoothing
model would provide for general structure of the subsurface based on operator and additional contrast minimization to obtain simple and
inverted resistivity distribution. rather smooth resistivity model (Kaminsky, 2016). The input data
A forward modelling is done for the lithologically constrained model. were apparent resistivity and phase data from Stratagem EH-4. Prior
The model is defined using a priori geological data or results from other to inversion, the CSAMT data need to be smoothed due to noise, since
geophysical methods. In that way the number of possible solutions is noisy data may cause unrealistic features to appear in the model. The
significantly reduced as the interpreted model is under control of the data were inspected visually for each station in order to remove spikes
F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63 53
Fig. 5. 2D inversion of determinant data along profile MP-1 at the Gotalovec area (Pannonian basin). Comparison of observed and calculated apparent resistivity is presented at the top, and
ERT profile (TP-1) is in the middle. Top of high resistivity carbonate rocks at the western part of profile can be interpreted at the same depth in both models and the lateral contact is almost
vertical.
and smooth the data. The strike angle has been determined according to by 1.2 relative to the thickness of the layer above it. It resulted in the
geological trends and the data were rotated to TE (electric polarization) depth of the bottom layer being between 5000 and 7000 m.
and TM (magnetic polarization) mode. The measured data are rather The computation was done by joint inversion of TE and TM data, and
noisy and Jones and Groom (1993) showed that the strike angle deter- also the determinant (D) which can be considered as the arithmetic
mination based on field data is unstable in the presence of noise and mean of TE and TM data. According to Pedersen and Engels (2005) the
local distortion. Since the determination of strike angle from the data determinant of the impedance tensor is independent of the assumed
themselves was questionable we have choose the aid of geological strike direction so the best fitting model is found to be practically inde-
trend. pendent of the starting model used. The inversion is done iteratively to
The starting model for inversion was constructed with constant re- fit the calculated model to measured CSAMT data, until the given criteria
sistivity and represented by a grid defined by the thickness of surface is reached, which can be either the maximum number of iterations or
layer, incremental factor and number of layers which determines the the maximum RMS error. We have used a number of iterations as stop
depth of the bottom layer. The block width is scaled to match the site criteria; in most cases 5 to 7, since a great number of iterations some-
spacing with two additional nodes between stations, and since the sta- times makes the model unnecessarily complicated and can result in a
tions were evenly spaced it produced almost equidistant mesh. The model with unrealistic details. Our goal was to invert for the model
starting thickness of the first layer was set to 1 m and number of the that will give a picture of the major structures.
layers 38 to 40, depending on the profile. The incremental factor was Based on the inverted resistivity models and other available data, the
1.2, which means that each subsequent layer's thickness is multiplied initial models for the forward modelling were constructed. The
54 F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63
resistivity models were created in a block mode with the sites spacing at a very shallow depth (20–30 m). At a distance between 200 and 250
from the field layout and taking into account the topography. In forward m resistivity is sharply decreased, and the border is steep, which can al-
modelling it is approximated that the field in each cell of the mesh ready be seen on the ERT profile (Fig. 5). In the second part of the profile
changes linearly. The response (resistivity and phase) for defined resis- (from 300 m to the end) resistivity is around 120 Ωm and contours
tivity model is calculated for both modes and also determinant (D). Be- show a wide trough. But, comparing the observed and calculated apparent
cause shallow layers of low resistivity can significantly influence the resistivity sections the greatest discrepancy can be seen exactly for the low
calculated data, the results from electrical resistivity tomography and frequency in the second part of the profile where the measured apparent
electrical soundings where available were used to define shallow layers. resistivities are higher (Fig. 5). To the depth of about 120 m CSAMT
In that way we could more reliably determine the deeper boundaries, model can be compared with the tomography profile TP-1 and agreement
i.e. the interface between clastics and carbonate rocks. is good. In the tomography model high resistivity can be observed at the
beginning of the profile at shallow depth (20 m), while in the second
6. Geophysical interpretation part of the profile resistivities are low. But in the second part of the tomo-
graphic profile, the basement probably couldn't be reached because of
6.1. Gotalovec area greater burial depth. Near surface the resistivity is very low (about 20
Ωm). High resistivities originate from carbonate rocks while low resistivi-
At the first micro-location in the Gotalovec area, the Occam inversion ties originate from impermeable clastic rocks. The lateral contact of high
method was applied to obtain 2D resistivity model. Along the profile and low resistivities is sharp and sub-vertical so a fault contact between
MP-1 that is 700 m long, 16 CSAMT soundings were measured. The final carbonates and clastics can be assumed.
resistivity model was obtained after six iterations (Fig. 5). In the resulting Based on MT inversion results and ERT model, MT forward model-
model, at the beginning of the profile resistivity is high (about 500 Ωm) ling was carried out. The ERT profile helped in defining the depth and
Fig. 6. Results of forward modelling along profile MP-1. At the top are observed and calculated apparent resistivity sections for the final model presented at the bottom. The unbroken line
represents the interface between clastics and carbonates in the final model. Two other possible solutions are presented with a dotted line and a dashed line.
F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63 55
lateral extent of shallow low-resistivity zone, which can significantly than the deepest part should be about 50 m wider (Fig. 6). But, for the
influence the depth and shape of buried carbonate interface. The narrower zone the depth could be as great as 280 m with small change
CSAMT inversion has been used to define deep parts and carbonate bed- in RMS error.
rock in the initial resistivity model. A final solution of the forward Comparison of MT inversion and forward models gives similar re-
modelling has been selected according to the RMS error and previous sults and very good agreement in the western part of the profile. But
investigations. The final model shows very shallow carbonate rocks the trough in the middle of the profile is more pronounced in the in-
(about 20 m) in the first 200 m of the profile (Fig. 6). Sharp, almost ver- verse model than in the forward model, and the transition from low to
tical contact between high resistivity carbonates (800 Ωm) and clastics high resistivity is quite wide so it is difficult to define the contact bound-
is defined at this distance. The depth of the basement is much greater in ary. The vertical uplift of the high-resistivity block at the beginning of
the second part of the profile and varies slowly, from 180 to 140 m, at a the profile also suggests a fault contact.
distance 320–700 m. However, greater depths of about 220 m are defined At the second micro-location in the area of Gotalovec electrical
in the narrow zone at a distance 200–320 m. The resistivity of carbonates soundings were measured in a wider area prior to CSAMT measure-
changes laterally, and in the second part of the profile is 500 Ωm. If the car- ments (Fig. 2). All electrical sounding curves show an increase in resis-
bonate interface is about 25 m shallower in the middle part of the profile, tivity in their deepest parts. The increase is caused by possible aquifers,
but the depth of the high resistivity body varies in a very wide range, Triassic dolomites below them were determined in the borehole. The
from 190 to 700 m (Fig. 2). An exploration borehole has been drilled water quality and recovery possibilities were better than expected. In
at the electrical sounding site with the shallowest interpreted depth order to get more precise data about the burial depth of carbonate
(GS-3). Exploration well was positive and confirmed the results of elec- rocks, the data from exploratory borehole have been used to calibrate
trical sounding. The Miocene limestones at a depth of 266.5 m, and the resistivity, thus reducing the ambiguity of interpretation. At the
Fig. 8. Forward CSAMT model along profile MP-2 (Gotalovec area). At the top are observed and calculated apparent resistivity sections for the final model presented at the bottom.
Unbroken black line represents the step boundary between clastic and carbonate rocks in the final model. The shift in boundary depth of ±10% resulted in RMS error increase by
around 0.2% (dashed lines). The model with greater depth corresponds better with the depth from DC soundings (black dots).
F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63 57
sounding site GS-3 the depth of carbonate rocks has been determined at depth gradually decreased till the end of the profile, where it was 225 m.
266.5 m as a reliable interface so the resistivity of carbonates of 367 Ωm The resistivity of carbonates was at first defined to 400 Ωm, as determined
was determined. According to these results, the resistivity of carbonates by electrical soundings. In the next step the resistivity has been increased
was interpreted as 400 Ωm. Reinterpretation of other sounding curves to 650 Ωm in order to reduce the RMS error (9.7%).
has been performed using this resistivity value, and more precise infor- The calculated apparent resistivity section for model with clastic resis-
mation on the burial depth of carbonate rocks was obtained. In that way, tivity of 55 Ωm and carbonate rock resistivity of 650 Ωm compared to ob-
the depths of aquifers, Triassic dolomites and Lithothamnium lime- served apparent resistivity section shows good agreement (RMS = 4.5%).
stones, on a wider area were generally defined by electrical soundings. However, if the top of the carbonates is 25 m shallower compared to the
The CSAMT data were measured at 15 stations along reflection pro- starting model, the RMS error is 8.1%. Thus, the final model shows the car-
file to acquire the information about resistivities. Water-bearing car- bonate rocks depth of 325 m at the beginning of the profile and 200 m at
bonates are clearly visible on the CSAMT-profile because of high the end of the profile, presented with a full line over the smoothed final
resistivities compared to impermeable deposits characterized by very model in the background (Fig. 8).
low resistivities (Figs. 7 and 8). The inversion of the CSAMT profile Since the resolution strongly decreases with depth, uncertainty anal-
MP-2 was performed using the Occam method and the final model on ysis has been performed within the forward modelling. If the depth of
Fig. 7 has been obtained after nine iterations (RMS = 9.5%). The depth carbonate boundary is shifted for −10%, which means it is shallower
of carbonate rocks is difficult to define, but it could be interpreted as be- for 20–33 m in regard to the final model, the fit to the observed data
tween the depths of 150 and 200 m. The resulting resistivity model does is almost the same with RMS error 4.5%. Similar results were obtained
not fit entirely the expected results, since the interpreted depth on the when the boundary depth was greater for 10% (RMS = 4.8%), Fig. 8. If
nearby electrical sounding sites is much deeper (327 at GS-10 and the depth of carbonates is even greater, i.e. 400 m at the beginning of
353 m at GS-9). Also, it is expected that the depth of interface becomes the profile and 275 m at the end of the profile, the RMS error is now in-
shallower towards the end of the profile but the depth is the greatest creased (5.2%). The fit is also less in the case of the model with a larger
around a distance of 250 m. If the depth of interface is defined between jump i.e. great change in the depth of carbonate boundary at a distance
100 and 150 m than its trend in the first part of the profile is as expected. of 150 m (Fig. 8; dotted line). Therefore, based on the forward model-
The forward modelling shows much better agreement with other ling it can be assumed that the depth of carbonate rock boundary is re-
available data (Fig. 8). An initial model was constructed based on inter- solved to a depth of ±10%.
pretation of DC soundings at GS-9 and GS-10. GS-9 is located 50 m from Additionally, more precise reflection measurements were per-
MT-2 station and the profile, and GS-10 is located on the CSAMT-profile formed at the identified location, which made it possible to precisely de-
at a distance of 215 m (Fig. 1). The top of carbonate rocks has been fine lithological relationships. On the reflection profile, three sets of
interpreted at the depth of 353 m at GS-9, while at GS-10 it is at the strong, mostly continuous reflections were noted in terms of depth, be-
depth of 327 m. In the initial 2D model the depth of carbonates was tween which there are zones with weak and discontinuing reflections
350 m and has been constant to the distance of 150 m. After that the (Fig. 9). At the beginning of the profile, there is the first set with a two-
Fig. 9. CSAMT and seismic reflection profiles at the Gotalovec survey area. Contact of clastic and carbonate rocks is defined more precise on the basis of seismic reflection and electrical
sounding data.
58 F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63
way travel time of 140 ms, the second one is in the 180–300 ms interval, Results of the MT forward modelling show very good agreement
and the third one in the 355–455 ms interval. If the boundary between with the seismic reflection data (Fig. 9). At the beginning of the profile
marl and carbonates is introduced from the GS-9 electrical sounding the depth is the greatest (325 m), and varies very little to the middle
(Fig. 9), and assuming the average seismic velocity of 2500 ms, the inter- of the profile, but then rapidly decreases towards the end of the profile
face will be set to the deepest reflection in the second set which is located (Fig. 8). Mapping of carbonate interface can be more precisely carried
at two-way time of 310 ms at the beginning of the profile. By following out in the reflection data (Fig. 9). Because of the lower resolution of
this reflection towards the end of the profile, the time of 180 ms can be the CSAMT method, the boundary cannot be as precisely defined as on
read. Since it is a well-known fact that clear reflections cannot be obtained the reflection profile. However, in terms of hydrogeology, the CSAMT
in carbonates, the interface between marl and carbonates defined in this method generally provides more complete data. Based on resistivities,
manner is completely logical. When transferring this horizon to the trans- conclusions can be drawn about the existence of an aquifer and its burial
versal reflection profile P-2, it can be observed that the same boundary is depth. On the other hand, seismic reflection provides more precise data
again the last deepest reflection of the second set, under which extends a about discontinuities and relative structural relationships, but it does
zone of weak and discontinuous reflections. not say anything about rocks at the contact of discontinuities, so its
Fig. 10. Resulting model of 2D inversion for profile MP-3 at the Baška (Dinarides); bottom. At the top are pseudo sections of measured and calculated apparent resistivity and in the middle
is ERT model along profile measured parallel to MP-3. Near surface high-resistivity body is clearly visible on both models, and to a depth of about 100 m resistivity is low.
F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63 59
potential can be completely explored only when it is combined with to reduce the interpretation ambiguities to define the depth of the lime-
electrical sounding and audio-magnetotellurics. Therefore, in such a stone aquifer.
model, the CSAMT method should be applied first in order to define po- The inversion of CSAMT data was performed using the Occam method
tential aquifers and indicate the most favourable micro-locations where and the final model was obtained after five iterations for combined TM
seismic reflection can be applied as the latter allows for a more precise and TE mode (Fig. 10). The inversion of determinant (D) data gives very
mapping of aquifer depths and potential fault zones. similar results with 10–20 m greater depth in the central part of the pro-
file. High resistivity originates from a carbonate basement. The model
shows high resistivities at a depth of about 120 m (isoline of approx.
6.2. Baška area 100 Ωm) to a distance of 200 m, and then the depth gradually increases
to about 150 m until a distance of 400 m. The boundary is shallower to-
In the third micro-location on island Krk, the Cretaceous limestones wards the end of the profile, and at the end is approximately 70 m. A shal-
are not affected on the tomographic profile, even in the central part with low high resistivity body is also observed to a depth of no more than 25 m,
the greatest depth of the penetration, which means the depths are in a distance range from 20 to 250 m.
greater than 100 m (Fig. 10). But the surface block of high resistivities, The initial model for the forward modelling has been constructed
with average thickness of 20 m, is clearly visible in the ERT data. This using the ERT results and the inversion CSAMT results. A shallow high re-
block is also introduced in the MT forward modelling, and contributed sistivity body at a distance from 0 to 280 m and to a depth of 20 m was
Fig. 11. CSAMT forward modelling in the Baška area (Dinarides). The observed and calculated apparent resistivity pseudo-sections are displayed at the top. The unbroken black line
represents contact of clastics and carbonates in the final model which is shown in the background as smooth model. The second possible model, with constant depth of boundary is
indicated with dashed line.
60 F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63
introduced, as well as the high resistivity body in the background of the modelling there is no need for introducing such a zone, so the fault
clastics with resistivities about 20 Ωm. Very good agreement of calculated zone in the basement was not assumed.
apparent resistivities and observed data (RMS = 8.2%) is obtained for the
model with gradual increase of carbonates depth from 75 to 115 m, up to 7. Geological models
the distance of 280 m. The greatest depth (around 120 m) is in the central
part of the profile, till a distance of 460 m. To the end of the profile, the In the area of Gotalovec, low resistivities near the surface and ex-
slope of carbonate interface increases, and the depth decreases to about tending to the greater depth in some parts of the profile, are interpreted
60 m (Fig. 11). However, almost as good agreement (RMS = 8.4%) is ob- as the effect of impermeable clastic rocks. High resistivity below them is
tained for a somewhat different model, with constant depth of carbonates caused by fractured and weathered carbonates which act as an aquifer.
(110–120 m) from the distance of 280 m to the eastern end of the profile. Both resistivity models, the inverse and the forward, along profile MP-1
But at the end of the profile, a body of 200 Ωm needs to be defined above (Figs. 5 and 6) suggest a fault contact at a distance of 200 m. The contact
the carbonate rocks (Fig. 11). is subvertical so a normal fault with a great jump (about 200 m) is as-
It can be shown that the interface depth in this model is determined sumed (Fig. 12). The lowest resistivity (about 20 Ωm) is near surface
within an accuracy of ±10 m. When the carbonate boundary in the final and these are interpreted as clay and marl deposits. Between those
model is shifted 10 m shallower, comparison of calculated and mea- layers and Triassic dolomites in the basement there is an alteration of
sured resistivity gives an RMS error of 8.9%, and if the boundary is impermeable and poorly permeable Neogene clastic layers: marl,
shifted 10 m deeper, the RMS error is 8.3%. But, if the shift is larger sandy marl and clayey sandstones. This model shows that great jumps
and amounts 20 m, the RMS error is significantly increased and is 9.8% of faults can be expected in the area as a consequence of tectonic activ-
for +20 m and 8.5% for −20 m. ities. It will help in the consideration of hydrogeological potential of the
Both CSAMT models, inverse and forward, show the similar configura- selected micro-location.
tions (Figs. 10 and 11). In the first half of the profile both models show shal- At the profile P-2 the depth of carbonate interface is greater, espe-
low high resistivity block (110 Ωm), which originates from weathered cially at the beginning of the profile where it is greater than 320 m
limestones within the flysch deposits. The layer of lowest resistivity (Fig. 13). Resistivity models and seismic reflection profiles clearly
(around 20 Ωm) extends to the depth of about 100 m and generally follows show uneven carbonate bedrock, and the depth decreases from the be-
the relief of the profile, except at the end of the profile where its thickness is ginning of the profile towards the end (Figs. 7–9). But, high resistivities
reduced. The low resistivities originate from impermeable package of flysch here may originate from Miocene Lithothamnium limestones and Trias-
deposits dominated by clay and marl. Below them there are high resistivi- sic dolomites, which cannot be resolved based on the interpreted resis-
ties (500 Ωm) arising from fractured, permeable carbonate rocks. tivity. As both of these rocks represent aquifers, it is essential to define
However, it must be emphasized that the forward model provides a the boundary of clastics and carbonates. So, although on the profile
significantly distinctive image than the inverse model. It is difficult to are defined Triassic dolomites, in some parts of the profile also can be lo-
define the boundaries in the inverse model since the area of transition cated Miocene limestone in reef development. The faults are interpreted
from low to high resistivity is very wide. Besides, a significantly lower on the basis of clear lateral breaking of reflections on the seismic profile.
resistivity in the basement was observed at a distance of 300–450 m, The geological model shows great depth variations of clastic–carbonate
which could indicate a fault zone. But in the context of forward interface, that is the aquifer depths. This makes it possible to determine the
Fig. 14. Geological cross-section along the MP-3 profile at the Baška area on island Krk.
62 F. Šumanovac, J. Orešković / Journal of Applied Geophysics 153 (2018) 47–63
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