GeolCarp_Vol63_No3_233

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA

, JUNE 2012, 63, 3, 233—239 doi: 10.2478/v10096-012-0017-3

The resistivity image of the Muráň fault zone (Central

Western Carpathians) obtained by electrical resistivity

tomography

RENÉ PUTIŠKA

, IVAN DOSTÁL

, ANDREJ MOJZEŠ

, VOJTECH GAJDOŠ

, KAMIL ROZIMANT

and RASTISLAV VOJTKO

Department of Applied and Environmental Geophysics, Faculty of Natural Sciences, Comenius University, Mlynská dolina G,

842 15 Bratislava, Slovak Republic; putiska@fns.uniba.sk

Department of Geology and Paleontology, Faculty of Natural Sciences, Comenius University, Mlynská dolina G, 842 15 Bratislava,

Slovak Republic

(Manuscript received June 9, 2011; accepted in revised form September 30, 2011)

Abstract: The paper describes the application of geophysical prospecting techniques for estimation of the fault’s incli-
nation. The field survey was carried out across the Muráň fault structure in the Slovenské rudohorie Mts (central
Slovakia). Three different geophysical methods were used to map the fault zone: Electrical Resistivity Tomography
(ERT), induced polarization (IP) and radon emanometry. All these methods have been used to locate the fault zone area,
but the principal aims of this research are to test the efficiency of the 2D ERT technique to recognize the geometrical
characterization of the fault and to improve our tectonic knowledge of the investigated area. For the synthetic cases,
three geometric contexts were modelled at 60, 90 and 120 degrees and computed with the l

norm inversion method, the

norm with standard horizontal and vertical roughness filter and the l

norm with diagonal roughness filter. In the

second phase this geophysical methodology was applied to fieldwork data. Our results confirm that the ERT technique
is a valuable tool to image the fault zone and to characterize the general geometry, but also the importance of setting up
the right inversion parameters. The main contribution of the geophysical investigations in this case was the determina-
tion of the location and confirmation of the inclination of the Muráň fault. The result of this study is the ability to make
a visual estimation of the direction and dip of the fault. Pursuant to this work the dipole—dipole electrode configuration
produces the best resolution, particularly for the location of vertical and dipping structures. The advantage of this array
is that it shows the ability to assess the trend of the dip and therefore it can be strongly recommended. The result is also
a case study of a small scale tectonic survey involving geophysical methods.

Key words: Central Western Carpathians, Muráň fault zone, geophysical prospecting, electrical resistivity tomography,
induced polarization, radon emanometry.

Introduction

The  study  area  is  located  in  the  central  part  of  the  Central
Western Carpathian Mountain Belt (the Slovenské rudohorie
Mts,  Fig. 1).  The  area  consists  of  the  Carnian  Wetterstein
limestone of the Muráň Nappe belonging to the Silicic Unit
(cf.  Kozur  &  Mock  1973;  Mello  1979),  Lower  Paleozoic
gneisses  of  the  Southern  Veporic  Unit  (cf.  Bystrický  1959;
Klinec  1976;  Hovorka  et  al.  1987)  and  Quaternary  consoli-
dated breccias (cf. Ložek 1960).

The main goal of this paper is to present a study that con-

tributes to the determination of the exact position, depth con-
tinuation and inclination of the Muráň fault structure. In the
study  of  fault  system  geometry,  a common  procedure  is  to
make  use  of  information  from  stratigraphic,  structural  and
geomorphological  studies.  This  information  could  be  ob-
tained  from  drilling  and  exploration  boreholes.  However,
these  methods  are  expensive  and  time  consuming,  which
prevents their use on a large scale. Moreover, these types of
data are spatially limited. In contrast, geophysical measure-
ments  can  provide  a  less  expensive  way  to  improve  our
knowledge.  In  many  cases,  geophysical  prospecting  tech-

niques can provide complementary data that enable geologi-
cal  correlation,  even  in  parts  where  there  are  no  data  from
boreholes. In this area an outcrop has been found, where it is
possible  to  see  the  contact  between  Mesozoic  sequence
(Wetterstein  limestone  and  dolomite)  and  Lower  Paleozoic
crystalline  basement  (gneisses)  Fig. 2.  This  place  was  used
as

electrical resistivity tomography (

ERT) reference profile.

The result of the reference measurement was used as the en-
try  parameter  to  prepare  models  for  the  forward  modelling
program Res2Dmod (Loke 2002). Three different geophysi-
cal methods were used to map the fault zone. Resistivity sur-
vey  (ERT)  was  complemented  by  the  measurement  of  IP
(induced polarization) and radon gas concentration in soil air
(radon emanometry). The principal aims of this research are to
test the efficiency of the 2D ERT technique with the different
arrays  to  recognize  the  geometrical  characterization  of  the
fault  using  smooth  inversion  methods  and  block  inversion
methods. These two methods were compared by Olayinka &
Yaramanci (2000) and Loke et al. (2003), who demonstrated
the  characteristics,  advantages  and  drawbacks  of  both  meth-
ods.  They  concluded  that  in  cases  where  the  resistivity  con-
trast  is  gradual,  smooth  inversion  is  more  suitable,  whilst

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GEOLOGICA CARPATHICA, 2012, 63, 3, 233—239

when there is a sharp variation in resistivity contrast, block in-
version is preferable. Checking and changing some mathemati-
cal  parameters,  such  as  damping  factors,  ratio  of  thickness  of
the  first  layer,  diagonal  filter  and  the  smoothness  matrix,  can
also  be  performed  (Cardarelli  &  Fischanger  2006).  Further-
more, modifying the inversion results by changing the starting
model appears to be the best way to obtain valid physical results
from the inversion. The results of measurement are also affected
by electrode arrays (Mendoza & Dahlin 2008). The resolution
and  penetration  depth  of  arrays  also  depend  on  the  geological
models (electrical properties, anomaly body geometry) and the
noise contamination levels, all of which may be efficiently sim-
ulated by numerical methods (Dahlin & Zhou 2004).

Geological setting

The Muráň fault is the most distinctive disjunctive structure

in the Western Carpathians which is evident by its geological
structure and terrain morphology. The Muráň fault was first
described by Zoubek (1935), according to the village of
Muráň. This fault is considered to be predominantly a strike-
slip fault with dominant left-lateral movement (e.g. Plašienka
1983; Pospíšil et al. 1989; Marko 1993; Vojtko 2003; Vojtko

et al. 2011). The NE—SW trending Muráň fault forms a very
straight discontinuity, which separates carbonates on the
north-western side from the crystalline basement on the south-
eastern side (Fig. 1). The Muráň fault represents a strongly de-
formed zone and the host rocks are intensively mylonitized.

The north-western block is composed of the Wetterstein

limestone  of  the  Muráň  Nappe  which  belongs  to  the  Silicic
Unit.  In  the  study  area  the  Wetterstein  limestone  is  dolo-
mitized  or  changed  to  dolomite.  In  this  dolomite  the  lime-
stone  forms  small  irregular  bodies,  mainly  lenses  or  layers.
The  dolomite  is  of  light  grey  to  grey  colour,  and  has  a
grained or massive fabric. The bedding is visible mainly as
alternating  dark  and  light  thin  beds.  The  thickness  of  the
Wetterstein Formation is from 75 to 375 m; about 250 m is
the average thickness (Bystrický 1959; Vojtko 2000).

The south-eastern block consists of Lower Paleozoic meta-

morphites  of  the  Southern  Veporic  Unit  (cf.  Hovorka  et  al.
1987;  Vozárová  &  Vozár  1988;  Hók  &  Vojtko  2011).  The
main  lithotypes  of  this  crystalline  basement  are  middle-
grained  gneisses  over  the  fine-grained  ones  occasionally
with  layers  of  amphibolites  (Klinec  1976;  Hovorka  et  al.
1987; Bezák et al. 2009).

The upper stratum in the study area consists of breccia.

The  Pleistocene  Muráň  breccias  are  composed  of  carbonate
detritus  cemented  by  calcareous  sinter.  The  cement  is  often
compact  and  occasionally  porous.  The  bedding  of  these
breccias is parallel to the slope (Ložek 1960).

Synthetic study

The reference profile was measured very closely to the un-

covered subvertical contact between the Wetterstein Forma-
tion and the crystalline basement (gneisses) (Fig. 2). The
purpose of the reference ERT measurement was to obtain
real values of resistivity to prepare synthetic models fitted to
real geological formation.

In this case, the result of the reference measurement was

used to prepare input data for the numerical model to simu-
late  the  geological  situation  along  the  synthetic  model.  The
synthetic  data  are  computed  using  the  forward  modelling
program Res2Dmod (Loke 2002). The synthetic models rep-
resent the tree geometrics of the contact of two environments
(a vertical 90° and a dipping 60° and 120°) and they repre-
sent  a  simplified  geological  and  structural  sketch  along  the
investigated  profile.  Synthetic  data  sets  were  generated  for
dipole—dipole, pole—dipole, Wenner alpha and Schlumberger
arrays. The electrode spacing was 5.5 m for simple compari-
son with real data.

Field study

The task of the field study was to define the exact position

and inclination of the Muráň fault. The inversion procedure
and the new knowledge from the modelling presented above
was applied to the field study.

The study profiles were located transversely on the Muráň

fault, close to the village of Muráň (Fig. 1) and the measure-

Fig. 1. Location map with geological situation of survey site in the
Muráň fault area (according to Klinec 1976, modified).

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GEOLOGICA CARPATHICA, 2012, 63, 3, 233—239

ment was performed with four different arrays: dipole—dipole
(DD)  (Fig.  3c,g,k),  pole—dipole  (PD)  (Fig.  3d,h,l),  Wenner
alpha (WA) (Fig. 3e,i,m), Schlumberger (SCH) (Fig. 3f,j,n)
to  enable  the  comparison  with  synthetic  models.  Profile 1
was  complemented  by  IP  (Fig. 3b)  and  radon  emanometry
(Fig. 3a). The IP result shows that this method was not very
successful in this particular case.

Three 2D electrical resistivity tomography lines “Profiles 1,

2 and 3” (Fig. 3) were collected using ARES instrument (GF
Instruments,  CZ).  The  ERT  profiles  were  oriented  roughly
perpendicular to the investigated fault trace and all three pro-
files  had  the  same  configuration.  The  distance  between  the
profiles  was  11 m.  For  localization  and  elevation  of  the  pro-
file, the GPS system Trimble, Pathfinder ProXH was used.

Radon emanometry is an atmogeochemical survey method

based on the measurement of alpha activity of soil air samples
taken from the same depth of rock weathering cover. The al-
pha activity is a result of the alpha disintegration process of
nuclei of radon isotope

222

Rn and its daughter products. As the

parent radium isotope

226

Ra commonly occurs in rock fabric

and

222

Rn is a gaseous element, the fault system is a very ap-

propriate way for upward moving not only for radon but also
for other Earth gases. Thus the volume activity (kBq · m

—3

) of

soil radon gas measurements along the profile crossing the as-
sumed fault zone could contribute considerably to its more ex-
act characterization (Gruntorád & Mazáč 1994; Giammanco et
al. 2009). The radon measurements were performed with the
portable radon detector LUK-3R (SMM, CZ) with sampling
from the same depth of about 0.8 m.

mean square (RMS) error for all profiles. During the model-
ling the PD array has shown capacity to recognize the dip of
the fault, but the result of the fieldwork was affected by a high
noise level in the deepest part. The PD array has the best depth
penetration and the result shows the ability to locate vertical or
dipping structures but with a lower image resolution.

The imaging resolution of the DD array (Fig. 3c,g,k) is

best for all three profiles and is much better than in case of
the other arrays, particularly for the location of vertical and
dipping structures. The result of the DD array was chosen for
3D modelling. The data from P1, P2 and P3 profiles were
collated in RES3DINV software. The l

norm with xy and yz

diagonal  roughness  filter  was  used  for  forward  modelling
calculation.  The  result  was  used  for  3D  visualization
(Fig. 4a),  where  the  contact  of  the  two  lithological  blocks
and  the  dip  of  the  fault,  which  is  almost  90°  toward  the
south-east,  can  be  clearly  seen.  The  geological  map  shows
the exact contact of the Veporic crystalline basement and the
Mesozoic sequence of the Silicic Unit from the geophysical
survey (Fig. 4b).

The location of the fault zone is also visible on the profile

curve of the radon volume activity in the soil air (Fig. 3a). The
fault zone seems to be much wider than it appears from the
ERT measurements. This is logical evidence of the higher
looseness of rock material around the fault zone on both sides
– over weathered Mesozoic carbonates as well as over Lower
Paleozoic gneisses, which resulted in higher gas permeability
and therefore in higher radon gas flux from the deeper parts.

Fig. 4. 3D results of electrical resistivity tomography with the direction of the fault and
map of the geological situation show the exact contact of the Veporic crystalline basement
and the Mesozoic sequence of the Silicic Unit from the geophysical survey.

Figure 3 shows the result of the field-

work,  where  the  north-western  part  is
characterized  by  high  values  of  inter-
preted resistivity, while the resistivity in
the  south-eastern  part  is  less  than
100  m.  A  fault,  which  can  be  clearly
seen  in  the  image,  separates  both  types
of  structures.  The  inversion  results
show,  that  if  there  is  a  good  resistance
contrast between two environments, it is
easy  to  determine  the  contact  of  these
two  environments  using  the  electrical
resistivity tomography.

As mentioned above, the measure-

ment was performed with four different
arrays  (DD,  WA,  SCH,  and  PD).  The
DD  may  provide  higher  image  resolu-
tion  than  PD,  WA  and  SCH.  The  WA
and  SCH  arrays  show  sharp  vertical
contact

for

all

three

profiles

(Fig. 3e,f,i,j,m,n), but after the compar-
ison  with  synthetic  models  it  is  possi-
ble to see that these two arrays show a
sharp,  almost  a  90°,  vertical  contact
also  for  the  dip  of  60°,  and  120°.  The
WA  and  SCH  configurations  show  a
perfect resistivity contact but the reso-
lution for dipping structure is low.

The results of the PD arrays

(Fig. 3d,h,l) have the highest root

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GEOLOGICA CARPATHICA, 2012, 63, 3, 233—239

Conclusions

On the basis of the former geological and geophysical

studies the Muráň fault can be identified as a deep-seated re-
gional crustal fault. The analysis of the total Bouguer anom-
alies  and  different  transformed  gravity  maps  indicates  that
the fault is characterized by significant gravity gradient (e.g.
Fusán  et  al.  1971,  1987).  On  the  set  of  new  gravity  maps
consisting  of  total  Bouguer  and  regional  gravity  anomalies
from the Carpathian-Pannonian Basin region the Muráň fault
can also be observed approximately from  ub ana to Lublin
(Bielik  et  al.  2006;  Bielik  &  Wybraniec  2007;  Bielik  &
Mikuška  2007;  Alasonati  Tašárová  et  al.  2008,  2009).  Tak-
ing into account the results published by Vozár et al. (2010)
in the Western Carpathians the evolution of the Muráň fault
could  also  be  related  to  the  evolution  of  another  regional
crustal fault, which is known as Diósjenő Line.

The main contribution of the geophysical investigations in

this case was the determination of the location and confirma-
tion  of  the  inclination  of  a  small  part  of  the  Muráň  fault
(Fig. 4).  The  result  of  geophysical  prospecting  is  showing
the exact contact of the Veporic crystalline basement and the
Mesozoic  sequence  of  the  Silicic  which  is  approximately
250 m north-west from the geological mapping (Fig. 4b).

The result of this study is the ability to make a visual esti-

mation for the direction and the dip of the fault. In this case
calculated inversion models from synthetic data have been
compared with calculated inversion models using real data
measurement. The data have been calculated by the l

norm

with diagonal roughness filter. The 2D inversion result of the
reference profile and resistivity profile correlated with syn-
thetic models.

In the context of this work, the DD electrode configuration

produces the best resolution, particularly for the location of
vertical and dipping structures. The advantage of this array is
that it shows the ability to assess the trend of the dip and
therefore it can be strongly recommended.

Acknowledgments:

The authors are grateful to the the Slovak

Research and Development Agency APVV (Grant Nos.
APVV-0194-10, APVV-0158-06) and the Slovak Grant
Agency VEGA (Grant Nos. 1/0095/12, 2/0067/12, 1/0747/11,
1/0468/10) for the support of their research.

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