GeolCarp_Vol60_No4_331

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA, AUGUST 2009, 60, 4, 331—338 doi: 10.2478/v10096-009-0024-1

Microseismic identification of geological and tectonic

structures in the Komjatice Depression

(Western Carpathians)

ANNA V. KALININA

, SERGEY M. AMMOSOV

, VIKTOR A. VOLKOV

, NIKOLAY V. VOLKOV

JOZEF HÓK

, LADISLAV BRIMICH

and MARTIN ŠUJAN

Institute of Physics of the Earth, Russian Academy of Sciences, B. Gruzinskaya 10, 123995 Moscow, Russia

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

Slovak Republic; hok@fns.uniba.sk

Geophysical Institute, Slovak Academy of Sciences, Dúbravská cesta, 845 28 Bratislava, Slovak Republic;

geofbrim@savba.sk

EQUIS Ltd., Račianska 57, 831 02 Bratislava, Slovak Republic

(Manuscript received July 24, 2008; accepted in revised form December 18, 2008)

Abstract: The microseismic survey method was applied to the study of the geological structures in the region around
the Mochovce nuclear power plant. The previous geological and geophysical investigations considered a contact be-
tween the Miocene volcanites and sediments as a neotectonic fault. The results of the microseismic investigations allow
us to interpret the zone of a supposed neotectonic fault as a transgressive contact of the sediments and the volcanic rocks
without a tectonic disruption.

Key words: Miocene, Western Carpathians, geological structure, microseismic survey, Rayleigh waves, microseisms.

Introduction

The  Western  Carpathians  form  a  mountain  range  arc  with
dominant  nappe  structures  with  a significant  zonal  arrange-
ment  and  orogen  polarity  processes  migrating  in  time.  It  is
one  of  the  essential  reasons  dividing  the  Western  Car-
pathians into the Outer Western Carpathians (OWC) and the
Inner  Western  Carpathians  (IWC).  One  of  the  typical  mor-
photectonic  features  of  the  IWC  are  Neogene  depressions
and  volcanites.  The  investigated  area  belongs  to  the  Ko-
mjatice  Depression  which  is  a  north-eastern  part  of  the
Danube Basin. Numerous studies focused on the geological
and  tectonic  structures  of  the  Komjatice  Depression  in  the
past  (e.g.  Gaža  &  Beihauerová  1976;  Zbořil  et  al.  1987;
Harčár  &  Priechodská  1988;  Priechodská  &  Harčár  1988;
Baráth  &  Kováč  1995;  Nagy  et  al.  1998;  Hók  et  al.  1999).
Detailed geological and tectonic studies were realized in the
vicinity of the Mochovce nuclear power plant (EMO) situat-
ed at the easternmost margin of the Komjatice Depression. In
the  EMO  vicinity  a  conspicuous  contact  was  described  be-
tween the Miocene volcanics and sediments. This contact is
situated  on  the  eastern  foothill  of  the  Dobrica  elevation
(Fig. 1). From the point of view of the conservative solution
approach  the  contact  was  interpreted  as  a  neotectonic  fault
with  potential  Quaternary  activity  (Hók  et  al.  2003).  The
fault  separates  the  Miocene  volcanites  from  the  Quaternary
and Miocene sediments. The application of the microseismic
survey  method  yielded  new  information,  which  helped  to
specify  the  geological  and  tectonic  structures,  as  well  as  a
supposed fault-like contact in the EMO vicinity, which is lo-
cated in the NE part of Komjatice Depression.

Geological setting

Miocene sediments of the Komjatice Depression overlay

an erosive divided pre-Tertiary substratum drilled in several
boreholes  (Biela  1978)  and  rising  up  in  the  Tribeč  Mts
(Fig. 1).  The  stratigraphic  range  of  Neogene  sediments  is
Middle  Badenian  (Middle  Miocene)  to  Pliocene.  The  sedi-
mentary fill of the Komjatice Depression during the Miocene
megacycle was characterized by a gradual decrease of salini-
ty  of  the  depositional  environment  upward.  There  are  three
lower-order  shallowing  upward  cycles  in  sedimentary
record.  The  depositional  environment  changed  from  marine
to  brackish,  caspibrackish  and  lacustrine-swamp  with  coal
deposition.  This  succession  is  overlain  by  a  Pliocene  cycle,
composed  of  deltaic  and  fluvial  deposits  (Hók  et  al.  1999).
The occurrence of the Middle Badenian sediments (the Poz-
ba Formation) is restricted to the central part of the depres-
sion.  The  Sarmatian  sediments  (Vráble  Formation)  are
transgressively spread over the whole depression. The Vráble
Formation comprises calcareous clays, sands and conglomer-
ates  with  volcanic  rocks  at  the  bottom.  The  volcanic  rocks
form  subaquaceous  lava  flows,  which  belong  to  the  distal
parts of the Štiavnica stratovolcano rock complexes (the Prie-
sil Formation sensu Konečný et al. 1998). The paleocurrents
of the lava flows were generally oriented in a NE—SW direc-
tion in the investigated area. Sediments of the Vráble Forma-
tion,  biostratigraphically  constrained  to  the  Sarmatian
period, were found in the parametric borehole ŠVM-1 drilled
about 5 km SW from the investigated area (Sliva in Hók et
al. 2003). The Lower Sarmatian sediments of the Vráble For-
mation  are  situated  directly  above  the  volcanic  lava  flows.

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MICROSEISMIC IDENTIFICATION OF GEOLOGICAL STRUCTURES (WESTERN CARPATHIANS)

The  Pannonian  sediments  of  the  Ivánka  Formation  overlay
the  Vráble  Formation.  The  angular  unconformity  is  present
between the Ivánka and the Vráble Formations. The Ivánka
Formation  sediments  are  predominantly  represented  by  cal-
careous  clays  and  claystones  with  admixture  of  sands  repre-
senting  fluvial  influence  in  sedimentary  basin.  The  Beladice
Formation (Pontian) is the last member of the Miocene mega-
cycle  sedimentation.  It  was  deposited  in  a  shallow  lacustrine
to swamp environment. The variegated clays with coal inter-
calations are typical for the Beladice Formation. The Pliocene
sediments (the Volkovce and Kolárovo Formations) are prod-
ucts of the deltaic sedimentation. The most typical sediments
are coarse-grained gravels and sands with clay admixture.

The northeast trending faults are most remarkable in the

tectonic  structures  of  the  Komjatice  Depression.  The  brittle
structures of the Mojmírovce and Šurany fault systems were
the  most  important  of  the  faults  activated  during  the  devel-
opment of the Komjatice Depression (Hók et al. 1999). The
Mojmírovce fault system bounds the Miocene deposits with
the  pre-Tertiary  rocks  of  the  Tribeč  Mts.  The  Šurany  fault
system  is  a  comparative,  partly  antithetic  fault  system  rela-
tive to the Mojmírovce fault system. It restrains the southeast
and  east  margin  of  the  Komjatice  Depression  depositional
area  and  also  the  horst-like  structure  of  the  neovolcanic
rocks comprising the Kozmálovské vŕšky Hills. We assume
that the Mojmírovce and Šurany fault was active during the
initial rifting of the Komjatice Depression and the rifting was
connected with the Pozba Formation, deposition (cf. Hók et
al. 1999). The paleostress field with NW—SE oriented exten-
sion accompanying subsidence of the depression and deposi-
tion  of  the  Vráble  Formation,  during  the  Sarmatian  period.
The  transgressive  characteristic  of  deposits  and  modelling
suggest a moderate subsidence during this period (Lankreijer
et  al.  1995).  The  next  phase  of  a  wide  rifting  manifests  the
angular unconformity between the Sarmatian and Pannonian
sediments  (the  Vráble  and  Ivánka  Formations).  The  Late
Miocene  paleostress  field,  with  a  NE—SW  to  ENE—WSW
compression  and  perpendicularly  oriented  extension  result-
ing  in  following  subsidence  of  the  depression.  Deposition
during the Pannonian and Pontian suggests a rapid transition
of  the  synrift  stage  evolution  to  final  thermal  subsidence
(postrift stage) of the whole Danube Basin.

The Pliocene Volkovce Formation deposits, reaching up to

1000 m in thickness toward the Danube Basin centre (Baráth
& Kováč 1995) points to a rejuvenation of the tectonic activi-
ty. However, the end of the Pliocene and the beginning of the
Quaternary  represents  structural  reworking  of  the  Komjatice
Depression  during  the  period  of  tectonic  inversion.  The  cen-
tral  part  of  the  depression  between  the  Tribeč  Mts  and  the
Kozmálovské vŕšky Hills is characterized by uplift resulting in
erosion of older sediments (Priechodská & Harčár 1988).

The area southeast of the Kozmálovské vŕšky Hills has

been subsiding since the Mindelian. This is shown by the age
of the oldest preserved terrace of the Hron River (Halouzka
1968), as well as by the 40 m thick accumulations of Quater-
nary sediments (Tkáč et al. 1996). It was one of the reasons
why the sharp contact between the volcanics and the Mio-
cene sediments was interpreted as a fault structure with po-
tential activity during the Quaternary.

Methodology

Seismic background oscillations (microseisms) have been

studied for more than a hundred years. Recently many investi-
gations  were  devoted  to  developing  methods  using  mi-
croseisms  as  sounding  signals.  Microseismic  signals  are
always present at every point on the Earth’s surface eliminat-
ing the necessity of artificial sources of registered signals.  A
number of experiments demonstrate the relations between the
amplitude-frequency characteristic of the microseisms and the
elastic properties of the medium (Bard 1999).

The approaches utilizing microseisms for the study of the

geological environment could be approximately divided into
two groups. The first group studies the experimental disper-
sion  dependencies  between  microseismic  wave  velocities
and  corresponding  frequencies.    The  main  purpose  of  these
measurements  consists  of  receiving  the  velocity  section  of
the  investigated  area  after  the  inverse  problem  solution  on
the  basis  of  the  experimental  dispersion  curves.  This  ap-
proach requires synchronous measurements with the help of
seismic  arrays  of  different  configurations  (Shapiro  &  Ritz-
woller  2002).  The  second  group  studies  the  composition  of
the  correlation  of  stable  statistical  properties  of  the  mi-
croseismic  field  and  the  structure  of  geological  heterogene-
ities  (Asten  1978;  Nakamura  1989).  In  that  case,  the
measurements could be performed using a single seismic sta-
tion. But in practice a number of assumptions regarding the
nature  of  the  microseisms  sources,  their  spectral  properties,
and the proportion of the content of different types of waves
are accepted based on previous experimental investigations,
both  in  the  study  area  and  in  other  regions.  This  group  is
characterized  by  the  simplicity  of  measurement  procedures
and  good  consistency  of  microseismic  investigation  results
with  other  geological  and  geophysical  methods  despite  the
set of initial assumptions.

The microseismic signal looks like interference by differ-

ent types of seismic waves, which propagate as separate
wave trains of limited duration, and which present the deter-
ministic signals within this duration. From the other side, the
microseismic signal is a random signal, because the propor-
tion of the content of waves of different type, initial phases,
amplitudes, and duration of wave trains are unknown. As an
illustration of the randomness of the process X(t), the follow-
ing example could be used:

X(t) = A cos (

ωt + ϕ),

where

Α and ω are constants, and ϕ is a random value with

defined probability distribution, so that for one realization
the random value

ϕ is equal for all values of t (for example

for the time interval of observation). In that case the random
variations take place only on the realization ensemble, but
not on time intervals.

The method of microseismic survey, where the spatial

properties of the spectral characteristics are used for the loca-
tion  of  the  geological  heterogeneities  (Gorbatikov  et  al.
2004;  Ammosov  et  al.  2007),  was  applied.  This  method
could  be  referred  to  as  a  statistical  approach.  The  measure-
ments above the investigated structures were performed us-

334

KALININA, AMMOSOV, V.A. VOLKOV, N.V. VOLKOV, HÓK, BRIMICH and ŠUJAN

ing separate mobile station (one or several units). For proper
interpretation of the received data we shall ensure that the re-
sulting values are stationary and do not vary during the day,
month, and do not depend on changing climatic conditions,
etc. The estimation of the stationary interval of the signal
dispersion in the increasing temporal window takes into ac-
count the known dependence between the spectrum and the
dispersion of a random signal:

where

and S

(f) are dispersion, and spectral density

correspondingly  of  the  random  microseismic  process  (Ben-
dat & Piersol 1966). The experimental investigations of the
stationarity  of  intervals  of  the  microseisms  for  different  lo-
calities  and  different  conditions  on  the  Earth’s  surface
showed that the signal dispersion begins to stabilize after the
signal accumulation during 15—20 minutes for the frequency
range 10—12 Hz and during 40—60 minutes for the frequency
range 0.1—1.0 Hz (Gorbatikov & Stepanova 2008). It is nec-
essary to notice that the stationary interval is limited. More-
over the time is changing for different frequency ranges and
for  different  observation  conditions.  To  separate  the  global
and local microseismic sources during the measurements, it
is necessary to install one seismic station (reference station)
for  continuous  recording  of  the  microseismic  signal  in  the
vicinity of the investigated area.

Physical background of the microseismic survey

method

The microseismic survey method is based on the analysis

of  the  spatial  distribution  of  the  vertical  component  of  the
microseismic field for all frequencies of the spectra. The an-
alytical  solutions  proved  that  in  the  Rayleigh  fundamental
mode  the  zone  of  maximum  shear  stresses  is  located  at  the
depth equal to half of their wave length. The zones of maxi-
mum amplitudes are situated close to the surface. The local
heterogeneities with different elastic modules lead to chang-
es  of  the  oscillation  character  of  the  microseism  and  their
amplitudes.  If  seismic  wave  velocities  in  the  heterogeneities
are higher than in the surrounding rocks the amplitude of the
microseismic waves above the heterogeneities decreases, and
vice versa (Gorbatikov et al. 2004; Kalinina et al. 2008).

The observations were performed at different points with

the step 100 m in the investigated area using mobile stations.
During processing the field data were corrected using the ref-
erence  station  data  records.  The  results  of  the  analysis  are
maps of the distribution of microseismic amplitudes reflect-
ing  the  fields  of  the  relative  velocity  changes  for  different
frequencies.  The  dependence  of  the  Rayleigh  wave  ampli-
tudes  on  the  depth  of  the  half-space  is  given  in  (Levshin  et
al. 1992). The maps of the distribution of microseismic am-
plitude  for  different  frequencies  give  information  about  the
velocity properties of the medium at different depths.

It was necessary to check the following circumstances dur-

ing processing:

1. Type of waves dominated in registered signal;
2. Statistical stability of the registered parameters.
The main assumption of the microseismic survey method

is the prevalence of the Rayleigh-type waves in the vertical
component of the low frequency (lower than 1 Hz) mi-
croseismic field. However, our working frequency band also
lies in the high frequency area, which contains a high per-
centage of body waves. The wave composition using the po-
larized analysis of the particle movement was studied, and it
was necessary to preprocess the data to remove the high am-
plitude noises (mainly the noises caused by transportation).

The control of statistical stability was realized by the inves-

tigation of the behaviour of the signal dispersion in the in-
creasing temporal window for each observation point. During
processing the data were corrected using the reference station
records, and the ratio of accumulated (stationary) power spec-
tra was obtained. The pictorial representations of the resulting
matrix are the horizontal and vertical slices at any chosen site.

Processing and results

The investigated area represented a rectangle with the size

approximately  0.5  by 2.2 km  (Fig. 2).  The  measurements
were  realized  along  six  profiles  100 m  apart.  The  average
distance between points of registration along the profiles was
100 m.  Close  to  the  contact  zone  between  volcanites  and
sediments near the Dobrica Hill the registration points were
denser (25—50 m). For the continuous registration the refer-
ence  station  was  installed  in  the  center  of  the  studied  area.
The  registration  time  interval  for  mobile  stations  at  each
point  was  45—50 min.  The  seismic  station  consisted  of  the
three-component velocimeter KMV and a registration block
UGRA (Marchenkov et al. 1997).

The main goal of the processing was the gathering and

analysis of the power spectra. In Fig. 3 the observed smooth-
ing power spectra of microseisms along profile 3 are shown.
The results of the microseismic survey are presented as verti-
cal  profiles  and  horizontal  layers  (Figs. 4  and  5).  The  axes
are  given  in  meters.  The  XY  projection  is  UTM  Zone  34
Northern Hemisphere (WGS 84). The zero depth on the ver-
tical  axis  corresponds  to  the  level  located  at  the  absolute
mark equal to 200 m. The colour spectrum reflects the inten-
sity of the relative amplitude of the microseisms in decibels.
The  microseismic  survey  method  allows  us  to  distinguish
rocks  by  velocity  properties.  It  was  meaningful  to  assume
that  at  least  two  types  of  lithological  rocks  –  the  Miocene
volcanites  (intensity  of  the  relative  amplitude  of  the  mi-
croseisms lg A < —2 dB ) and the Miocene and Pliocene sedi-
ments (lg A > 2 dB) – could appear in the resulting maps.

The vertical profiles and horizontal layers presented in

Figs. 4 and 5 exhibit the following characteristic features of
the investigated area:

1. The zones of low amplitudes lg A < —2 dB (blue colour)

interpreted as volcanic rocks are well distinguished down to
600—650 m. The sediments lg A > 2 dB (red colour) are lo-
cated between them.

2. The bottom stratum, —2 dB < lg A < 0 dB, (green co-

lour) has velocity properties, which are nearly average be-

336

KALININA, AMMOSOV, V.A. VOLKOV, N.V. VOLKOV, HÓK, BRIMICH and ŠUJAN

Fig. 4. The variations of spectral amplitudes of microseismic signals of investigated media volume; (a) the relief map with points of obser-
vations, the dashed lines show the positions of vertical sections presented on the maps (b—f).

tween volcanites and sediments; it looks homogeneous
enough and this hinders the identification of any fault fea-
tures there.

3. In the upper part of profiles 4 and 5 it is possible to dis-

tinguish the difference of sediment velocities, —2 dB < lg A
< 0 dB and by A > 0, (the green colour above the red one).

The more contrasting zones are displayed separately in Fig. 6

as isosurfaces bounding the difference values of microseismic
amplitudes. In Fig. 6a the isosurface bounds the high amplitudes
area (

Α > 9 dB), which corresponds to a low velocities area (the

sedimentary deposits). In Fig. 6b the volcanic rocks are present-
ed, their velocity properties increase (amplitudes of microseisms
decrease A < —2 dB). This figure allows us to investigate the
range of the softening and weathering of the volcanics.

The interpretation of the geophysical data shows two dif-

ferent horizons in the geological structure in the EMO vicini-
ty (Figs. 4, 5). The lower horizon could be interpreted as the
pre-Tertiary basement rocks (green colour). The upper hori-
zon represents the Miocene volcanites and sediments be-
tween them. The Miocene volcanic rocks (blue colour) are

338

KALININA, AMMOSOV, V.A. VOLKOV, N.V. VOLKOV, HÓK, BRIMICH and ŠUJAN

remnants of lava flows. The lava flows are situated directly
over  the  pre-Tertiary  basement  rocks.  A  similar  situation  is
described at borehole GK-6 drilled about 10 km NE from the
investigated  area  (Biela  1978).  The  Miocene  and  the
Pliocene sediments (red colour) are placed partly on the pre-
Tertiary  basement  rocks,  partly  on  the  volcanic  lava  flows.
This arrangement is in good agreement with the lithological
and sedimentological character of the Vráble Formation.

Between the Miocene volcanites and the sediments a nor-

mal fault (Fig. 2) was formerly supposed, separating the vol-
canites from the sediments at the east foothill of the Dobrica
elevation (Hók et al. 1999). The interpretation of the mi-
croseismic investigation results did not prove a fault contact
between the volcanites and the sediments. This result is also
supported by no existence of adequate offset along the sup-
posed fault in the pre-Tertiary basement (see Fig. 4).

Due to the above mentioned facts, it is possible to interpret

this structure as the transgressive contact of the sediments
overlaying the volcanic lava flows.

Conclusions

The interpretation of the geophysical investigation results

allows us to recognize two floors in the geological structure in
the EMO vicinity. The lower horizon contains the pre-Tertiary
rock sequence. The upper horizon belongs to the transgressive
formation of the Miocene volcano-sedimentary sequence. Ac-
cording to previous investigations (e.g. Priechodská & Harčár
1988;  Hók  et  al.  1995)  the  upper  horizon  represents  the
Vráble Formation. The uppermost part of this horizon most
probably belongs to the Volkovce Formation (e.g. Baráth &
Kováč  1995).  The  results  of  the  geophysical  investigation
show  a  transgressive  contact  without  a  tectonic  disruption
(fault)  between  the  volcanic  rocks  and  the  sediments  at  the
eastern foothill of the Dobrica elevation.

Acknowledgment: This work was supported by the Slovak
Research and Development Agency under the contract
No. APVV-0158-06, APVT-51-002804 and by VEGA Grant
Agency under Projects No. 2/0107/09, No. 1/0461/09.

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