ELECTRICAL RESISTIVITY TOMOGRAPHY AS A TOOL IN GEOLOGICAL MAPPING
Szymon OSTROWSKI*, Grzegorz PACANOWSKI, Marcin LASOCKI PBG GEOPHYSICAL EXPLORATION CO., LTD Warsaw - PL
e-mail: s.ostrowski@pbg.com.pl
References
Biernat, S. [1955] Szczegó³owa Mapa Geologiczna Polski, arkusz Wojkowice. Instytut Geologiczny, Warszawa.
Birkenmajer, K. and Oszczypko, N. [1989] Cretacerous and Palaeogene lithostratigraphic units of the Magura Nappe. Krynica subunit, Carpathians. Annales Societatis Geologorum Poloniae, 59(1-2), 145-181.
Cieszkowski, M. and Golonka J. [2006]. Olistostroms as Indicator of the Geodynamic Process (Northern Carpathians). GeoLines20, 27- 28.
Chrz¹stowski, J.‚ Nieœcieruk, P. and Wójcik A. [1991] Szczegó³owa mapa geologiczna Polski, arkusz Muszyna. Pañstwowy Instytut Geologiczny, Warszawa.
Golonka, J., Krobicki, M., Matyszkiewicz, J., Olszewska, B., Œl¹czka, A. and S³omka T. [2005] Geodynamics of ridges and development of carbonate platform within the Carpathian realm in Poland. Slovak Geological Magazine, 11, 5-16.
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700
-28 -24 -20 -16 -12 -8 -4 0
-28 -24 -20 -16 -12 -8 -4 0
Przeczyce earth dam is located on the Czarna Przemsza river, in northern part of Silesia region. Spread of the dam is above 600 m and its height reach 12 m in the centre of the valley. Deformations of the dam motivated its operator to apply geophysical investigation of the dam body and basement to identify possible hazards. Set of geophysics tools applied revealed that the cause of deformation is exterior to the dam body, thus geology of the basement must be investigated. ERT imaging was performed to determine the resistivity of the basement, and interpret its geology.
The valley of Czarna Przemsza river is cut in Middle Triassic beds composed of limestone and dolomite. Triassic beds form monocline of gentle regional dip toward north-east (Biernat, 1955), locally undulated. In the direct vicinity of dam, Triassic strata are composed of thick-bedded dolomite covered by thin-bedded intercalations of limestones and marls.
Lower Jurassic deposits composed of bauxite-like clays filling paleokarst cavities are known from the region. The valley is partly filled by Quaternary alluvial sands and gravels of thickness not exceeding 10 m.
The medium in the analysed area was divided to four categories of different resistivity characteristics:
Near-surface, homogenous complex of resistivity exceeding 200 Ùm, corresponding to lower part of dam body and alluvial infill of the valley.
Complex of resistivity ranging from approximately 50 to 100 Ùm, interpreted as Triassic thin-bedded limestones and marls
Complex of resistivity exceeding 100 Ùm, reaching locally over 300 Ùm interpreted as Triassic thick-bedded dolomites Complex of resistivity bellow 50-70 Ùm, composing irregular bodies up to 30 m in diameter, penetrating basement.
Such bodies are interpreted as Lower Jurassic bauxite-like clays filling paleokarst cavities.
ERT imaging in the area of Przeczyce dam allowed to recognise the geology of valley basement. The main features of the basement geology are:
Slightly undulated, two layer composition of the basement Occurrence of fault zones in the Triassic rocks
Occurrence of paleokarst cavities filled by Lower Jurassic bauxite-like clays
Existence of paleokarst cavities filled with ductile clay, surrounded by rigid Triassic carbonate rocks may explain the deformation of the dam. Until our survey, possibility of existence of basement heterogeneity was not recognised as a cause of dam deformations.
- dam body and alluvial deposits
- bauxite-like clays and karst residuum, Lower Jurassic
- thin-bedded marly limestone and marly dolomite, Triassic - thick-bedded dolomite, Triassic
- fault ERT section
Interpretation of the ERT section
5 5
5
5
5 5 5
5 5
5 5
5 5
5
20 20
20 20
20 20
20
20 20
20
20
20 20
20
20
20
20 2
0 20
20 20
20
20
35 35
35 35
35 35
35 35
35 35
35 35
35
35
35 35
35 35 35
35
50 50
50
50
50
50
50 50
50
50 50
50
50
50 50
50
70 70
70
70
70 70
70 70
70
70
70 70
70 70
70 70
70 70
70
70 100 100
100
100
10 0
100
100
100
100 100
100
100
100
100
100 100 100
100 100 100
100
150 1
50
150
150 150
150 150
150
150 150
150
150
150 150
15 0
150
150
150 15
150 0
150
150 150
200 2
00
200
200 200
200 200
200 200
200
200 200
200
200
200
20 0
200
200 200
200
200 200
200 200
300 300
300 300
300 300
300 300
300 300
30 0
300 300
300
300
30 0 300
300 300
500
500 500
500 500
500
500
500 1000
10
00 1000
1000 1000
1000
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480
330 340 350 360 370 380 390 400 410 420 430 440
[m. asl.]
[m.]
SW SW S N
Subsilesian nappe
secondary thrust zone
secondary thrust zone Silesian nappe - Lgota beds
Silesian nappe - Andrychów Klippen Olistholite
fault
fault graben in frontal part of the Silesian nappe
limestone olistholite
Silesian thrust
20 20
35
35 50
50 70
70 70
70
100
100
100 100
100 100
150
150 150
150
200
300 30
0
500 500
100 0
1000
1000
20 40 60 80 100 120 140 160 180 200 220
380 390 400 410 420
[m]
[m asl.]
SW NE
gneiss olistholite
Silesian nappe - Lgota beds AKB olisthostrome
steepened secondary thrust
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300[Ùm]
The Andrychów Klippen Belt (AKB) is located in the Western Carpathians, in the base of Silesian nappe. The Belt is latitudinally elongated. Its length is several kilometres and width does not exceed the quarter of kilometre. AKB is composed of blocks of Upper Jurassic to Lower Cretaceous reef (stramberg facies) limestone (Olszewska et al., 2008; Waœkowska-Oliwa et al., 2008) and Proterozoic metamorphic rocks (Ksi¹¿kiewicz 1972). Size of the blocks range from tens to one hundred meters in diameter. Blocks of AKB lay on siliciclastic siltstone and mudstone composing Subsilesian nappe and infill of the Carpathians foredeep and are overlain by sequence of flysch type alternations of sandstone and siltstone beds, composing the Silesian nappe. Both underlying and overlying flysch rocks are Cainozoic in age (Ksi¹¿kiewicz, 1972).
The nature of the AKB is the matter of debate. It is widely accepted, that the Belt is an olistostrom and was formed by redeposition of rock blocks from the edge of the Silesian flysch basin to its centre (Cieszkowski, Golonka 2006; Golonka et al. 2005). The alternative explanation of the occurrence of exotic blocks is, that they were tectonically incorporated into the base of Silesian nappe during orogenesis. Although limestones of the AKB were exploited, and mapped carefully, its inner composition, and size and shape of blocks are poorly recognised.
The aim of the ERT surveys, was to detail inner composition of the AKB – size and shape of blocks, nature of contacts between blocks and surrounding rocks, and definition of borders of the Belt.
Three parallel ERT sections were measured near the Inwa³d village where AKB is prominently exposed.
The medium in the analysed area can be divided into three main categories of different resistivity characteristics:
Homogenous zone of high resistivity ranging from 70 up to 300 Ùm interpreted as flysch sandstone containing minor intercalations of siltstone and mudstone. Zones of lowered resistivity correspond to rocks of increased content of fine grained beds. This type of medium represent rocks of Silesian nappe.
Homogenous complex of low (below 20 Ùm) resistivity, interpreted as siltstone and mudstone of Subsilesian nappe and the Carpathians foredeep infill.
Complex of highly variable resistivity, containing irregular blocks with resistivity exceeding 100 Ùm and matrix of resistivity ranging from 20 to 70-100 Ùm. Blocks of high resistivity were interpreted as blocks of exotic rocks in Andrychów Klippen belt, while medium of reduced resistivity is interpreted as fine clastic matrix of the belt.
Zones of high resistivity gradient on the basis and top of the variable resistivity medium were identified as thrust or fault zones.
Interpretation of ERT imaging allowed to:
Estimate the size of exotic blocks, which range between 20 and up to 100 in diameter
Ilustrate the irregular shape of exotic blocks, that exclude simple tectonic origin of the AKB
Demonstrate the variability of the matrix of the AKB caused by admixture of exotic clasts smaller than ERT resolution
llustrate local steepening of trust zone below AKB caused by mechanic heterogeneity of the belt
Indicate the possibility of occurrence of secondary trust between AKB and Silesian nappe
llustrate the effect of brittle deformations (tectonic graben) in front of the Silesian nappe
The quality of ERT imaging and its usefulness in detailed geological mapping was shown by comparing geological interpretation of the ERT image with geological data obtained from archival drill cores and section logs of old quarry (figure 1 bottom).
5
5
10
10 10
10
20
20 20 2
0 35
35
35 35
50
70
100 200
20 40 60 80 100 120 140 160 180
340 350 360
[m. asl.]
SW NE
[m]
Silesian thrust AKB
matrix
AKB olistholite
Subsilesian nappe
1-Silesian nappe - sandstones 2-Andrychów klippen marls (matrix) 3-Limestones (klippes)
4-Brecciated limestone
5-Andrychów klippen marls (matrix) 6-Subsilesian nappe - marls
1
2 3
4 3 4
3
5
6
2 2 2
Geological sketch from Inwa³d quarry (Ksi¹¿kiewicz 1972)
Borehole
A - ERT section through Andrychów Klippen belt and adjacent Silesian and Subsilesian nappes in the Inva³d village. Central part of the section is composed of blocks of olistholites in low resistivity matrix (olisthostrome). Note small graben in the front of the overlying Lgota beds
B - ERT section through Andrychów Klippen belt and adjacentpart of the Silesian nappe in the town Andrychów. The thrust zone is locally steepened due to occurence of incompetent block of gneiss olistholite.
C - ERT section through Andrychów Klippen belt within old Inwa³d quarry compared to geological section based on drill core and field data (after Ksi¹¿kiewicz 1972). Notice general similarities in internal composition and the exact match with the drill core.
A
B
C
5
5 5
5
20
20 20
20
35
35 35 35
35 35
35
35 35
35
35
50 50
50 50
50
50
50
50 50
50
50 50 50
50 50
50 50
70 70
70
70
70
70 70
70 70
70
70 70 70
70 70
70 70
70
70 70
70 70
70
70
100 100
100
100
100
100 100
100 100
100
100 100
100 100
100
100
100 100
100 100
100 100
100 100 100
100
100
100
100
100
100 150
150
150
150
150 150 150
150
15 0 150
150
150
150
150 150
150 150
150
150 150
150 150
150
150 150 200
200
200
200
200 200
200
200 200
200
200
200
200 200
200
200 200
200
200 200
300
300 300
300 300
300
300
300
300
300 300
30 0
300 500
500
500 500
500
500
500
500
500
500 1000
50 100 150 200 250 300 350 400 450 500 550 600 650
480 500 520 540
Magura nappe - Krynica Formation
W E
section 9
20 35 3535
35
50 50
50
70
70
70 70
70 70
100 100
100
100 100
100
100
100
100
150
150 150
150
150
150
150 150
200 150
200
200
200 200
200
200
200
200
200
200 200
200
300
300 300
300
300 300
300 300
500 50
0
500 500
500 500
500
1000
1000
50 100 150 200 250 300
440 460 480
Magura nappe - Krynica Formation
NW SE
section 19
25
503 35
35
50 50
50
70
70
70 70
70
100 100
100
100 100
100
150 150
150
150 150
150
150 150
150 150
150 150 200
200
200
200 200
200
200 200
300 300
300
50 100 150 200
540 560 580 600
Magura nappe - Krynica Formation
W E
section 6
The Muszyna region (central part of the Polish Carpathians) is well known for its mineral springs. Intensive exploitation of aquifer demanded new estimation of water resources. Location and spread of horizon of marls confining sandstone aquifer were regarded as a key factor in aquifer recognition, thus ERT survey was employed.
Examined area is located within the Magura nappe, Outer Carpathians. Rocks are composed of Paleogene thick bedded flysch type sandstone (Chrz¹stowski et al. 1991). Two distinct horizons of marls and marly shales reaching a few tens of meters are known from the lower part of the sandstone sequence (Birkenmajer, Oszczypko 1989). Beds are generally dipping gently to south- west and are undulated locally. The system of regional faults cuts Magura nappe from NE to SW.
A set of parallel ERT sections were measured in the area of interest that allowed to recognise three main categories of medium:
Homogenous complex of high resistivity exceeding 100-150 Ùm interpreted as sandstone beds
Homogenous complex of low resistivity not exceeding 50 Ùm interpreted as impermeable marly horizon confining the aquifer
Complex of variable resistivity ranging between 35 and 100 Ùm, corresponding to water saturated sandstones or complexes of-thin bedded sandstones and shales.
Zones of high resistivity gradient were interpreted as fault zones.
Analysis of the obtained results allowed to:
Determine the position of impermeable marly horizon
Precise the location and geometry of regional faults and describe unknown faults featuring geology of the investigated area
Indicate the existence of tectonic horst in which confining horizon is uplifted Demonstration of block tectonics of the investigated area, and relationship of tectonic features and aquifer geometry allowed to better understand hydrogeology of the aquifer.
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
[Ùm]
faults
fault faults
regional fault regional fault
regional fault fault zones
fault zone
marls and marly sandstone sandstone dominated flysh
sandstone dominated flysh
sandstone dominated flysh sandstone dominated flysh
sandstone dominated flysh
A - Geological map of the area covered with ERT study. Map provided by the investor is obtained by surface mapping.
Impearmable marls are gray; Krynica Formation sandstone - brown and red;
alluvial deposits - green and white; solid black lines - faults of the regional importance. ERT sections marked with arrowed lines
B - Map of tectonic features of area presented on figure A. Shaded part on the map covers tectonic horst where impermeable marly horizon is uplifted.
The edges of this block are perspective locations of productive wells.
Solid red lines - faults of regional importance of certain strike; dashed, thick lines - fragments of regional faults with uncertain strike; thin dashed lines - minor faults
C - Three selected ERT sections from Muszyna area. Section 9 - uplifted marly horizon in tectonic block; section 19 - fault zone of regional fault and minor faults associated with regional fault, note ca. 10 m dislocated bed of lowered resistivity; section 6 - regional fault zone in predominantly sandstone flysh, note abrupt decrease of resistivity in the fault zone.
