2D Interpretation of Vertical Electrical Soundings: Application to the Sarantaporon Basin (Thessaly, Greece)

IOP PUBLISHING JOURNAL OF GEOPHYSICS AND ENGINEERING J. Geophys. Eng. 9 (2012) 50–59 doi:10.1088/1742-2132/9/1/006 2D interpretation of vertical electrical soundings: application to the Sarantaporon basin (Thessaly, Greece)

A Atzemoglou1 and P Tsourlos2

1 Institute of Geology and Mineral Exploration, 1 Fragon str., 54626, Thessaloniki, Greece 2 Department of Geophysics, School of Geology, Aristotle University of Thessaloniki, 54124, Downloaded from https://academic.oup.com/jge/article/9/1/50/5128347 by guest on 23 September 2021 Thessaloniki, Greece E-mail: [email protected] and [email protected]

Received 12 May 2011 Accepted for publication 3 November 2011 Published 12 December 2011 Online at stacks.iop.org/JGE/9/50

Abstract A large-scale vertical electrical sounding (VES) survey was applied at the basin of Sarantaporon, Elassona in order to study the tectonic and hydrogeological setting of the area. A large number of VES was obtained on a near-regular grid and data were initially processed with 1D inversion algorithm. Since some of the dense measured soundings were collinear, it was possible to combine 1D sounding data and produce 2D data sets which were interpreted using a fully 2D inversion algorithm. 2D geoelectrical models were in very good agreement with the existing drilling information of the area. 2D interpretation results were combined to produce pseudo-3D geoelectrical images of the subsurface. Resulting geoelectrical interpretations are in very good agreement with the existing geological information and reveal a relatively detailed picture of the basin’s lithology. Further, the results allowed us to obtain new, and verify existing, structural information regarding the studied area. Overall, it is concluded that 2D interpretation of 1D VES measurements can produce improved subsurface geophysical images and presents a potential useful tool for larger scale geological investigations especially in the case of reprocessing existing VES data sets.

Keywords: large-scale geoelectrical survey, pseudo-3D geoelectrical images, 2D geoelectrical inversion, Thessaly Greece

1. Introduction 2D geoelectrical images (Bobatchev et al 2001).Duetothe independent inversion approach and the posterior combination Vertical electrical sounding (VES) techniques are traditionally of the produced 1D model, lateral geoelectrical layer used for the prospection of 1D structures (Koefoed 1979). continuation is not often achieved using this approach. In order VES interpretation is based on fully automated 1D inversion to tackle this problem Gyulai and Ormos (1999) proposed the algorithms (Inman 1975, Rijo 1977, Zhody 1989, Basokur 1.5D inversion by performing 1D interpretation of collinear 1990, 1999). Standard VES inversion algorithms assume VES which were interconnected by deﬁning linking functions that subsurface structure is 1D, a fact that is not always the to constrain lateral resistivity variation. The technique of case. Further, a known problem of the 1D VES interpretation laterally constrained inversion (LCI) presented by Auken approach is the lack of solution uniqueness (Kunetz 1966, et al (2005) perform 1D inversion of individual soundings Koefoed 1979). but also special lateral constraints are applied to the model Assuming that several collinear soundings are collected, in order to produce smoothly varying layered pseudo-2D standard processing approach involves processing soundings sections. Wisen et al (2005) advocated that the use of the independently and then combining them to formulate pseudo- LCI approach in layered structures in combination with prior

1742-2132/12/010050+10$33.00 © 2012 Nanjing Geophysical Research Institute Printed in the UK 50 2D interpretation of vertical electrical soundings borehole information can improve interpretation, yet in the 2.2. Geological setting case of lateral resistivity variations LCI interpretation must be The study area belongs to the Pelagonian zone which is complemented with 2D inversion approaches. structured mainly by marbles, gneisses and schists, with lens- El-Qady (2006) combined 1D VES sounding data and shaped limestone layers and sedimentary formations (figure 2). performed 2D inversion on the data set in order to study a The Pelagonian cover consists of pre-alpine and alpine geothermal reservoir. Sultan and Santos (2008) applied a fully geological formations intruded by Palaeozoic or younger 3D inversion to a set of 35 regularly gridded 1D geoelectrical plutonic veins (Mountrakis 1983, Kilias and Mountrakis soundings for geotechnical site investigation. 1987). The Palaeozoic crystalline basement consists mainly In this work we apply a full 2D inversion to interpret of compact gneisses followed by early Middle Triassic collinear large-scale VES with typical AB/2 spacings of 800 m formations of gneiss-schists, schists and gneisses. These coming from the area of Sarantaporon (Thessaly, central formations include thin layers of ortho-gneisses and marbles. Greece). The 2D inversion approach is standard in processing The above-mentioned marbles (layers) have a mean thickness electrical resistivity tomography (ERT) data (Loke 1996); of 200 m, while the total formation thickness is about however, its application in collinear 1D soundings is not 600 m. Downloaded from https://academic.oup.com/jge/article/9/1/50/5128347 by guest on 23 September 2021 common. This is due to the totally irregular VES electrode The carbonic formations of the region are present as layers spacing which is not well suited to the regular modelling into the basement metamorphic rocks and as overlying marbles mesh structures used by most geoelectrical 2D interpretation of Triassic–Jurassic age. They occupy a small extent and are schemes. located in the regions of Dolichi, Kokkinogeia and Azoros, Here the modelling mesh was modified accordingly to while the marbles cover a bigger extent of the mountainous accommodate the VES electrodes and subsequent inversions space of the wider Sarantaporon region. At the SE margins of resulted in obtaining fully 2D geoelectrical sections of the the Sarantaporon basin, the marbles are intensively karstified subsurface deriving from the interpretation of collinear VES. and folded exhibiting a thickness of more than 500 m. Further 2D images were combined to produce pseudo-3D There are also basic metamorphic volcanic rocks at images. Geoelectrical results verified and enriched the current the upper metamorphic background formations. Across the geological knowledge of the area. Pelagonian zone’s background there is a large granite intrusion named after the place of occurrence (Kalithea granite). There is more than one phase of this granite intrusion. 2. Study area The ophiolites are situated across the margins of the Pelagonian zone. These ophiolites are allochtonous, coming 2.1. Geographic and geomorphologic setting from the oceanic regions of the Vardar and Sub-Pelagonian zones which are situated on both sides of the Pelagonian zone. The study area (Sarantaporon basin) is situated in the prefecture of Larissa, Thessaly, central Greece. It is a sub-area that is located between the municipality of Sarantaporon and 2.2.1. Sarantaporon basin sediments. The main research the villages Milea, Vouvala, Lykoudi, Mikro Eleutherochori, region is a tectonic graben. The tectonic grabens, created Kalithea, Lofos and Dolichi (figure 1). during upper Miocene extensional deformation, have been The wider research area is directly connected to the filled up with Neogene and Quaternary sediments which cover the bigger area of the basin while the basin margins are pre- geological formations and the intense active tectonic regime. Tertiary geological formations. Morphologically the region can be divided into three units: The Sarantaporon basin sediments are classified into two • The alluvial unit of the basin, categories. • The hilly unit region, that includes peaks of limestone and • The first category includes the Neogene formations gneiss-schist rocks and located at the southern part of the studied area (Milea– • The mountainous region that occupies the bigger extent Gerania–Petroto–Kalithea) and they are, for the most part, and is constituted by the compact basement rocks of the shingles of schist-gneiss origin with intermediate amounts Pelagonian zone (gneiss, schists and marbles). of water. The base of the Tertiary (Neogene) formations is conglomerates of gneiss and limestone origin and its The geomorphologic relief of the research area is the result ceiling consists of green-grey clays. of the erosion and the influence of human activities. • The second category consists of newer quaternary alluvial The broader studied area has two distinct hydrological sediments (sands, gravel and shingles in alternation basins: the Sarantaporon hydrological basin and the Phythion- with clays) that are products of weathering which Gerania basin. The mean elevation at the Sarantaporon filled the central and northern part of the studied hydrological basin is 884 m, while at the Phythion-Gerania area (Sarantaporon–Milea–Kokkinogeia–Dolichi). These hydrological basin is 796 m. sediments host a significant aquifer. The quaternary The region is seeped by torrents and streams with deposits are placed on top of the Neogene formations particularly deep slopes at points. All torrents lead to the and cover the biggest part of the basin. The quaternary Voulgaris river, which drains the region and has only seasonal formations are constituted in the lower part by sands and flow. gravel that become layers of marls and mud and the upper

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Figure 1. Map showing part of the Sarantaporon basin including the study area, the VES centres and the drill holes used to calibrate the geophysical interpretations.

layers by sands, gravel and shingles. The quaternary while at the western and eastern margins are diminished deposits exhibit a maximum thickness of more than and at the southern edge are absent. The petrological 200 m at the central–NW part of the Sarantaporon basin, composition of these quaternary deposits shows that the

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Figure 2. Geological formations at the Sarantaporon hydrological basin, the geophysical research area (blue dashed line) and the geoelectrical lines considered in this work. Simpliﬁed geological map from the IGME geological sheets of Elasson (1985) and Livadion (1982) following also Manakos (2001).

conglomerates ingredients originated mainly from the (FYROM)–Florina–Ptolemais–Kozani. The geological Olympus and the Kamvounia mountains. These have formations of the wider region were subjected to intense been certiﬁed by the water drill holes GT1–95, GT1–96, tectonism which is responsible for the evolution of the research GT1–98, GT1–99, GT2–99 and GT4–99 constructed by area. The main tectonic Miocene lineaments have NW– the Greek Institute of Geology and Mineral Exploration SE directions and are responsible for the formation of the (IGME). The alluvials are the quaternary formations Sarantaporon basin. More recent tectonism (Plio–Pleistocene) consisting of the contemporary deposits and screes across is related to E–W, N–S and NE–SW faults which gave the basin the Sarantaporon basin. its ﬁnal shape.

2.3. Tectonic setting 2.4. Hydrogeological conditions The main tectonic stress regime has a NW–SE orientation at the Sarantaporon basin. The Sarantaporon basin is The alluvial aquifers of (1) the Sarantaporon (western part of the prolongation of the big tectonic trench of Monastiri the studied area) and the (2) Phythion–Gerania (eastern part of

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Figure 3. Simpliﬁed geological map. the studied area) sub-basin are developed at the southern part formations while the western margin coincides with the of the previous mentioned tectonic trench. eastern margin of the Sarantaporon aquifer. In general, the structure of the Sarantaporon and (1) The western margin of the Sarantaporon aquifer coincides Phythion–Gerania aquifer appears to be relatively uniform with the large fault of Sarantaporon–Milea while its with successive aquifer layers interrupted by a combination eastern margin is limited by a fault situated in the centre of clay layers and layers of sands, gravels and conglomerates. of the studied area (see ﬁgure 2). The bottom surface of Extensive drilling program results suggest that three main the Sarantaporon aquifer is the upper horizon (marls) of hydrogeological formations exist: the lignites of the region. (1) Unconsolidated sands and gravels mainly in the central, (2) The hydrogeological sub-basin of Phythion–Gerania SE and S part of the aquifer. has an elongated shape. Northern, southern and (2) Impermeable clay lenses and layers. eastern margins coincide with the basement metamorphic (3) Conglomerates.

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3. Measurements and data processing columns were used to calibrate the geoelectrical interpretation results. The exact position of the wells is also depicted in The geophysical survey is part of a larger research programme figure 1. realized by the Greek Geological Survey (IGME) aiming Data quality was particularly good as indicated by the at studying the general geological and hydrogeological low standard deviations (0.5–1%) recorded in the field. The conditions of the wider Sarantaporon area. relatively smooth behaviour of all recorded sounding curves The aim of this extensive geophysical survey that was indicates that the geology of the area fits the assumptions of realized by applying the VES method was to map the 1D interpretation. This was also verified later by the low RMS lithological structure of the wider Sarantaporon region in errors (<1–5%) obtained by all the sounding interpretations. relation to the tertiary and quaternary deposits. The further Initially all soundings were interpreted using the IPI2WIN aim was to decide the depth of the bedrock as well as to get program (Bobatchev et al 2001). The software performs additional information about the tectonics of the region. iterative minimization of the misfit between real and modelled The total extent of the area studied in this work is data based on a least number of layers initial model using approximately 110 km2 and includes 95 VES, all measured Tikhonov’s regularization. In figure 4, four typical sounding with the Schlumberger array. The majority of VES have AB/2 curves and the IPI2WIN interpretations are presented. In ranging from 500 to 800 m, depending on the local conditions. soundings of figures 4(a) and (b) the resistive bedrock appears The sounding centres are positioned on seven lines (line relatively shallow while in the sounding curves of figures 4(c) 1 to line 7 as shown in figure 2) which have a NW–SE and (d) the resistive bedrock appears to be much deeper below orientation and an in-between distance of 1000 m. The typical alternations of sand and clay sediments. inter-distance between the VES centres was 1000 m for all The IPI2WIN software allows the user to combine several lines except line 1 which has a denser VES centre (500 m) independent sounding curve interpretations in one single distribution as shown in figure 2. section. An example of the resulting resistivity image for Some of these VES were realized in the vicinity of the the case of line 7 (see figure 2), which consists of ten VES, is local water drills (Varvarousis et al 1998, Manakos 2001). depicted in figure 5. Actually independent 1D interpretations They proved particularly useful since the available lithological of line 7 were extracted in a quasi-2D mode and are plotted

55 A Atzemoglou and P Tsourlos m 600 550 500 450 400 350 300 250 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000 m

Log(ρα) Ωm -1 0.2 1.4 2.6 3.8 5 6.2 7.4 8.6 9.8 11 Figure 5. Quasi-2D resistivity image for the case of line 7 (see ﬁgure 2). Image was formulated by plotting the independent 1D interpretation of all ten soundings of line 7. Elevations are above sea level.

Inversion Parameter Downloaded from https://academic.oup.com/jge/article/9/1/50/5128347 by guest on 23 September 2021

electrode positions

Finite Element

Sounding Center Sounding Sounding Sounding Center Center Center

Figure 6. 2D inversion parameter and mesh structure.

using the same logarithmic rainbow colour scale. As can be where p is the model resistivity vector, Wd is data error seen from the results of figure 5, combining independent 1D variance, J is the Jacobian matrix, C is the smoothness matrix, inversion results in a single line is not necessarily very helpful: and λ is the Lagrangian multiplier. different resistivity levels may generate a rather complicated A proven 2.5D finite element method (FEM) scheme was geoelectrical image which is often difficult to interpret and this used as the platform for the forward resistivity calculations. seems to be more important in cases of mapping complicated The core of the algorithm is based on the generation of a very geological structures. dense finite element mesh, which can practically accommodate every current or potential electrode of every individual The fact that the study area presents a relatively VES measurement. Subsequently, many neighbouring finite complicated geological structure suggests that the standard elements are clustered to form an inversion parameter (see 1D interpretation procedure may be inadequate for imaging the figure 6 top). The width of the inversion parameters is subsurface structure. To cope with the problem, a 2D inversion automatically adjusted to be consistent with the distance of procedure was applied to the 1D sounding curves. In particular the sounding centres (figure 6). Note that topography can also 7 NW–SE lines, composed of arbitrarily spaced collinear 1D be incorporated into the inversion procedure. geoelectrical soundings, were selected to be processed in a Finally results from the 2D inversion of the 1D fully 2D mode by a customized 2D inversion algorithm (Kim soundings were combined to formulate georeferenced quasi- 2009). 3D resistivity images of the subsurface. The algorithm tries to find the best 2D model that fits the data using a smoothness constrained inversion algorithm 4. Interpretation (Tsourlos 1995). Optimization involves minimizing the L2 All seven interpreted subsurface resistivity sections are norm of field and model data misfit e subject to minimizing presented in figure 7 starting from the northernmost line 7 the model roughness. Minimization yields the final normal and ending with the southernmost line 1. All images are under equations for the iterative model resistivity update for the kth identical rainbow scale. Furthermore, all available drilling iteration: information, although not used to constrain inversion results, is also presented within the relevant sections in order to allow = T T −1 T pk+1 pk + (J Wd J + λC C) J Wd e (1) comparisons.

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Figure 7. 2D inversion images of lines 1–7 (see ﬁgure 2). Relevant drilling information is also included.

Based on the sounding interpretations, the following (b) the main sedimentary formation consists of clays and correlation between geoelectrical and geological formations alterations of relatively thin layers of sands and gravels can be seen: which are not always easy to identify as individual geoelectric layers. (a) the topsoil has varying resistivities due to high variability (c) the bedrock formation is clearly identiﬁed as geoelectrical of composition. layer and has resistivities more than 1000 m. Distinction

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Figure 8. Quasi-3D distribution of resistivities in six different elevation slices (from 550 to 300 m). Existing tectonic information as well as structural features inferred by the geoelectrical images is noted in every slice.

between the different formations (schist-gneiss, marbles material (green colour) which is more clear in the parts of the and granite) is impossible because of similar resistivity sections where bedrock appears to be very deep. range. Further tectonic features can be seen, since bedrock seems to exhibit sharp boundaries. This is in line with the graben The geoelectrical method was able to delineate the structure of the studied area which can be clearly seen in different materials quite effectively since resistivity contrasts lines 5, 4 and 3. proved high. In particular, in all sections, the bedrock The intense differentiation of the resistivity from line to formation (resistive structures with red colours) as well as the line suggests that we are dealing with a clearly 3D geological thickness of the soft sediments (blue, green colours) is clearly setting. Therefore, in order to further interpret the geophysical depicted. In general, the agreement between the drilling images the 2D inversion results were combined appropriately information and the geoelectrical images is very good. In in order to form georeferenced quasi-3D resistivity slices for many cases there is an almost exact geoelectrical contrast with different elevations. the bedrock appearance. Such cases are spotted in line 7 (drill The different resistivity slices which are depicted in GT1-98, GT3-97), line 5 (drill S-5), line 3 (GT2-97). Overall, figure 8 suggest that a graben structure exists. Main tectonic the above developments suggest that the interpretation of the features of NW–SE directions are clearly depicted as well as high resistivity region (red colour), as the bedrock formation, faults of NE–SW orientations. This is in line with the tectonic is quite assured. setting of the area and the already mapped faults depicted in A further differentiation in resistivity can be seen within the geological map (figures 3 and 2). the soft sediments between the fine grained, mostly clay, Although further interpretation and correlation with material (blue colour) and the more coarsely grained, sandy existing data is required, it is quite evident that the present

58 2D interpretation of vertical electrical soundings interpretation approach provides a signiﬁcantly improved References imaging capability compared to standard 1D interpretation. Overall it is considered as a very useful tool for larger scale Auken E, Christiansen A V, Jacobsen B, Foged N and Sorensen K geological investigations. 2005 Piecewise 1D laterally constrained inversion of resistivity data Geophys. Prospect. 53 497–506 Basokur A T 1990 Microcomputer program for the direct interpretation of resistivity sounding data Comput. 5. Conclusions Geosci. 16 587–601 Basokur A T 1999 Automated 1D interpretation of resistivity Geophysical results alone cannot interpret the Sarantaporon soundings by simultaneous use of the direct and iterative sub-basin structure, but in association with geological, methods Geophys. Prospect. 47 149–77 geomorphological and borehole data can establish the shape, Bobatchev A, Modin I and Shevnin V 2001 IPI2WIN v.2.0, User’s the dimensions and the ﬂoor (depth) of the basin. The manual El-Qady G 2006 Exploration of a geothermal reservoir using resulting geophysical images are in very good agreement geoelectrical resistivity inversion: case study at Hammam with the existing geological and tectonic information for the Mousa, Sinai, Egypt J. Geophys. Eng. 3 114–21

area and indicate the ability of geophysical surveys to aid Gyulai A and Ormos T 1999 A new procedure for the interpretation Downloaded from https://academic.oup.com/jge/article/9/1/50/5128347 by guest on 23 September 2021 the geological interpretation. The geophysical interpretations of VES data:1.5-D simultaneous inversion method J. Appl. allow us to verify the continuation of known faults and obtain Geophys. 64 1–7 Inman J R 1975 Resistivity inversion with ridge regression new structural information for the study area. Geophysics 40 798–817 As can be seen from the results of the geophysical Institute of Geology and Mineral Exploration (IGME) 1982 investigation, two main faulting systems can be identified. The Geological map, Livadion sheet, Athens, Scale 1:50000 first has NNW–SSE direction and the second faulting system Institute of Geology and Mineral Exploration (IGME) 1985 has NE–SW direction. The results of figure 8 areinvery Geological map, Elasson sheet, Athens, Scale 1:50000 Koefoed O 1979 Geosounding Principles, 1, Resistivity Sounding good agreement with the typical tectonic model of the studied Measurements (Amsterdam: Elsevier) area and verify the existing tectonic regime of the region. Kunetz G 1966 Principles of Direct Current Resistivity Prospecting There is also very good agreement between the geological (Berlin: Gebruder Borntaeger) faults (as they have been identified, Manakos 2001) and the Kilias A and Mountrakis D 1987 Zum tektonischen Bau der possible faults as they are concluded by the interpretation of Zentral-peloponnischen Zone (Kamvounia-Gebirge, N. Griechenland) Z. Dt. Geol. Ges. 138 211–37 the geoelectrical images. Kim J H 2009 DC2DPro-2D interpretation system of DC resistivity We claim that despite the application and increased tomography User’s Manual and Theory KIGAM, S. Korea effectiveness of ERT measurements, still VES measurements Loke M H and Barker R D 1996 Rapid least-squares inversion of can be considered, in some cases, as a preliminary apparent resistivity pseudosections by quasi-Newton method investigation tool for geoelectrical imaging at large scales Geophys. Prospect. 44 131152 Manakos A 2001 Hydrogeological survey at Sarantaporon basin and depths. Of course the presented methodology due to sedimentary formations IGME, internal report (in Greek) the inherent sparseness of the sounding network will result in Mountrakis D 1983 Structural Geology of the North Pelagonian images of reduced lateral resolution compared to the resolution Zone. S.1 and Geotectonic Evolution of the Internal Hellenides obtained by the ERT measuring mode. (Macedonia, Greece) Habil.–Schr. University Thessaloniki As shown in this work, increased effectiveness is achieved Rijo L 1977 Modelling of electric and electromagnetic data PhD Thesis University of Utah by interpreting collinear VES data in a fully 2D mode. Sultan A S and Santos F 2008 1D and 3D resistivity inversions for Resulting fully 2D images are certainly more informative than geotechnical investigation J. Geophys. Eng. 5 1–11 the quasi-2D images produced by combining the independent Tsourlos P 1995 Modeling, interpretation and inversion of 1D interpretations. Results indicate the ability of the presented multielectrode resistivity data DPhil Thesis University of York, approach to aid and verify geological interpretations. UK Varvarousis G, Metaxas A, Kotis Th, Ploumidis K and Vrettos K The VES technique has been globally used for several 1998 Report for the lignite research at the Sarantaporon basin decades as the main geoelectrical measuring tool for different IGME, internal report, in Greek types of investigations involving among other geological, Zhody A 1989 A new method for the interpretation of Schlumberger hydrogeological and geothermal research. This fact suggests and Wenner sounding curves Geophysics 54 245–53 that there is a large data base of existing VES data sets Wisen R, Auken E and Dahlin T 2005 Combination of 1D laterally constrained inversion and 2D smooth inversion of resistivity which can be reprocessed on the basis of the proposed data with aprioridata from boreholes Near Surf. Geophys. methodology. By applying this low-cost operation-enhanced 2005 71–9 interpretation, the quality of the existing VES data sets can be improved.