Đang tải... (xem toàn văn)
In this paper, we have suggested a finite difference algorithm for resistivity forward and inverse modeling with electrically anisotropic media in applied geophysics. Electrical anisotropy affects the surface measurement in a fashion that may make interpretation erroneous.
Turkish Journal of Earth Sciences http://journals.tubitak.gov.tr/earth/ Research Article Turkish J Earth Sci (2018) 27: 152-166 © TÜBİTAK doi:10.3906/yer-1702-6 Resistivity inversion of transversely isotropic media 1, Ertan PEKŞEN *, Türker YAS Department of Geophysical Engineering, Faculty of Engineering, Kocaeli University, Kocaeli, Turkey Department of Marine and Environmental Researches, General Directorate of Mineral Research & Exploration, Ankara, Turkey Received: 08.02.2017 Accepted/Published Online: 27.12.2017 Final Version: 19.03.2018 Abstract: In this paper, we have suggested a finite difference algorithm for resistivity forward and inverse modeling with electrically anisotropic media in applied geophysics Electrical anisotropy affects the surface measurement in a fashion that may make interpretation erroneous This means that the resistivity section obtained by an inversion method that does not incorporate electrical anisotropy gives the wrong subsurface structure We used a classical multielectrode dataset of a profile for estimating not only horizontal but vertical resistivity as well Thus, the electrical anisotropy can be calculated Finally, the finite difference mesh can be corrected by using the estimated anisotropic coefficient The result of the developed algorithm was verified with a 2-layered analytic solution Furthermore, the present method was also tested on a field dataset Key words: Electrical resistance tomography, electrical anisotropy, 2D resistivity modeling, 2D resistivity inversion, direct resistivity method, transversely isotropic media Introduction Electrical conductivity of a geological formation may vary in different directions depending on the formation’s properties This type of directional property of conductivity gives rise to electrical anisotropy (Maillet, 1947) Ignoring the electrical anisotropy of a geological formation or assuming that the formation is isotropic may yield to a misleading interpretation (Nguyen et al., 2007; Wiese et al., 2009; Greenhalgh et al., 2010) Electrical anisotropy detection in the field can be determined by using a square array, since the square array is more sensitive to electrical anisotropy in a geological formation than the most commonly used Schlumberger and Wenner arrays (Habberjam, 1972, 1975; Matias, 2002; Yeboah-Forson and Whitman, 2014; Yeboah-Forson et al., 2014) The second approach for determining electrical anisotropy in the field based on the surface measurement is an azimuthal resistivity measurement (Busby, 2000) However, the square array requires more fieldwork than the more commonly used electrode arrays to get a strike direction and to determine electrical anisotropy of the medium If we need to estimate what the strike direction is in an electrically anisotropic formation, we can apply the square array (Yeboah-Forson and Whitman, 2014; YeboahForson et al., 2014) Otherwise, we often prefer the most commonly used collinear arrays rather than a square one * Correspondence: ertanpeksen@kocaeli.edu.tr 152 In some situations, such as one we came across in an archaeological site in Bathonea in Turkey, there was a shift in depth estimation due to possible electrical anisotropy To show this misleading interpretation as an example, one can compare inversion results obtained using the same dataset with and without electrical anisotropy assumptions To this, we compared both results The datasets used in this study were collected at an archaeological site in Turkey in 2013, during the field season The dataset was then inverted with the assumption of the earth as a 2-dimensional (2D) isotropic and anisotropic model, respectively Comparison of the excavated site and inverted resistivity sections suggests that electrical anisotropy affects the depth information, which is directly related to the electrical anisotropy coefficient as expected In this study, we considered the 2D direct current (DC) method in anisotropic media This method is also known as electrical resistance tomography (ERT) or imaging (ERI) The method used in the present study can also be called 2.5D due to an infinite strike direction normal to the resistivity profile Here, we developed a 2D finitedifference (FD) code for forward and inverse modeling The 2D code validation was tested against an analytical solution for the 1-dimensional DC method in a layered anisotropic medium introduced by Pekşen et al (2014) The electrical anisotropy coefficient of a geological formation can be estimated by using an inversion based PEKŞEN and YAS / Turkish J Earth Sci on the least-squares method For this purpose, electrical data as mean apparent resistivity values collected using a multielectrode system along profiles on the surface can be used We then convert the data for estimating vertical and horizontal resistivity of geological formations in the corresponding field, based on surface electrical measurements In general, it is impossible to distinguish horizontal and vertical resistivity values from a single surface measurement without constraint, due to the principle of equivalence To overcome this difficulty, we assume that the vertical resistivity value is greater than or at least equal to the horizontal resistivity value Anisotropic inversion is not new in resistivity methods: Pain et al (2003), Herwanger et al (2004), LaBrecque et al (2004), Kim et al (2006), and Wiese et al (2015) have studied the inversion method within electrically anisotropic media for different types of applications LaBrecque et al (2004) studied 3D resistivity inversion using surface DC data In our inversion process, we solve the vertical and horizontal resistivity values of model mesh iteratively After we estimate the anisotropy coefficient, the last step of our algorithm modifies the mesh for the depth correction The proposed method was tested on synthetic models and the field dataset successfully 2D DC FD modeling 2.1 Forward modeling in anisotropic media Electrical conductivities of formations can vary in different directions In such formation, Ohm’s law can be written as: (1) where is the current density (A/m2), is the electric field (V/m), and is the conductivity tensor (S/m), which has the following form: (2) The conductivity tensor is a positive-definite and symmetric matrix with respect to the main diagonal in the Cartesian coordinate (Marti, 2014) In this paper, we used the right-handed coordinate system that is positive downward In the anisotropic earth model, electrical current is not parallel to the electric field However, if the medium is isotropic, the conductivity is a scalar In an isotropic model, current flows in the same direction as the applied electric field In sedimentary sequences, the conductivity can be assumed to be equal in the x and y directions, while it is different in the z direction In our assumption, the formation is generally isotropic in the horizontal plane (x–y) However, it is anisotropic in the vertical plane (x–z) This kind of anisotropic model is known as transverse isotropy in the vertical direction (Figure 1) The conductivity tensor given by Eq (2) can be rotated through the Euler angles, which makes the principal axes and the recording frame coincide with each other This diagonalization can be applied using the following: (3) where R is the Euler matrix The superscript T stands for a transposition of the Euler matrix Note that the Euler matrix is an orthonormal matrix Thus, the inverse of the Euler matrix is equal to its transposition, which is RT=R-1 Based on our assumption, the conductivity tensor can be written as: (4) where the prime denotes the principal axis values The conductivity tensor can be easily transferred from the primed to the unprimed or the unprimed to the primed coordinate system through the Euler matrix without any error with respect to the mathematical point of view, since the Euler matrix is orthonormal To perform this rotation, Eq (3) can be used However, this requires a priori information about the Euler angles, such as strike and dip directions In this study, we assume that the Euler angles are known More specifically, the strike and dip angles are zeros, which means that the primed and the unprimed coordinate systems coincide For anisotropic media, the forward response can be obtained by a numerical solution of the following equation: (5) where v(x,y,z) is the potential field distribution in a 3D domain, I is a point source with its location indicated by subindex s, is the conductivity tensor, and δ is the Dirac delta function To develop the forward response of the isotropic model, we used the theory introduced by Dey and Morrison (1979) They solved Poisson’s equation in a 2D arbitrarily shaped model by the FD method, assuming the earth to be isotropic while employing a point source and mixed boundary condition In this paper, we follow the same methodology for the forward modeling To solve Poisson’s equation using the FD method, we need to discretize the corresponding domain in the x and z directions (Figure 2) This discretization determines the model size as well In general, a finer mesh design gives more accurate results We cannot continuously increase the number of cells in the x and z directions, since the computational time increases (Pidlisecky and Knight, 2008) A typical value of the matrix dimension is 100 × 20 in our cases This coarse mesh design can also be divided into 2, 4, 8, etc Our experience suggests that using divisions of a single cell gives much better results, and it is 153 PEKŞEN and YAS / Turkish J Earth Sci Transverse isotropy in the vertical direction σh electrodes z(m) σh -5 -10 40 σv 20 y(m) -20 -40 -40 -30 -20 -10 10 x(m) 20 30 40 Figure Sketch shows transversely isotropic medium Red and blue colors represent various formations of the corresponding earth models Horizontal conductivities are in the x and y directions (σh) Vertical conductivity is in the vertical direction (σv) Arrows show the location of potential and current electrodes along the measuring profile X Z i=1 ΔZ1 ΔX1 i=2 ΔX2 ΔXN-1 i=N j=1 j=2 ΔZM-1 j=M Figure The rectangular grid displays the discretization of the finite difference method suitable for us with respect to accuracy and computational time Thus, a cell was divided into or subcells to get a finer grid in the 2D conductivity domain in this study Once the mesh design was completed, we assigned each intersection of the discretized domain with a conductivity σn(x,z) value (Figure 3a) Therefore, a capacitance matrix can be set up properly by employing a proper boundary condition such as Neumann, Dirichlet, or mixed Using matrix notation, a discrete system of equation can be written as: Cv=s (6) where C is the capacitance matrix, v is a vector with unknown potential values in the modeling domain, 154 and s is a point source vector The capacitance matrix depends on the model geometry and physical properties of the domain Eq (5) can be solved by the Cholesky decomposition method (LU), or it may be solved with other methods, such as QR factorization (Haber and Oldenburg, 2000; Candansayar, 2008) For each source position, Eq (5) must be solved Even though we assume that a geological subsurface model is 2D, the distribution of potential is in the 3D domain Domain transformation from 3D to 2D can be achieved by integration in the Fourier domain (Dey and Morrison, 1979; Pidlisecky and Knight, 2008; Xu et al., 2000) Finally, from this 2D model’s potential values, PEKŞEN and YAS / Turkish J Earth Sci ΔXi-1 ΔXi ΔXi-1 i,j-1 ΔZj-1 ΔZj i-1,j i,j σi-1,j i,j-1 σi,j-1 σi-1,j-1 ΔXi i+1,j σi,j λΔZj-1 λΔZj i-1,j v i-1,j-1 σ i,j v σi,j-1 h s i-1,j i+1,j σi,jh v i,j σ σvi-1,j i,j+1 i,j+1 (a) h σi,j-1 h σi-1,j-1 (b) Figure (a) A rectangular grid discretization of a cell, assuming the earth to be electrically isotropic (b) A rectangular grid discretization of a cell, assuming the earth to be electrically anisotropic one can calculate apparent resistivity for any electrode configuration Apparent resistivity calculations can be classically achieved by multiplication of a geometric factor, with potential difference normalized by injected currents (Dey and Morrison, 1979) Furthermore, we also normalize these apparent resistivity values by a homogeneous response in the model introduced by Marescot et al (2006) as: , (7) where Δv is the potential differences measured in the field, ΔvH is the potential differences in a homogeneous model, and ρH is the resistivity value of a homogeneous model, e.g., ohm-m (Marescot et al., 2006) In our anisotropic model, we assume that the earth consists of arbitrary shapes with electrical anisotropy Figure 3b shows a cell in an anisotropic medium Based on our assumption that the horizontal conductivity values are in the x direction and the vertical conductivity values are in the z direction, the vertical conductivity values are always smaller than the horizontal value in each cell (Maillet, 1947) The geometric mean of the conductivity values is given as (Maillet, 1947): , (8) where σh(x,z) and σv(x,z) are conductivities in the horizontal and vertical directions, respectively The subindices h and v stand for horizontal and vertical directions Note that From these conductivity values, one can calculate the electrical anisotropy coefficient of each cell of the medium Thus, the anisotropy coefficient can be calculated with the following equation (Maillet, 1947; Grant and West, 1965): (9) Laboratory measurements show in most sedimentary rocks The range of the coefficient of the anisotropy value is between 1.0 and 7.5 (Carmichael, 1989; Negi and Saraf, 1989) Without electrical anisotropy assumption, interpretation may be erroneous (Nguyen et al., 2007) Electrical anisotropy should be considered even when the anisotropy coefficient is (Wiese et al., 2009) Electrical anisotropy can even be detected in alluvium (Greenhalgh et al., 2010) Based on our assumption given above, the forward response of an isotropic earth model can be extended to an anisotropic earth model by replacing the conductivity tensor with the geometric mean of the conductivity values Thus, we have: (10) where v(x,y,z) is the potential field distribution in the 3D domain, I is a point source with its location indicated by a subindex s, σn(x,y) is the geometric mean of conductivity values calculated by Eq (8), and δ is the Dirac delta function The FD method requires a mesh designed by dividing rectangular cells in the region as mentioned previously The corresponding 2D region consists of dimensions (assuming that the conductivity does not change along the y direction in a Cartesian coordinate system) This region can be divided into Nx by Mz cells (Figure 2) Here, Nx is the number of cells in the x direction with step size Δx Similarly, Mz is the number of cells in the z direction with step size Δz In cases where electrical anisotropy exists, the vertical step size is divided by the electrical anisotropy coefficient with Δz / λ However, we not know initially what the 155 PEKŞEN and YAS / Turkish J Earth Sci anisotropy coefficients are Thus, we need to determine the anisotropy coefficient so that the mesh design can then be corrected Our inversion algorithm iteratively finds not only horizontal but also vertical resistivity values At the end of the inversion process, we also need to calculate electrical anisotropy coefficients by using Eq (9) so that the electrical anisotropy correction can be employed Note that any type of FD discretization requires mesh design as mentioned above In our model, the mesh does not change during iteration as usual The pseudosection is represented by using median depth of investigation (Edwards, 1977; Loke, 2016) This is a crucial point: if electrical anisotropy exists in a geological formation, interpretation using 2D inversion code may be erroneous without considering anisotropy We developed a new inversion code by using MATLAB The code can be used for inverting resistivity sections of 2D isotropic and anisotropic earth models Note that here we assume that the measurement profile and layers of the formation are parallel to each other The newly developed forward code was validated against 1D analytical solutions on anisotropic examples (Pekşen et al., 2014) Comparison of the results for an analytic solution for a 2-layered earth model and a 2D DC FD anisotropic model are illustrated in Figure The results are very similar The purpose of numerical comparison with analytic results is to be a useful control for numerical modeling (Greenhalgh et al., 2009a.; Yan et al., 2016) 2.2 Inversion Estimation of subsurface conductivity distribution from measurement requires inversion There are many algorithms and methods available in the literature for this purpose (e.g., Pain et al., 2003; Herwanger et al., 2004; LaBrecque et al., 2004; Kim et al., 2006; Greenhalgh et al., 2009b; Wiese et al., 2015) The forward 2D DC FD method can be formulated as the following matrix notation: d=G(m) (11) where G is a nonlinear forward operator, m is a parameter vector, and d is an observation vector (Meju, 1994) Various approximations can be followed in order to estimate model parameters from observations In this study, we applied the singular value decomposition (SVD) method for solving Eq (11) The sensitivity matrix of the system is calculated for each iteration to solve Eq (11) The partial derivative of the sensitivity matrix was calculated by using the method proposed by Tripp et al (1984) The sensitivity matrix of the model consists of the partial derivative of the model parameters; the dimensions of the matrix are N by M N is the number of measurements and M is the number of parameters Depending on the condition (N > M, N < M, or N=M), we may meet some difficulties while inverting the matrix Problems based on the number of parameters and observation points can be categorized as follows: when N > M, it is called an overdetermined problem; when N=M, the problem is known as evenly determined; finally, when Figure Comparison of the results of an analytic solution for a 2-layered earth model and a 2D DC FD anisotropic model 156 PEKŞEN and YAS / Turkish J Earth Sci N < M, it is called an underdetermined one (Menke, 1989) This problem is ill-posed from the mathematical point of view; the solution may not be unique These kinds of inversion problems can be coped with by using different algorithms, such as a damped-least squares method (Meju, 1994), Tikhonov regularization (Tikhonov and Arsenin, 1977; Zhdanov, 2002), or the Occam method (DeGrootHedlin and Constable, 1990; Aster et al., 2005) In this study, we used the SVD method The details of the SVD method can be found in the work of Golub and Loan (1996) The problem can be solved iteratively with the SVD algorithm given by Meju (1994) in the following matrix notation: (12) , (13) where Δp is a vector consisting of model parameters, V and U are orthonormal matrices, λi is the ith eigenvalue of the system, and ε is a constant m stands for a parameter vector and k denotes an iteration number To clarify the inversion steps we have used here, flowcharts of the algorithms for isotropic and anisotropic media are shown in Figures and We have followed these inversion steps As initial values, we use the geometric mean of the apparent resistivities of a profile Since the initial value is a single apparent resistivity value, it is a sort of homogeneous model for isotropic cases As for the anisotropic model, the initial value we begin with for an electrical anisotropy coefficient is 1.1 This is a kind of threshold value for inversion with the anisotropic model It can be between 1.1 and Without a small perturbation of anisotropy, the method suffers in finding the correct model Using a classical inversion algorithm, the horizontal and vertical resistivity values cannot be distinguished from surface resistivity measurement, due to the principle of equivalence This can also be seen in Eq (8) To overcome this problem, we use the constraints during iterations We also apply a Laplacian and filter operator to the data as a smoothing factor (Sasaki, 1989) Note that the problem is an underdetermined one, since the number of parameters is smaller than the number of observations In spite of these difficulties, we have found the subsurface resistivity distribution of the isotropic and anisotropic models Once the inversion algorithm reaches a predetermined RMS threshold value (e.g., 3%), the process ends For an anisotropic model, we further calculate the electrical anisotropy coefficient, and then we finalize the process by incorporating the vertical shift due to electrical anisotropy into the mesh discretization by Δz/λ Micro- and macroanisotropy In general, electrical anisotropy can be classified as macro or micro For geological studies, macroanisotropy exists in homogeneous stratified rocks The presence of successive parallel and different conductivities with long linear structures causes macroanisotropy The layered internal structure of shale results in microanisotropy (Negi and Saraf, 1989) Electrical anisotropy is a scaling problem An anisotropic layer can equally be represented by a stack of isotropic layers (Maillet, 1947) Furthermore, a number of isotropic layers can cause pseudoanisotropy Elongated small particles in the sedimentation such as clay or sand prefer certain directions, which also gives rise to electrical anisotropy (Kunz and Moran, 1958; Rey and Jongmans, 2007) Numerical example Figure shows a synthetic 2D model The background of the model is anisotropic The anisotropy coefficient is with The resistivity of the buried body is 200 ohm-m in the horizontal and vertical directions Therefore, it is an isotropic block The depth of the upper side of the block is 1.5 m The inverted resistivity sections with isotropic and anisotropic models are given in Figures 8a and 8b The white color shows the exact position of the block in the middle of Figures 8a and 8b The vertical shift can be seen with and without incorporating electrical anisotropy when comparing both results Based on Figures 8a and 8b, one can state that inversion without electrical anisotropy can make interpretation erroneous regarding the depth of the anomalous body’s location Inversion results of the isotropic and anisotropic assumptions give some differences in the vertical body position These differences should be related to the anisotropy coefficient of the medium Thus, we can say that if any geological formation has electrical anisotropy properties, we should use the 2D inversion process with incorporated electrical anisotropy The second synthetic model consists of electrically anisotropic blocks embedded in a model The exact model is shown in Figure The anisotropy coefficients of the blocks are and 2.5, respectively We assume that the background of the model is isotropic in this synthetic example Figures 10a and 10b show the inverted results with and without anisotropy assumptions It can be seen from Figures 10a–10c that there is no significant vertical shift in the depth, since the background of the model is isotropic Our inversion algorithm successfully found the upper block close to the surface; however, the lower block was not located precisely Comparison of both inverted results with and without incorporating anisotropy shows a relatively small enhancement of the result with anisotropy (see Figures 10a and 10b) 157 PEKŞEN and YAS / Turkish J Earth Sci Figure The flow chart of inversion steps, assuming the earth to be electrically isotropic Field example Bathonea is a prehistoric city in İstanbul Figure 11 shows the location of the Kuỗukỗekmece Lake district in İstanbul 158 Many archaeological objects and structures in Bathonea have been found and identified, such as harbor structures, roads, buildings, and smaller artifacts The periods of these PEKŞEN and YAS / Turkish J Earth Sci Figure The flow chart of inversion steps, assuming the earth to be electrically anisotropic 159 PEKŞEN and YAS / Turkish J Earth Sci 0 10 20 30 Depth (m) ρh=200Ω.m λ =1 50 1.5 m 40 ρh=2.5Ω.m ρv=10Ω.m λ =2 ρvv =200Ω.m 2m 5m Figure A synthetic 2D resistivity model is shown There is a rectangular body (electrically isotropic) in the middle of the model The background of the resistivity model is assumed to be electrically anisotropic Figure (a) Estimated 2D model obtained from 2D inversion; the model is assumed to be electrically isotropic The white rectangular shape in the inverted resistivity section shows the exact location of the body (b) Estimated 2D model obtained from 2D inversion assuming the model to be electrically anisotropic The white rectangular shape in the inverted resistivity section shows the exact location of the body findings are Hellenistic, Roman, and Byzantine Excavations and research in the ancient city continue to the present day (Aydıngün, 2007) More information about and photos of Bathonea can be found at http://www.bathonea.org Here, we show ERT profiles measured with 30 electrodes Each profile was collected with a multielectrode instrument The profile length was 58 m, with 2-m electrode spacing As an array, we used the Wenner alpha electrode configuration Apparent resistivity pseudosections are displayed in Figures 12a–12c for each profile Each profile was inverted by using the presented inversion methods with and without electrical anisotropy assumptions Inverted resistivity sections are given in Figures 13a–13c without electrical anisotropy assumption Figures 14a–14c shows the inverted resistivity sections for each profile 160 incorporating electrical anisotropy The vertical shift can be seen when we compare Figures 13a–13c and 14a–14c Figure 15 shows the field before and after excavations At the beginning, we interpreted the resistivity section without considering electrical anisotropy We found some anomalies that were related to structures such as ancient remnants, possible ancient water channels, and some remains of ancient walls However, after excavation, we realized that there was a difference between the interpreted depth based on 2D inversion without electrical anisotropy and the archaeological findings The application of the 2D inversion method with electrical anisotropy gave much better results in this field study We estimated the location of the archaeological wall based on our inversion results The archaeologist found PEKŞEN and YAS / Turkish J Earth Sci 10 20 Depth (m) 1.4m 40 50 ρ =100Ω.m 2.9m h ρϖv=625Ω.m λ =2.5 5.5m 30 1.1m ρ =100Ω.m h ρv =100Ω.m λ =1 ρ =10Ω.m h ρϖv=40Ω.m 2.5m λ =2.0 5.5m Figure A synthetic 2D resistivity model is shown There are rectangular bodies with electrical anisotropy embedded in the model The background of the resistivity model is assumed to be electrically isotropic Isotropic Section Figure 10 (a) Estimated 2D model obtained from 2D inversion; the model is assumed to be electrically isotropic The white rectangular shapes in the inverted resistivity section show the exact location of the bodies (b) Estimated 2D model obtained from 2D inversion assuming that the model is electrically anisotropic The white rectangular shapes in the inverted resistivity section show the exact location of the body (c) The exact model the wall and some other archaeological objects as we expected The depth of the wall was approximately between 0.75 m and 2.00 m Before any excavations, it is vitally important to know the exact location of any buried objects in an archaeological area with respect to the depth Our numerical and field experiences suggest that if electrical anisotropy exists in an area, we should always expect that the depth of layer boundaries or buried objects appearing on an inverted resistivity section is closer to the earth’s surface than what the actual depth is Conclusions In this paper, we have developed and investigated an inversion method in electrically anisotropic media In general, we not expect electrical anisotropy to have an effect on the data acquired in an archaeological area However, the background formation of archaeological objects can be electrically anisotropic due to sedimentation Some small elongated particles prefer a certain direction, which is a well-known fact 161 PEKŞEN and YAS / Turkish J Earth Sci Kỹỗỹkỗekmece Lake Kỹỗỹkỗekmece Lake Profile-2 Profile-3 Profile-1 Figure 11 Location of the study area in Kuỗukỗekmece district in Turkey A circle in the upper left corner of the figure indicates the study area in Turkey The aerial photo in the background shows Kuỗukỗekmece Lake (Image Landsat Data SIO, NOAA, US Navy, NGA, GEBCO © 2016 Başarsoft, Google Earth) Electrical resistivity tomography (ERT) profiles are illustrated on the right of the figure as profiles 1–3 (http://www.google.com/maps) Figure 12 Pseudosections of profile-1 (a), profile-2 (b), and profile-3 (c) 162 PEKŞEN and YAS / Turkish J Earth Sci Figure 13 Estimated 2D model obtained from 2D inversion assuming electrically isotropic earth for profile-1 (a), profile-2 (b), and profile-3 (c) The lines with white in the sections indicate that the areas were excavated by archaeologists Figure 14 Estimated 2D model obtained from 2D inversion assuming electrically anisotropic earth for profile-1 (a), profile-2 (b), and profile-3 (c) The lines with white in the sections indicate that the areas were excavated by archaeologists 163 PEKŞEN and YAS / Turkish J Earth Sci Lake Lake Profile-2 Profile-3 Profile-1 Lake Figure 15 Photo of the ERT profiles in the field before and after excavations next to Kuỗukỗekmece Lake There are some archaeological findings shown: wall and channels (to see more pictures: http://www.bathonea.org) The study shows that the depth estimated based on inversion without electrical anisotropy can be erroneous Thus, interpreting the data with consideration of electrical anisotropy gives much better results regarding the actual depth of archaeological finds When considering geological formation complexity, it is difficult to say that a formation is electrically isotropic To enhance the interpretation of an ERT profile, we may incorporate the electrical anisotropy effect to the data We show that it is possible to estimate the anisotropy coefficient of any cells in the FD mesh The mesh can then be corrected by using the anisotropy coefficient This can be done after inversion is completed After the vertical mesh step size is corrected by dividing all of them by the electrical anisotropy coefficient, we finally get the corrected mesh From our synthetic examples and field 164 study, we can say that the proposed method can be useful for interpretation of ERT data This approximation was successfully tested on synthetic and field datasets The method we used here requires running inversion codes twice If one finds the two sets of results to be very close to each other, one may say that the medium is isotropic Otherwise, the medium may be electrically anisotropic Acknowledgments We thank Prof Dr Şerif Barış for permission to use his data We also thank Associate Prof Dr Şengül G Aydıngün for providing us with archeological information from Bathonea and for her support during the fieldwork We appreciate the efforts of Demirkan Baylar, Erdinỗ Duman, and Güngör Doğan during the fieldwork PEKŞEN and YAS / Turkish J Earth Sci References Aster RC, Borchers B, Thurber CH (2005) Parameter Estimation and Inverse Problems London, UK: Elsevier Kunz KZ, Moran JH (1958) Some effects of formation anisotropy on resistivity measurements in boreholes Geophysics 4: 770-774 Aydıngün ŞG (2007) A New Prehistoric Settlement near Kuỗukỗekmece Lake in stanbul: Avcılar-Firuzköy İstanbul, Turkey: T.C Kültür ve Turizm Bakanlığı Available online at http://www.bathonea.org/yayin/Main-Frames/yay003.pdf LaBrecque DJ, Heath G, Sharpe R, Versteeg R (2004) Autonomous monitoring of fluid movement using 3-D electrical resistivity tomography J Environ Eng Geoph 9: 167-176 Busby JP (2000) The effectiveness of azimuthal apparent resistivity measurements as a method for determining fracture strike orientations Geophys Prospect 48: 677-698 Candansayar ME (2008) Two-dimensional individual and joint inversion of three- and four-electrode array dc resistivity data J Geophys Eng 5: 290-300 Carmichael RS (1989) Practical Handbook of Physical Properties of Rocks and Minerals Boston, MA, USA: CRC Press DeGroot-Hedlin C, Constable SC (1990) Occam’s inversion to generate smooth, two-dimensional models from magnetotelluric data Geophysics 55: 1613-1624 Dey A, Morrison HF (1979) Resistivity modeling for arbitrarily shaped two-dimensional structures Geophys Prospect 27: 106-136 Edwards LS (1977) A modified pseudosection for resistivity and induced-polarization Geophysics 42: 1020-1036 Golub GH, Van Loan CF (1996) Matrix Computations Baltimore, MD, USA: Johns Hopkins University Press Grant FS, West GF (1965) Interpretation Theory in Applied Geophysics New York, NY, USA: McGraw-Hill Greenhalgh SA, Marescot L, Zhou B, Greenhalgh M, Wiese T (2009a) Electric potential and Frechet derivatives for a uniform anisotropic medium with a tilted axis of symmetry Pure Appl Geophys 166: 673-699 Greenhalgh SA, Zhou B, Greenhalgh M, Marescot L, Wiese T (2009b) Explicit expressions for the Frechet derivates in 3D anisotropic resistivity inversion Geophysics 74: 31-43 Greenhalgh S, Wiese T, Marescot L (2010) Comparison of DC sensitivity patterns for anisotropic and isotropic media J Appl Geophys 70: 103-112 Habberjam GM (1972) The effects of anisotropy on square array resistivity measurements Geophys Prospect 20: 249-266 Loke MN (2016) Tutorial: 2-D and 3-D Electrical Imaging Surveys Gelugor, Malaysia: Geotomo Software Available online at http://www.geotomosoft.com/downloads.php Maillet R (1947) The fundamental equations of electrical prospecting Geophysics 12: 529-556 Marescot L, Rigobert S, Lopes SP, Lagabrielle R, Chapellier D (2006) A general approach for DC apparent resistivity evaluation on arbitrarily shaped 3D structures J Appl Geophys 60: 55-67 Marti A (2014) The role of electrical anisotropy in magnetotelluric responses: from modelling and dimensionality analysis to inversion and interpretation Surv Geophys 35: 179-218 Matias MJS (2002) Square array anisotropy measurements and resistivity sounding interpretation J Appl Geophys 49: 185194 Meju MA (1994) Geophysical Data Analysis: Understanding Inverse Problem Theory and Practice Tulsa, OK, USA: SEG Menke W (1989) Geophysical Data Analysis: Discrete Inverse Theory San Diego, CA, USA: Academic Press Negi JG, Saraf PD (1989) Anisotropy in Geoelectromagnetism New York, NY, USA: Elsevier Nguyen F, Garambois S, Chardon D, Hermite D, Bellier O, Jongmans D (2007) Subsurface electrical imaging of anisotropic formations affected by a slow active reverse fault, Provence, France J Appl Geophys 62: 338-353 Pain CC, Herwanger JV, Saunders JH, Worthington MH, de Oliveira CRE (2003) Anisotropic resistivity inversion Inverse Probl 19: 1081-1111 Pekşen E, Yas T, Kıyak A (2014) 1-D DC resistivity modeling and interpretation in anisotropic media using particle swarm optimization Pure Appl Geophys 171: 2371-2389 Pidlisecky A, Knight R (2008) FW2_5D: A MATLAB 2.5-D electrical resistivity modeling code Comput Geosci 34: 1645-1654 Habberjam GM (1975) Apparent resistivity, anisotropy and strike measurements Geophys Prospect 23: 211-247 Rey E, Jongmans D (2007) A 2D numerical study of the effect of particle shape and orientation on resistivity in shallow formations Geophysics 72: F9-F17 Haber E, Oldenburg D (2000) A GCV based method for non-linear ill-posed problems Computat Geosci 4: 41-63 Sasaki Y (1989) Two-dimensional joint inversion of magnetotelluric and dipole–dipole resistivity data Geophysics 54: 254-262 Herwanger JV, Pain CC, Binley A, de Oliveira CRE, Worthington MH (2004) Anisotropic resistivity tomography Geophys J Int 158: 409-425 Tikhonov AN, Arsenin VY (1977) Solutions of Ill-Posed Problems New York, NY, USA: Halsted Press Kim JH, Yi MJ, Cho SJ, Son, JS, Song WK, (2006) Anisotropic crosshole resistivity tomography for ground safety analysis of a high-storied building over an abandoned mine J Environ Eng Geoph 11: 225-235 Tripp AC, Hohmann GW, Swift CM (1984) Two-dimensional resistivity inversion Geophysics 49: 1708-1717 Wiese T, Greenhalgh SA, Marescot L (2009) DC sensitivity for surface patterns for tilted transversely isotropic media Near Surf Geophys 7: 125-139 165 PEKŞEN and YAS / Turkish J Earth Sci Wiese T, Greenhalgh S, Zhou B, Greenhalgh M, Marescot L (2015) Resistivity inversion in 2-D anisotropic media: numerical experiments Geophys J Int 201: 247-266 Xu S, Duan B, Zhang D (2000) Selection of the wave numbers k using an optimization method for the inverse Fourier transform in 2.5D electrical modeling Geophys Prospect 48: 789-796 Yan B, Li Y, Liu Y (2016) Adaptive finite element modeling of direct current resistivity in 2-D generally anisotropic structures J Appl Geophys 130: 169-176 166 Yeboah-Forson A, Comas X, Whitman D (2014) Integration of electrical resistivity imaging and ground penetrating radar to investigate solution features in the Biscayne Aquifer J Hydrol 515: 129-138 Yeboah-Forson A, Whitman D (2014) Electrical resistivity characterization of anisotropy in the Biscayne Aquifer Groundwater 52: 728-736 Zhdanov MS (2002) Geophysical Inverse Theory and Regularization Problems New York: Elsevier ... studied the inversion method within electrically anisotropic media for different types of applications LaBrecque et al (2004) studied 3D resistivity inversion using surface DC data In our inversion. .. geometric mean of the apparent resistivities of a profile Since the initial value is a single apparent resistivity value, it is a sort of homogeneous model for isotropic cases As for the anisotropic... since the number of parameters is smaller than the number of observations In spite of these difficulties, we have found the subsurface resistivity distribution of the isotropic and anisotropic models