An Assessment of Electrical Resistivity Soundings Data by Different Interpretation Techniques

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1 International Journal of Biological, Ecological and Environmental Sciences (IJBEES) Vol. 1, No. 3, 212 ISSN An Assessment of Electrical Resistivity Soundings Data by Different Interpretation Techniques Aditya Kumar Bhoi Abstract The Vertical Electrical Sounding (VES) is the most widely used geophysical technique for subsurface exploration. It can be interpreted by different technique to get subsurface profile, their layer thickness and true resistivity. However, confusion occurs during selection of suitable method for interpretation, as the accuracy of result largely depends on interpretation technique. Twelve VES data were collected from different sites and interpreted by different technique for finding the depth of soil formations. The interpreted results were compared with the actual subsurface profile. The comparative study shows that the Barnes layer method is most suitable among all these method. Keywords Electrical Resistivity, Geophysics, Interpretation, Negative Slope, T I. INTRODUCTION HERE are various approaches available to gather information about the subsurface, and the best is undoubtedly the direct observation of earth materials. However, this approach is of course rarely possible to the extent that people would like. A much more common scenario is the need to acquire physical measurements on the surface and complement these measurements with whatever direct geological observations are available to deduce the subsurface geology. The value of geophysics is therefore its ability to acquire information about the subsurface over a substantial area in a reasonable time frame and in a cost effective manner. There are a number of different geophysical in-situ tests that can be used for stratigraphic information and in the determination of engineering properties. Such as Seismic Methods, Electrical Methods, Gravity and Magnetic methods, near sub surface nuclear method and Borehole methods. Electrical resistivity studies in geophysics may be understood in the context of current flow through a subsurface medium consisting of layers of materials with different individual resistivity. For simplicity, all layers are assumed to be horizontal. The resistivity of a material is a measure of how well the material retards the flow of electrical current. Hence different materials possess different range of resistivity. Due to this characteristic of material, electrical resistivity method is widely used for groundwater investigation, ground water contamination survey, subsurface cavity, karst and fault, archaeological survey, characterization of fibres distribution in a steel fibre reinforced concrete, to study delineation of seepage zones, to find out soil layer and their thickness and so forth. The resistivity data are collected and interpreted by any of the following interpretation technique. Such as Curve matching, Barnes layer [4], Direct slope [1], Moore s cumulative with Hummel s extension [11], Inverse slope method [1] and some software which are basically based on above interpretation technique. But confusion arises in selection of suitable technique for interpretation. Each and every interpretation technique having some merits and demerits; hence no single method can give satisfactory result in all subsurface conditions. A comparison between different interpretation techniques [6] showed that the Moore s cumulative with Hummel s extension is more reliable than other techniques. Baig (198) reported that Moore s cumulative with Hummel s extension yields resistivity value with negative sign for certain condition, which cannot be interpreted. II. DATA ACQUISITION Many specific field techniques have been used in DC resistivity survey, but only three of these are suitable for all condition, namely Wenner, Schlumberger and Polar-dipole [7]. Wenner array is most suitable for collection of field data, as data acquired from it can be interpreted by any interpretation technique [12]. Wenner array has been deployed for collection of resistivity data. The resistivity data were collected from Ballukraya and Sakthivadivel (1984) and presented in table I. Aditya Kumar Bhoi, formerly Master of Technology student, Department of Civil Engineering, Indian Institute of Technology Delhi, New Delhi , India (phone: ; aditya_ce@student.iitd.ac.in). 18

2 International Journal of Biological, Ecological and Environmental Sciences (IJBEES) Vol. 1, No. 3, 212 ISSN TABLE I ELECTRICAL RESISTIVITY SOUNDING DATA OF VARIOUS SITES Electrode Resistance (R), ohms Spacing (a),m Site A Site B Site C Site D Site E Electrode Spacing x Apparent resistivity (ohm-m-m) Fig.1 Interpretation of data by Direct slope method for site A Electrode Spacing /Apparent resistivity (mho ) Fig. 4.1 Subsurface Profile for Site A Fig. 4.2 Subsurface Profile for Site B Fig. 4.3 Subsurface Profile for Site C Fig.2 Interpretation of data by Inverse slope method for site B Cumulative resistivity (ohm-m) Fig.3 Interpretation of data by Moore cumulative method for site C Fig. 4.4 Subsurface Profile for Site D Fig. 4.5 Subsurface Profile for Site E 19

3 International Journal of Biological, Ecological and Environmental Sciences (IJBEES) Vol. 1, No. 3, 212 ISSN III. RESULT AND DISCUSSION A. General Interpretation of the results obtained in the field is somewhat difficult, because the earth resistivity variation is great and complex. Analysis is made from the calculated apparent electrical resistivity. The resistivity data were interpreted by Barnes layer, Direct slope, Inverse slope method, Moore s cumulative with Hummel s extension, SSR MP AT software and IX1D v.3 software. The results have been presented as table II and fig.1, 2,3, and 4. B. Comparison between Manual interpretation and software interpretation The plot showing variation of subsurface profile with depth has been presented as fig. 4. In this following Legend and symbol have been used. Soil Decomposed rock Weathered rock Hard rock Infinite resistivity layerer I- Interpreted subsurface profile by Barnes layer method II- Interpreted subsurface profile by Moore's cumulative with Hummel's extension III- Interpreted subsurface profile by direct slope method IV- Interpreted subsurface profile by inverse slope method V- Interpreted subsurface profile by SSR MP AT software (auto joining) VI- Interpreted subsurface profile by SSR MP AT software (manual joining) VII- Interpreted subsurface profile by IX1D software VIII-subsurface profile obtained from bore hole Site A B C D E It has been observed from the plot that Barnes layer method and IX1D v.3 software are giving nearly same result in terms of number of layer; layer thickness and layer (true) resistivity (see fig.4). It can be concluded that one can use IX1D v.3 software for rapid calculation in place of manual calculation by Barnes layer method, as it is based on Barnes layer method. The result obtained from SSR MP AT software is showing close resemblance with Inverse slope method. Site A, C and D are showing similar subsurface profile (see fig.4) and similar value of h/ρ (See Table III). For site E, subsurface profile made by inverse slope method and SSR MP AT software (see fig. 4) showing different layer but similar value of h/ρ (see Table III). It is clearly evident that for site A, C, D, and E Hummel s equation is giving same result for Inverse slope method and SSR MP AT software; hence one can use SSR MP AT software for rapid inversion of resistivity data. C. Comparison and correlation between different interpretation techniques In order to arrive suitable result one need to compare and correlate similar and dissimilar properties of two or more object. The comparison and correlation of different techniques are discussed in following paragraph with the help of certain properties, results and formulae. 1) Sequence, thickness and number of strata From fig.4, it is clear that interpreted subsurface profile may not match with actual ground truth. The number of layer, their thickness and true resistivity may vary tremendously. For site A interpreted profile by Barnes Layer method, SSR MP AT auto joining and IX1D software shows close similarity with actual ground truth, except the hard rock layer (see fig. 4). TABLE II INTERPRETED VALUES OF THICKNESS(t), AND TRUE RESISTIVITY(ρ) BY VARIOUS TECHNIQUES AND SOFTWARE Barnes Moore s cumulative Direct Inverse SSR MP AT Software IX1D Remark Layer With Hummel s Extension Slope Slope Auto Manual Software joining joining t ρ t ρ t ρ t ρ t ρ t ρ t ρ (m) (Ω-m) (m) (Ω-m) (m) (Ω-m) (m) (Ω-m) (m) (Ω-m) (m) (Ω-m) (m) (Ω-m) Q infinite Type curve 5 infinite A Type infinite curve 3 19 Negative A Type resistivity curve Negative H 9 27 resistivity Type curve H Type curve 11

4 International Journal of Biological, Ecological and Environmental Sciences (IJBEES) Vol. 1, No. 3, 212 ISSN TABLE III COMPARATIVE TABLE OF HUMMEL S EQUATION LAYER THICKNESS Σ (mho) TRUE RESISTIVITY Site Site Site Site Site A B C D E Barnes layer method Moore's cumulative with Hummel's extension Direct slope method Inverse slope method SSR MP AT software (auto joining) SSR MP AT software (manual joining) IX1D software Average of all method In case of site C result of all method except Moore s cumulative with Hummel s extension show similarity with actual subsurface profile (see fig. 4). The interpreted result by Direct Slope method is quite similar to actual subsurface profile for site D, here actual subsurface profile having two layers (see fig. 4.2).. From above comparisons, it is clear that sequences of strata are maintained, but thicknesses of strata are varying some times. Hence author conclude that Barnes layer method (both manual calculation and IX1D software) gives nearly same subsurface profile as actual ground truth for site A, and C, followed by inverse Slope method (including both manual calculation and SSR MP AT software), which gives similar result for site C. Direct slope method is suitable, when actual ground having less number of strata as in site D(see fig.4). From fig.4, it has been observed from the plots Barnes Layer method (both manual and IX1D software) and Inverse slope method (both manual and SSR MP AT Software) give nearly same result in terms of number of layer and true resistivity value. Because both methods reflect Hummel s equation as in built one (refer appendix 1). The resistivity of each layer increase of decrease as it is approached to non-conducting or conducting layer respectively. In Moore s cumulative with Hummel s extension method resistivity are arranged by integration, hence resistivity of each layer eliminated. That means it lead to equivalence principle. Due to which a sandwiched layer (with lower resistivity or h/ρ equal to other layer) will not influence the cumulative resistivity line. Hence the layer may not be recognized. Same thing is happen if the thickness of that layer is small. Hence it is giving less number of layers. 2) Maximum and minimum resistivity value It can be noticed from the Table II, that Moore s cumulative with Hummel s extension method (for site E) and Direct slope method (for site D) are giving less number of layer compare to other method and also low resistivity value. The maximum true resistivity values is 247 ohm-m corresponds to site D, when interpreted by Direct slope method and 3 ohm-m correspond to site E, when interpreted by Moore s cumulative with Hummel s extension method. This is too low compare to true resistivity value interpreted by other techniques. The variation of layer thickness and corresponding true resistivity of same site by different technique based on Hummel s equation are presented as Table III. From this table, it can be noticed that minimum values of h/ρ for sites A, B, C, D, and E are 1.12,.19,.16,.73, and.49 mho correspondingly. The above minimum values are coming from Barnes layer method (for site A, and E, it is from IX1D software, which is based on Barnes layer method). From the Table III, it is clearly evident that Direct Slope method gives maximum value of h/ρ for site C (.32 mho), and D (.296 mho) followed by Moore s cumulative with Hummel s extension method for site B (.26 mho), and E (1.7 mho). The maximum value of h/ρ for site A is 4.64 (SSR MP AT auto joining). Barnes layer method gives minimum values of h/ρ for all sites; simultaneously Direct Slope method and Moore s cumulative with Hummel s extension method gives maximum value of h/ρ for maximum sites. That means for same set of electrical resistivity data Barnes layer method gives higher true resistivity value and Moore s cumulative with Hummel s extension method gives lower true resistivity value. 3) Negative resistivity value and negative slope Moore cumulative with Hummel s extension method sometimes gives negative true resistivity value, as ohmm for second layer of site C (see fig.3), and ohm-m for third layer of site D(see table II). This does not have any physical significance in geological situation. If any layer give negative resistivity value, then one cannot measure resistivity of next layer; as resistivity of each and every layer are correlated to each other by slope of cumulative resistivity and Hummel s equation directly. Direct slope (see fig. 1) and Inverse slope method (see fig.2) sometimes give negative slope; which mean negative resistivity. But it has been suggested to be infinitely high resistivity layer by Ramanuja Chary (26). Here one can ignore this layer and can calculate resistivity of next layer as negative slope does not influence true resistivity of successive layer. Negative resistivity and negative slope are generated, because above method unable to interpret such resistivity data. This limitation are method specific, as one can see (refer fig.4, and table II) other method are giving positive result for these sites. It is clearly evident from fig. 4, and Table II that, Barnes Layer method (both manual and IX1D software) and Inverse slope method (both manual and SSR MP AT Software) give nearly same result in terms of number of layer and resistivity. IV. CONCLUSION From the study of assessment of electrical resistivity data following conclusions can be made. Moore s cumulative with Hummel s extension method gives less number of layers and sometimes it also give negative resistivity value. The IX1D v.3 software and Barnes layer method gives 111

5 International Journal of Biological, Ecological and Environmental Sciences (IJBEES) Vol. 1, No. 3, 212 ISSN similar result in terms of number of layer, layer thickness and layer resistivity. Similar trend also shown by SSR MP AT software and Inverse Slope method. Direct Slope method and Inverse Slope method gives negative slope for certain case. Direct Slope method and Moore s cumulative with Hummel s extension method gives low resistivity value and Barnes layer method gives maximum resistivity value. Barnes Layer method and Inverse slope method give nearly same result in terms of number of layer, layer thickness and true resistivity value. Barnes layer method followed by Inverse Slope method gives nearly same subsurface profile as actual ground truth. From the above points it can be conclude that Barnes layer method is most suitable for inverting resistivity data, followed by Inverse Slope method. APPENDIX I Similarities between Barnes layer method and Inverse Slope method In case of Barnes layer methods following equation are used. 1 1 (1) Rl Rn Rn 1 And ρ l = (2) 2π.al 1 Rl = 2π. al. Rl Hummel s equatio hm = h1 ρm ρ1 +h2... (3) ρ2 Equation 1 and 2 are not other than Hummel s equation 3. hm ρm = h1 ρ1 +h2 ρ2 => hn = hn 1 ρn ρn 1 +al ρl => al = hn - hn 1 ρl ρn ρn 1 hn = - hn 1 2π.hn.Rn 2π.hn 1.Rn 1 ( 1 1 ) 2π Rn Rn π Rl => ρl = 2π. al. Rl In case of Inverse slope method (see fig.a.1), it is clear that (a/ρa)1+(a/ρa)2...= (a/ρa)m h1 => ρ1 +h2... = hm ρ2 ρm ACKNOWLEDGMENT The support and cooperation of friends is gratefully acknowledged. REFERENCES [1] M. Y. A. Baig, Direct slope technique of determining absolute resistivity. Journal of civil engg. Division, Institution of Engineers, 61, [2] P. N. Ballukraya, and R. Sakthivadivel, Analysis and Interpretation of electrical resistivity data from hard rock areas for groundwater exploration. Research report, Centre for Water Resources, Anna University of Technology, Madras, and India [3] H. E. Barnes. Soil investigations employing a new method of layer value determination for earth resistivity investigation. Highway Research Board Bull., 65, [4] H. E. Barnes. Electrical subsurface exploration simplified. Roads and Streets, 97, [5] J. N. Hummel, A theoretical study of apparent resistivity in surface potential methods. Trans. A.I.M.E., Geophys. Prosp., 97, [6] J.M. Kate, and Kichchu Mal Comparative study of resistivity interpretation technique Proc., IGC- 83 (1), VII [7] G.V. Keller, Engineering application of electrical geophysical methods. Proc. ASCE Specialty conf. on subsurface exploration for underground excavation and heavy construction, Henikar,UK [8] W. Moore, An Empirical Method of Interpretation of Earth Resistivity Measurement. AIMME Tech. Publ., [9] K.R. Ramanujachary, Workshop/orientation program on electrical resistivity techniques. IGIS, Hyderabad. 26 [1] P. V. Sankarnarayan, and K.R. Ramanujachary, Short note an inverse slope method of determining absolute resistivities. Geophysics, 32(6), [11] V. V.J. Sarma. An extension to Moore s method of interpretation of earth resistivity measurements. Transactions, Society of Mining Engineers, [12] A. Singh,. Soil Engineering in Theory and Practice, Asia Publishing House. Delhi, Aditya Kumar Bhoi was born in Orissa, India, on May, He received the B.tech. degree in Civil Engineering from IGIT, Satang, Orissa, India and M.Tech.degree in Geotechnical and Geoenvironmental Engineering from IIT Delhi, India in 28 and 21, respectively. His recent emphases are geophysical method, green energy, soil properties. Fig. A.1 Interpretation by Inverse slope method 112