NEAR-SURFACE STUDIES USING GEOELECTRICAL AND EM TECHNIQUES IN NORTHERN INDIA

Abstract

Geophysics is the branch of Physics, which deals with the study of physical properties of earth interior by measuring related physical field on or near the earth surface. The commonly used physical fields are seismic wave, gravity, magnetic, thermal, electrical and electromagnetic (EM) etc. Characteristics of these fields are governed by the properties of the sources they are generated as well as the properties of the medium they permeate. It is the property of the medium, which geophysicists utilize to study the earth interior. Near-surface geophysics is an emerging area of geophysics, which is used to solve wide variety of problems related to the near-surface usually referred to the depth less than 200 m [14,69,79,147,158]. Electrical and EM techniques have been used for solving various problems related to the near-surface investigations: archaeological investigations, civil engineering purposes for geotechnical characterization of foundation soil [28,40]. These techniques have extensively been used for aquifer characterization, study of groundwater contamination due to waste disposal and geothermal investigations [33,34,69,138,144,180,182,206,231]. The ambiguities in resistivity interpretation were discussed by Kumar et al. [112]. Integrated resistivity survey was used for fracture delineation for groundwater exploration [49,230] and for estimating hydraulic parameters [180,229]. Electromagnetic methods were used for shallow basin studies [1], groundwater prospecting and hydrogeological exploration [66]. The study on theoretical modeling of the use of the earth's thermal energy as a source of heat was carried out by Ghoshal et al. [62]. In the present thesis, integrated electrical and electromagnetic measurements have been used to study the electrical resistivity of subsurface material and its characterization. The investigations are limited to shallow depth (< 200 m) and therefore the entire work is presented under the title, "Near-surface studies using geoelectrical and EM techniques in northern India". Vertical Electrical Soundings (VES), Electrical Resistivity Tomography (ERT), and the two electromagnetic (transient electromagnetic (TEM) and radiomagnetotelluric (RMT)) measurements were conducted at three different sites in north Indian region. Geologically these sites are characterized by different soil and rock formations. Systematic measurements were carried out by using the state of art equipments: multi-electrode imaging systems for ERT, iii GDP32 Zonge NanoTEM for TEM [240] and RMT device developed at University of Cologne, Germany [207]. The processing and inversion of recorded ERT data were performed using Res2dinv [121] and DC2DInvRes [76] codes. TEM data were analyzed using 1D inversion code: EMUPLUS, developed at University of Cologne, Germany [175] and RMT data were analyzed using the 2D inversion code developed by Rodi and Mackie [164]. A detailed analysis of recorded data has been carried out using singular value decomposition (SVD) and forward and inverse modeling. Resolution analysis of MT data and forward and inverse modeling of RMT data were presented by Kalscheuer and Pedersen [104], and Kalscheuer et al. [105]. Noise characterization and correction in EM data were studied by Junge [101]. Geotechnical characterization of soil was carried out by correlating the results of resistivity measurements with the SPT (Standard Penetration Test) and DCPT (Dynamic Cone Penetration Test) parameters (N-values). A new empirical relationship is presented between the transverse resistance of soil column and N-values. The relationship is analytical and site specific. It has a potential for its application in various civil engineering purposes for soil characterization using surface resistivity methods. Another problem attempted is related to the study of aquifer characterization and groundwater contamination due to waste disposal practice in and around Roorkee area. The effect of untreated waste disposal practice on groundwater has been investigated using ERT, TEM and RMT data recorded in and around waste disposal sites near Roorkee. Third site was located in central Himalayan region in district Chamoli, Uttarakhand, India. The site is dotted with both hot-water and cold-water springs. For this study TEM and ERT measurements were carried out in and around the hot-water spring zones. It has been observed that TEM recorded from Himalayan foothills (around Roorkee) and from central Himalayan regions show different characteristic features. The data recorded from central Himalayan region, shows a sign reversal in the intermediate time range (z 10 [is), whereas no sign reversal is observed in the data from Roorkee region. To interpret the sign reversal in TEM data an extensive 2D/3D modeling experiment was performed using the SLDM (Spectral Lanczos Decomposition Method) code [47,48,83,85]. Subsurface electrically conducting zones were correlated with geothermal effects and temperature distribution [2]. Geothermal structures were delineated in Laddakh Himalaya using MT method by Harinarayana [81]. The qualitative iv study on the changes in spring water flow in Chamoli, Uttarakhand area was carried out by Sarkar and Chander [170]. The measurements used in the present work were carried out during my stay at IIT Roorkee, India, whereas most of the processing and interpretation were performed during my stay at the University of Cologne, Germany under DAAD Sandwich Program. The entire work is presented systematically in the form of following seven Chapters. Chapter — 1 is an introduction, which describes the background of exploration geophysics and various geophysical methods especially used for near-surface investigations. Application of electrical and electromagnetic methods for shallow investigations has been discussed briefly. A brief literature review of the studies on groundwater contamination, aquifer characterization, soil characterization, geothermal investigations is presented. Subsequently, the plan of the thesis is given in the end of this chapter. Chapter — 2 deals with the methodology and details of experimental techniques used for the measurements. The field procedures, configuration used for the data acquisition in ERT, TEM and RMT methods have been discussed. A comprehensive outline of data processing and interpretation procedure followed for these methods have also been included. The theory of the individual and joint inversion of DC and TEM data is also discussed. Chapter — 3 describes the study of groundwater contamination and aquifer characterization in and around Roorkee, Uttarakhand, India. ERT and TEM soundings data were recorded and analyzed in and around the two untreated sewage-disposal sites: Saliyar and Khanjarpur. The data were recorded in the vicinity of the waste-disposal sites and at a far remote site, and were interpreted in terms of amount and areal extent of contaminants. Lithology data were used for correlating the resistivity values, which were derived from the 2D inversion of resistivity and 1D Occam inversion of TEM data, with the subsurface formation. These two methods were used jointly to increase the reliability of the results. It was thus observed that the sewage pollutants, infiltrated through the soil, reach the shallow unconfined aquifer, in and around the polluted area up to a limited distance from the waste disposal sites. The present study suggests that the groundwater present in the shallow unconfined aquifer (< 10 m depth), close to waste disposal site is contaminated. It was observed that the decrease in the resistivity of shallow unconfined aquifer is up to 50% in comparison with that of the shallow unconfined aquifer at remote site (uncontaminated) located about 20 km northwest of the disposal sites. The extent of the polluted aquifer zone was delineated by comparing its resistivity with that of the same aquifer at a point away from the disposal sites. Thus, the extent of groundwater pollution and relative direction of its flow was also determined qualitatively [201]. To explore the use of Radiomagnetotelluric (RMT) method for the study of groundwater contamination, integrated surveys of ERT, TEM and RMT were carried out on two sites (Khanjarpur: contaminated site and Sherpur: reference site). Finally ERT, TEM and RMT results were integrated to present a comprehensive conductivity structure of the area. ERT method has also been used for aquifer characterization and to define the geometry of aquifer system in Pathri-Rao watershed situated in the Piedmont zone of Himalayan foothill region, Uttarakhand, India [98]. For this purpose, 9 ERT profiles were recorded in the area to define horizontal and vertical geometry of aquifer system and to infer the local groundwater flow condition. On the basis of resistivity values it has been found that shallow and deeper aquifers have different degree of interaction in the area. In Chapter — 4, the study has been further extended to the joint inversion of DC and TEM data to validate the results obtained by individual inversion. Since the DC data are sensitive to resistive layers and insensitive to conductive layers. On the contrary, TEM data is sensitive to conductive layers and insensitive to resistive ones. Therefore, the joint inversion of both data sets is likely to improve the reliability of the resistive and/or conductive structures delineated by individual inversion. Synthetic model studies were carried out for H, Q, K and A types layered resistivity distribution of the surface. The inversion results indicate that the model parameter derived from the joint inversion of DC and TEM data are able to resolve both structures with better resolution. After validating the code on synthetic data, the joint inversion of DC and TEM data was tried on field data collected from the contaminated area. It has been observed that the joint inversion effectively resolved the model parameters and in general the models are consistent with the results presented in Chapter-3. ERT and TEM data recorded in and around the geothermal area in the central Himalayan region, India have been analyzed in Chapter — 5. The study area is dotted with hot-water springs with the water temperature of 45° - 55°C at the surface and is located around Helang on either side of Alaknanda River. To assess the geothermal potential and its lateral and vertical geometrical configuration in and around the hot-water springs in the study area, 7 ERT profiles and TEM data at 7 locations were recorded and analyzed. 2D inversion of ERT data vi indicates a low resistivity (< 50 Clm) zone in the vicinity of hot springs appears to be associated with the underground water-channel through the fractured rock. Bedrock resistivity is very high (> 1000 em). The resistivity of weathered near-surface soil at a far distant location from hot spring is also low (< 100 em). A common feature in all TEM data in this area is the sign reversal, observed at nearly intermediate time stages ( 10 ps). Consistent sign reversal in all TEM data at intermediate time stages supports the existence of multi-dimensionality of geoelectrical structure. Therefore, 2D/3D TEM responses were computed using SLDM code and were compared with the field data. The SLDM code is based on finite difference method and enables us to reproduce the observed induced voltage of the TEM-technique as a function of time. Based on the analysis of ERT and TEM data set, it was concluded that the geothermal anomalies associated with the hot spring in the central Himalayan region is local feature and appeared in the zone represented by low resistivity (< 50 flm) at shallow depth (< 100 m) in the vicinity of hot spring region. Application of electrical and electromagnetic method for geotechnical investigations is discussed in Chapter — 6. The results of ERT were correlated with SPT and DCPT data at two sites in Uttar Pradesh (UP), India. Two ERT profiles, each measuring 355 m long, were recorded using 72 electrodes deployed at 5 m spacing. SPT and DCPT data were obtained from the same locations at 28 points. Electrical characterization of subsurface soil was done using borehole data and grain size analysis of the soil samples collected from boreholes. The concept of electrical resistivity variation with soil strength, related to the grain size distribution, cementation, porosity and saturation has been used to correlate the transverse resistance of soil column with the number of blow counts (N-values) obtained from SPT and DCPT data. An empirical linear relationship has been obtained between the transverse resistance of soil column and number of blow counts (N-values) at these sites [200]. The linear relationships are site-specific and the coefficients of linear relation are sensitive to the lithology of subsurface formation, which was verified by borehole data. The study demonstrates the usefulness of the ERT method in geotechnical investigations, which is economic, efficient and less time consuming in comparison to the other geotechnical methods, such as SPT and DCPT, used for the purpose. Chapter — 7 summarizes the entire work presented in the thesis. Conclusions, limitations and suggestions for future work have also been given. vii