E. Chandrasekhar

Wavelet Analysis of Geophysical Well-log Data of Bombay Offshore Basin, India

Geophysical well-log (bore-hole) data facilitate understanding of the physical properties of the subsurface formations as a function of depth measured in a well. In the present study, the wavelet transformation technique was applied to the well-log data of three wells in the Bombay High oil field, India, in order to identify depths to the tops of oil and/or gas formation zones (pay zones). Continuous wavelet transformation (CWT) was performed on gamma-ray, resistivity, neutron porosity and velocity log data sets in order to determine the space-localization of the oil and/or gas formation zones. The choice of a mother wavelet is important and largely depends on the data under investigation. We have applied a variety of wavelets to the different log data sets to not only identify the depths to the tops of formation zones, but also to determine the optimum wavelet that best characterizes the pay zones. On examination of scalogram plots of each log corresponding to each wavelet for their better resolution in identifying the formation boundaries, we have found that the scalograms corresponding to the Gaus1 wavelet appeared to give the best resolution in identifying the depths of pay zones in all the well-log data sets of all three wells. To further validate the above observation, a histogram analysis of CWT coefficients is made. This showed that, of all the wavelets considered for the present study, Gaus1 wavelet is the most appropriate and optimum for determining the space-localization of pay zones in all the well-log data sets considered in the present study. The depths of pay zones estimated from scalogram plots of logs agree well with those provided by the Oil and Natural Gas Corporation Ltd., India.

Regional Electromagnetic Induction Studies Using Long Period Geomagnetic Variations

Electromagnetic (EM) induction in the Earth by time-varying geomagnetic field variations at different frequencies facilitates to look into different layers of the Earth from surface up to upper mantle depths and beyond using electrical conductivity as a diagnostic parameter. Because of their ability to penetrate to greater depths, long period geomagnetic variations of periods of 1 day and above act as unique tools to probe the Earth’s interior up to upper/lower mantle depths in the depth range of 200–1500 km. Thus, to provide a deeper insight in to the electrical behaviour of the upper mantle beneath Indian region, geomagnetic Sq, stormtime variations and 27-day variation and its harmonics data have been analyzed to obtain an integrated conductivity-depth profile up to upper mantle depths. The data were recorded at a dense network of 13 observatories situated along the narrow 150° geomagnetic longitude band extending from dip equator at the southern tip of India to the northern parts of Russia. Results of the regional deep mantle conductivity interpreted in the light of mantle mineral phase changes and compared with other geophysical parameters are discussed.

Use of Wavelet Transformation for Geophysical Well-Log Data Analysis

Geophysical well-log (bore-hole) data represent the rock physical properties as a function of depth measured in a well. They aid in demarcating the subsurface horizons, identifying abrupt changes in physical properties of rocks and locating cyclicity (if any) in stratal succession. Rocks document their depositional history, which geoscientists unfold by analyzing different geophysical data, such as well-log data, seismic data, etc. We have applied continuous wavelet transformation technique on one type of well-log data (gamma ray log) from three wells located in Bombay High oil field, India. Our analyses have helped to identify the discontinuities, detect the horizons and check the presence of cyclic pattern (if any) in deposition. Among various wavelets that we have used, the Morlet wavelet has proved to be most appropriate to resolve the marker horizons in gamma ray log data. We also conclude that the choice of wavelet is dependent upon the rock intrinsic properties that vary greatly in the space domain (with varying depth).

Geo-electrical structure of the mantle beneath the Indian region derived from the 27-day variation and its harmonics

Estimates of the C-response function were determined by Z: H method to obtain conductivity and depth values for a substitute perfect conductor (conductosphere) beneath the Indian region, utilizing geomagnetic variations at periods of 27-day and its harmonics. Two and half years of continuous geomagnetic data were utilized. These data were recorded during 1975–77, at a chain of 13 stations confined to the 150° geomagnetic longitude band, which extended from the dip-equator at the southern tip of India, to the northern parts of Russia. Complex demodulation technique was employed to determine the electromagnetic (EM) responses. Taking advantage of the dense latitudinal distribution of the observatories, the demodulates of all the stations were tested statistically to check the validity of the P10 approximation for the inducing field. Single-station response estimates, for a 27-day period, computed by a robust method have shown that reliable EM responses (consistent with P10 source dependence and with local 1-D Earth structure) could be obtained for only 6 stations, all situated in the mid-latitude region. The depth estimates at all 6 stations are consistent, including Sabhawala (SAB) which is situated close to the Himalayan collision zone. The negligible differences in the depth estimates of these mid-latitude stations do not show any latitudinal dependence, as against such an observation reported for the European and the North American regions. The mean depth of the conductosphere is found to be 1200 (±200) km, with an average conductivity of 0.7 (±0.3) S/m. Comparison of the mean geo-electrical structure with those of other regional models shows that the presence of a mid-mantle conductor at 850 km depth could be considered to be a global phenomenon.

Assessment of groundwater salinity in Nellore district using multi-electrode resistivity imaging technique

Multi-electrode resistivity imaging survey with 48 electrodes was carried out to assess the extent of salinity inland, in the shallow subsurface in Nellore district, Andhra Pradesh, in the Eastern Ghats Mobile Belt (EGMB) region. Resistivity data were recorded using Wenner–Schlumberger configuration at nine sites along a profile of about 55 km in length, laid perpendicular to the coast. An average spacing of 6 km is maintained between each site. Assessment of groundwater salinity in the study area was made by joint interpretation of the two-dimensional (2D) geoelectrical models of all the sites together with the geochemical analysis results of water samples and geology. At sites closer to the coast, 2D geoelectrical models of the subsurface indicate low resistivities (2–50 Ωm) in the depth range from surface up to 15 m. Such low resistivities are due to the high salinity of the groundwater. Geochemical analysis results of water samples at six locations close to the electrical resistivity survey sites also suggest high salinity and high concentrations of total dissolved solids and other chemicals at sites closer to the coast. Away from the coast, the resistivities in the depth range from surface up to 15 m vary in the range of 50–150 Ωm. Accordingly, the chemical analysis of water samples collected at these sites also showed relatively low levels of salinity and salt concentrations in them. However, away from the coast, the resistivities vary in the range of 150–1500 Ωm in the depth range from 20–40 m. While the aquaculture and agriculture activities may contribute to high salinity at the sites closer to the coast, the presence of deep-seated paleochannels aiding in transporting seawater inland, and water–rock interactions are suspected to be the chief causes for notable salinity at places away from the coast at shallow depths. We opine that the high salinity at shallow depths, coupled with the deep-seated paleochannels transporting seawater, could pose problems to probe further depths particularly using electromagnetic induction methods in the study region.

On the role of oceans in the geomagnetic induction by Sq along the 210°magnetic meridian region

We have utilized solar quiet daily variation (Sq) data recorded at a network of temporary and permanent magnetic observatories, operated along the western Pacific coast, in the 210° Magnetic Meridian (MM) region, spanning both hemispheres. The selected data sets correspond to solar-quiet year 1996. We have determined the ionospheric source current systems for all three Lloyd’s seasons prior to calculating the Sq-induction response. As the selected data sets encompass the whole range of the western Pacific, we studied the role of self-induction effect (SIE) of the ocean at 24-hr period of Sq on the induction response, following the theory developed by Rikitake (1960). Our calculations show a considerable influence of the SIE on the induction response at 24-hr period, implying that ignoring the SIE in induction studies in oceanic regions leads to erroneous interpretation of the results. We have observed a large regional bias in the derived induction response estimates along the 210°MM region. As the ocean effect also depends on the regional subsurface structure and its conductivity, we believe that in addition to the SIE, the bias in the estimated response functions could be due to the strong influence of the coupling of the electric currents induced in the ocean and the highly heterogeneous upper mantle that has resulted from the active lithospheric convergence between the Pacific and Philippine Sea plates all along the western Pacific. We interpret the derived induction responses in the light of the tectonic significance of the 210°MM region. We also discuss the well-defined seasonal differences in ionospheric source current systems by comparing them with those reported for the East Asian sector.

Detrended Fluctuation Analysis of Geophysical Well-Log Data

Geophysical well-log data provide a unique description of the subsurface lithology, as they represent the depositional history of the subsurface formations, vis-a-vis the variation of their physical properties as a function of depth. However, a correct identification of depths to different lithostratigraphic units is possible only by using effective data analysis tools. In the present study, detrended fluctuation analysis (DFA) technique has been applied to gamma-ray log, sonic log and neutron porosity log of three different wells, A, B and C, located off the west-coast of India (i) to discuss the statistical characterization of different subsurface formation properties based on their fractal behavior and (ii) to identify the depths to the tops of formations by comparing the results of DFA with those of wavelet analysis. The DFA technique primarily facilitates to understand the intrinsic self-similarities in non-stationary signals like well-logs by determining the scaling exponent in a modified least-squares sense. In the present study, DFA was carried out in two ways using (i) non-overlapping window method to determine the global scaling exponent and (ii) overlapping window method to determine the local scaling exponent. In the non-overlapping window method, data segments of different windows, each having equal length, were first used to estimate the average fluctuations. The linear least-squares regression between the logarithm of average fluctuations and the logarithm of window lengths then defines the global scaling exponent. For gamma-ray logs of all the three wells, the non-overlapping window method shows two distinct ranges of global scaling exponents, in the ranges 0.5–1.0 and 1.0–1.6. While the former signifies the presence of persistent long-range power-law correlations, indicating the stochastic nature of the sedimentation pattern in the data, the latter indicates the existence of short-range correlations of non-stochastic nature but cease to be of power-law form. On the other hand, the sonic and neutron porosity logs of wells A and C show a single global scaling exponent value of greater than 1.0, signifying the non-stochastic nature of the interval transit time (primary porosity) and neutron porosity, respectively, in the entire data sequence as a function of depth. However, in case of well B, the sonic and neutron porosity logs show two distinct ranges of global scaling exponents, one in the range 0.5–1.0 and the other between 1.0 and 1.5, probably suggesting the effect of different diagenetic conditions in well B, compared to those in wells A and C. Choosing a particular window length and sliding it with unit shifts over the entire length of data for estimating the continuous variation of local scaling exponents as a function of depth defines the overlapping window method. This has been applied on all the log data sets of all the wells to generate the plots of variation of local scaling exponents as a function of depth. Comparison of such plots of variation of local scaling exponents of all the logs with the wavelet scalograms of respective logs revealed that the obtained depth estimates agree well with the known lithostratigraphy of the study region.

Imaging near-surface defects using step-frequency ground-penetrating radar

Step-Frequency GPR (SFGPR) investigations were carried out at the location of a crude oil storage tank at a petroleum refinery. The storage tank was founded on an elevated platform (tank-pad). Subsidence of a portion of the tank-pad led to cracking of its bottom steel plates and subsequent leakage of crude oil. SFGPR imaging was done within and outside the tank, in the frequency range of 10–260 MHz, to understand the cause of the subsidence. Complex signal analysis was useful in identifying a series of cavities in the subsurface, in the depth range of 2–15 m, close to the location of subsidence of the tank-pad. In order to stabilize the foundation of the tank, the subsurface area infested with cavities was grouted systematically. SFGPR imaging was done again after grouting, in the same area in the same manner to evaluate the efficacy of grouting and check for presence of remnant cavities. Results of the SFGPR investigations, before and after grouting, which aided restoration of the foundation of the oil tank, are discussed.

External field characterization using CHAMP satellite data for induction studies

Knowledge of external inducing source field morphology is essential for precise estimation of electromagnetic (EM) induction response. A better characterization of the external source field of magnetospheric origin can be achieved by decomposing it into outer and inner magnetospheric contributions, which are best represented in Geocentric Solar Magnetospheric (GSM) and Solar Magnetic (SM) reference frames, respectively. Thus we propose a spherical harmonic (SH) model to estimate the outer magnetospheric contribution, following the iterative reweighted least squares approach, using the vector magnetic data of the CHAMP satellite. The data covers almost a complete solar cycle from July 2001 to September 2010, spanning 54,474 orbits. The SH model, developed using orbit-averaged vector magnetic data, reveals the existence of a stable outer magnetospheric contribution of about 7.39 nT. This stable field was removed from the CHAMP data after transforming to SM frame. The residual field in the SM frame acts as a primary source for induction in the Earth. The analysis of this time-series using wavelet transformation showed a dominant 27-day periodicity of the geomagnetic field. Therefore, we calculated the inductive EM C-response function in a least squares sense considering the 27-day period variation as the inducing signal. From the estimated C-response, we have determined that the global depth to the perfect substitute conductor is about 1132 km and its conductivity is around 1.05 S/m.

Aeromagnetic and Magnetotelluric Studies in Guaribas Region of Parnaiba Basin in North East Brazil for Groundwater Assessment

Aeromagnetic and Magnetotelluric (MT) surveys have been conducted over the intra cratonic sedimentary Parnaiba basin located in the semi-arid northeastern part of Brazil for sub-surface structural mapping and ground water resource investigation. From aeromagnetic data several faults in the sub-surface have been identified that are conducive to be potential groundwater resource regions. Broad band (0.001sec – 1000sec) MT soundings along a profile within the area surveyed aeromagnetically, mapped two localized high conductivity zones (of resistivities <10 Ω-m) representing sedimentary aquifer zones. These zones lie close to the faults identified from the aeromagnetic survey whose top lies at depth of 150m.