Shashi Prakash Sharma

VFSARES-a very fast simulated annealing FORTRAN program for interpretation of 1-D DC resistivity sounding data from various electrode arrays

Employing the very fast simulated annealing (VFSA) global optimization technique, a FORTRAN program is developed for the interpretation of one-dimensional direct current resistivity sounding data from various electrode arrays. The VFSA optimization depicts various good fitting solutions (models) after analyzing a large number of models within a predefined model space. Various models that yield reasonably well fitting responses with the observed response lie along a narrow elongated region of the model space. Therefore, instead of selecting the global model on the basis of the lowest misfit error, it is better to analyze histograms and probability density functions (PDFs) of such models for depicting the global model. In a multidimensional model space, the most appropriate region to select suitable models to compute the mean model is the one in which the PDF is larger in comparison to the other regions of the model space. Initially, accepted models with misfit errors less than the predefined threshold value are selected and lognormal PDFs for each model parameter are computed. Subsequently, mean model and uncertainties are computed using the models in which each model parameter has a PDF more than the defined threshold value (>68.2%). The mean model computed from such models is very close to the actual subsurface structure (global model). It is observed that the mean model computed using models with a PDF more than 95% for each model parameters yields the actual model. Moreover uncertainty computed using models with such a high PDF and lying in a small model space will be small and it will not be considered as the actual global uncertainty. Resistivity sounding (synthetic and field) data over different subsurface structures are optimized using the VFSA program developed in the present study. Optimization results reveal that the actual model always locates within the estimated uncertainty in the mean model. Since the approach requires much less computing time (a few minutes) using an ordinary PC, results with smaller uncertainty can be obtained using repeated computations with a smaller search range in comparison to the results obtained in a large search range. The efficacy of the program is demonstrated by interpreting data from various layered earth structures. Field examples associated with groundwater and mineral exploration are also presented. Interpreted model parameters show excellent correlation with drilling results. The optimization program can be used for various case studies like those associated with groundwater, mineral exploration, subsurface pollution studies, and saline water incursion in coastal areas.

Delineation of subsurface structures using self-potential, gravity, and resistivity surveys from South Purulia Shear Zone, India: Implication to uranium mineralization

The unexplored South Purulia Shear Zone (SPSZ) at the north of Singhbhum Shear Zone (SSZ) in Eastern India is a prospective zone for structural-guided hydrothermal mineralization. We carried out an integrated geophysical study using self-potential (SP), gradient-resistivity profiling (GRP), and gravity study across the SPSZ to identify the near-surface structural features and probable correlation with the uranium mineralization of the region. We studied a broad low SP, anomaly zone correlated with corresponding low-gravity and lowresistive zone across the same part of the study area. This conductive and low-density zone was identified as the width of the brittle-to-ductile and highly altered SPSZ. The 2D modeling of SP and residual gravity data along a northeast–southwest profile across the shear zone between Raghunathpur and Barabazar localities revealed the northerly dipping shear zone with an average width of ∼4.5 km. However, the 2D modeling of the SP data suggested numerous thick, sheet-type vertical and/or inclined structures intervening the shear zone, which were well correlated with the vertical structures delineated by the 2D gravity inverse model. The vertical alteration zones (density and conductivity) at ∼40-, 200-, and 400-m depths have been identified over this region. These alteration zones are likely to be mineralized zone because a hydrouranium anomaly has also been reported from those locations earlier. We studied the efficacy of an integrated approach using GRP, SP, and gravity surveys for the investigation of near-surface vertical to dipping conducting structures associated with uranium mineralization in such shear zone regions.

A comparison of performances of linearized and global nonlinear 2-D inversions of VLF and VLF-R electromagnetic data

The performances of linearized (local) and global nonlinear joint 2-D inversions of very low frequency (VLF) and VLF resistivity electromagnetic measurements are analyzed. A stable iterative inversion scheme is used in linearized inversion while the very fast simulated annealing approach is used in global nonlinear inversion. Synthetic noise‐free and noisy data due to three different models in complexity and two field examples are considered. Synthetic examples show that linearized inversion reveals the subsurface structure better than global nonlinear inversion provided the model has only a few parameters under inversion. Both linearized and global nonlinear inversions must be performed combining all available data in order to obtain the most reliable estimates of the subsurface parameters. Complex models with a large number of parameters are better to invert using global nonlinear inversion although the CPU time needed is always much longer than the one used in linearized inversion. Contrary to global n...

Resolution of multiple sheet-type structures in self-potential measurement

The resolution of self-potential anomalies due to closely spaced multiple sheet-like bodies by the potential difference and potential gradient is studied in this paper. Self-potential anomalies due to several synthetic models were inverted through a very fast simulated annealing (VFSA) global optimization. Increase in depth to the top, polarization constant and depth extent of the body decreases resolution at a particular target separation. It has been observed that depth to the top and separation between two targets play an important role in the resolution. Vertical sheets at equal depth can be resolved in the potential difference measurement only if they are separated by at least four times their depth, while they can be resolved in the gradient method, if they are separated by twice the depth. Resolution using potential difference becomes more difficult for dipping sheets, although the potential gradient method can resolve them efficiently. Efficacy of potential gradient data in the inversion is demonstrated in the study using synthetic data as well as field measurement from South Purulia Shear Zone related with uranium investigation.

Geophysical anomalies associated with uranium mineralization from Beldih mine, South Purulia Shear Zone, India

Beldih mine at the central part of the South Purulia Shear Zone (SPSZ) has been reported with low grade uranium-bearing formation within quartz-magnetite-apatite host in kaolinized formation. Therefore, the present integrated geophysical study with gravity, magnetic, radiometric, very low frequency electromagnetic (VLF) and gradient resistivity profiling methods around the known mineralized zones aimed at identifying the exact geophysical signatures and lateral extent of these uranium mineralization bands. The closely spaced gravity-magnetic contours over the low to high anomaly transition zones of Bouguer, reduced-to-pole magnetic, and trend surface separated residual gravity-magnetic anomaly maps indicate the possibility of high altered zone(s) along NW-SE direction at the central part of the study area. High current density plots of VLF method and the low resistive zones in gradient resistivity study depict the coincidence with low gravity, moderately high magnetic and low resistivity anomalies at the same locations. Moderate high radioactive zones have also been observed over these locations. This also suggests the existence of radioactive mineralization over this region. Along profile P2, drilled borehole data revealed the presence of uranium mineralization at a depth of ∼100 m. The vertical projection of this mineralization band also identified as low gravity, low resistivity and high magnetic anomaly zone. Thus, the application of integrated geophysical techniques supported by geological information successfully recognized the nature of geophysical signatures associated with the uranium mineralization of this region. This enhances the scope of further integrated geophysical investigations in the unexplored regions of SPSZ.

A practical solution in delineating thin conducting structures and suppression problem in direct current resistivity sounding

In hard rock areas, conventional apparent resistivity measurement using Schlumberger resistivity sounding fails to detect thin conducting structures (2-D and 3-D fractures filled with groundwater and mineral aggregate) concealed at a large depth. In the present study, an attempt is made to way-out the detection problem of deep seated thin conducting layer. It is proposed to study the apparent conductance simultaneously with resistivity sounding to detect such conductive zones qualitatively. Apparent conductance is defined as the magnitude of current flowing in the subsurface for a unit applied voltage through current electrodes. Even though such measurement is of qualitative importance, it gives extremely valuable information for the presence of conductive zones at depth in challenging hard rock terrain. It has been observed that apparent conductance increases significantly when groundwater bearing fractures and conductive bodies are encountered in the subsurface. Field data from different locations are presented to demonstrate the efficacy of such measurement. The measurement assists to the conventional resistivity sounding for successful prediction of groundwater zones at large depth in different hard rock areas and is of enormous importance. The approach is also used for possible solution of suppression problem in the DC resistivity sounding when intermediate layer is not reflected in the resistivity sounding curve. Finally, the approach can be used together with resistivity sounding to solve many practical problems.

Interpretation of self-potential anomaly over idealized bodies and analysis of ambiguity using very fast simulated annealing global optimization technique

An efficient and reliable approach is developed for the interpretation of self-potential anomaly measured over idealized bodies (sphere, horizontal and vertical cylinder) using a very fast simulated annealing (VFSA) global optimization method. Since VFSA optimization lends itself to a number of good-fitting models in a vast multi-dimensional model space, the nature of ambiguity in the interpretation has also been investigated simultaneously. The study reveals that, while optimizing all model parameters (electric dipole density, horizontal location, depth, polarization angle and shape factor) together, the VFSA approach yields a number of equivalent solutions. It has been observed that the shape factor plays an important role in finding a reliable estimate of other model parameters. The analysis of ambiguity shows that a small change in the shape factor produces a large change in the estimated electric dipole density. Accordingly, inaccurate estimates of other model parameters have also been obtained. It has been observed that the optimization method is able to determine all the model parameters accurately when shape factor is fixed. Therefore, interpretation of Self-potential data is carried out by adapting a two-step procedure. In the first step, all the model parameters are optimized. The inversion results obtained after the first step indicates the value of shape factor is around 1.5, 1.0 or 0.5. Subsequently in the second step, the shape factor is fixed to 1.5, 1.0 or 0.5 and other model parameters are optimized. In this way, the most reliable result has been obtained, and ambiguity in the interpretation has become insignificant. The efficacy of this approach is demonstrated using noise-free and noisy synthetic data and three field examples from different areas. One field example is interpreted using multiple targets to show the efficacy of the developed approach in dealing with optimization of a large number of model parameters. The computation time of the two-step procedure is very short (35 s for each step). It is highlighted that, even if the shape factor is known either from a priori geological information or anomaly contour map, interpretation should be performed in two steps to obtain the most reliable estimate of various model parameters as well as confirmation of geometrical shape of the subsurface structure.

Delineation of extension of uranium mineralization zone using resistivity and Very Low Frequency electromagnetic surveys around South Purulia Shear Zone, India

During a random radiometric survey, nuclear radiation was detected near exposed rocks in an open-pit phosphate-mine at Beldih (Purulia, W.B.), India. Radiometric analysis of rock samples reveals a significant concentration of uranium. Geological study suggests that Beldih phosphate mine lies near the South Purulia Shear Zone (SPSZ). Shear zones in the region are E-W striking elongated zone and associated with uranium mineralization (famous Singhbhum shear zone for several uranium mines). Electrical resistivity (profiling and sounding) and Very Low Frequency (VLF) electromagnetic surveys were performed around Beldih mine to delineate the suitable conducting zones which may be associated with uranium mineralization. Significant lateral extension of possible source is delineated; it could be exploited later on a large scale for commercial utilization. It is observed that gradient resistivity profiling (GRP) survey is the most suitable technique to explore the nearly vertical conducting structure. The GRP measurements were carried out at different locations. A good correlation of low resistive zones on various GRP was observed from the measured data. The study concludes that subsurface structures are nearly vertical and have alternate resistive and conducting bands. An inclined bore-hole drilled confirms the presence of alternate resistive and conducting structures and uranium mineralization is detected in conducting bands. This suggests that GRP can be conducted for precise location of the target zone. This will help in planning bore holes for drilling to find the exact thickness, lateral and vertical extent of the deposit for commercial utilization. VLF measurements were also carried out along various profiles. Even though VLF observation was quite noisy due to presence of power lines in the area; this method can be used to map the area and model the data for actual sub-surface conducting zones in less noisy areas as a support for drilling works.

Integrating Apparent Conductance in Resistivity Sounding to Constrain 2D Gravity Modeling for Subsurface Structure Associated with Uranium Mineralization across South Purulia Shear Zone, West Bengal, India

South Purulia Shear Zone (SPSZ) is an important area for the prospect of uranium mineralization and no detailed geophysical investigations have been carried out in this region. To delineate the subsurface structure in the present area, vertical electrical soundings using Schlumberger array and gravity survey were carried out along a profile perpendicular to the SPSZ. Apparent conductance in the subsurface revealed a possible connection from SPSZ to Raghunathpur. The gravity model reveals the presence of a northerly dipping low density zone (most likely the shear zone) extending up to Raghunathpur under a thin cover of granitic schist of Chotanagpur Granite Gneissic Complex (CGGC). The gravity model also depicts the depth of the zone of density low within this shear zone at ~400 m near Raghunathpur village and this zone truncates with a steep slope. Integration of resistivity and gravity study revealed two possible contact zones within this low density zone in the subsurface at depth of 40 m and 200 m. Our study reveals a good correlation with previous studies in Raghunathpur area characterized by medium to high hydro-uranium anomaly. Thus the conducting zone coinciding with the low gravity anomaly is inferred to be a possible uranium mineralized zone.

Fast imaging of subsurface conductors using very low‐frequency electromagnetic data

The study presents a fast imaging technique for the very low-frequency data interpretation. First, an analytical expression was derived to compute the vertical component of the magnetic field at any point on the Earth’s surface for a given current density distribution in a rectangular block on the subsurface. Current density is considered as exponentially decreasing with depth, according to the skin depth rule in a particular block. Subsequently, the vertical component of the magnetic field due to the entire subsurface was computed as the sum of the vertical component of the magnetic field due to an individual block. Since the vertical component of the magnetic field is proportional to the real part of very low-frequency anomaly, an inversion program was developed for imaging of the subsurface conductors using the real very low-frequency anomaly in terms of apparent current density distribution in the subsurface. Imaging results from the presented formulation were compared with other imaging techniques in terms of apparent current density and resistivity distribution using a standard numerical forward modelling and inversion technique. Efficacy of the developed approach was demonstrated for the interpretation of synthetic and field very low-frequency data. The presented imaging technique shows improvement with respect to the filtering approaches in depicting subsurface conductors. Further, results obtained using the presented approach are closer to the results of rigorous resistivity inversion. Since the presented approach uses only the real anomaly, which is not sensitive to very small isolated near-surface conducting features, it depicts prominent conducting features in the subsurface.