Arkoprovo Biswas

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.

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.

Shoreline changes in response to sea level rise along Digha Coast, Eastern India: an analytical approach of remote sensing, GIS and statistical techniques

Shoreline is one of the rapidly changing linear features of the coastal zone which is dynamic in nature. The issue of shoreline changes due to sea level rise over the next century has increasingly become a major social, economic and environmental concern to a large number of countries along the coast, where it poses a serious problem to the environment and human settlements. As a consequence, some coastal scientists have advocated analyzing and predicting coastal changes on a more local scale. The present study demonstrates the potential of remote sensing, geospatial and statistical techniques for monitoring the shoreline changes and sea level rise along Digha coast, the eastern India. In the present study, multi-resolution and multi temporal satellite images of Landsat have been utilized to demarcate shoreline positions during 1972, 1980, 1990, 2000, and 2010. The statistical techniques, linear regression, end-point rate and regression coefficient (R2) have been used to find out the shoreline change rates and sea level change during the periods of 1972–2010. Monthly and annual mean sea level data for three nearby station viz., Haldia, Paradip and Gangra from 1972 to 2006 have been used to this study. Finally, an attempt has been made to find out interactive relationship between the sea level rise and shoreline change of the study area. The results of the present study show that combined use of satellite imagery, sea level data and statistical methods can be a reliable method in correlating shoreline changes with sea level rise.

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 gravity and magnetic anomaly over thin sheet-type structure using very fast simulated annealing global optimization technique

A Very Fast Simulated Annealing (VFSA) global optimization algorithm is developed for interpretation of gravity and magnetic anomaly over thin sheet type structure for ore exploration. The results of VFSA optimization show that it can uniquely determine all the model parameters without any uncertainty. Inversion of noise-free and noisy synthetic data for single structures as well as field data demonstrates the efficacy of the approach. The technique has been vigilantly and efficaciously applied to two real data examples from Canada with the presence of ore bodies. In both Model examples, the model parameters acquired by the present method, mostly the shape and depth of the buried structures were found to be in respectable agreement with the actual parameters. The present method has the proficiency of evading highly noisy data points and enhances the interpretation results. The technique can be extremely appropriate for mineral exploration, where the gravity and magnetic data is observed due to ore body of sheet like structure embedded in the shallow and deeper subsurface. The computation time for the whole process is very short.

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.

Very Low-Frequency Electromagnetic Method: A Shallow Subsurface Investigation Technique for Geophysical Applications

The very low-frequency (VLF) electromagnetic (EM) method is the simplest EM method to delineate shallow subsurface conducting structures. Since the approach utilizes signals transmitted from worldwide transmitters located in coastal areas in the 5–30 kHz frequency band, it is suitable to depict conducting structures up to 200 m depth in highly resistive terrain. Freely and readily available primary field signals anywhere around the Earth make the VLF method very convenient and efficient for field data collection. Further, VLF data processing using digital linear filtering is quite accurate and very efficient in depicting the qualitative information about subsurface conductors, even though quantitative interpretation of VLF data is as complex as other EM data interpretation. In the present study, various aspects of the VLF method such as basic theory, worldwide VLF transmitters, quantities measured, and interpretation procedures are discussed in detail. Finally, the efficacy of the VLF method for groundwater investigation, mineral investigation, and landslide and subsurface pollution monitoring studies has been demonstrated. Even though the VLF method is a rapid technique for subsurface investigation, use of complementary geophysical methods such as gravity, direct current (DC) resistivity, self-potential, radiometric, etc., reduces the ambiguity in the interpretation and yields reliable subsurface information.