Tapas Acharya

Analysis of lineament swarms in a Precambrian metamorphic rocks in India

Addressing the geologic significance of lineaments and their correlation with joints/fractures is still unclear. The present study attempts to analyse the lineament swarms developed in a Precambrian metamorphic terrain in India using both unfiltered and filtered techniques. The unfiltered analysis technique shows that the major lineament and fracture trends are oriented along EW and NS directions respectively, thus failing to provide any correlation between them. The application of domain-based filtering techniques identifies a highly predominant fracture-correlated lineaments in mica schist constituting the EW trending shear zone in the area. This correlation is not evident in the areas north and south of the shear zone, where the lineaments are consistently oriented along the foliation planes of the rocks and are designated as ‘foliation correlated’. The present analysis indicates that the fracture frequency and the strain history may have played significant roles for the formation of fracture-correlated lineaments in the metamorphic terrain.

Hydraulic significance of fracture correlated lineaments in precambrian rocks in Purulia district, West Bengal

Filter analysis of lineaments in Precambrian metamorphic rocks was used to delineate fracture-correlated lineaments and hydraulically significant fractures. The unfiltered analysis technique fails to show correlation between major lineaments and fractures. Domain-based and discrete filtering techniques successfully identify fracture-correlated lineaments within the brittle-ductile shear zone in conjunction with fractures characterized by high fracture frequencies (>10/m). The locales of hydraulically significant fractures can thus be assessed if the geological controls governing the spatial distribution of fracture frequencies are computed using structural domain approach. The concurrence of fracture-correlated lineaments and hydraulically significant fractures within the brittle-ductile shear zone is evident.

A very fast simulated annealing method for inversion of magnetic anomaly over semi-infinite vertical rod-type structure

A very fast simulated annealing (VFSA) global optimization method is used to interpret and modeling of magnetic anomaly over a vertically magnetized semi-infinite vertical rod-type structure. The results of VFSA optimization reveal that various parameters show a number of equivalent solutions when shape of the target body is unidentified and shape factor is also optimized together with other model parameters. The study reveals that restricting the shape factor to its actual value gives the utmost reliable results. Inversion of noise-free, noisy synthetic data and field data demonstrates the efficacy of the approach.

Study of fractures in Precambrian crystalline rocks using field technique in and around Balarampur, Purulia district, West Bengal, India

Location of recharge zone in Precambrian crystalline rock is still unclear. The present study attempts to perform a detailed analysis of the joints/fractures developed in a Precambrian metamorphic terrain in and around Balarampur in Purulia district of West Bengal, India using bedrock data. The analysis shows that the orientations of major fracture trends are variable along with varying lithological units and structural affinities. The application of lithology-based analysis technique identifies highly predominant fracture frequency and fracture aperture in mica schist and phyllite in the area. This property is not evident in the granite gneiss and epidiorite. The moderate to high fracture permeability value is also associated with the fractures occurring in the shear zone. Mica schist and phyllite associated with the shear zone may represent a permeable recharge zone in the region.

Delineation of potential groundwater recharge zones in the coastal area of north-eastern India using geoinformatics

Integration of satellite-based remote-sensing data and the geographical information system (GIS) for the exploration of groundwater resources has developed a revolution in the field of groundwater research, which assists in evaluating, observing, and preserving groundwater resources. In the present paper, various groundwater potential zones for the assessment of groundwater recharge zone availability have been delineated using remote-sensing and GIS techniques. Landsat 8 satellite imageries were used to prepare various thematic layers such as: lithology, altitude, slope, and groundwater table map from the inventoried tube well data. These layers were then transformed into raster data using the feature to raster converter tool in the ArcGIS 9.3 software. The raster maps of these factors are allocated a fixed score and weight computed from multi-influencing factor (MIF) technique. Subjective weights are assigned to the respective thematic layers and overlaid in GIS platform for the identification of potential groundwater recharge zones within the study area using weighted overlay index analysis (WOIA). The groundwater potential zones thus obtained were divided into five categories, viz., very poor, poor, good, moderate, and excellent zones. The results reveal that the areas of excellent groundwater potential are estimated to permit precipitated water percolates into subsurface and ultimately contributes to recharge the groundwater. These results suggest that the high potential zones will have a key role in future expansion of drinking water and irrigation development in the study area.

Hydrodynamic Study of Weathered Zone of Hardrock Aquifers Using Dugwell Data

The present study discusses a technique of locating high-discharging dugwells in Precambrian metamorphic rocks in Balarampur, Purulia district, West Bengal, India and describes the hydrodynamic properties of hardrock aquifers using bedrock joints/fractures, lineaments and weathered zone. Dugwells with higher discharges are mainly concentrated in metapelites, rich in biotite. In groups of dugwells, high-discharging wells exhibit linear spatial distribution, denoted as “well-line,” and consistently show high well discharge along each well-line in every dugwell group. The spatial distributions of high-discharging wells are associated with thick weathered-zones. Well-lines, comprising abruptly high-discharging wells, are spatially correlated to the lithostratigraphic contacts and lower elevation zones.

Evaluation of regional fracture properties for groundwater development using hydrolithostructural domain approach in variably fractured hard rocks of Purulia district, West Bengal, India

Estimation of geohydrologic properties of fractured aquifers in hard crystalline and/or metamorphosed country rocks is a challenge due to the complex nature of secondary porosity that is caused by differential fracturing. Hydrologic potentiality of such aquifers may be assessed if the geological controls governing the spatial distribution of these fracture systems are computed using a software-based model. As an exemplar, the Precambrian metamorphics exposed in and around the Balarampur town of Purulia district, West Bengal (India) were studied to find out the spatial pattern and consistency of such fracture systems. Surfer and Statistica softwares were used to characterize these rock masses in terms of hydrological, structural and lithological domains. The technique is based on the use of hydraulically significant fracture properties to generate representative modal and coefficient of variance (Cν) of fracture datasets of each domain. The Cν is interpreted to obtain the spatial variability of hydraulically significant fracture properties that, in turn, define and identify the corresponding hydrolithostructural domains. The groundwater flow estimated from such a technique is verified with the routine hydrological studies to validate the procedure. It is suggested that the hydrolithostructural domain approach is a useful alternative for evaluation of fracture properties and aquifer potentiality, and development of a regional groundwater model thereof.

Lithostratigraphic contact – a significant site for hydrogeological investigation in crystalline fractured-rock terrains

Estimating the hydrogeologic control of fractured aquifers in hard crystalline and metamorphosed rocks is challenging due to complexity in the development of secondary porosity. The present study in the Precambrian metamorphic terrain in and around the Balarampur of Purulia district, West Bengal, India, aims to estimate the hydrogeologic significance of lithostratigraphic contacts using fracture characteristics obtained from surface bedrock exposures supported by hydrological data from the existing dugwells. This study involves the domain-wise analysis of the frequencies of fractures that control the fracture-porosity. The domain-wise study reveals higher fracture-frequencies adjacent to the lithostratigraphic contacts. The concurrence of lithostratigraphic contacts with the occurrences of high-discharging wells and also with the deep weathered zone in low-lying areas is clearly established, thus assigning the lithostratigraphic contact as hydrogeologically significant. An increase in frequencies of the fractures within the ‘influence zone’ of the lithocontact, is clearly visible. Among those fractures, particularly, which make the angle greater than the ‘limiting angle’ with the lithocontact are characterised by increased frequencies. However, brittle rocks like quartz biotite granite gneisses, phyllite and epidiorite show high porosity of fracture, within the ‘influence zone’ of the lithostratigraphic contact. Enhanced deepening of the weathered-zone at lower topographic region may perhaps be a plausible explanation for this increased fracture-porosity at lithocontact to assign it as a hydrogeologically significant transmissive zone within fractured rocks.

Influence of Rock Foliation on Fracture Permeability: The Case of Precambrian metamorphics (Purulia, Eastern India)

Influence of Rock Foliation on Fracture Permeability: The Case of Precambrian metamorphics(Purulia, Eastern India) Hydrological influence of foliation in variably fractured Precambrian metamorphic rocks is usually difficult to predict. As an exemplar, the Precambrian metamorphic exposed in and around the Balarampur town of Purulia district, West Bengal (India) were studied to find out the influence of rock foliation on fracture permeability using a hydrostructural domain-based model. These Precambrian metamorphic rocks, characterized by foliated fabric, resulted from the combination of (a) brittle granite gneisses rich in quartz and garnet, (b) an intense brittle-ductile shear zone, and (c) ductile metapelites with micaceous minerals. Pre- and post-monsoon depth-to-groundwater table contour maps, mode, coefficient of variance (Cv) and contour maps of frequencies and apertures of fractures demonstrate inter- and intra-domain variable qualitative fracture permeability’s.