Subash Chandra

Geophysical Characterization of Hard Rock Aquifers

Geophysics plays a major role for characterizing the hard rocks for groundwater studies. The qualitative and quantitative application has increased since past few years due to rapid development and advancement in microprocessors and associated numerical modelling solutions. Although geophysics has ability to probe deep earth interior (say >1000 m), but its application for groundwater studies is usually restricted to depths less than and around 250 m below the surface. These include mapping the depth and thickness of aquifers, mapping aquitards or confining layers, locating fractures and fault zones and mapping contamination to the groundwater such as that from saltwater intrusion. The theoretical and practical background to geophysics has been extensively reviewed and can be studied in standard texts on the subject, for example Kearey & Brooks (1991); Telford et al. (1976); Parasnis (1979); Dobrin (1976); Grant and West (1965); Reynolds (1997); Miller et al. (1996); Murali et al. (1998); etc.

Source speciation resolving hydrochemical complexity of coastal aquifers.

There is a growing concern of seawater intrusion to freshwater aquifers due to groundwater overexploitation in the eastern coastal belt of Southern India. The problem becomes complex in the regions where industrial effluents are also contaminating the freshwater aquifers. In order to understand the hydrochemical complexity of the system, topographic elevation, static water level measurements, major ion chemistry, ionic cross plots, water type contours and factor analysis were applied for 144 groundwater samples of shallow and deep sources from Quaternary and Tertiary coastal aquifers, located within the industrial zone of 25 km(2) area near Cuddalore, Southern India. The ionic cross plots indicates dissolution of halite minerals from marine sources and seawater mixing into inland aquifers up to the level of 9.3%. The factor analysis explains three significant factors totaling 86.3% of cumulative sample variance which includes varying contribution from marine, industrial effluent and freshwater sources.

Investigating Geophysical and Hydrogeological Variabilities and Their Impact on Water Resources in the Context of Meso-Watersheds

Abstract In India, particularly in semi-arid and hard rock regions, the declining water table and the reduction in bore well yield as well as net irrigated area are becoming a common situation to the extent that even implementation of rainwater harvesting programs has been unsuccessful. This is mainly attributed to the adoption of ad hoc management strategies and techniques without considering the geometry and nature of the aquifer together with area-specific parameters, such as soil cover and lithological units. Knowledge of net groundwater recharge (R) and groundwater draft, the two components responsible for changes in groundwater storage (ΔS), is important for assessing the groundwater resources and for the implementation of watershed programs. In the present study, geophysical and hydrogeological investigations were performed to decipher the aquifer geometry and its extent to estimate the groundwater resources and selection of suitable sites for rainwater harvesting.

Quantitative characterization to construct hard rock lithological model using dual resistivity borehole logging

Borehole logging is a very robust tool to accurately locate transitions between weathered layers and fractures in hard rock settings; therefore, it can help substantially in the construction of regional and local hydrogeological models. A simple and low-cost resistivity probe, named dual resistivity logger (DRL), was experimented to map the formation resistivity at two investigation distances by means of three active electrodes. Forward response of DRL was analysed on the synthetic data generated for a conceptual hard rock hydrogeological model as well as tested at two field sites in hard rock aquifers of southern India. The DRL was proven efficient in demarcating the aquifer into successive hydrogeological zones, i.e. the laminated-fissured (L-F), the fissured-semi-fissured (F-SF), and semi-fissured-basement (SF-B) layers. The results were verified by comparison with well lithologs based on rock cuttings and temperature logging. The DRL has proven its ability to locate the hydraulically active fractures as well as contacts between weathered layers. It offers a simple but efficient way to acquire underground data for building hydrogeological models.

Mapping of 3D Hydrogeological Framework of the Deccan Basalt Groundwater Systems Using AEM

Summary The SkyTEM data provides a comprehensive three-dimensional picture of the subsurface. The low moment data ensured the high resolution mapping of the near surface and the high moment data to the deeper level. The dual moment AEM data provided high resolution mapping of subsurface from top to ∼ 250 m depths. The results revealed significant contrast between the B-G layers that has a resistivity range of 10–15 Ωm and generally has groundwater yields of 6–8 lps. It is also noted that the groundwater yield (> 6 lps) increases for deeper (<150 m) B-G contact. The potential aquifer zones such as intertrappeans, fractures in vesicular basalts and Basalt-Gondwana contact are clearly mapped in the resistivity section. The regional fault (potential aquifer) which is abutting in the SW part with its orientation SE-NW is successfully delineated in the mean resistivity maps. The aquifer systems in basalts and Gondwanas and their extensions are also clearly demarcated on the 3D map using AEM data.

Time lapse tracer and SP measurements to characterize the hydrodynamics of fractured granite aquifer: A case study

An attempt is made to identify and delineate the groundwater flow direction and rate in a fractured hard rock aquifer in Maheshwaram granite watershed in Andhra Pradesh using multiple tracers such as bromide, iodide and rhodamine-B under both natural and induced conditions. A main well in the center and three observation wells at 25 m in north-northwest, southeast and southwest directions respectively were constructed based on 222Rn anomalies and 4He measurements. Streaming potential survey was also carried out in the area before, during and after pumping test. The tracer tests indicated preferential groundwater flow in N-S direction. The groundwater flow velocity estimated based on arrival of tracer under natural and induced conditions are 0.52 m/d and 375 m/d respectively.

Societal application of geophysics as an aid to a rescue operation at Jaipur

Geophysical techniques are normally employed to explore the subsurface and determine the anomalies but they are always much larger in dimension. However, a very precise measurement was needed to pin point a misaligned horizontal tunnel made at 47 m depth to intercept an old bore well and rescue a four year child fallen accidently on 9th November at Jagatpura village in Jaipur, India. Due to the greater depth and presence of iron casing, electric power, etc., no equipment could work to determine the orientation of the tunnel that has lost its direction. The dimension of the site and many other constrains didn’t allow conventional geophysical methods to be used. An innovative method was then designed and successfully applied, which determined 1.92 m deviation of the tunnel end point to the east from the old bore well. The digging was redirected accordingly and the bore well was intercepted. This paper describes the theoretical background, forward simulations and the field experiment of the newly designed geophysical method.