The main objective of geophysical investigations is acquisition of maximum sub-surface information through a systematic coverage of the entire area. In hard rocks, vertical electrical soundings (VES) got the maximum application. Other techniques like Self Potential (SP), Induced Polarization (IP), Mise-a-la-masse of electrical method, magnetic, seismic refraction , electromagnetic (the horizontal loop and very low frequency - VLF) methods have also been used as and when required.

Aeromagnetic, seismic and deep borehole data of other agencies were also analysed for their effective utilization in ground water exploration.  CGWB has so far carried out about 2,000 VES and electrically logged 560 bore holes apart from few VLF profiles. These surveys are being carried out using the time tested geophysical equipment. These enormous data have been used in getting the standardized resistivity values for various electrical layers in different hydrogeological environments vis-à-vis occurrence of ground water.

A large part of Maharashtra state and the Union Territories of Dadra and Nagar Haveli is underlain by hard rock strata particularly by Deccan Basaltic flows [81% of the total area] with localized occurrences of sedimentary and alluvial formations. The surface geophysical investigations, in the region are being carried out mainly for

1) The site selection for exploration drilling

2) Artificial recharge studies

3) Site selection for water supply schemes for local bodies, defense establishments and other governmental agencies etc.

4) Special investigations like pollution studies and supportive studies for systematic/reappraisal hydrogeological surveys.

5)  Special investigations for drought affected areas in order to reduce well failures and rejection of negative areas.

Apart from the above, subsurface geophysical investigations like bore hole logging is also carried out as a post drilling operation for recommending well design etc. Though most of the surveys are demand based, as far as ground water exploration is concerned, Board has the geophysical capabilities of mapping bed-rock topography in rocks, locating fresh water aquifers in saline environments, identifying the direction of ground water movement and seepage, locating favourable areas for artificial recharge schemes, mapping pollution plumes and locating the ground water potential zones for restoring water supply as a part of drought / disaster management.

Methodology: The aim of Geophysical investigations is to look for the contrast between the physical properties of the target and its surroundings. As such more the contrast/anomaly better would be the geophysical response and hence identification of the target. There are different techniques of which some are responsive to hydrogeological heterogeneities while others are not. Accordingly, one or more suitable techniques/methods are employed for effective delineation of the target – in this case the water bearing formations, for example a weathered/fractured zone in hard rock, sand/gravel zones in alluvial formations. The most commonly used geophysical method in ground water exploration is electrical resistivity method as the response is both qualitative and quantitative.

The gneisses and granites of older Precambrian, capped by basaltic flows with intertrappean beds of upper cretaceous to lower Eocene period form the major geological formations of the district. Extensive outcrop of the granites are found in the Southeastern parts of the district along its border with Andhra Pradesh. Thin beds of nodular limestone and gray sandstone occur below the Deccan trap.

In this village a Gradient Resisitivity Profile (GRP) was carried out with the current electrode separation (AB) of 500m. and the potential electrode separation of 20m. The central part of the array, having a length of 180m was scanned by measuring resistivities at an interval of 10m. The orientation of the profile was kept NW-SE. On the same line of the GRP a Wenner Resistivity Profile (WRP-a) was run from 50m south east of zero station of the GRP. Also a parallel Wenner Resistivity Profile (WRP-b) was taken 36 m NE of WRP-a. All the three profiles (Fig.) have shown the same pattern of resistivities, increasing from southeast to northwest. Vertical Electrical Soundings (VES) were also conducted at the zero station of GRP (VES-1), on WRP-a at 200m northwest from the zero station of the GRP (VES-2) and at 40m.SE of the zero station of the GRP on WRP-b (VES-3). On the basis of resistivity lows on GRP and WRP-a, the site was recommended at the zero station of the GRP. The borehole drilled down to a depth of 120 m at this point has yielded 12.18 lps. However an observation well drilled at 20m NE of the exploration well, which is on a resistivity high of WRP-b has yielded only 0.14 lps.

A short-term investigation was taken up for the augmentation of water supply to Nagpur Municipal Corporation (NMC) at a site in Knhan River alluvium near Kamptee town. Hydrogeological and geophysical investigations were carried out at this site in order to delineate the nature and thickness of the alluvial sediments in the river and for selecting a suitable site for the construction of collector well. In all 11 VES were carried out in 3 sq.km area (Fig.). The VES-1 and 3 are located on a sand bar forming a small island towards the bank. VES-4 is located at the adjoining sand bar, VES-6 and 7 are located on another adjoining sand bar. The VES-5, 8 and 9 are located on the northern edge of Kanhan river. VES-10 and 11 are located upstream side of the weir on Kanhan river.

Based on the resistivity data a fence diagram (Fig.) was prepared to see the disposition   of the sand layer and to recommend the sites for the collector wells. This diagram indicates that the sand thickness vary from 5.6m (at VES-6) to more than 35m (at VES-11). Similarly the fence diagram shows that the sand thickness is maximum at VES-2 (38m), which falls on the sand island adjoining the southern bank after the confluence or the rivers. The sand thickness decreases towards east. The site at VES-2 is considered as the suitable site for the construction of the collector well. The collector well , however, is to be located in the river bed, so that the radials are directly below the river bed.

Some of the criteria adopted are high water percolation rate, presence of suitable unsaturated granular zone for effective water storage and shallow depth to compact rock at the site of artificial recharge structure. Electrical resistivity and electromagnetic methods can differentiate water saturated and unsaturated portion of the formations based on their resistivity contrasts. 

Refraction seismic techniques posses high success rate in identifying fractured part of the rock and map compact rock topography. This is based on the velocity contrast of the elastic waves (generated artificially) in different subsurface formations. However in unconsolidated sediments electrical resistivity methods are quite effective in locating the permeable granular zone and its lateral extent. The method is also suitable to map interface between fractured and compact rock formations. At shallow depth Very Low Frequency (VLF) electromagnetic technique is very successful in mapping the lateral extent of consolidated and unconsolidated parts of the formations. Similarly the movement of the ground water can be ascertained by carrying out Self Potential (SP) surveys. Combination of different techniques is finally the requirement for selection of suitable sites for construction of artificial recharge structures.

Surface geophysical investigations were carried out in the watershed to supplement the sub surface data about the aquifer geometry for artificial recharge with the objective of:

a)      Determining the regional aquifer geometry

b)      Determining the granular zone above the water table

c)       Investigate the presence of granular zones at the injection well sites.

Schlumberger Electrode Array was used for determining the regional aquifer geometry. VES were taken along two traverses from North to South at an interval of 2 km. Apart from these a few VES were also taken in the remaining part of the watershed. In total 95 VES were taken (Fig.). It is observed that in the watershed that the clay content is more between 70-80mbgl and above it 3-6 granular horizons are encountered. Further, there is great heterogeneity and granular horizons show pinching and swelling at depths.

In order to know the percentage of granular horizons above the water level, Wenner configuration was used with an increment of 0.5m for the electrode separation (a) to pick up thin horizons. In all, 68 sites were investigated. 3-4 granular horizons were inferred above the water table. The area having maximum percentage of granular content is ideal for ground water recharge. From the perusal of Fig., the following inferences are made.

1. The percentage of granular zone is the least (<50%) along the northern bank of Tapi River.

2. The percentage of granular zone increases from <50% (in the South of the study area) to >80% (in the central part) and again starts decreasing towards north to a low of 50%.

Geophysical logging is the measurement of depth wise variations of electrical and physical properties in a borehole. It is the post drilling approach to optimize the design and development of well. In ground water exploration, the commonly measured parameters are Self Potential (SP), Point Resistance (PR), Short and Long Normal resistivities, Lateral and natural Gamma.  However other techniques like Caliper  logging (for diameter of borehole), Neutron-Neutron logging and flow logging were also carried out as and when required. Logging of bore holes is carried out to obtain information on

·          The occurrence, and quality of water in the geological formation

·          The physical characteristics of geological formation through which the bore hole is drilled

·          The dimension, construction and physical condition of the bore hole

·           Percentage contribution of ground water from each aquifer

Depending on the instrumentation involved, three types of measurements are in vogue viz.

1.       Discrete measurements where parameters are measured at regular intervals of depth, at points of interest and plotted on linear scale. 

2.       Analog measurements where continuous depth wise recording of parameters is done on a chart paper .

3.       Digital measurement, where the parameters are recorded digitally. The advantage with this type is the data can be stored on digital storage media for further analysis on computer and for reproducing logs at desired scales.

Central Region is equipped with all the above three types of loggers and typical geophysical logs acquired with them are presented in Fig.1, Fig. 2  and  Fig 3

Case   Studies III                                                       

A. The exploratory well drilled at the village Khandala, Nagpur district was logged using the Digital logger- (ABEM Terra meter SAS 200 SWEDEN) to a depth of 185m. The Self Potential (SP) and Normal resistivities were recorded at intervals of 1 m and the plotted log was presented in the (Fig.). From the analysis of the logs it is inferred that the granular zones occur between the following depth ranges.

8-19m, 24-42.5m, 49-51m, 53-57m(mixed with shale), 65-73m, 77.5-86.5m, 91.5-101m, 105-114m, 116-120m(mixed with shale), 120-126.5m, 143-146m, 148-153m, 160-164.5m, 171-176m and 179-182. mbgl.

B. A pilot borehole drilled at the village Pathansawangi, Savner Taluk, Nagpur district was logged with the UPTRON analog logger. The Self Potential (SP), and Normal resistivities were recorded and the log was presented in the Fig. From the analysis of the logs it is inferred that the granular zones occur between the depth ranges.

32-45m, 47-50m, 54-72m(61-62m clay ) 82-89m, 93-100m, 108-115m, 124-129m, 136-139m, 144-150m, 164-168m and 176-178mbgl

C. The bore hole drilled at Vavarde, Jalgaon district under Out Sourcing Exploratory Drilling programme was logged down to a depth of 200m by OYO portable digital logger and the parameters measured were Self Potential, Short and Long Normal Resistivities and Natural Gamma. (Fig.) As the log was recorded in hard rock terrain, the response of SP was not amenable for interpretation. As such, the interpretation is based on resistivity and gamma logs only. The values observed for different litho-units are as follows.

                       Litho unit                  Resistivity  (Ohm m)                          Gamma counts (CPS)

                 Weathered basalt                   30-80                                                      10-25

         Amygdaloidal/jointed basalt            80-150                                                      5-10

                   Massive basalt                    100-250                                                     2-10

     Based on these values, more than 10 different flows were demarcated and the flow tops have been demarcated based on the presence of Weathered basalt/Amygdaloidal basalt immediately below the massive basalt. These can be clearly seen at the following depths.

     7-9m, 14m, 39-43m, 46-48m, 63-65m, 71-73m, 97-98m, 105-106m, 110m, 117m, 126-127m, 137-138m, 150-151m, 157-158m, 164-167m, 194-196m