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Geophysical Studies
 
Surface Geophysical Methods
 
Surface geophysical methods include electrical, electromagnetic, magnetic, seismic and gravity. The most commonly used method is electrical, followed by electromagnetic, as they directly give information on the quantity and quality of water present in the primary and secondary pore spaces of the formations. Therefore these two methods are used in the pilot project areas.
 
a. Electrical methods
The main purpose of electrical surveys is to identify groundwater yielding zones, their geometry, variation in quality (in terms of salinity), and direction of groundwater movement.
Groundwater, with the various dissolved salts it contains, is ionically conductive and enables electric currents to flow into the ground. Thus, the measurement of subsurface resistivity gives information on the presence of water as well as on the lithology.
In the simplest form of electrical resistivity method, a known amount of electrical current is passed into the ground through a pair of electrodes. The potentials developed due to the current within the ground are measured across another pair of electrodes on the ground.
 
Schematic view of electrical resistivity method
Source: P. Chandra
 
The following electrical techniques are applied in the pilot project areas:
i. Resistivity and induced polarization sounding and resistivity profiling
  Resistivity sounding, also known as Vertical Electrical Sounding (VES), is conducted to get information on depth-wise resistivity variation. In this technique, the current electrode / potential electrode separation is increased in a sequence. It yields depth-wise resistivity variations, which are interpreted in terms of aquifer aquitards at depth and salinity of water in the aquifers.
  Resistivity profiling is conducted by shifting the fixed set up of current electrode from one station to another along a profile line to get lateral variation of resistivity. Gradient resistivity profiling is a special set up for resistivity profiling in which current electrodes are kept at large separation and the one-third central space is scanned by moving the potential electrode pair.
  Induced polarization sounding is conducted to measure the chargeability associated with clays. It is mainly to differentiate between the conductive clays and conductive saline water saturated zones
  Example of an electrical resistivity sounding curve from a hard rock area interpreted for depth-wise variations in resistivity and saturated fracture zones (high discharge zone confirmed through drilling).
 
  Source: CGWB
   
  Example of a gradient resistivity profile showing lateral variations in resistivity and demarcation of relatively low resistivity saturated fracture zones (high discharge zone confirmed through drilling)
 
  Source: CGWB
   
  Example of a subsurface lithological section based on VES results
 
  Source: CGWB
   
ii. Resistivity Imaging
  Resistivity imaging technique is a new development in recent years to image the subsurface having moderately complex geology and wide variations in aquifer occurrences. It combines resistivity sounding and profiling, incorporates the effects of lateral variations in resistivity on sounding, and produces a two- or three-dimensional subsurface resistivity image, thus leading to the three-dimensional geometry of the aquifers. Such surveys are carried out automatically using a large number of equally spaced collinear array of electrodes connected to a multi-core cable. A laptop microcomputer together with an electronic switching unit is used to automatically select the relevant electrodes for each measurement. The major time is taken only in planting the electrodes.
  In the pilot study, electrical resistivity imaging is used to substantiate the sounding results across the geologic structures where conventional approach of sounding or profiling may not yield fruitful results.
   
  Schematic view of mutli-electrode arrangement for resistivity imaging.
 
  Source: Loke, http://www.georentals.co.uk/Lokenote.pdf
   
  Example of an electrical resistivity image.
 
  Source: NGRI
   
iii. Ground electromagnetic method
  Measurement of subsurface resistivity variations by electrical methods (direct current resistivity) to delineate aquifers is useful and popular. However, it has some procedural and technical constraints, such as the injection of current into ground through electrodes, the difficulty in current penetration if the surface layer is extremely resistive, e.g., exposed rocks, dry sands, desert sand dunes, etc, and shallow near-surface in-homogeneities affecting the measurements. Also, non-availability of the large open stretch required to spread the current electrodes, about 3 to 5 times the desired depth of exploration, prevents its application at places.
   
  In electromagnetic (EM) methods, the subsurface resistivity or conductivity variations are measured inductively. Advantages of EM methods as compared to electrical methods are the following:
 
The ratio of depth of exploration to surface spread in EM is comparatively high.
EM is relatively less affected by near-surface lateral changes in resistivity.
EM surveys are faster in data acquisition than the electrical resistivity surveys.
EM methods are in general more effective in detecting conductive layers than resistive layers so highly resistive overburden does not hinder the detection of deeper conductive targets.
   
  The disadvantages of EM methods are as follow:
 
In presence of conductive overburden, like clay or saline water saturated layer, the deeper thin conductive freshwater saturated fracture zones may not get detected.
The effect of anthropogenic features like power line, rail, buried cables, fencing etc., is relatively pronounced in EM.
   
  The EM surveys can be conducted in frequency domain (FEM) as well as Time domain, also know as Transient (TEM). While FEM is a continuous excitation method, in TEM measurements are made at different times of the order of micro to milli seconds after the transmitter current is put off.
  Applications of FEM and TEM methods in mineral exploration are quite common and popular. In applying EM methods for groundwater investigations, one of the differences observed is that the subsurface conductivity contrasts encountered are much smaller than that in mineral prospecting.
  EM methods, and particularly FEM, are used for reconnaissance and for delineating saturated weathered and fracture zones having conductivities higher than the compact and dry surrounding, and also for selecting sites for water well drilling. Generally, EM methods are used in conjunction with electrical resistivity method.
  In the pilot study of the aquifer mapping programme, Time-Domain Electromagnetic soundings are carried out at selected places in addition to the direct current resistivity soundings and imaging to confirm the results and jointly invert the data to reduce the ambiguities. As of now, TEM soundings for aquifer delineation have not been in regular practice in India, except for sporadic work carried out by NGRI. If TEM soundings yield better results than direct current resistivity sounding for a particular hydrogeological terrain, or help reduce the ambiguities, conducting TEM sounding, which is quite fast, will become the standard approach for aquifer mapping in that terrain by surface geophysical technique.
   
  Schematic view of TEM sounding and diagram
 
  Source: Adapted from North Carolina Department of Environment and Natural Resources, http://www.ncwater.org/Education_and_Technical_Assistance/Ground_Water/TDEM/