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Geophysical Studies
 
Airborne Geophysical Methods
For fast and close-spaced coverage of the pilot areas, Helicopter-borne Time Domain Electromagnetic surveys are carried out. In airborne geophysical surveys, the physical properties of the subsurface are measured from the air.
The advantages of Heliborne EM surveys are continuous and fast data-acquisition with high accuracy and resolution, and access to any type of terrain. They enable to acquire large volume of data and are relatively economical as compared to surface surveys in terms of quantum of data obtained in a set time.
In the past, Heliborne EM surveys have generally been conducted for mineral exploration. Recently, they have been used for aquifer mapping in Europe, USA and Australia. This is the first time that heliborne geophysical surveys are carried out in India for aquifer mapping.
 
Source: Aarhus University
 
The SkyTEM is a heliborne hook on time-domain electromagnetic system. There is no operator and instrument in the helicopter. The pilot can fly the helicopter independently and the data acquisition is automatic. It is a dual moment system facilitating measurements for shallow and deeper subsurface information separately.
SkyTEM can measure from very early time to late times (in the order of micro and milli seconds), which provide information on near surface aquifers to deep aquifers and the geological structures.
Hard rocks areas having wide ranging hydrogeological heterogeneities within a short distance are covered by close-spaced (100 to 200m) flight lines heliborne surveys, whereas sedimentary areas are covered by relatively wider flight line spacings. Confirmation of heliborne EM anomalies is done by integrating them with ground-based geophysical and borehole data. Thus, the basic objective of heliborne surveys is to prepare shallow to moderately deep physical property maps and also to narrow down the zone of interest for fracture zone identification and other terrain specific objectives like identification of fresh-saline interface, aquifer boundary or confining clay layer, etc.
 
Schematic view of groundwater condition in hard rock area.
Source: P. Chandra, J. Roux
Under the pilot study, heliborne surveys are conducted in all the pilot project areas. Prior to the actual heliborne field surveys, a forward modeling exercise is carried out to define precise survey parameters, guide the surveys, assess the limitations, and manage the expected targets.
 
Example of forward modeling for heliborne TEM response.
Source: NGRI
Depending on the results obtained through heliborne surveys and their validation with ground geophysics and borehole information, heliborne surveys will be adopted as a methodology for the upcoming National Aquifer Mapping Programme in the entire country.