ELECTRICAL RESISTIVITY METHODS
PRINCIPLE: Ohm’s law. In an electrical circuit, the current (I) is proportional to the potential difference or the electromotive force, (V), and the resistance R. That is, I = V/R or R = V/I. The materials are classified as Good Conductors and bad conductors or insulators. Good conductors allow the electric current to pass through them e.g., metals, water, etc., and bad conductors or Insulators do not allow electric current to pass through them e.g., rocks, wood, etc.
Application of Resistivity methods.
- Location of Dugwell, Borewell, Tubewell sites.
- Movement of GW flow
- Granular zone in sedimentary areas
- Water quality
- Sea water intrusion studies
- Deep aquifers
- Artificial recharge studies
SURVEY METHODS: 1. Electrical profiling: Horizontal variation in resistivity in the subsurface is measured. Profiling curve (Resistivity Versus Distance)
Electrical sounding: Vertical variation in resistivity with depth is measured. The curve is called as VES curve. xx – hard rock (un-saturated formation)
fractured rock (saturated formation)
VLF METHOD
Basically, it is an electromagnetic method. It works with a freq of 15-30 kHz of em waves transmitted from a distant navigation transmitter. The em waves are induced into ground, in the presence of conductor or saturated body, secondary em waves are generated.
The instrment used for this survey is called VLF WADI.
The instrument is used to delineate worthy and unworthy areas.
1. Case study.
Aquifer delineation work was carried out in GP 9 watershed under AQMP PROJECT by conducting resistivity surveys in a grid pattern. A grid of 1 km x 1 km was prepared for the entire watershed and at every intersection point, VES was conducted.
Blue colour – Low resistivity zone (Aquifer)
Red colour – High resistivity zone (Un-worthy area or unsaturated area)
Aquifer delineation work was carried out in Jalna district under MWSIP Project (GP 20) and iso-resistivity zone was prepared for AB/2 – 15m separation.
The Electrical resistivity surveys play an important role from Groundwater development to Groundwater management.
All regional offices Konkan Bhavan, Pune, Nashik, Aurangabad, Amravati and Nagpur are well equipped with Geophysical units. The survey charge for cultivators is Rs. 1500. The amount should be deposited in nearest District Senior Geologist GSDA Office.