## Geophysical Methods

### Electrical Resistivity Method:-

Electrical resistivity ( ρ ) of a conductor is

ρ = RA / L

Where R = Resistance ( Ω ), A = cross section area of the conductor  cm2

L = length of conductor ( cm ) , ρ = resistivity ( Ω– cm )

Resistivity of the material depends upon the types of materials and its moisture content i.e rocks and dry soil have higher resistivity as compared to saturated soil.The electrical resistivity methods are of two types

Electrical profiling method:-

It is also known as resistivity mapping method. In this method 4 electrodes ( made of metal spikes ) are used at a equal spacing. Two outer electrodes are known as current electrodes and inner electrodes are known as potential electrodes.The mean resistivity of the strata is determined by applying a D.C current at outer electrodes and calculating the voltage drop between the inner electrode by using given formula.

ρ = 2 π d V / I

Where I = current , d = spacing of electrodes and V = voltage drop

Resource:- slidesharecdn.com

### Electrical sounding method:-

In this method four electrodes expanded about a fixed location say P as shown in fig below. Spacing of the electrode in first setting is a1, which is increased to a2 in 2nd setting and so on. Thus the spacing of the electrodes is gradually increased to a distance equal to the depth of exploration. As the depth of current penetration is equal to the electrode spacing, changes of resistivity is correlated to the changes in soil strata.The method is useful in studying the changes in the strata with increase in depth at a point.