Four-point method is also called as
Right Answer is:
To measure earth resistivity, a four-terminal instrument is used, along with four small-sized electrodes driven down to the same depth and equal distances apart in a straight line. Four separate lead wires connect the electrodes to the four terminals on the instrument, as shown. Hence the name of this test: the four-terminal method.
Small electrodes are inserted into four small holes in the earth all to a depth of b meters and spaced along a straight line at intervals of meters, and making electrical contact with the earth only at the bottom. A test current I is injected through the earth between the two outer electrodes and the potential E between the two inner electrodes is measured with a potentiometer or high-impedance voltmeter. The ratio between the observed potential and the injected current is referred to as the apparent resistance, which is a function of soil resistivity and the electrode geometry. The apparent soil resistivity is computed by multiplying the apparent resistance by a geometric multiplier. The apparent soil resistivity down to a depth equal to a S.
If the electrode depth (B) is kept small compared to the distance between the electrodes (A), the following formula applies:
ρ = 2πAR
ρ is the average soil resistivity to depth A in ohm-cm,
A is the distance between the electrodes in cm,
R is the Megger earth tester reading in ohms.