Appendix L

General Formula for Apparent Resistivity

If there are two current electrodes and two potential electrodes on the surface of a half-space, the configuration can be represented as follows:

It is not necessary that the four electrodes be in a straight line.

If apparent resistivity is Ra  and V is the difference in voltage between M and N then

   

Most modern earth resistivity instruments report V/I as resistivity, R. Therefore


  

For a clear derivation of the above equation, see:

Reynolds, J.M., 1997, An Introduction to Applied and Environmental Geophysics: New York, John Wiley & Sons, Inc. pp. 424-426

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