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Fig. 6-5. Potential field due to electrical current passing from electrode A to electrode B
in a very large volume of homogeneous material.

the lateral depth of investigation. The greater the spacing, the greater the
“depth” of investigation.
Rock sequences are never electrically homogeneous: they consist of layers
of different resistivities, and it is the purpose of electrical resistivity logging
to measure the resistivities of the various layers by measuring the potential
at a fixed distance close to the current electrode as they are moved up the
hole. However, it will be evident that the resistivity of a bed cannot be
measured if the bed is thinner than the spacing. In fact, the apparent resis-
tivity of a bed must be corrected for thicknesses up to a few times the
spacing because beds of different resistivities above and below affect the
potential field. Other corrections concern the resistivity of the mud and the
diameter of the borehole. If a large diameter hole is filled with a saline, con-
ductive mud, most of the current passing between the electrodes will pass
down the hole.