A resistivity-measuring system using a “normal” electrode configuration. A constant current is passed between a current electrode on the sonde (A electrode) and one at the surface (B electrode) while the potential difference is measured between another on the sonde (M electrode) and a reference electrode (N electrode). The “spacing” is the diference between the A and M electrodes. Usually spacing of about 16 inches is used for a the short normal and 64 inches for the medium or long normal. The measure point is midway between the A and M electrodes.
A normal device has a depth of investigation said to be about twice the AM spacing. The normal is an unfocused device which produces a symmetrical curve which has been particularly useful in correlation and indetermination of lithology. Formation detail can be increased by decreasing the AM spacing, but depth of investigation suffers.
