A geophysical exploration system uses a single conductor cable connected to a current source, the cable having a number of earth probes coupled to it at spaced points with addressable switching units. The switching units are activated in sequence so that the probes inject current into the earth. At locations spaced from the probes the resistivity is measured and recorded for each injection. Resistivity measurements are compared with anomaly models to find a match of patterns, indicating the presence, size and location of an anomaly.