Abstract: A method and apparatus for investigating the electro-magnetic characteristics of the sub-soil. At least four and normally five directional magnetic field detectors placed on the ground. A first set of two detectors is spaced apart and oriented in the same general direction, a second set of two detectors is also spaced apart and oriented in a second direction transverse to the first direction, and the fifth detector is positioned on the ground transversely to the first four detectors. The output signals generated by the detectors, which are characteristic of portions of the sub-soil underlying the detectors, are simultaneously recorded for subsequent analysis to identify sub-soil characteristics, discontinuities, mineral deposits and the like.