Abstract: A method of electromagnetic geophysical exploration of the earth's subterranean formations wherein an array of magnetometers are deployed at reference sites to obtain a measure of horizontal components of the earth's electromagnetic field and magnetometers are simultaneously deployed at a plurality of exploratory sites within a region of interest to obtain a measure of the vertical component of the earth's electromagnetic field. Resultant correlations of the horizontal and vertical components of the electromagnetic field can provide a multidimensional representation of the earth's resistivity structure.

Abstract: A tuned borehole gravity gradiometer comprises a dipole mass system having means for selectively tuning the sensitivity of the gravity gradiometer to detect variations in the earth's gravity gradient within a borehole. The tuning means includes varying the metacentric height of the dipole mass system by selectively varying the center of gravity of the dipole mass system with respect to the metacenter of the dipole mass system.