1. A technique for investigating the region between two or more boreholes by measuring the transit times and/or amplitudes of P- and/or S-waves from a source located in one borehole to geophones in other boreholes. Usually implies cross- hole tomography (q.v.). Three-component geophones may be used. 2. A technique for resistivity or electromagnetic measurements between boreholes, used for fracture and cavity detection, reservoir studies, and orebody delineation.