A method for detecting the seismic discontinuity in acoustic impedance caused by an acoustically hard, reflective object buried a few feet below poroelastic soil using seismic activity induced through acoustic coupling with a remote sound source. The abrupt change in the soil impedance caused by the buried object causes sound to reflect between the object and the surface and increase the amplitude of the seismic vibrations induced by the incident acoustic energy. The change in the seismic displacement of the soil is on the order of angstroms which can be detected using remote optical test equipment such as a laser-doppler vibrometer (LDV) commonly used in nondestructive testing. A sound source emits sound at frequencies that induce significant seismic coupling with the poroelastic soil. Part of a beam of laser light of an LDV is scanned over the ground. The laser light is shifted in frequency from its source frequency by an amount intended to approximate the frequency of the anticipated seismic vibrations. The seismic vibrations of the soil frequency modulate the laser light to form upper and lower side bands. The amplitude of the side bands increases in the presence of an acoustically hard object due to the greater seismic vibrations over the acoustically reflecting surface. Laser light that is scattered back is combined with unshifted laser light in the photodetector of the LDV so as to eliminate the optical frequency effects of the laser and to cause the carrier frequency and side bands to emerge as distinct signals.
