SYSTEM AND METHOD FOR EARTH PROBING WITH DEEP SUBSURFACE PENETRATION USING LOW FREQUENCY ELECTROMAGNETIC SIGNALS

摘要

An earth probing system uses deep penetration of electromagnetic waves intosoil and other media. Advantage is taken of lower attenuation of radar wavesin soil by frequencies of about three megahertz or less. Bursts ofelectromagnetic energy of various frequencies in this range are consecutivelytransmitted. The transmitting antenna (118) is continuously tuned, so as tomaintain resonance during each burst, allowing large circulating currents andhigh power output. In a receiving antenna system, a dual antenna arrangement(111, 112) is providing for obtaining improved reception. A corresponding dualantenna circuit employs "spatial notch filtering", automatic adjustment ofantenna gain-frequency variations, as well as compensation for transmittergain variation. The system may be implemented in a totally analog, totallydigital, or hybrid manner. Preferably, a signal processing method detects anddigitally samples signals reflected from subsurface layers and buried objects(316, 316'). The invention provides means for removing system distortions andinterfering signals, for compensating for aliasing errors and frequency-dependent antenna gain and phase variations, and for avoiding masking errorsintroduced by strong reflections.