Abstract: In accordance with the invention, there is provided a system for determining the location of an acoustic impedance boundary within the earth. A seismic signal is generated at a first location and detected at a second location so that the signal travels from the source to the acoustic impedance boundary and from the acoustic impedance boundary to the sensor. In a first embodiment, the travel time from source to receiver and the direction, in three dimensions, of the arrival path of the signal at the sensor are determined. Utilizing the determined direction of travel to the sensor and known information regarding the velocity profile of the earth strata between the source and the acoustic impedance boundary and between the receiver and the acoustic impedance boundary, a calculated travel path from the source to receiver is determined over which the calculated travel time substantially equals the measured travel time.