A system and method for generating low frequency seismic source signal by the non-linear combination of two higher frequency energy signals to form a shear-wave signal in geologic formations. The differential between the higher frequencies operates in a nonlinear medium to generate a highly directive, low frequency seismic energy source. A shear-wave signal is produced in the geologic formations by the interaction of two P-wave beams focused on a volume of such geologic formations. Each beam propagates the same signal but in opposite polarity to the other beam. At the beam intersection the vertical component cancels and the horizontal component is amplified. The system and method facilitates application of P-wave and shear-wave energy to subsurface geologic formations underlying soil or water.
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