System and method for investigating sub-surface features of a rock formation using compressional acoustic sources

A system for investigating rock formations outside a borehole, the system comprising:

• a first acoustic source configured to generate a first acoustic beam in a first direction relative to a longitudinal axis of the borehole, the first acoustic beam comprising a first train of pulses at corresponding first frequencies and separated in time, each pulse in the first train of pulses being modulated in frequency;
• a second acoustic source configured to generate a second acoustic beam in a second direction relative to the longitudinal axis of the borehole, the second acoustic beam comprising a second train of pulses at corresponding second frequencies and separated in time, each pulse in the second train of pulses being modulated in frequency, wherein the first and the second acoustic sources are arranged within a localized area of the borehole and wherein the first and the second acoustic beams intersect in an intersection volume outside the borehole;
• a controller configured to steer the first acoustic beam at a first beam direction and the second acoustic beam at a second beam direction, wherein the first and second beam directions are selected such that the first and second acoustic beams intersect at the intersection volume away from the borehole and the first train of pulses and the second train of pulses are timed to interact non-linearily at the intersection volume to generate a shear acoustic wave comprising a third train of pulses at corresponding third frequencies and separated in time, each pulse in the third train of pulses being modulated in frequency, the shear acoustic wave having a third direction relative to the first beam direction and the second beam direction, the shear acoustic wave returning to the borehole due to a non-linear mixing process in a non-linear mixing zone within the intersection volume, the third frequencies being equal to a difference between the corresponding first frequencies and the second frequencies;
• and
• a receiver array disposed in the borehole and positioned in the borehole to receive the shear acoustic wave comprising the third train of pulses at the third direction from the intersection volume,
• wherein the first and second acoustic sources and the receiver array are arranged to be independently moveable within the borehole in at least two degrees of freedom including translation along a longitudinal axis of the borehole and rotation in azimuth about the longitudinal axis of the borehole, wherein the first and second acoustic sources and the receiver array are positioned within the borehole so as to maximize a magnitude and detectability of the shear acoustic wave that propagates back from the intersection volume.