Data acquisition and processing system and method for investigating sub-surface features of a rock formation

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

• a first acoustic source configured to generate a first acoustic signal at a first frequency;
• a second acoustic source configured to generate a second acoustic signal at a second frequency;
• wherein the first acoustic source and the second acoustic source are arranged and positioned within the borehole and are controllable such that the first and the second acoustic signals intersect in an intersection volume outside the borehole and a start time difference is provided between the second acoustic signal and the first acoustic signal;
• a receiver arranged within the borehole, the receiver being configured to receive a detected signal returning to the borehole having a frequency equal to a difference between the first frequency and the second frequency, the detected signal being generated by a non-linear mixing process from the first acoustic signal and the second acoustic signal in a non-linear mixing zone within the intersection volume;
• and
• a recording system configured to record the detected signal and store the detected signal in a storage device and to record measurement parameters including a position of the first acoustic source, a position of the second acoustic source, a position of the receiver, elevation angle and azimuth angle of the first acoustic signal and elevation angle and azimuth angle of the second acoustic signal.