System and method for investigating sub-surface features of a rock formation with acoustic sources generating coded signals

A system for investigating rock formations outside a borehole using coded signals from acoustic sources, the system comprising:

• a first acoustic source configured to generate a first acoustic signal comprising a first plurality of pulses arranged as a time sequence, the first plurality of pulses being separated in time, each pulse comprising a first modulated signal at a central frequency, wherein central frequencies of two consecutive pulses are different;
• a second acoustic source configured to generate a second acoustic signal comprising a second plurality of pulses arranged as a time sequence, the second plurality of pulses being separated in time, wherein a separation in time between centers of two consecutive pulses is the same as a separation in time between centers of two corresponding pulses in the first plurality of pulses, wherein a start time difference is provided between a start time of a broadcast of the second plurality of pulses and a start time of a broadcast of the first plurality of pulses, wherein each pulse comprises a second modulated signal, and a central frequency of the second modulated signal within each pulse in the second plurality of pulses is a selected fraction of the central frequency of the first modulated signal within each corresponding pulse in the first plurality of pulses;
• wherein the first acoustic source and the second acoustic source are arranged within the borehole and are controllable such that trajectories of the first and the second acoustic signals intersect in an intersection volume outside the borehole;
• a receiver arranged within the borehole, the receiver being configured to detect a received signal returning to the borehole, the received signal including a 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 processor configured to perform data processing on the received signal to extract the signal generated by the non-linear mixing process over noise or over signals generated by a linear interaction process, or both.