Detection of near subsurface voids that are resistive or contain conductive elements within the void space

A non-transitory medium on which is stored software for detecting near-subsurface voids in the earth, the software comprising instructions that when executed cause one or more processors to:

• capture signals received by a plurality of controlled source electromagnetic receivers responsive to signals generated by a controlled source electromagnetic transmitter, wherein the signals comprise pseudo-random numeric codes that are over 8191 chips long that create a broad spectrum signal that is not coherent with noise sources;
• detect bi-static doublets in the captured signals, comprising:
• cross-correlate the signals generated by the controlled source electromagnetic transmitter with the signals received by the plurality of controlled source electromagnetic receivers, producing a cross-correlated data;
• detect peaks and valleys within the cross-correlated data;
• sort the cross-correlated data by significance;
• and
• identify bi-static doublets in the sorted cross-correlated data;
• and
• identify near-subsurface voids corresponding to the detected bi-static doublets.

A method for detecting near-subsurface voids in the earth, comprising:

• positioning a controlled source electromagnetic transmitter on a surface of the earth;
• distributing a plurality of controlled source electromagnetic receivers across the surface of the earth relative to the controlled source electromagnetic transmitter, wherein the plurality of controlled source electromagnetic receivers are synchronized with the controlled source electromagnetic transmitter;
• generating a first signal into the earth by the controlled source electromagnetic transmitter, wherein the first signal comprises pseudo-random numeric codes that are over 8191 chips long that create a broad spectrum signal that is not coherent with noise sources;
• receiving a second signal by the plurality of controlled source electromagnetic receivers responsive to the first signal;
• generating a third signal by cross-correlating the second signal to the first signal;
• detecting bi-static doublets in the third signal, comprising:
• cross-correlating the signals generated by the controlled source electromagnetic transmitter with the signals received by the plurality of controlled source electromagnetic receivers, producing a cross-correlated data;
• detecting peaks and valleys within the cross-correlated data;
• sorting the cross-correlated data by significance;
• and
• identifying bi-static doublets in the sorted cross-correlated data;
• and
• identifying near-subsurface voids in the earth corresponding to the bi-static doublets.