Nonlinear electroseismic exploration

A method for electroseismic prospecting of a subterranean formation, said method comprising the steps of:

• (a) selecting a periodic waveform;
• (b) generating said waveform as an electrical signal and transmitting it into said subterranean formation at a preselected frequency;
• (c) detecting and recording seismic signals resulting from conversion of the electrical energy into seismic energy in said subterranean formation;
• (d) Fourier transforming said seismic signals from the time domain to the frequency domain;
• (e) collecting the transformed data at twice the signal frequency, and extracting amplitude and phase information;
• (f) repeating steps (b)-(e) for a plurality of different signal frequencies;
• and
• (g) inverse Fourier transforming the extracted amplitude and phase information of step (e) back to the time domain.

An electric signal generator for generating waveforms for use as source signals to be injected into the ground in electroseismic prospecting for hydrocarbons, comprising:

• (a) a signal generator adapted to take AC input and convert it to a preselected output waveform suitable for electroseismic prospecting, said generator having a plurality of switches to shape said output waveform and at least one transformer to control the voltage of said output waveform;
• (b) a controller means having a computer and being connected to and controlling said switches and being connected to and monitoring said AC input, said computer being programmable to cause said switches to select phase and voltage levels from said AC input to produce said preselected output waveform and a corresponding reference waveform;
• and
• (c) an interface means connected to said controller means and having terminals for connection to a seismic data acquisition system, said interface means adapted to (1) synchronize the start of seismic data acquisition with the start of the waveform generated by said signal generator, and (2) send said reference waveform to the seismic data acquisition system for use in data processing.

An electric signal generator for generating waveforms for use as source signals to be injected into the ground in electroseismic prospecting for hydrocarbons, said generator having a positive output terminal, a negative output terminal and a common output terminal, comprising:

• (a) three transformers, each having a primary winding and a secondary winding, said primary windings having terminals for connection to, respectively, the three phases of a three-phase AC power source, said secondary windings each having two ends and a center tap;
• (b) six controllable switching means, each switching means having one input and two output terminals, each switching means being connected at its input terminal to a different end of the three secondary windings, one output terminal of each switching means being connected together and to said positive generator output terminal, the other output terminal of each switching means being connected together and to said negative generator output terminal, and said common output terminal of the generator being connected to the center taps of the three secondary windings;
• (c) a controller means connected to each of the six switching means, said controller means having computer means sending signal voltages to each of said switching means causing each output terminal of said switching means to be connected or not to the input terminal of said switching means according to said signal generated by said computing means, said computing means being programmable to define a desired waveform and a processing reference waveform, said controller means being also connected to said primary winding terminals with said computing means monitoring such input information to select phase and voltage levels needed to produce the desired waveform;
• and
• (d) an interface means connected to said controller means and having terminals for connection to a seismic data acquisition system, said interface means adapted to (1) synchronize the start of seismic data acquisition with the start of the waveform generated by said signal generator, and (2) send said processing reference waveform to the seismic data acquisition system for use in later data processing.

A method for electroseismic delineation of a subterranean petroleum-bearing formation comprising the steps of:

• (a) selecting a source waveform and a reference waveform, said source and reference waveforms being selected (I) to reduce amplitudes of side lobes produced by correlation of the square of the source waveform with the reference waveform, and (ii) to reduce interference between said correlation and a correlation of the source waveform with the reference waveform;
• (b) generating the source waveform as an electrical signal and transmitting it through at least two electrodes into the subterranean formation;
• (c) detecting with one or more seismic receivers and recording seismic signals resulting from conversion of the electrical energy into seismic energy in the subterranean formation;
• and
• (d) correlating the recorded seismic signals with the reference waveform.

A method of producing hydrocarbons from a subterranean formation comprising steps of:

• (a) selecting a source waveform and a reference waveform, said source and reference waveforms being selected (i) to reduce amplitudes of side lobes produced by correlation of the square of the source waveform with the reference waveform, and (ii) to reduce interference between said correlation and a correlation of the source waveform with the reference waveform;
• (b) generating the source waveform as an electrical signal and transmitting it into the subterranean formation;
• (c) detecting and recording seismic signals resulting from conversion of the electrical energy into seismic energy in the subterranean formation;
• (d) correlating the recorded seismic signals with the reference waveform;
• (e) using the information from step (d) to assess the commercial hydrocarbon potential of the subterranean formation;
• and
• (f) producing any hydrocarbons identified in step (e).