Patent EA-005193-B1 (EA)
Title: Nonlinear electroseismic exploration
Inventor: HORNBOSTEL SCOTT C
IPC: G01V1/02
Filed: 2002-04-23
Abstract: 1. A method for electroseismic prospecting of a subterranean formation, said method 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 into the subterranean formation; (c) detecting 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. 2. The method of claim 1, wherein said source waveform and said reference waveform are constructed from a single element, said element consisting of a single full cycle of a preselected periodic waveform, said elements being pieced together with polarities sequentially specified by a preselected digital code, said periodic waveform having a frequency predetermined to give desired depth penetration of said subterranean formation. 3. The method of claim 2, wherein the waveform element is a single cycle of a 60 Hz sinusoid. 4. The method of claim 2, wherein the waveform element is constructed from selected phases of a three-phase power supply to have a desired frequency less than or equal to 60 Hz. 5. The method of claim 2, wherein said digital code is a maximal length shift-register {1,-1} sequence with the resulting source waveform modified to a {1,0} sequence by zeroing the negative polarity elements; said reference waveform is the square of the source waveform before the negative polarity elements are zeroed, said squared wave then undergoing polarity reversal of segments corresponding to "-1" terms in said maximal length shift-register sequence; and said correlation is circular correlation. 6. The method of claim 5, wherein said interference reduction is accomplished by constructing a second source waveform by reversing the polarity of the source waveform selected in step (a), repeating steps (b)- (d) with said second source waveform, and then adding together the correlated seismic signals resulting from the two source waveforms. 7. The method of claim 5, wherein said interference reduction is accomplished by replacing some pre-selected "1" terms in said {1,0} source wave coding sequence with "-1" terms, said replacement being designed to substantially maximize the time separation between said correlation of the source waveform and said correlation of the square of the source waveform. 8. The method of claim 7, wherein which "1" terms to replace with -1 are determined by multiplying said {1, 0} sequence by a circularly rotated version of said maximal length shift-register {1,-1} sequence, thereby generating the desired {1,-1, 0} code. 9. The method of claim 5, wherein the degree of said maximal length shift-register sequence is sufficiently large to reduce said side lobe amplitudes to a predetermined level. 10. The method of claim 5, wherein said transmitting of said source waveform into said subterranean formation is repeated a sufficient number of times to reduce loss of information due to said circular correlation to a predetermined level. 11. The method of claim 7, wherein said interference reduction is further accomplished by constructing a second source waveform by reversing the polarity of the source waveform selected in step (a), repeating steps (b)- (d) with said second source waveform, and then adding together the correlated seismic signals resulting from the two source waveforms. 12. The method of claim 1, wherein said reference waveform is bandpass filtered to conform to the expected frequency content of said recorded seismic signals. 13. The method of claim 1, further comprising the following additional step: (e) ordering the seismic signals from the subterranean formation by amplitude, and interpreting any substantially larger amplitudes to represent hydrocarbons. 14. 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. 15. An electrical signal for use in electroseismic prospecting of a subterranean formation, said signal having a waveform constructed from a single element, said element consisting of a single full cycle of a preselected periodic waveform, said periodic waveform having a frequency predetermined to give desired depth penetration of said subterranean formation, said elements being pieced together with polarities sequentially specified by a preselected digital code, said digital code being chosen to substantially minimize side lobes resulting from correlation of the square of the signal waveform with a reference waveform, said reference waveform also being chosen to substantially minimize said side lobes, said digital code and said reference waveform also being chosen to reduce interference with said correlation from a correlation of the signal waveform with said reference waveform. 16. The electrical signal of claim 15, wherein said waveform element is a single cycle of a 60 Hz sinusoid. 17. The electrical signal of claim 15, wherein said waveform element is constructed from selected phases of a three-phase power supply to have a desired frequency less than or equal to 60 Hz. 18. The electrical signal of claim 15, wherein said digital code is a maximal length shift-register {1,-1} sequence with the resulting signal waveform modified such that the negative polarity elements are zeroed, said reference waveform is the square of the signal waveform before the negative polarity elements are zeroed, said squared wave then undergoing polarity reversal of squared elements corresponding to "-1" terms in said maximal length shift register sequence to yield said reference waveform, and said correlation is circular correlation. 19. The electrical signal of claim 15, wherein said digital code is a {1,-1, 0} sequence derived from a maximal length shift-register {1,-1} sequence by first modifying said {1,-1} sequence to replace all "-1" terms with 0, then modifying the resulting {1, 0} sequence to replace some preselected"1"terms with-1, said replacement of "1" terms with-1 being determined so as to reduce said interference by substantially maximizing the resulting time separation between the correlations of the signal waveform and of the square of the signal waveform with said reference waveform, said reference waveform being constructed by repeating said element the same number of cycles as in said signal waveform, then squaring the resulting waveform, then reversing the polarity of those squared elements corresponding to "-1" terms in said {1,-1} sequence; and said correlation is circular correlation. 20. The electrical signal of claim 19, wherein the replacement of "1" terms with -1 is accomplished by multiplying said {1, -1, 0} sequence term-by-term by a circularly rotated version of said {1,-1, 0} maximal length shift-register sequence. 21. The electrical signal of claim 18, wherein the degree of said maximal length shift-register sequence is sufficiently large to reduce said side lobe amplitudes to a predetermined level, and said reference waveform is modified by subtracting any low frequency components, including DC. 22. 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. 23. 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