AN EXPERIMENTAL EVALUATION OF CHAOTIC WAVEFORMS FOR CROSSWELL SEISMIC TOMOGRAPHY

SEISMIC TOMOGRAPHY PROVIDES THE EXPLORATION INDUSTRY WITH FUNDAMENTAL INFORMATION ON THE STRUCTURE AND HYDROCARBON POTENTIAL OF A SUBSURFACE. HOWEVER, AS EXPLORATION TARGETS GET SMALLER AND MORE COMPLEX, TOMOGRAPHIC IMAGING TECHNIQUES BECOME LIMITED BY THE FUNDAMENTAL RESTRICTION OF SAMPLING AND RESOLUTION. THE USE OF HIGHER-FREQUENCY SOURCES PROVIDES LITTLE HELP, BECAUSE ATTENUATION LIMITS THE DISTANCES OVER WHICH HIGH-FREQUENCY SEISMIC WAVES CAN PROPAGATE. BASED ON RESEARCH WITH SIMILAR PROBLEMS IN THE AREAS OF UNDERWATER ACOUSTICS AND RADAR, AN EXPERIMENT IN CROSSWELL TOMOGRAPHIC IMAGING IS PLANNED USING A NEW CLASS OF WAVEFORMS KNOWN AS CHAOTIC WAVEFORMS. A CHAOTIC WAVEFORM REPRESENTS THE RESPONSE OF A NNLINEAR SYSTEM THAT IS APERIODIC AND DEPENDS ON THE INITIAL CONDITIONS OF THE SYSTEM. UNLIKE TRADITIONAL FREQUENCY MODULATED (FM) SWEEPS WHICH ARE NARROWBAND AT ANY GIVEN INSTANCE OF TIME, A CHAOTIC WAVEFORM IS BROADBAND OVER ITS ENTIRE DURATION, RESULTING IN A SHARPER AMBIGUITY FUNCTION. UNLIKE RANDOM OR PSEUDORANDOM NOISE, A CHAOTIC WAVEFORM CAN BE GENERATED BY A SEISMIC SOURCE AND TUNED FOR A SPECIFIC FREQUENCY BAND. MODEL STUDIES INDICATE THAT THE CHAOTIC SOURCE WAVEFORM WILL IMPROVE THE RESULTING SEISMIC IMAGE. THIS EXPERIMENT WILL COMPARE THE RECONSTRUCTED TOMOGRAPHIC IMAGES PRODUCED BY THE CHAOTIC WAVEFORM, AN FM SLIDE, AND AN IMPULSE. WELLS SELECTED FOR THIS EXPERIMENT ARE SEPARATED BY 330 FT. THE TEST WILL COVER THE DEPTH RANGE OF 2400 TO 2700 FT, WHICH INCLUDES CRETACEOUS LIMESTONES AND SHALES FROM THE AUSTIN CHALK, EAGLE FORD, AND BUDA FORMATIONS.