ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

2aUW7. Real time signal processing for high-frequency long distance crosswell tomography.

Murat Kuru

Tokuo Yamamoto

Geoacoustics Lab., RSMAS, Univ. of Miami, 4600 Rickenbacker Cswy., Miami, FL 33149

Acoustic crosswell tomography was used for determining the sediment porosity, permeability, and shear strength from travel-time measurements by means of travel-time inversion and rock mechanics [Yamamoto et al., Geophysics (1994) (1995)]. These properties can be used to model the propagation and scattering of acoustic waves in saturated sediments. Due to the effects of intrinsic attenuation, transmission loss, and scattering, crosswell measurements usually are limited to ranges of 50--100 times the wavelength of the signals. It was believed that the diffusive nature of scattering put the coherent limit of acoustic wave propagation in sediments at approximately 1000 wavelengths. To extend the measurements of the propagation of acoustic waves to longer ranges, a real time processing and data acquisition system was employed. A crosswell tomography experiment with a crosswell distance of 540 m was performed at a limestone aquifer at Sanibel Island, Florida. Pseudorandom binary sequences modulated with carrier frequencies up to 6 kHz, which puts the range around 1400 wavelengths, were successfully transmitted by using pulse compression and coherent averaging techniques. The hydraulic structure of the Earth with a cross section of 540 mx150 m has been imaged at spatial resolution of a few meters. [Work supported by ONR.]