Morley, Mike1, Ross Chapman1, Tom McGee2, J.
(1) University of Victoria, Victoria, British Columbia, BC
(2) University of Mississippi, University, MS
ABSTRACT: Estimation of Seismic Velocities in the Hydrate Stability Zone Using Ship Noise at a Vertical Hydrophone Array
Vertical arrays have been used extensively in underwater acoustics for estimating velocities in geoacoustic models of the ocean bottom. The estimation method is based on matched field processing, a technique in which calculated replicas of the acoustic field are compared to the measured signal at the array. The replica fields are based on a geoacoustic model of the ocean bottom that generally consists of velocities and densities for each layer. The estimation process searches for the set of parameter values that generates the best match between the calculated and measured fields. Vertical arrays are used in the integrated hydrate monitoring station to detect changes in the seismic velocities that may be related to gas venting or hydrate dissociation. The paper describes a modification to the technique that uses broadband noise radiated by ships passing near the array as the sound source. Spectral components of the noise within the band 50-200 Hz are processed separately, and the results are averaged over the band. The method is applied first to a synthetic ship signal in a geoacoustic environment that simulates the site of the DoE/JIP borehole at Atwater Valley Block 14 in the Gulf of Mexico. The simulation is used to test the performance of the techniques for different intensity levels of the ship noise, and to test the sensitivity to changes in the velocity in the layers.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.