^PSEstimating Geoacoustic Properties of Marine Sediments by Matched Field Inversion Using Ship Noise*

By

Michael G. Morley ¹, N. Ross Chapman ¹, Tom McGee ², and Bob Woolsey ³

Search and Discovery Article #40164 (2005)

Posted August 20, 2005

*Poster presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005.

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¹University of Victoria, Victoria, BC ( [email protected])

²University of Mississippi, University, MS

³University of Mississippi, Oxford, MS

Abstract

Geoacoustic inversion for seabed properties using ambient noise sources in the ocean has received a great deal of interest recently. A project is currently underway to establish a remote gas hydrate monitoring station in the Gulf of Mexico. A major goal of the project is to detect changes that occur in the acoustic properties of gas-hydrate-bearing sediments due to the formation and dissocation of hydrates. It is hoped that temporal monitoring of these properties can be achieved by Matched Field Inversion (MFI) using noise from passing ships of opportunity as a sound source. In MFI, an optimization scheme is used to determine the geoacoustic model that produces the best match between the observed acoustic data and replica acoustic data calculated by a propagation model. Once a best-fit model is obtained, a Bayesian inversion approach is applied to estimate the Posterior Probability Distributions (PPD) of the geoacoustic model parameters. The moments of the PPD yield estimates of uncertainties, correlations, and sensitivity of the model parameters in the inversion. In this study, the method is investigated using both synthetic data and real ship noise data collected with a prototype vertical line array deployed in Mississippi Canyon region of the Gulf of Mexico.

Map of the Mississippi Canyon 798 test site showing position of vertical line array (red star) relative to recorded ship tracks. The green triangles indicate the positions of piston core samples taken on a subsequent cruise in January, 2004.