AVO Analysis of Multibeam Backscatter, an Example from Little Bay, NH and Skjalfandi Bay, Iceland

Abstract

In the seismic reflection method, it is well known that seismic amplitude varies with the offset between the seismic source and detector, and that this variation is the key to the direct determination of lithology and pore fluid content of subsurface strata. Based on this fundamental property, amplitude-versus-offset (AVO) analysis has been used successfully in the oil industry for the exploration and characterization of subsurface reservoirs. Multibeam sonars acquire acoustic backscatter over wide range of incidence angles, and the variation of the backscatter with the angle of incidence is an intrinsic property of the seafloor. With the necessary changes being made, a similar approach to seismic AVO analysis can applied to the acoustic backscatter. To illustrate this approach, AVO analysis was applied to a Simrad EM3000 (300kHz) multibeam sonar dataset from Little Bay, NH, and to a Simrad EM300 (30kHz) multibeam sonar dataset from Skjalfandi Bay, Iceland. The analysis starts with the backscatter time series stored in raw Simrad datagrams, which are then corrected for seafloor slope, insonification area, time varying and angle varying gains. Then, a series of AVO attributes (near, far, slope, gradient, fluid factor, product etc) are calculated from the stacking of a number of consecutive time series. Based on the calculated AVO attributes and the inversion of a modified Jackson et al (1986) acoustic backscatter model we estimate the acoustic impedance, the roughness, and consequently the grain size of the insonified area on the seafloor. In Little Bay, the estimated impedance and the grain size were compared to in-situ measurements of sound speed and to the direct analysis of grain size in grab samples, showing a very good correlation. In Skjalfandi Bay, the AVO attribute of fluid factor was calculated, which presented an estimate of the gas/fluid content in the sediment structure. The areas with high fluid factor anomalies correlated to regions that showed evidence of gas in seismic profiles.

Department

Center for Coastal and Ocean Mapping

Publication Date

12-2004

Volume

85, Issue 47

Journal Title

Fall Meeting, American Geophysical Union (AGU)

Conference Date

Dec 13 - Dec 17, 2004

Publisher Place

San Francisco, CA, USA

Publisher

American Geophysical Union Publications

Document Type

Poster

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