HR: 0800h
AN: OS41C-0483    [Abstracts]
TI: AVO ANALYSIS OF MULTIBEAM BACKSCATTER, AN EXAMPLE FROM LITTLE BAY, NH AND SKJALFANDI BAY, ICELAND
AU: * Fonseca, L
EM: luciano@ccom.unh.edu
AF: CCOM/JHC University of New Hampshire, 24 Colovos Road, Durham, NH 03824 United States
AU: Mayer, L
EM: larry.mayer@unh.edu
AF: CCOM/JHC University of New Hampshire, 24 Colovos Road, Durham, NH 03824 United States
AU: Kraft, B
EM: bjkraft@cisunix.unh.edu
AF: CCOM/JHC University of New Hampshire, 24 Colovos Road, Durham, NH 03824 United States
AU: Brandsdottir, B
AF: University of Iceland, Oskju, Sturlugata 7, Reykjavik, 101 Iceland
AU: Richter, B
EM: br@isor.is
AF: Icelandic Geosurvey, Grensasvegi 9, Reykjavik, 108 Iceland
AB: 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.
UR: http://ccom.unh.edu
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3045 Seafloor morphology and bottom photography
DE: 3200 MATHEMATICAL GEOPHYSICS (New field)
DE: 3210 Modeling
DE: 0933 Remote sensing
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting