AVO investigations of shallow marine sediments

Amplitude-variation-with-offset (AVO) analysis is based on the Zoeppritz eq uations, which enable the computation of reflection and transmission coeffi cients as a function of offset or angle of incidence. High-frequency (up to 700 Hz) AVO studies, presented here, have been used to determine the physi cal properties of sediments in a shallow marine environment (20 m water dep th). The properties that can be constrained are P- and S-wave velocities, b ulk density and acoustic attenuation. The use of higher frequencies require s special analysis including careful geometry and source and receiver direc tivity corrections. In the past, marine sediments have been modelled as ela stic materials. However, viscoelastic models which include absorption are m ore realistic. At angles of incidence greater than 40 degrees, AVO function s derived from viscoelastic models differ from those with purely elastic pr operties in the absence of a critical angle of incidence. The influence of S-wave velocity on the reflection coefficient is small (especially for low S-wave velocities encountered at the sea-floor). Thus, it is difficult to e xtract the S-wave parameter from AVO trends. On the other hand, P-wave velo city and density show a considerably stronger effect. Attenuation (describe d by the quality factor Q) influences the reflection coefficient but could not be determined uniquely from the AVO functions. In order to measure the reflection coefficient in a seismogram, the amplitudes of the direct wave a nd the sea-floor reflection in a common-midpoint (CMP) gather are determine d and corrected for spherical divergence as well as source and streamer dir ectivity. At CMP locations showing the different AVO characteristics of a m ud and a boulder clay, the sediment physical properties are determined by u sing a sequential-quadratic programming (SQP) inversion technique. The inve rted sediment physical properties for the mud are: P-wave velocity alpha = 1450 +/- 25 m/s, S-wave velocity beta = 90 +/- 35 m/s. density rho = 1220 /- 45 kg/m(3), quality factor for P-wave Qp = 15 +/- 200, quality factor fo r S-wave Qs = 10 +/- 30. The inverted sediment physical properties for the boulder clay are: alpha = 1620 +/- 45 m/s, beta = 360 +/- 200 m/s, rho = 13 80 +/- 85 kg/m(3), Qp = 790 +/- 660, Qs = 25 +/- 10.