AVO investigations of shallow marine sediments

Abstract

AVO/AVA (Amplitude Versus Offset/Angle) analysis has been used to determine the physical characteristics o f shallow marine sedi ments. The parameter that can be constrained, are P-and S-wave velocity, bulk density and acoustic attenuation. Shear modulus and other geotechnical properties such as grain size can be inferred from these parameters. The theory o f AVO investigations is given by the Zoeppritz equations (Zoeppritz, 1919), which allow the com putation o f the reflection and transmission coefficients as a function o f angle o f incidence. AVO has been w idely used in the petroleum industry, usually for frequencies o f 20-100 Hz. High frequency shallow sediment AVO studies require special analysis including careful geometry and source and receiver directivity corrections. In the past, marine sediments have been m odeled as elastic materi als. However, viscoelastic models with absorption are more realis tic. At large angle o f incidence, AVA-fimctions derived from vis coelastic models differ significantly from those with purely elastic properties. The influence o f S-wave velocity on the reflection coef ficient is rather small (especially for the low S-wave velocities encountered at the seafloor). On the other hand, P-wave velocity and density show a considerably stronger effect (Fig. 1). Thus, it will be difficult, or nearly impossible, to extract the S-wave param eter from AVA trends, especially when the signal-to-noise ratio is low.In order to measure the reflection coefficient in a seismogram, the peak amplitudes o f the direct wave and the seafloor reflection in a CMP (Common Mid Point) gather are determined and corrected for spherical divergence, and source and receiver directivity (Fig. 2). At CMP locations showing different AVA characteristics, the sedi ment parameters P-and S-wave velocity, density and absorption are determined by using a Sequential Quadratic Programming (SQP) inversion technique. The use o f the viscoelastic model within the inversion gives better results in fitting the data than the elastic model without absorption.The introduction o f constraints, e.g. empirical relationships given by core investigations, can improve and stabilize the inversion results.In Fig. 2, a soft mud layer, with a typical low P-wave velocity close to w ater velocity, forms the seafloor at CM P-Bin 570 whereas at CMP-Bin 4000 the seafloor contains boulder clay, a mixture of sand and gravel with a higher P-wave velocity and density. In both cases the inversion gives reasonable results for the first three sediment parameters. Note that the value o f the S-wave veloc ities have an uncertainty o f more than 100% due to the insensitivi ty o f the reflection coefficient to this parameter.The investigation o f deeper sediment layers by AVA requires a good estimate of the absorption (described by the Q-factor) within the first sediment layer.The influence of Q on the reflection coefficient is enhanced in the wide-angle domain. However, our measurements were restricted to a maximum angle of incidence of 60°, which...

Thin-layer effects in glaciological seismic amplitude-versus-angle (AVA) analysis: implications for characterising a subglacial till unit, Russell Glacier, West Greenland

“…Although lower quality factors have been recorded for sea-floor sediment (e.g. Best et al, 1994;Ayres and Theilen, 2001;Riedel and Theilen, 2001;Sain and Singh, 2011), we allocate the highest values to minimise the Q-contrast (and hence its reflectivity contribution) across the glacier bed.…”

Studying converted waves in shallow marine environment

“…From the Zoeppritz equations [2] the following expression for the reflection conversion coefficient R PS is obtained from Aki and Richards [3] …”

Acquisition and Inversion of Dispersive Seismic Waves in Shallow Marine Environments

“…The investigation of the variation of seismic amplitudes with angle of incident of the seismic waves has been successfully applied to derive reservoir characteristics and sediment properties, sometimes also the shear wave velocity in consolidated sediments and rocks (Carcione & Tinivella, 2000;Whitcombe et al, 2002). While it can help to determine the den-sity and impedance of sediments, the sensitivity of the reflection coefficients on shear wave velocity is weak for shallow marine sediments (Riedel & Theilen, 2001). Methods to directly measure the shear wave velocity at the sea floor have been tested successfully (Gimpel, 1987;Davis, 1996).…”