The Determination Of Shallow Marine Sediment Lithology Using High-Resolution Multi-Offset Seismic Data

Abstract

Abstract A method is presented to determine the cormpressional velocity and bulk density of unlithified marine sediments directly from multi-offset, high resolution seismic data. These measurements can be used to estimate the Iithology and elastic moduli of marine sediments. The method provides a means to rapidly evaluate extensive areas of the seafloor. Introduction This paper presents a method for determining the compressional velocity and bulk density for marine sediments using multi-channel, high resolution seismic data. The seafloor interface presents a unique case because the velocity and density of the water layer are known. This knowledge, combined with the amplitude response of the seismic data at nonnormal angles of incidence, provides an opportunity to determine the normal incidence reflection coefficient quantitatively. The use of multi-fold common mid-point (CMP) acquisition methods allows the measurement of the angle of incidence at which the critical angle occurs. The critical angle is defined from the relationships given by Snells Law. Snell's Law relates the compressional and shear wave velocities of two isotropic, homogeneous layers to the angles of reflection and refraction for compressional and shear waves which result when a plane wave impinges on a boundary. If the critical angle can be accurately determined, the velocity of the sediment layer can be calculated given the velocity of the water layer. Empirical evidence presented by several authors suggests that the porosity, mean grain size, and bulk moduli of unlithified marine sediments are strongly correlated to compressional velocity' and bulk density. The determination of the in-situ compressional velocity and bulk density using high resolution, multi-channel seismic data makes possible the rapid determination of other physical properties of marine sediments for a variety of engineering, environmental, and exploration applications. This paper presents a short review of the existing research on this subject, including the most often applied theoretical and empirical methods. In order to illustrate the multi-offset CMP method, we computed the reflection coefficients with increasing angles of incidence for a variety of continental shelf and terrace clastic sediments using the Zoeppritz equations (1919). The input data for the offset models is given by Hamilton (1971a). The Zoeppritz models provide insight into the maximum reflection amplitudes at the critical angle. Having determined the compressional velocity of the sediment layer, the reflection coefficient can be determined using the average maximum amplitude as measured from the Zoeppritz models. We show how the only remaining undetermined physical parameter, the bulk density of the sediment layer, can be computed using the normal incidence limit of the Zoeppritz equations. Review of Previous Work Since the early 1950's several researchers (Akal, 1972, Breslau, 1965, Buchan et al., 1972, Horn et al.,1968, Keller and Bennett, 1970, McCann, 1972, and Sutton et rd., 1957) have presented empirical evidence suggesting that many useful physical parameters of unlithified marine sediments including porosity, mean grain size, and the bulk modulus are strongly correlated to compressional velocity and bulk density. For this reason the measurement of velocity and density has been of great importance in the study of the physical properties of marine sediments.