Earth Resources Laboratory :: Abstracts

Imaging of Large Offset Ocean Bottom Seismic Data

Edmund C. Reiter

Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on February 8, 1991 in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Abstract

This thesis describes techniques to image deep crustal velocity and reflectivity structure from large offset Ocean Bottom Hydrophone (OBH) data. We choose to emphasize methods which image the data, rather than methods which forward model the data, to better utilize the increased data volumes associated with modern large offset marine seismic experiments.

We first describe a method for determining a two-dimensional (2-D) velocity field from refraction data that has been decomposed into some function of slowness. The most common decomposition, intercept time - slowness or T-p, is used as a n intermediate step in an iterative wavefield continuation procedure previously applied to one dimensional (1-D) velocity inversions. We extend the 1-D approach to 2-D by performing ht downward continuation along numerically computed raypaths. Synthetic data are used to demonstrate how this approach can compensate for the effects of known lateral inhomogeneities while determining an underlying 1-D velocity field. We also use synthetic data to show how multiple refraction lines may be used to determine a general 2-D velocity model. Large offset field data collected with an OBH are used to illustrate this technique in an area of significant lateral heterogeneity caused by a sloping seafloor. At present, limitations of this 2-D approach are caused primarily by the sparseness of typical refraction surveys, but hopefully may be overcome in the future with more appropriate acquisition geometries.

Next, we show that data from an on-bottom hydrophone recording a near-surface source provide an opportunity to treat water column multiples as useful signal. A ray-equation based Kirchoff pre-stack depth migration is used to image primary reflections and deep water multiples recorded on an Ocean Bottom Hydrophone (OBH). We use synthetic data to examine the difficulties in identifying the trued path of the water column multiple. For flat-layered media there are two different multiple paths which have identical travel times: one that reflects beneath the source, and one that reflects over the receiver. However, they do not have the same amplitude, and it can be shown that their amplitudes differ sufficiently to allow a reliable image to be extracted form the energy that reflects over the receiver. As a final step, the image obtained from the multiple is corrected for the ª phase shift from the free surface and then added to the image from the primary reflection. Application of the technique allows the utilization of coherent deep water multiples as signal, and this results in an increased signal-to-noise ratio in the final image.

Finally, we discuss the application of both the 2-D velocity inversion/imaging method and the dep water multiple migration method to field data collected across the Carolina Trough off the East Coast of the U.S. Migration of large offset Moho reflections result in an image of the Moho shallowing from approximately 80 km of offset which clearly shows the Moho shallowing from approximately 37 km to approximately 25 km. Comparison of a velocity image correspond well with highly reflective depths in the reflectivity image.