Using layer replacement to improve velocity analysis beneath near-surface reefs

Abstract

Exploration in the Timor Sea is hampered by the presence of many near-surface limestone reefs. Reef structures form complex velocity anomalies because of their internal velocity structure and because they produce a rapidly varying sea-floor profile. Distortion of seismic data recorded over such reefs comprises event pull-up and poor imaging. Conventional common mid-point processing is not adequate to interpret prospective targets below the reefs. Seismic distortion associated with reefs can be reduced using replacement techniques of varying complexity. Wave-equation datuming is an accurate method but computationally expensive, and replacement statics is an efficient method but only approximate. All replacement methods require an estimate of the velocity structure inside the reef itself, although this is not normally attempted in production processing. The seismic image beneath surface reefs can be improved by integrating layer replacement with velocity analysis. The seismic data are first modified by replacing the water velocity with an average velocity of the reef. This involves two steps: downward continuing to the sea floor, and then upward continuing to the surface. A sufficiently accurate value for an average velocity can usually be obtained using conventional velocity analysis. This approach removes most of the velocity inhomogeneity associated with the water/reef interface and renders subsequent velocity analysis more accurate. A model of a typical reef from Browse Basin, which includes strong lateral velocity inhomogeneity on the reef flanks, shows that velocity analysis post-replacement gives much more accurate RMS velocities inside and immediately below the reef. Events below the reef are imaged more faithfully. Conventional velocity analysis is not accurate enough to give interval velocities for the reef because the CMP method cannot image such strong lateral velocity changes. The same model can be analyzed using reflection tomography based on the algebraic reconstruction method (ART). ART is sufficiently accurate to give useful estimates of the interval velocities within the model reef. Comparing results before and after replacement indicates that replacing the water layer prior to ray tracing gives closer estimates of the high-velocity reef flanks and reduces velocity smearing across the sea floor to water interface. Estimates of the interval velocities inside the reef are important for accurate imaging. These can be used to continue downward through the reef structure for a second pass of wave-equation replacement, or for applying a variety of pre-stack depth migration methods.