Quantitative evaluation of dynamic time-shifts for correcting seismic data

Abstract

Large lateral velocity changes, particularly those associated with sea-floor topography, can severely distort seismic stack sections, for example, under the submarine canyons in the Gippsland Basin in Bass Strait. Conventional replacement statics partially correct these problems, but are usually inadequate in situations where the sea-floor variation is severe or the water is deep. Dynamic corrections calculated by ray-tracing through the region of strong lateral velocity variation give improved results. Dynamic corrections derived by ray-tracing can be evaluated by analyzing t2 -y2 plots where t is time and y is offset within CMP gathers. This facilitates making the following comparisons: (1) the zero-offset time before and after dynamic correction; (2) the RMS velocities (calculated using zero-offset rays traced in the model) with normalized stacking velocities after dynamics; (3) hyperbolic moveout before and after dynamics. A similar evaluation of conventional replacement statics provides a reference for evaluating the ray-tracing derived 'replacement dynamics'. Ray-tracing requires a subsurface model which can be difficult to construct in areas of structural complexity. This is often the case near submarine canyons. A simple model using water depth and a single marker horizon or datum when ray-traced to derive replacement dynamics gives significant improvement with respect to the conventional statics solution in submarine canyon areas. Errors in velocity can be reduced by as much as 50% and errors in hyperbolic moveout reduced by about 70%.