Layers and bright spots

Abstract

Seismic amplitudes are affected by both geometric and lithologic features of reflecting layers. Observation of “bright spots” is a function of not only pore fluid saturations, but also of layer thickness and surrounding rock type. The numerical model developed uses a finite sum of reflected and mode‐converted waves to evaluate the application of wave analysis in determining pore fluid types. Amplitude analysis alone is not sufficient to define realistically pore fluid type, but when amplitude analysis is combined with phase and mode‐converted shear‐wave behavior, it is possible to differentiate between pore fluid types. Various configurations which lead to bright spots, including coal and limestone layers, are evaluated. An algorithm is developed, based on the numerical examples, which can differentiate gas zones from other bright spot data. Thickness of the plane layer is an important factor in defining the overall reflectivity due to interference of the reflected wave components. Gas saturation of a layer is not a sufficient condition for a bright spot, but can lead to “dull spots” for layers in the right thickness range (relative to the incident wavelength). The lithology of the boundary material determines the extremes in amplitude variation which result when thickness or wavelength is varied. If a priori knowledge of the lithology or thickness of the structure is available, the amplitude‐phase‐converted wave algorithm can be applied to other than bright spot data to characterize the structure. This indicates there are important applications of the amplitude‐phase‐converted wave algorithm to field development and to bright spot analysis for exploration.