Characteristics of Reflectivity Strength on a Thin Bed

Abstract

From the point of view of resolution, an existing thin layer may not be detected by seismic wavelets. By numerical experiment and using the reflectivity strength, it is illustrated that the existence of a thin layer with a thickness of less than one-eighth of the dominant wavelength of the propagating seismic wavelet can be detected.

An observed seismic wavelet consists of sub surf ace reflectivity, i.e. the composite wavelets are a function of the separations of individual reflectivity alone. For a thin layer, the shape of a composite seismic wavelet is a function of layer thickness. Using plane wave theory and assuming no energy is dispersed, the authors calculate a synthetic seismogram for a geologically pinchout model based on an input Ricker wavelet. The calculated composite wavelets are then cross-correlated with the derivative of the input wavelet. From Widess's (1973) studies, the resolvable ability of a seismic wavelet is clearly defined and understood by the correlation. To examine the effects of the thickness of a thin layer on reflectivity strength, the Hilbert transform is then used to transfer synthetic wavelets. By destructive interference, reflectivity strength shows a minimum when the layer thickness is less than one-eighth of the dominant wavelength of the wavelet. The minimum no longer occurs as the layer thickness exceeds the above criterion. This phenomenon of reflectivity strength on the layer thickness of a ''real'' thin layer can be considered as an indication of its existence.

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