Investigating the velocity‐depth ambiguity of reflection traveltimes

Abstract

Reflection seismic data contain a long wavelength ambiguity making it difficult to separate traveltime information into velocity and reflector depth components. The existence of this velocity‐depth ambiguity is a feature of the geometry of the subsurface and is not caused by the particular inversion algorithm being used. Factors that control the occurrence of velocity‐depth ambiguities include the effective width of a potential velocity anomaly; i.e., its spatial wavelength, its height above a reflector, and its thickness. Factors that do not affect velocity‐depth ambiguities are the magnitude of the anomaly (the difference in velocity between it and the background) and the cable length with which data were recorded. A thin velocity anomaly induces an ambiguity at a wavelength approximately equal to 4.44 times the height of the anomaly above the reflector. A thick anomaly that spans the entire space from surface to reflector induces an ambiguity at a wavelength approximately equal to 2.57 the depth to the reflector. These are wavelengths that are significant in size, and therefore are of exploration interest. Through Fourier analysis, any subsurface velocity field can be decomposed into spatial frequency components. Thus the wavelength dependent velocity‐depth ambiguity adversely affects all velocity distributions.