Vertical seismic profile polarization method for determining reflector orientation

Abstract

The strike and dip of the reflecting interfaces in the vicinity of a well can be derived from VSP data recorded with a three‐component downhole seismometer. The method relies on measuring the polarizations in the direct and reflected compressional waves. The mathematical relation which is the basis of the method is exact only when the seismometer is positioned at the depth where the interface intersects the well. At that depth the polarization of the reflected wave cannot be measured because of interference with the direct wave. The orientation of the normal to the reflector can be determined to within 2° when the polarizations of the direct and reflected waves are determined within 100 m of the reflector. If the direct and reflected waves can be identified without multidepth processing, only measurements at a single depth are required to determine the reflector orientation. Error analysis and statistical refinement can be achieved by measuring the polarizations at several seismometer depths, source azimuths, or source offsets. The method is robust in the sense that the error in the interface orientation cannot exceed the error in the measurement of the polarizations. The method should be useful in structurally complex areas provided the reflections can be observed at depths above the depths of generation.

Polarization measurement uncertainty on three‐component VSP

“…The uncertainty found in the azimuthal angle for the field data was compared with the uncertainty as a function of signal-to-noise ratio (S/N) in a model consisting of a synthetic signal contaminated with band-limited noise having a Gaussian amplitude distribution (Spencer et al, 1988;Knowlton, 1990). In this model, a sinusoidal three-component signal was contaminated with band-limited Gaussian noise of varying amplitudes.…”

“…Applications of polarization information include finding the dip and strike of reflecting horizons (Spencer et al, 1988), separating P and S wavefields (Dankbaar, 1987), and detecting anisotropy (Crampin, 1984). For polarization information to be used to its full potential, particle polarization must be measured accurately, and the uncertainty in measurement must be known.…”