Phase offset indicator (POI): A study of phase shift versus offset and fluid content

Abstract

There is limited published experimental and theoretical work related to offset‐dependent reflection and phase changes at the interface between porous media. An appropriate relationship between seismic parameters and phase shift has not been well‐established. This investigation measures variations of phase for nonnormal incidence reflection as a function of frequency at the interface of a horizontally layered porous (perforated) medium, using a two‐dimensional Plexiglas model. In both parts of this study, phase‐matched filters (PMF) are used to analyze the data. Phase‐matched filters previously have been applied to digital records of Rayleigh waves and Love waves. The purpose of the first part of the study is to demonstrate application of the PMF process to reflected compressional waves and to use this technique in discriminating multiples from primaries. By using this linear process, the complex spectrum of the primary signal can be recovered from the multiple arrivals. The reduction of reflected compressional pulse wavelets to a zero‐phase wavelet provided a tool to examine the change in phase (and amplitude) as the wave travels through a lossy medium. The second part of the study deals with the relationship of phase shift Δϕ at an interface of a horizontally layered porous medium, as a function of fluid content, angle of incidence (offset), and frequency. The result indicates that, at midband frequency of 60 kHz, an air‐ (gas‐) saturated medium would cause a large increase in Δϕ with offset, whereas a water‐saturated medium would show less change in Δϕ with offset. The phase shift‐versus‐offset response for oil is assumed to be intermediate between the responses for gas and water.