Synthetic Acoustic Logs Over Bed Boundaries And Horizontal Fissures

Abstract

The finite difference method is used to predict the effects of wave propagation in boreholes with vertically varying elastic properties. Specifically horizontal interfaces between rock types (for example shales and sandstones) and between rock and horizontal liquid fissures of varying width are considered. The amplitudes of the transmitted compressional head waves and Stoneley waves are only slightly affected by thin horizontal fissures and stringers. On the other hand, the pseudo-Rayleigh wave, when it is present (e.g., in sandstones), can be almost totally blocked by a very thin (1 em) horizontal fissure. Both compressional head waves and pseudo-Rayleigh waves show significant reflections at horizontal discontinuities. Stoneley waves and PL modes are not significantly reflected. In general, mode conversion is not a significant effect in the bed boundary and horizontal fissure models considered. Although some very strong effects can be observed on the transmitted and reflected waves none of the models showed significant scattering (for example, from pseudo-Rayleigh waves to compressional head waves). The effects observed at bed boundaries in the field (for example, Paternoster and Larrere, 1985) are adequately simulated by the synthetics. Adjusting for the differences in offsets of the two cases, arrival times and amplitudes correspond well. The only exception to this is the absence of identifiable mode conversions. Larger offset models will be necessary to clearly distinguish any mode conversions that may be occurring.