ABSTRACT

Reflections in acoustic borehole-logging data can be used to image near-borehole geologic structures, using either monopole or dipole measurements, the latter of which resolve azimuthal ambiguities. Dipole methods in particular can characterize small and subtle fractures in reservoir environments. Accurate tools for wavefield simulation are required to account for wave modes in and out of the borehole, and for the receiver responses to these modes. We have evaluated a 3D elastic staggered-grid finite-difference method for isotropic and anisotropic media in the context of the borehole acoustic imaging problem. Based on the convolutional perfectly matched layer and the multiaxial perfectly matched layer schemes, a hybrid perfectly matched layer method was developed and implemented to reduce artificial boundary reflections. The superiority of this hybrid perfectly matched layer beyond the convolutional perfectly matched layer and the multiaxial perfectly matched layer was proven based on numerical benchmarks in isotropic and anisotropic media. Numerical simulation of radiation, reflection, and multipole reception of elastic waves, excited by a dipole source, was carried out. To optimize azimuthal detection, the relationships between S-wave polarization as well as source-receiver offset and source-reflector angle was analyzed. Results indicated that the S-S reflection is most sensitive to the angle between the incident ray and the normal to the reflector. Its maximum amplitude occurs as the incident angle reaches its critical value, a fact that can be used to calculate the total propagation distance of the S-S wave. The critical angle as well as the SH-wave velocities of geologic structures outside the borehole can thus be determined.

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