A 3-D plane-wave beamforming method for borehole azimuthal acoustic reflection imaging

Abstract

SUMMARY In the exploration and development of carbonate reservoirs, borehole azimuthal acoustic reflection imaging can be used to survey the near-borehole geological structures such as fractures, faults or caves in the formation, but the accuracy of azimuth measurement and imaging quality are usually deteriorated due to the amplitudes of event signals being often much weaker than those of borehole mode waves. This study proposes a data processing method for borehole azimuthal acoustic reflection imaging to improve the accuracy of azimuth measurement and imaging quality of the near-borehole geological structures. First, three adjacent receiving sensors in the vertical plane closest to the given azimuth are selected to form a linear phased array receiver subarray, and the elevation angle of the event signals can be obtained by comparing the amplitudes of the stacked waveforms in the vertical plane for different elevation angles. Further, three receiving sensors, closest to the given azimuth, are selected from the arcuate phased array receiver, where the central sensor of the linear phased array receiver subarray is located, to form an arcuate phased array receiver subarray, and the 3-D stacked waveforms with the known elevation and azimuth angle can be calculated. In the incident direction of the event signals, the event signals will be significantly enhanced because the event signals in waveforms of each sensor in the subarray satisfy the in-phase stack condition, so as to improve the migration image quality and interpretation accuracy fundamentally. We confirmed this method by processing azimuth reflection acoustic data from a field experiment including two adjacent fluid-filled artificial boreholes. The comparison of the field data processing results with and without 3-D waveform stacking demonstrated that 3-D waveform stacking significantly improves the accuracy of azimuth interpretation and imaging quality.