High-resolution adaptive beamforming for borehole acoustic reflection imaging

Abstract

Acoustic reflection imaging in boreholes is believed to be a vast improvement in the evaluation of heterogeneous reservoirs. However, only 2D azimuth information of reflectors can be achieved by conventional methods. To obtain 3D information, a new high-resolution adaptive beamforming method has been developed to apply to the borehole environment, using an array acoustic logging tool with azimuthally distributed receivers. Moreover, a migration method based on the Fourier finite difference method is introduced into the beamforming to effectively reduce computational complexity from 3D to 2D and to reduce the effects of noise caused by velocity changes in the vertical and horizontal planes. Resolution analysis of the imaging results indicates that the tool diameter and source frequency are the key factors in increasing the azimuth resolution. Furthermore, increasing the resolution with the new method can help improve the fidelity of reconstructed objects in the reflection image. A comparison of results obtained by the new method and the conventional method with simulation data demonstrates that the new method can achieve a reflection image of high azimuth resolution with extremely noisy signals. Application to field data also found a good agreement between the reflection image and the borehole resistivity image. Therefore, detailed information regarding the azimuth and the radial distance of reflectors to the borehole can be determined in 3D space.