Identifying reflector azimuth from borehole multicomponent cross-dipole acoustic measurement

Abstract

The borehole dipole shear-wave reflection imaging method has great potential in heterogeneous reservoir explorations because of its deep investigation depth and relatively large reflection amplitude. However, the generally used shear horizontal (SH) reflection approach can only indicate the reflector strike and has an inherent defect in azimuth ambiguity. We have developed a multicomponent cross-dipole array acoustic measurement with four azimuthally distributed receiver arrays and a method using reflected dipole P-waves to eliminate the azimuth ambiguity caused by the SH reflection. The recorded data include cross-dipole waves with four components and two combined dipole-monopole waves that stack the data of the four azimuthally distributed receivers induced by each dipole source. A theoretical analysis indicates that the dipole compressional reflection is sensitive to the reflector azimuth. Therefore, the cross-dipole waves are first used to determine the reflective interface strike with the SH reflection. The compressional reflections obtained from the cross-dipole data and the combined dipole-monopole data are then processed to identify the correct azimuth. The effectiveness and accuracy of the method are validated via synthetic and field data examples in a soft formation. Our method may potentially solve the azimuth ambiguity problem in borehole acoustic reflection imaging and fully use cross-dipole acoustic measurements.