Experimental studies of reflected nearborehole acoustic waves received in borehole models

Abstract

Laboratory experiments are performed with an isotropic Lucite borehole model to measure the acoustic wave fields generated by a monopole source in the borehole. The source not only generates acoustic waves propagating along the borehole, but also in the surrounding formation. If there is a reflection interface in the formation, the acoustic waves can be reflected back to the borehole. We measure the acoustic waves on the model surface to investigate the acoustic fields in the formation. Acoustic measurements record both waves reflected from an interface or fracture and those propagating in the borehole. The frequency of the reflection is higher than that of the borehole waves, and the apparent velocity of the reflection is higher than the P-wave velocity. In this paper, we extract the waves reflected from an interface—which is parallel or declined to the borehole axis—with a high-pass filter. If the apparent velocity of the acoustic wave recorded in a borehole is faster than that of the P-wave propagating along the borehole, the wave must be reflected by a surface outside of the borehole. With these measured waveforms we calculate the distance between the borehole and the reflection surface and the P-wave velocity in the direction perpendicular to the borehole axis. When the reflection is declined to the borehole axis, the reflected waves recorded at both sides of the source have different first-arrival-times and different apparent velocities. Laboratory measurements show a new method to determine an interface out of a borehole, the formation anisotropy, and the distance of a reflection.