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 are 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 a 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 inclined 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. When the reflector is dipping relative 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 method to determine an interface and the distance of a reflector.
Directional drilling is one of the most important developments in petroleum exploration in the past decades. Compared with the traditional vertical drilling, directional drilling can increase the exposed section length through the reservoir by drilling at an angle through the reservoir, increasing oil production. To investigate formation properties, acoustic logging while drilling (LWD) was developed. The monopole LWD system is applied to measure the compressional wave (Tsang and Rader, 1997). The quadrupole acoustic logging system avoids the wave propagating along the tool and measures the shear wave velocity in the formation. The traditional acoustic logging system records the acoustic waves propagating along the borehole to investigate the formation properties around the borehole. The amplitude and frequency of the borehole acoustic waves depend on the borehole size and the acoustic properties of the surrounding formation (Cheng and Toksöz, 1981; Pallet and White, 1982; Kurkjian and Chang, 1986). During the directional or horizontal drilling, we not only want to measure the acoustic properties of the formation, but we also want to determine or forecast the tool position in the reservoir. We want to maintain the drilling in the reservoir in order to determine the distance between the borehole and the reservoir boundary (Hornby 1989; Esmersoy et al., 1998). The logging tool not only records the acoustic waves propagating along the borehole, but it can also receive the acoustic waves reflected from the boundary. In a vertical borehole, we call the acoustic wave entering the formation a leaking mode, because this wave cannot be reflected from the horizontal boundary (Cheng and Toksöz, 1981).
