Borehole acoustic reflection survey (BARS) is the name given to the data acquisition and processing for imaging near-borehole structures. The waveforms are acquired by a sonic tool using monopole and dipole sources whose frequency ranges are approximately 8 and 3 kHz, respectively. The main objectives of BARS are to image caprocks for confirmation of geosteering and well placement, to observe extensions of fractures for efficient production of gas and oil, and to find other wells for avoiding hazardous situations. The event signals used for imaging are the reflected P- and S-waves and the transmitted PS- and SP-waves. Before applying the imaging method, the direct waves, such as the P-, S-, Stoneley and flexural waves, are removed, and the reflected signals are extracted. Because the amplitudes of direct waves are significantly larger than those of reflected waves, the waveform separation is not straightforward and customized methods are used. In the imaging process, conventional Kirchhoff migration is often used. In this paper, a new coherency-based weighting method is applied and tested to obtain high-resolution images. This paper reviews parts of the processing workflow, such as waveform separation and migration, also tested on fracture imaging examples.
Near-borehole structures are imaged using the sonic waveforms acquired by a sonic logging tool (Hornby, 1989; Esmersoy et al., 1998; Tang et al., 2007; Tang and Patterson, 2009; Haldorsen et al., 2010). The main objectives of a borehole acoustic reflection survey (BARS) are to image caprocks for confirmation of geosteering and well placement (Borland et al., 2007; Haldorsen et al., 2010), to observe extensions of fractures for efficient production of gas and oil (Yamamoto et al., 1999; Hirabayashi et al., 2010), and to find other wells (Jervis et al., 2012). The source types and event signals to be processed are selected depending on the imaging targets. Because the central frequencies of monopole and dipole sources of the sonic tool are approximately 8 and 3 kHz, respectively, the monopole images have higher resolutions and the dipole images have deeper depths of investigation.