Abstract

VSP has long been a tool for investigating the location of salt flanks. Traditionally a salt proximity survey has been used to estimate the location of salt boundaries relative to a wellbore; however, this technique depends on accurate knowledge of the shape of the salt and the local sediment velocities. In addition, this method does not work well with complicated 3D salt structure. Beyond the traditional salt proximity survey, many researchers have developed various methods and strategies for improving the locating and/or imaging of salt flanks (O'Brien et al., 2002; Brandsberg-Dahl et al., 2003; Zhao et al., 2006). With these methods, the quality of the resultant image depends on a prior knowledge of the (possibly anisotropic) overburden velocity field for very long offsets and also is concerned with multi-pathing through salt invalidating a 2D assumption. In looking for a simpler approach to this problem, a single-well imaging concept may be attractive. Using downhole sources and receivers, it is possible to image near-borehole structure directly, with the significant advantage of having to know only the velocity field locally around the borehole. In the case of a vertical well near a salt flank, the main concern is only about the sediment velocities near the well, and a 2D assumption is likely appropriate. Unfortunately, though encouraging results have been seen using sonic frequency tools (sonic logging tools used as imaging devices, see Hornby, 1989; Esmersoy et al., 1998; Tang, 2004), to date no one has developed a single-well imaging configuration that enables direct imaging of near-borehole structure to an offset of 500 m or more as is needed to image salt flanks. Here primary issues are that, at the required low frequencies, most (almost all) of the energy excited by an impulsive source in a borehole is converted to tube wave energy, and also there is the challenge of trying to generate significant seismic frequency in, say, an 8-inch borehole.

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