129 3. Borehole geophysical methods 3.4 Borehole seismic method Vertical Seismic Profile or VSP (Hardage 1985, 1992; Mari et al., 1999; Mari and Coppens, 2003; Mari and Vergniault, 2018) is the most used form of well seismic surveying conducted in vertical wells. VSP is a well seismic method for which the source and the receiver are approximatively on the same vertical. The VSP vertical resolution ranges from meters to tens of meters and its lateral range of investigation can reach a few tens of meters (Fresnel zone). After processing, a VSP provides a seismic trace, that is directly comparable to a surface seismic section recorded in the vicinity of the well. The lateral range of investigation of a VSP is increased by doing acquisition in deviated wells or can be improved by offsetting the source with respect to the well in case of vertical well. This technique is called Offset Vertical Seismic Profiling (OVSP). The image obtained after processing is thus a single-fold seismic section. A Seismic Walkaway is a series of offset VSPs, with the surface source situated at several locations corresponding to successively increasing offsets with respect to the borehole. The image obtained after processing is a section with a low degree of multiple fold coverage. For VSP acquisition, the sources are vibrators or weight droppers for on-shore surveys, air guns or sparkers for off-shore surveys. The borehole sensor can be a single-component geophone (vertical geophone) or a three-component geophone (a vertical component and two orthogonal horizontal components). The borehole sensor can also be a hydrophone, or even a four-component sensor: a three-component geophone and a hydrophone. The receiver can also be a string of borehole sensors, allowing the acquisition of data at several depth levels simultaneously (between 4 and 12 levels). Distributed Acoustic Sensing (DAS) is an established technology for recording seismic response using optical fiber cables (Willis, 2022). The DAS technology is being used with increasing success in VSP, especially due to the selective sensitivity of the fiber to axial deformations. Mestayer et al. (2011), Mateeva et al. (2013, 2014), Lesnikov and Allanic (2014) demonstrated that DAS data provides VSP results comparable with conventional VSP acquisition. However, current DAS systems have a much higher noise floor than geophones meaning that small events may be harder to detect (Baird et al., 2024). DAS technology can be deployed in high temperature, highly deviated or horizontal wells. Meantime the current limitations of the DAS VSP are also well known. Directivity pattern, attenuation of the signal with the length of the fiber cable, uncertainty of the depth determination are among the observed problems (Lesnikov and Allanic, 2014). DAS VSP recorded with fiber cable, which can be deployed behind casing (Didraga, 2015) or production tubing, can provide a much denser spatial sampling than a geophone string at a relatively low cost per sensor. A VSP record is a two-dimensional record, with a vertical axis which represents the recording time and the horizontal axis which represents the depth locations of the borehole sensor. In case of vertical well, the horizontal axis is the vertical depth expressed in m. In case of deviated well, it represents the cable length. The borehole deviation must be measured and considered in the processing sequence. The
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