75 2. Well seismic surveying • Providing a detailed image in the area around the well (taking into account the offset of the source) • In-depth tying of seismic reflection profiles • Identifying primary and multiple reflections • Providing structural information, estimating reflector dip (3 component recording) • Locating fracture zones (hydrophone measurements) and fault plans (oriented 3C receiver) • Predicting the presence of reflectors or anomalous zones (e.g. under compaction) ahead of the drill bit, i.e. below the well for a vertical well, and ahead of the drilling front for a horizontal well The following elements must also be considered: • The main limitation is the lateral investigation around the well. In classic VSP, with a source offset by several meters, it is limited between the vertical resolution (quarter of wavelength) and around ten meters • A poor coupling of the probe to the formation leads to the presence of noise, especially for the horizontal components of the well receiver. • The anchoring force of the well receiver may be insufficient in the case of large diameter wells, resulting in poor coupling. • Well conditions, such as with poorly cemented cased wells, can make measurements difficult. A good coupling of the well sensor to the casing does not guarantee a good coupling of the receiver to the formation. It is then necessary to make a cementation log before carrying out a VSP. • The presence of guided waves (Stoneley waves) can be detrimental to the extraction and analysis of volume waves (mainly for hydrophone acquisition). • Acquisition is sensitive to industrial, human and natural vibrations. It is important to avoid cable transmissions by slackening the cable when measuring. • The acquisition duration can be estimated at 5 to 10 mn/depth, for a classic VSP. The offset VSP and the seismic walkaway enable an extension of the lateral investigation variable with depth and provide a detailed seismic survey of the vicinity of the studied objective. Although the well seismic lateral investigation is in any case limited, having receivers close to the objectives provides a good vertical resolution, due mainly to the fact that the surface weathering zone is crossed just once.
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