Air gun near the sea floor as shear-wave source?
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Abstract
The feasibility of using an air gun near the sea floor as shear-wave source has been investigated. With an air gun near the sea floor, an evanescent P-wave in the water becomes a propagating S-wave in the sea floor, such that it seems that a pure shear-wave source has been used at the sea floor. This type of wave has been called a P*S wave. An experiment with such a set-up has been carried out at the Valhall field. For that case, modelling shows that shear-wave related event of the type of P*S waves can be expected with such a set-up. Especially at larger offsets, P*S waves can be expected. When analysing the field records and Constant-Velocity Stacks, it is hard to find P*S-wave reflected events. On the other hand, P*S-wave refracted events can be discerned in records. These events can be brought up to stack level and imaged, as shown in this paper.