Vector Fidelity of OBC Data and Seafloor Coupling of the Vertical Component

Abstract

Abstract Ocean-bottom cables (OBCs) are the most practical and economic means of deploying seismic sensors on the sea floor. However, differences in coupling of the three-component (3C) data and impulse responses of the measuring devices can prevent a proper combination of the vector wavefields during processing of 3D converted-wave (PS-wave) surveys, particularly for the horizontal components. Since hydrophones are perfectly coupled to the fluid medium, their response is an optimal reference of true compressional-wave (P-wave) reflectivity. Operators can be designed from common-receiver gathers for removal and evaluation of vertical coupling effects. A least-squares operator design is applied to data from the Gulf of Mexico to minimize the difference between the hydrophone and vertical responses after taking into account water-trapped energy. Another way of doing this is by applying the equation of motion and minimizing the down-going energy in a window containing only primary reflections (see Schalkwijk et al., 1999). Picking such a window may be difficult for long source signatures, or when operating in shallow water. A similar calibration approach based on a window containing up-going energy from critically refracted waves does not suffer from these shortcomings. This method is demonstrated on a real data set from the North Sea and is shown to give good results. Introduction Vector fidelity of 3C geophone data is essential for obtaining an accurate P-wave and S-wave response from multicomponent surveys. On the ocean bottom it is expensive to bury cables or plant them with robotic devices to ensure good coupling. Although OBCs appear to be the most practical and economic means of deploying seismic sensors on the sea floor, inconsistent coupling of the three-geophone components or different impulse responses of the measuring devices can prevent a proper combination for vector-wavefield processing, i.e., the response of the two horizontal components are not identical and the vertical does not match the hydrophone. This can lead to difficulties when removing multiples by hydrophone-vertical (PZ) summation, or separating different wave-field constituents, such as the upand down-going waves, or the P- and S-waves. Compensating 3C data for effects of sub-optimal geophone coupling is not new, and there has been much theoretical work on the subject (e.g., Garmany, 1984; Duennebier and Sutton,1995; Schalkwijk et al., 1999; Gaiser et al., 2000; and Dellinger et al., 2001). In this paper, methods designed to measure and correct the coupling response of the vertical component are discussed. One approach is similar to Gaiser (1998) in that a reference wavefield (hydrophone response) is used to correct for coupling on a different component (vertical component). Thus, the vertical component P-wave response is matched to the hydrophone. Spectral balancing (deconvolution) operators are designed for the vertical geophone response for 3D OBC data and applied in a receiverconsistent manner (common-receiver gathers), ensuring that the vertical component can be optimally combined with the hydrophone component to remove water-borne multiples.