Vector Fidelity of Ocean-Bottom Seismic Data
Authors: Hokstad Ketil , Mjaaland Svein , Maaø Frank , Dong Hefeng 
Published: October 2008

Vector fidelity is an important issue in applications of oceanbottom seismic (OBS) data that involves shear-wave splitting, azimuthal anisotropy and amplitude analysis. Here, we discuss characterization and compensation for vector infidelity. Two component (2C) vector infidelity, involving only the horizontal components of the OBS data, can be characterized and corrected for based on geometrical properties. Vector fidelity problems involving 3C and 4C data are more difficult to analyze because in general, the interaction of the wavefield with the ocean bottom, and the physical properties of the ocean bottom must be considered as well.

Introduction

Ocean-bottom seismic (OBS) data has proven to be a valuable tool for reservoir characterization and imaging. The fourcomponent (4C) OBS receiver stations include a 3C geophone, recording particle velocity, and a hydrophone recordingpressure. With 4C OBS acquisition, both compressional (P) and shear (S) waves can be measured directly. PS-converted waves provide an independent set of measurements that can be used in situations where P-wave imaging fails. Excellent examples from the North Sea include imaging through gas clouds at Tommeliten (Granli et al., 1999) and Valhall(Thomsen et al., 1997), and the OBS survey at the Alba field (Hanson et al., 1999) where the converted waves provide a clear image of the top of the reservoir sands. Another important application of OBS data is attenuation of multiplesby means of PZ-summation (Barr, 1997), which involves the vertical component of the geophone and the hydrophone. In simple 2D applications of OBS data, only the kinematical properties of the data are utilized for imaging pusposes. Often, the multicomponent data are processed as a set of independent scalar fields, assuming that the vertical component records only P-waves, and the horizontalcomponents record only S-waves. Because of ray bending and seismic velocities increasing with depth, this is sometimes an acceptable assumption. In more advanced application of OBS data, including S-wave splitting, azimuthal anisotropy andAVO analysis, the amplitude and vector properties of the OBS data become important. Consequently, the vector fidelity of OBS data is an issue of major interest to the oil and gas industry.

Currently, two different types of OBS acquisition systems are in use:Cable-based OBS systems.Node-based OBS systems.

Each type has its advantages and limitations. Cable-based systems allows for inexpensive and easy deployment of a large number of receiver stations. High fold is obtained in the inline direction. The geophone-ground coupling can be poor, in particular in the crossline direction. Node-based systems are more expensive and tedious to deploy. Vector fidelity is usually better than for cable-based system. Uniform azimuthal coverage can easily be obtained. Different from cable-based systems, there is no preferred direction introduced by the system design. One of the pioneering North-Sea OBS data sets was acquired with a node-based acquisition system (Berg et al., 1995).