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Imaging sub gas has been shown to be a robust application of 4-componentseabed seismic. We review the acquisition and processing of an orthogonal(source lines perpendicular to receiver lines) 4-component 3D survey over the Lomond Field in the North Sea. The geophysical objectives were to improve theimaging of faults and the crest of the structure under the gas cloud, where theconventional seismic is distorted. Conventional streamers record only the pressure waves and suffer from verylow signal/noise ratio and pull-down under the gas cloud. It is recognized thatconverted shear waves are largely unaffected by fluids and the success ofearlier 2D lines e.g. Tommeliten (Berg et al.1) provided some of the motivationfor acquiring the 3D Lomond survey. A multi-component cable system was laid on the seabed to record bothpressure and converted wave data in a cross spread geometry. In total, fivepairs of cables were laid giving a seabed footprint of 36 sq.km. The cablesystem uses array's of hydrophones with 3 orthogonal velocity sensors. In processing, the hydrophone and vertical velocity sensor are summed togive an improved result. The horizontal velocity components are rotated intoradial and transverse components and the radial data is processed as aconverted shear wave. Multi-component methods have been developed, including NMO, DMO and binning, to improve converted wave processing. The orthogonalgeometry records ray-paths from all azimuths presenting some challenges toprocessing. Processed results will contrast the final migrated pressure wave andconverted wave images. The first shows good imaging of the steep flanksegments, distortion under the gas-invaded zone and poor event continuity inthe crestal feature. The second shows similar structure and good continuityunder the gas. Introduction to the Lomond 4C Project The use of 4-component seismic to solve the problem of imaging beneath gaszones has been shown to be a robust application. However orthogonally acquireddata presents some new challenges to processing. Conventional seismic records pressure waves (or Primary waves), the seabedsystem can also record transverse shear waves (or Secondary waves), these arecommonly called P-waves and PS-waves.

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