Multicomponent Seabed Seismic Data: A Tool for Improved Imaging and Lithology Fluid Prediction (1999)

ABSTRACT Multicomponent seabed seismic data acquisition, processing and interpretation of pressure and shear data are bringing new insight into seismic exploration and reservoir characterization. The reason we want to record "dual-wave" data is at least twofold. First, to improve seismic lithology and fluid prediction, as shear waves carry additional information about lithology, pore fluids, and fractures in the subsurface. Second, to improve seismic imaging, as joint processing and analysis of both pressure and shear data reduces the ambiguity of structural and stratigraphic interpretation. Independently of the use of multicomponent sensors, seabed seismic data recordingoffers the generic advantage of flexibility of the acquisition geometry. As virtually any pattern of shots and receivers is possible, data acquisition can be optimized to provide the most revealing subsurface image. In this paper, we present results from analysis of multicomponent seabed seismic data from the Tommeliten Alpha Field and the Statfjord Field, offshore Norway. Introduction Multicomponent marine technology holds a promising potential for risk reduction in seismic exploration. While oil and gas from large and geologically simple fields have constituted a substantial part of the world's hydrocarbon production up to now, new resources are to a greater extent found in small traps in geologically complex areas or in subtle traps that are difficult to identify. Petroleum reserves thus become progressively more difficult to discover and develop and the financial risks increase while the economic margins tend to decrease. The demand is therefore high for new technology that can reduce risk through more accurate prospect definition. The use of converted shear (PS) waves in addition to conventional pressure or compressional (P) waves offers a possibility to better address the risk elements in petroleum exploration work. In the 1970-80's, attempts were made to extract shear wave information from marine seismic data acquired in surveys with conventional sources and pressure-sensitive hydrophones located in the water column. The methods rely on double mode conversions at or just below the water bottom, giving PSSP reflections. For a given angle of incidence, mode conversion from compressional P-wave to S-wave is quite efficient in "hard" water-bottom environments. The water bottom shear velocity is the most critical parameter affecting the generation of observable PSSP reflections. Their amplitudes can be comparable to normal P-wave reflections when the S-wave velocity is greater than one-third of the water-bottom P-wave velocity1,2. In most areas, however, as the sea floor shear velocity is much lower, this is not a viable technique to record high quality shear wave data in the marine environment. Other solutions thus were investigated. Unless shear waves were generated by source devices on the sea floor, the methods necessarily had to rely on mode conversion by reflection from P-wave to S-wave at reflectors in the subsurface. In the early 80's, a few oil companies started to investigate how shear waves could be recorded on the seabed. Forinstance, Conoco Inc. considered the use of a marine shear wave vibrator source for shear wave excitation, while Shell Offshore Inc. investigated the potential of recording shear waves by using a "marine shear wave cable". However, to our knowledge, no results from these analyses were published in the open literature. In the late 80's, Statoil developed a concept for acquiring fourcomponent (4C) seismic data directly on the sea floor. Known as SUMIC (SUbsea seisMIC) the technique would enable the recording of both shear and compressional waves by planting into the seabed se