giving so-called PSSP reflections. For large angles of incidence, mode conversion from


P-wave to S-wave is quite efficient in ‘hard’ water-bottom environments. Te water-bottom shear velocity is the most critical parameter affecting the generation of observable PSSP reflections. Teir 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 velocity. In most areas, however, the sea-floor shear velocity is much lower. Terefore, the use of PSSP reflection data is not a viable technique to record high-quality shear-wave data in the marine environment. Other solutions were thus 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, so-called PS-reflections. In the late 1980s, Eivind Berg at Statoil led the development


of the SUMIC (SUbsea seisMIC) system, whereby both shear and pressure waves were recorded by sensors implanted in the seabed. Te sensor system was called 4C, with one hydrophone to measure P-waves and a three-component geophone to measure the particle-velocity vector. In 1992, after the development of the prototype SUMIC


sensor array, several extensive field equipment tests were carried out. The data quality from the SUMIC sensor layouts was judged to be remarkably good, and demonstrated that SUMIC was a viable system for acquisition of high-fidelity 4C data. Te first full-scale SUMIC data acquisition of a multifold


2D seismic line was conducted in late 1993 over Statoil’s Tommeliten Alpha structure in Block 1/9 in the southern part of the Norwegian sector of the North Sea. Te principal objective of the survey was to demonstrate the potential of the SUMIC technique for imaging subsurface structures through and below gas chimneys (see Figure 2.19). Te chosen exploration target had a reservoir which lay


cable hydrophone


connector


geophone housing


spike 30 cm


Figure 2.18: The original SUMIC sensor used in the 1993 Tommeliten 2D-4C survey. It consists of a 30cm- long, 6cm-diameter ‘spike’ which was planted into the sea bottom by an ROV (remotely operated vehicle) to achieve good coupling. Above the spike is the geophone housing, approximately 40 cm long. On the top left is the hydrophone housing. The top right device is a cable connector. An array of 16 SUMIC sensors was used in the acquisition. During the Tommeliten survey, 375 common receiver gathers were recorded. The ROV crew took approximately half an hour to plant each SUMIC detector (see Figure 2.14).


6 cm


beneath gas rising in a chimney within the overlying shales. Previous conventional seismic surveys, which rely on P-wave propagation only, produced unusable images in some regions because of the distortion and misfocusing introduced as the P-waves passed through the gas chimney. A small percentage gas saturation in the chimney introduces strong attenuation and heavily distorts the P-ray paths. Because shear waves are much less affected by fluids


than compressional waves, it was expected that the 4C technology would be suited to ‘seeing through’ the distorting gas chimney, enabling a reliable image of the target to be produced from shear waves. A continuous and regular 2D-4C profile of 12 km length passing over two wells was acquired in late 1993. In general, the quality of the 4C data was excellent at all locations along the line as the sea bottom, geological conditions, and water depth varied. Te processed PS-wave time section showed a good-quality image of the


Figure 2.19: (a) The structure of the Tommeliten Alpha structure is obscured by escaping gas (gas chimney) on the conventional pressure wave section. The top of the structure over a horizontal distance of 3 km between well 1/9-1 (left vertical line) and well 1/9-3R (right vertical line) cannot be mapped due to the gaseous overburden. The reflector disruption is so severe that no stratigraphic or structural interpretation can be made between the wells. The targets of interest are the reflections between 3 and 3.5s in the Top Ekofisk chalk interval and possible Jurassic prospects below 3.7s in the mid-part of the section. In the flank areas, the seismic data quality is considered to be very good. The PS-mode-converted shear-wave section (b) considerably reduces the area of uncertain structural interpretation, especially in the deeper part of the section (for the Top Ekofisk Fm and the Top Lower Cretaceous Fm). The reservoir zone lies between 5.5 and 6s on the PS-wave section, and the faulted pattern can be partly followed across the crest of the dome. (Adapted from Ikelle and Amundsen, 2005.)


P-data Top reservoir Gas chimney ? a (a) 1/9 – 1 3 km 1/9 – 3R b 3 km 67 PS-data


Statoil


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