Ekofisk Field exhibits a classic case of gas in the overborden destroying the image obtained from surface seismic by delaying and distorting the propagation of wavefronts. This is a very long standing problem: the field was developed with rigs set up on the northern and southern ritus, around the apparently collapsed centre. The first production well drilled into the middle came in fifteen hundred feet high to prognosis, showing the full site of the field and the effects of the gas. 30 years later about 60% of the crestal area bas been successfully imaged by undershooting the gas with 3D borehole profiles. The complete image, however, will be obtained only when a more complete onderstanding of the velocity structure of the gas has been achieved.


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