f Planning, equipment and execution of crosswel surveys

Schlumberger acquired a cross-well survey in Sellafield, Cumbria for UK Nirex Ltd in 1993 between BH2 and BH4, to establish the acoustic character which may be used to address uncertainties in the the understanding of the geological structure. The principle lithology consists of volcanic tuffs overlain by Breccias and fluvial sediments The volcanic rocks have both low porosity and permeability with localised fracture zones, primarily associated with faults. The Q estimated from VSPs conducted in the wells was 33. BH2 and BH4 are near vertical and are separated by 120 metres at surface, being respectively 1600 metres and 1240 metres in depth. The wells have been fully cored and extensively logged. Following successful acquisition of this data set , a second survey between BH2 and BH5, spaced at 590 metres was acquired. This paper describes the equipment and planning for these surveys, and is complemented by a discussion of the tomographic inversion results in two additional papers, Parker et al which discuss methodology and results of isotropic inversions , and Kragh et al on the anisotropic inversion results. Both are submitted for presentation in these proceedings.