THE OBLIQUE SEISMIC EXPERIMENT IN OCEANIC CRUST- EQUIPMENT AND TECHNIQUE R. A. STEPHEN* Department of Geodesy and Geophysics, Madingley Rise, Madingley Road, Cambridge, CB3 0EZ, England (Accepted 14 June, 1979) Abstract. In an Oblique Seismic Experiment (OSE), shots which have been generated at the surface from small (<1 km) to large (> 10 km) ranges, are received in an oceanic crustal borehole. The objective is to study the shallow structure of the oceanic crust in the vicinity of the hole. A three component borehole geophone with a discrete variable gain preamplifier was developed for the experiment by modifying a commercially available tool. The first successful OSE in oceanic crust was carried out on DSDP Leg 52 in March, 1977. The experiment demonstrated the feasibility of instrumenting oceanic crustal boreholes. Introduction In an Oblique Seismic Experiment (OSE), shots, which have been generated at the surface from small (< 1 km) to large (> 10 km) ranges, are received in an oceanic crustal borehole (Figure 1). The specific objectives of the experiment are: (i) to determine the lateral extent of the structure intersected by the borehole, (ii) to analyse the role of cracks in the velocity structure of oceanic crust, (iii) to determine if shallow basement structure is anisotropic, and (iv) to measure attenuation in oceanic crust. The first successful OSE in oceanic crust was carried out on DSDP Leg 52 in March, 1977 (Stephen et al., in press). This paper describes the equipment and technique used in this novel experiment. Equipment GENERAL DESCRIPTION The basic objective of the experiment is to obtain seismic recordings from shots at various ranges from the borehole. This requires two ships: a drilling vessel, which remains stationary over the hole, and a shooting ship, which fires the charges. The signals to be recorded are the shot instant, the direct arrival of * Present address: Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. Marine Geophysical Researches 4 (1979) 213-226. 0025-3235/79/0042-0213 $02.10. Copyright 9 1979 by D. Reidel Publishing Company.