Research note The Oblique Seismic Experiment on DSDP Leg 52

Summary. The Oblique Seismic Experiment (OSE) has been proposed to increase the usefulness of the IPOD crustal borehole as a means of investi-gating layer 2 of oceanic crust. Specific objectives are: to determine the lateral extent of the structure intersected by the borehole, to analyse the role of cracks in the velocity structure of layer 2, to look for anisotropy which may be caused by large cracks with a preferred orientation and, finally, to measure attenuation in oceanic crust. The first successful Oblique Seismic Experiment in oceanic crust was carried out in 1977 March in a hole 400 miles north of Puerto Rico. An adequate study of lateral velocity variations was impossible because the hole was not deep enough, the hole was inadequately logged, and the small scale basement topography was not known. In general both P- and S-wave velocity profiles suggest that the crack density decreases with depth in layer 2. Velocities at the bottom of layer 2 are the same as matrix velocities for basalt, implying that crack density may be negligible at this depth. No con-vincing evidence for anisotropy in either layer 2 or 3 is found from travel time analysis. The hole was not deep enough to measure attenuation from normal incidence shots and amplitudes were not consistent enough to obtain a measure of attenuation from long range shots. 1 Objectives of the OSE In an OSE shots which have been generated at the surface from small (< 1 km) to large (> IOkm) ranges are received in a borehole (Fig. 1). In oceanic crust, the direct or 'oblique' ray paths impinge on the receiver at angles from 0 " to almost 90". The OSE has been proposed to increase the usefulness of IPOD (International Phase of Ocean Drilling) crustal drilling as a means of investigating layer 2 in oceanic crust. The experiment has four objectives: (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