The oblique seismic experiment in oceanic crust

Abstract

The Oblique Seismic Experiment (OSE} is proposed to increase the usefulness of the IPOD crustal borehole as a means of investigating 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 to measure attenuation in oceanic crust. Both travel time and amplitude techniques are used to plan the experiment and to interpret the data. The reflectivity method for computing synthetic seisDDgrams is · developed for the case of· the receiver within the reflectivity zone and ray method results 'are shown for comparison. A three-component borehole geophone w1th discrete variable gain pre-amplifiers was developed for the experiment. The first successful Oblique Seismic Experiment in oceanic crust was carried out in March 1977 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. Wyllie's relation, self-consistent theory, and non- interactive theory are applied to the observed velocity profiles in an attempt to quantitatively determine the crack structure. In general both P-wave and S-wave 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 convincLng evidence for anisotropy in either layer 2 or 3 is found. Since the large fissures observed in the FAMJUS area should produce anisotropy it appears that large fissures are not present in the studied crust (110 My) . The results agree with the theory that large fissures are less prevalent in older crust, perhaps sealing with age, and that the density of small cracks decreases with depth. The hole was not deep enough to measure attenuation from normal incidence shots. Auplitudes were not consistent enough to obtain a measure of attenuation from long range shots. The Oblique Seismic Experiment in March 1977 was a tenuous operation and a higher priority should be given to the experiment before it is attempted again.