What Will We See Beneath The Sea?

Abstract

ABSTRACT If present trends in marine geophysical exploration continue, and if the rate of new technology advance is not inhibited by political or other external factors, great advances may be expected in what can be seen beneath the sea?s bottom. The bright spot technique is in its infancy and is evolving lines of advance in the extraction of stratigraphic information from seismic reflection data. The next five years should see the doubling of the resolving power of the seismic method together with a reasonably reliable method for determination of gross lithology. In the fine detailing of individual prospects, methods should be available for fine-tuning lithologic identification. TEXT That a question such as the one posed by our title can be a serious subject of a technical meeting is in itself a remarkable sign of the growth of science in this century. To the 19th-century scientist, the depths of the sea were a great mystery. He had a few soundings made with a rope or wire line to tell him tantalizing bits of information about the topography of the sea bottom. But even so prominent and ubiquitous a submarine topographic feature as the sea mount or guyot had to wait until World War II to be discovered. Only within the last three decades have we really been able to hope to map, in any detail, the interfaces beneath the sea bottom. The seismic refraction method was the first technique used to map submarine layers. As with the old soundings, however, the refraction method brought only isolated, discontinuous bits of information about some submarine layers. It was not until the seismic (or acoustic) reflection method was introduced into submarine mapping that some continuous map data became available. Submarine seismic reflection profiling has two distinct and distinctive ancestral lines. One group of marine technologists had developed the technique of mapping the ocean bottom with sound waves--sonar. With time, the sonar transmitters had become increasingly powerful to cope with deeper or more poorly reflecting ocean bottoms. Inevitably, sonar records (fathograms) were found to depict reflecting boundaries beneath the bottom. From this beginning, it was quite natural to continue to increase the power of the sonar system to achieve ever greater penetration beneath the bottom. All of the successors or descendants of sonar had a common configuration. On or near the survey ship was a transmitter of some sort. Almost anything that could (or might) be used to make an underwater sound was pressed into service. Devices ranged from piezoelectric crystals and magnetostrictive metal rods, through rubber balls inflated to bursting, to spark generators and electro-mechanical implosion devices. To receive the return signal, a single hydrophone or single hydrophone array was mounted on the hull or towed close to the ship. Penetrations of the sea bottom of up to one or two thousand feet were routinely obtained. When the late, great Maurice Ewing substituted half-pound blocks of TNT for the other sound sources, the results were spectacular.