Construction of a seismic profiler

Abstract

The design and construction of a continuous seismic profiling system is described. A review of some of the seismic sound sources currently in use shows how the phenomena associated with underwater explosions are made use of in the production of sound waves for seismic profiling. A full discussion is given of the pneumatic sound source which has been constructed for use with the Victoria University seismic program. The directional characteristics of ship-towed line hydrophone arrays enhances the detection of bottom reflected sound waves by reducing the output resulting from noise received along the axis of the array. Piezo-electric hydrophones are particularly suitable for use in such arrays. An elementary consideration of linear arrays shows that acoustic length plays an important part in determining array response and directionality. The directivity pattern at three different wavelengths is calculated for an array of sixteen elements. This shows that some knowledge of the energy spectrum of the sound source is necessary beofre a suitable receiving array can be built. The construction of two arrays employing crystal hydrophones is described. Preliminary experimentation with the airgun during the oceano-graphic cruise of USNS Eltanin from March to May, 1968, revealed inadequancies in the high pressure seals. The factors influencing the efficiency of these seals are discussed. The subsequent modifications that were made to the airgun led to its successful operation during the oceanographic cruise of M.V. Taranui in October 1968. During the 1968-69 summer Antarctic supply cruises of H.M.N.Z.S. Endeavour the complete profiling system was operated along the east coast of New Zealand, and in the region of the Campbell Plateau. Some of the results obtained are discussed in terms of equipment performance, and some suggestions are given for future work on the profiler.