Pacific Echo: A deep ocean collaborative experiment

Abstract

This paper summarizes experiments carried out using towed line arrays in the Pacific Echo sea trials. Although John Preston was not a participant, the experiments are examples consistent with his efforts in promoting collaborative research at sea with horizontal arrays. The overall goal of Pacific Echo was to study the impact of thin sediment ocean bottom environments on sound propagation in deep water. The objective of the experiments described here was to study the evolution of young oceanic crust. The hypothesis was that the sound speed of young basalt increased with age of the crust. Sound speed was inferred from low frequency measurements of the reflection coefficient versus grazing angle at sites of increasing distance from the deep ocean spreading center. The experiments introduced a novel design for measuring the reflection coefficients using two ships and small explosive charges. The signals at the array were spatially filtered to resolve the specular reflections as the ships opened range on set courses. The data provided estimates of both the compressional and shear wave speeds of the basalt. Results from the broadside reflection technique showed that sound speed increased with crustal age and were consistent with measurements obtained from conventional seismic reflection surveys.