Ocean-bottom seismograph

Abstract

In one program of Project VELA UNIFORM, the Air Force Cambridge Research Laboratories and Texas Instruments developed the ocean-bottom seismograph as a research tool for the study of acoustical wave energy at the solid earth-water interface generated either by earthquakes or by explosive sources. The seismograph described by this article is a third-generation unit having progressed through more than four years of development, design, and field experiments. The present configuration is spherical, 40 inches in diameter. It has been tested successfully in various locations to a depth of 24 000 feet. The primary components, in addition to the three-component seismometer system, contained within the seismograph are a digital clock having a 40-day capacity, with 0.1 second accuracy, seismic amplifiers capable of detecting noise level of 0.1 microvolt or less, a magnetic tape transport having a 30-day recording capacity for signals of less than 10 c/s frequency, and internal battery power supply for all components. In operation, the seismograph descends to the desired depth free fall, remains unattended and untethered, and is retrieved on sonar code command or preset time by decoupling the anchor base through a cocked spring mechanism, which is triggered by fusion of a small steel wire. Results to date have proved the feasibility of such an ocean-bottom data system, including the important capability to recall the package by sonar code command.