Abstract
Explosive charges are fired on the earth's surface at points in Arizona, California, and San Clemente Island. The resulting sound waves centering at 2 cps are detected by an array of sensing units in Southwestern Arizona. This array extends 108 miles along an east-west segment of a great circle with units placed at 4-mile intervals. From these units transmission lines are routed along the ground to recording stations near Dateland and Gila Bend. A single twin-conductor field line serves from one to three units and may be as long as 40 miles. Over such a line the sensing units are turned on by relays, and the signals are transmitted to the recording station. The use of carriers has permitted the separation of multiplexed signals by means of tuning controls. Push-pull condenser-coupled amplifiers drive the Brush recording pens. Sensitivity is controlled in 6-db steps through a range from 5 to 160 dynes per sq cm peak-to-peak for full scale deflection. A pass band from 0.2 to 5 cps is set by an acoustic filter in the sensing unit. Several years of use and development have resulted in a system which has proved to be quite satisfactory.
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