Hydroacoustic Calibration With Imploding Glass Spheres

Calibration and validation of acoustic source and propagation models plays an important role in the hydroacoustic component of Comprehensive Nuclear-Test-Ban Treaty CTBT monitoring. This validation requires that we have a loud acoustic source with sufficient low-frequency energy that could be easily deployed in the Sound Fixing and Ranging SOFAR channel. SUS charges Signal Underwater Source, 0.82 kg of explosive have been used for this purpose for years, but safety and environmental concerns make these sources impractical. Light bulbs with sinkers have long been used as shallow acoustic sources, but are too small for the purposes stated. Here we describe investigations into the use of thin-walled glass spheres as calibration sources. Since glass becomes stronger under pressure, a method is needed to reliably initiate the failure of the sphere at a specified depth. A prototype-smashing device, called a spherecracker, was designed for this purpose. It consists of a 4-inch-diameter piston with a 14-inch diameter ram. The end cap on the devices cylinder tapers to a 1-inch diameter opening with a rupture disk calibrated to fail within 5 of the failure pressure. The ram initiates failure by punching a hole into the glass sphere. In March 1999 we conducted an experiment with a 10-inch-diameter glass sphere at Dabob Bay, Washington. The sphere was tethered and lowered to a depth of 600 ft. A metal pipe was then slid down the tether line as a smashing device. This implosion produced a pressure pulse with peak amplitude of 9x1011 microPa at 1 meter and duration of 1 msec. The signal was recorded at a sampling rate of 100 kHz, and the spectrum shows energy from 50-50000 Hz. The low-frequency source level is approximately 200 dB re 1-microPa. In February 2000 a test was conducted with a 22-liter glass sphere off the coast of San Diego at a depth of 685 meters in water that was 1200 meters deep.