To ensure safe operation of underground salt caverns -- whether they're used for storage or brine production -- cavity surveillance via sonar data to detect changes in the cavities is a routine procedure for prudent operators.
These data can be acquired by lowering an ultrasonic transducer into the borehole to emit sonic signals at specific depths. The signals are reflected off the cavern walls and then detected by the same transducer.
The time lapse between signal emission and reflection is measured and, with knowledge of the acoustic velocity of the medium -- oil, brine, gas, etc., the distance to the cavern wall can be calculated.
To obtain the data to measure distances to the wall with regard to the cavern floor and roof and certain surface irregularities in the wall, the horizontally-aligned transducer is tilted.
During the survey process, computers ascertain if various zones have been surveyed adequately.
Given the multitude of underground cavern storage sites worldwide, specialists in sonar data monitoring procedures are plenty busy, according to Gary McCool, who heads up Sonarwire Inc., which he founded in 1983.
The sonar business, however, is not all work and no play.
Indeed, the German-based cavity and cavern monitoring company, SOCON, once used an underground salt cavern to record music -- albeit not exactly the toe-tapping kind.
The idea for the artistic endeavor was triggered in 1994 when Swiss sculptor and electroacoustic music composer Oscar Wiggli queried SOCON general manager Hartmut von Tryller as to how sound would behave in methane gas stored in a cavern under high pressure.
Intrigued, von Tryller set out to implement an experiment that would provide the answer.
A cavern in northern Germany that was due to be surveyed was selected for the project, which included participation by the company's main shareholder as a partner.
To restrain costs, an out-of-service sonar tool was chosen for the job. The ultrasonic transducer was replaced by a microphone and loudspeaker, and the sonar receiver and transmitter were replaced with a standard high-fidelity receiver and transmitter units.
Wiggli prepared hundreds of different sounds to introduce into the gas-filled underground space, and the complex sound sequences were altered by the cavern environment in varying degrees -- depending on the position of the equipment -- and recorded. The cavern had numerous protrusions and recesses as well as many leached pockets of varying sizes, providing the ideal setup to modify sound.
However, it was quickly determined that these caverns are not an acceptable venue for transforming just any kind of sound.
At the request of the SOCON team members, a Beatles recording was sent down into the cavern. But when the sound came back over the loudspeaker, it was distorted and undefinable -- a travesty for the die-hard fan.
The jury is out as to whether it was the gas or the cavern space that modified the sounds sent underground during the experiment. Whatever the case, the project that was triggered by an artist's curiosity yielded some unanticipated scientific benefits, according to von Tryller.
For instance, if a break should occur in the tubing that connects such a gas-filled cavern to the surface, the sound made by the gas escaping through the fissure could be detected by the acoustic tool.
Wiggli, meanwhile, has produced a CD of original electroacoustic music compositions comprised solely of the altered sounds that were recorded during the experiment. But don't race to the nearest music store -- it's not available commercially.