Underwater Altimeter Helps Protect Expensive Instruments

Tags: ua2, sonar

Anton Smirnov and Job Bello of EIC Labratories with their instrument that can locates deposits of submerged oil.
Fishers UA-2 underwater altimeter is attached to the instrument to show distance from the seafloor.

Towing expensive oceanographic equipment close enough to the ocean bottom to be effective, yet far enough away to prevent damage, has always been a challenge. Instruments like magnetometers and metal detectors must be towed fairly close to the seafloor to detect buried objects. The height of sonar systems off the bottom is critical to producing the highest quality images. To help solve this problem JW Fishers developed the UA-2 underwater altimeter. The altimeter allows underwater detection and survey equipment to be towed at a precise altitude above the seabed. The altimeter's transducer mounts on the underside of the towed device, and a cable up to 300 meters in length connects the downstairs electronics to a topside control box. The surface unit uses an LCD display to show the distance between the towed system and the bottom.

The US Army Corps of Engineers (USACE) has found a rather unique application for the altimeter. Among its many duties the corps is responsible for monitoring dredge disposal sites. Each year approximately 120 million cubic yards of dredged material are deposited at designated aquatic disposal sites located in 20 to 150 feet of water. According to a report issued by the USACE Waterways Experiment Station, monitoring of the deposits is necessary to evaluate and determine the long term physical and chemical stability of the deposit and to assist in determining the remaining disposal site capacity. One common monitoring technique is sediment sampling using a vibrocore system. There are a number of design variations, but a typical sampler consists of a steel base assembly that lays flat on the bottom, a pipe which is driven down into the bottom through a hole in the base, and steel head assembly at the surface. The pipe is pushed into the seabed by both mechanical force on the head, and by vibration. As the pipe descends, it captures a sample of the various layers of the sea floor. The pipe is then brought to the surface and an inner liner holding the sediment is removed.

A problem occurs if the head is driven too far and comes into contact with the base. This can damage the vibrocore. To eliminate the possibility of this happening, the corps is now experimenting with the use of underwater altimeters. Instead of attaching the altimeter's transducer to the head side of the vibrocore, the sonar transmitter is attached to the base and aimed at the surface. As the pipe is driven into the bottom and the head descends, the altimeter measures the distance between the upper and lower assembly. Watching the altimeter's display, the operator knows exactly how deep the pipe has penetrated the ocean floor, and also the distance between the head and base. The machine can then be stopped before the two run into each other, preventing any damage. Thomas Wyche with USACE reports, We tested the altimeter here in port. I noticed that as the vibrator started digging in with a 15 foot core tube, the UA-2 would show the progression of the penetration into the substrate. We saw the distance close from the head to the base as it went from 15, 14, ...8,..4, to the 1 reading. It looks like it will do the trick, and prevent the equipment from getting banged up.

Another group using the altimeter in an unusual application is EIC Laboratories, a research and development corporation in Massachusetts. EIC specializes in converting basic research into new products and processes. One of its newest products is a towed underwater instrument that uses a laser beam to locate oil spills in the water column and on the bottom. There is tremendous interest in using this device in the Gulf of Mexico in the wake of the BP disaster. Scientists at the lab have equipped their instrument with a UA-2 altimeter which inputs the exact distance from the sea floor to automatically focus the laser. Fishers SCAN-650 scanning sonar, which can see submerged oil, has been added to the equipment package along with a DDW-1 deep dive wing to assist in maximizing tow depth while minimizing the amount of tow cable required.