Tools and techniques for deep water sediment sampling around a ship wreck

In August 1997, a series of near-bottom operations were conducted in an acoustic transponder net deployed around the wreck of the ex USS-Agerholm, about 130 nm offshore Southern California in 800 m of water depth. The Marine Physical Laboratory\´s Deep Tow Control Vehicle equipped with two orthogonal horizontal thrusters, a sector scanning sonar, cameras, lights, a navigation sonar, and an altimeter sonar was used to first locate the wreck in a debris field spanning about 40,000 m ². Unlike conventional neutrally buoyant ROVs the control vehicle can handle payloads of a few thousand kilograms, depending on the water depth. The 0.680" electromechanical tow cable from which the vehicle and payload are suspended carries the full load and provides electrical power. The vehicle provides lateral position control. We used this capability to suspend a 0.25 m² box core below the vehicle and to take sediment samples at locations determined via transponder navigation. In spite of moderately rough weather conditions with 20 knot winds and 1.5-3 m seas on average, and in spite of operations from a ship without dynamic positioning, eight usable core samples were taken at ranges of 10 m to 1 km from the wreck. The vehicle was also used to deploy four sediment traps and an ADCP at precisely located bottom positions on a 50 m radius around the wreck. The relative merits and drawbacks of the tools and techniques used for these operations are discussed in this paper