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An autonomous profiling system is being developed to measure physical and optical properties in ocean-bottom boundary layers. System sensors will include electromagnetic current meters, temperature sensors, transmissometers, and water sample bottles affixed in a vertical array to a bottom-supported instrument frame at heights ranging from 0.25-5.00 m (1-16 ft) above its base. The instrumentation will measure high-frequency property fluctuations (5 Hz) as well as mean values. High-capacity tape recorders will permit unattended deployments for up to 3 months. Field tests will be conducted in the Gulf of Mexico.
The stress exerted by currents on the sea bottom is an important parameter in sedimentologic studies. The level of bottom stress governs the occurrence and the mode (suspension or bed load) of sediment transport. Of particular importance to the determination of bottom stress is the accurate quantification of stress components containing a vertical turbulent velocity term. Direct measurements of this and other high-frequency quantities that control the rates of erosion, deposition, and scour will provide a better understanding of sediment transport dynamics in modern environments and permit the development of rigorous criteria for interpreting ancient marine deposits.
Ocean-bottom boundary layers are characterized by high turbulence levels. An understanding of the flow dynamics thus requires a knowledge of the turbulence field. A large body of evidence indicates that suspended sediment modifies flow dynamics by changing the characteristics of both the mean and turbulence fields. An important change is the reduction in the magnitude and change in direction of the bottom stress. The profiling system will measure turbulence and suspended-sediment quantities contemporaneously. This sampling scheme will allow the determination of flow-sediment interactions and, for the first time, will provide data necessary for the validation of theoretical models of particle-laden flow.
The profiling system is a third-generation device that has an improved capability over any other existing profilers. It is the only one with the capability of evaluating the effects on currents and bottom stress of suspended sediments or temperature gradients very near the sea bed.
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