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Three telemetering probes designed to rest on or in fine-grained sediments of the sea floor and to make in-place measurements were used successfully at sea from the USC&GS ship Davidson during the summer of 1967. The test and evaluation site was a pocket of clay in the Wilkinson basin, Gulf of Maine; water depths ranged from 240 to 290 m. A tower about 4 m high and 2.4 m wide at the base was lowered to the sea floor. Within the tower was contained a motor-driven shaft to which was attached either a torque assembly and a vane for measuring shear strength, or a two-legged probe containing a Cs-137 isotope source in one leg and a scintillation detector in the other for measuring bulk density by gamma-ray transmission. The motor was operated remotely by command from the s ip. Vane rotation measurement made by strain gauges was telemetered through 3.6 km of electrical cable to the surface and recorded on an x-y plotter aboard the vessel. Pulses from the gamma-ray apparatus were transmitted directly over the electrical cable to the ship, where they were counted.
Shear strength was measured at eight localities in successive 30-cm intervals to a depth of 2.5 m below the bottom. Bulk density was measured at five localities in successive 2.5-cm intervals to a depth of 1.5 m below the bottom. The in-place results have been compared with laboratory measurements of shear strength and bulk density made on sediment cores collected in 1966 from the same localities. In-place vane shear strength distinctly is higher than laboratory vane shear strength and lower than laboratory Swedish fall-cone shear strength. All bulk density values determined by in-place and laboratory gamma-ray transmission methods and by the weight/volume laboratory method are nearly identical.
Pore pressure was measured successfully in place at one location by a differential piezometer probe that telemetered results over the electrical cable to a shipboard computer. The results obtained from this probe are not conclusive because of problems with the cable.
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