The penetrator is a torpedo-shaped object which is allowed to free fall through the water column and penetrate the sea bed by virtue of its own momentum During the past five years, the Building Research Establishment (BRE), in collaboration with the EEC Joint Research Centre (JRC) and various oceanographic institutions, has been developing the use of penetrators for the measurement of sediment properties in deep water (ref 1) Sediment strength can be deduced from the measured deceleration in the sediment, and other properties can be measured by onboard sensors and relayed to the ship acoustically.

The instrumented penetrator has been particularly relevant to the site investigation of the areas being studied to assess the feasibility of deep ocean disposal of radioactive waste, because large scale penetrators have been suggested as a possible technique for burying the waste (ref 2).

INSTRUMENTATION SYSTEMS

The acoustic doppler shift system (ADSS) uses the observed doppler shift of a constant 12 kHz transmitter to measure the velocity of the penetrator ADSS was used for the first deep ocean tests of large, instrumented penetrators (ref 2), which demonstrated that a 1800 kg penetrator can reach burial depths of over 30 m and can be tracked in 5400 m of water using acoustic instrumentation Further deep ocean trials, performed in 1984 and 1985 (refs 3 and 4), used ADSS to measure the behaviour of penetrators of different sizes and shapes.

The signal from the ADSS transmitter is attenuated as the penetrator enters the sediment; this reduces the quality of the data and in some cases results in total loss of signal Measurements made during the 1983 tests, which used a commercially available transmitter, indicated that the signal strength after the penetrator had come to rest in the sediment was marginal for successful operation of the system To improve the performance of ADSS, a more powerful transmitter was designed and built by the University of East Anglia (ref 5)

The pulsed acoustic telemetry system (PATSY) is a more versatile, 3.5 kHz system, which encodes data as the time intervals between a series of pulses so that up to eight channels of data can be transmitted PATSY, which is being developed by the Institute of Oceanographic Sciences (ref 6), was tested during the 1984 and 1985 penetrator trials (refs 3 and 4)

PATSY consists of three packages a communication package at the tail, a sensor package at the centre of gravity, and an optical switch (used to detect the instant of impact) at the nose For the Tyro 86 experiments, the sensor package comprised two accelerometers, two inclinometers, and two pressure transducers to measure the pore pressures in the surrounding sediment Data collection, storage, and transmission are controlled by a microprocessor, there are four modes of operation, which are entered sequentially (i) quiescent mode, where no data are collected or transmitted, (ii) slow data collection (SDC) mode, where no data are transmitted, but all sensors are read 10 times every second and the data stored in memory.

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