Towards a Multiple Station High Frequency Shallow Water Acoustic Tomography System

Acoustic Tomography System, " a thesis by Justin Michael Eickmeier. Abstract Title: Acoustic tomography studies have traditionally been conducted in deep water, implementing high power, low frequency pulses transmitted over long distances to map large scale current events. Conducting acoustic tomography in a shallow water environment using high frequency low power pulses represents both unique engineering challenges and opportunities for advancements in hydroacoustic measurement techniques. A two station acoustic tomography system has been designed, prototyped, tested and deployed in the field to evaluate multiple facets of system performance and the feasibility of deploying a large network of stations to define a shallow water current field. Each station is capable of transmitting a 1.5 ms continuous wave of 983 kHz sound and opening a 3.98 ms sample window collecting 512 samples for signal reception and definition. The key benefit of operating at a high frequency is that a 983 kHz signal is clearly discernable from lower frequency background noise. Due to the large attenuation coefficient associated with this high frequency signal, station separation is set at 0.92 m in sea water. The critical factor in acoustic tomography is determining the precise arrival time of an acoustic pulse sent from one station to another with known station-to-station separation. Station synchronization is presently accomplished by hardwiring the stations together and using an input-capture routine executed by a Microchip dsPIC microcontroller conducting instruction cycles at 29.48 MHz. The routine is accurate to within two instruction cycles. By measuring the arrival times of two pulses sent in opposite directions along the same ray path, a time of flight analysis can be conducted. Through finite impulse response (FIR) interpolation and window filtering, a differential analysis eliminates the impacts of dynamic background conditions and yields current measurements with a resolution of 2.88 cm/s.