Meet the Teacher:

Doug Wilson

The overarching objective of the cruise will be to study sound propagation with a view towards establishing an underwater acoustic navigation system optimized to track yellowtail flounder near and at the bottom around Georges Bank. Yellowtail flounder reside at depths principally between 20 and 40 fathoms (1 fm = 1.8288m). Such a fish tagging study may come as soon as next year. For this purpose we will need to understand better the limits of sound propagation up the slopes of Georges Bank, both on the steep northern slope and the more gradual southern slope. To do this we will deploy a sound source north and south of the bank and position a pair of hydrophones on a lightweight mooring at various locations around and on the bank to measure how well sound reaches different locations up the bank.

The main objective is, as stated, to measure transmission losses along various paths. We will have flexibility in terms of where we deploy the source and where we do the listening. We can listen straight up the bank slope and on top. We can also listen at locations at greater distances to either side. For planning purposes we will pong (=send an acoustic signal) every 6 minutes. We have a lowered hydrophone, calibrated, with which we can check the source level.

In order to put the measurements into a larger context and predict acoustic performance at other times of the year, the data will be compared ('predicted') with an acoustic propagation model. Dr. Gopu Potty will be in charge of this effort. The model will be used at sea so that results can immediately be compared. In order to set up the model we will need to know the sound velocity structure and the easiest way to obtain this will be by taking hydrographic stations (CTDs) along the sound propagation lines. Knowing temperature and salinity as a function of depth gives us the sound velocity profile.  We also hope to collect mud samples to get an idea of bottom sediment properties for these play a decisive role in how far sound can propagate in shallow water.

During the day the main activities will center around the mooring work and repositioning of the hydrophones. During the night we will do most of the hydrographic work along the lines of interest as well as collect mud samples. We have also developed a 'poor man's' thermistor chain which we plan to use to examine in greater detail the thermal structure of fronts around the bank. At all times the ship will be operating an acoustic Doppler current profiler to measure currents from the surface down.