ROBOT SENSES THE MOVEMENT OF THE SEAS

An autonomous underwater robot bristling with sensors is helping researchers to find out what goes on in turbulent water. Using a new sonar system, it can analyze the water over scales ranging from millimeters to tens of meters to answer questions such as how the momentum of a tropical cyclone is transferred from the wind of the water, how sewage effluent dumped into the ocean mixes with the surrounding water, or how organic wastes disperse and feed unwanted algae.

The device called the Microstructure Flux Profiler (MFP) was developed by scientists. It contains a suite of instruments, some new and some off the shelf, which collect detailed information on turbulent processes in lakes, estuaries and coastal areas. The MFP is the first instrument that can provide direct measurements of turbulent processes over a wide size range. This is critical because the transfer of energy in water is caused by a combination of advection-the water’s large scale horizontal motion-and mixing which may be on a minute scale.

The MFP successfully completed its first scientific investigation. It resembles a small tree-like cactus called a Cholla, weighs roughly 100 kilograms, and is 2 meters from top to toe. In the water it operates independently. Researchers pre-program its movements and what measurements it should take at each depth and then lift it into the water with a crane. The vertical position of the MFP in the water column is controlled by a ballast system similar to that of a conventional submarine. A computer directs a piston to take in water to descend, or eject water to ascend. There is also a fail safe mechanism which will cause the MFP to rise if the electronics fail.

An acoustic imaging system, or sonar, was developed specifically for the MFP. Transducers on each of its two arms emit high frequency sound and then measure the frequency and the amplitude pf signals bounced off the tiny particles moving in the water. By correlating data collected by the two sensors, the MFP reconstructs a two-dimensional image of the water’s turbulence in the vertical plane. The 2-D picture is especially valuable because it puts all the data gathered by the profiler into the context of the overall flow.

The other novel instrument created for the MFP is the laser Doppler velocimeter, which measures the speed of particles in the water. The device splits beam from a helium-neon laser in two and then crosses the beams in front of the MFP. At the point where the beams cross, a fringe pattern of alternating light and dark is produced because the light waves interfere with each other-canceling each other out where a peak meets a trough and reinforcing each other where a peak meets a peak.

As particles pass through the fringes, they reflect the light-dark pattern. The speed of the particles-and thus the speed of the water-is determined by measuring with a video camera the frequency if illumination of the particles as they move through the bright fringes.

The velocimeter detects the direction of the particles in the water by shifting the frequency of the laser beams which makes the fringe pattern move in one direction. The frequency of the particles moving through the fringes will increase or decrease depending on whether the particles are moving in the same direction as the moving fringes or the opposite direction.

The principle behind the velocimeter is the fact that the instrument is extremely small and that it takes readings, digitizes and analyses the data, stores its findings and then throws out the raw data to save on data storage space. This is possible because an on-board computer had been produced which can process the data nearly as quickly as a Cray-2 supercomputer, but fits into a space only 30 centimeters by 30 centimeters by 20 centimeters.

A standard strain gauge, mounted on the dome of the MFP, measures the pressure of the water above it and thus the depth. Similarly, commercially available thermostats measure the water temperature 100 times a second. Salinity of the water is determined electrically. Four electrodes pass a current through the water. The voltage required is proportional to the conductivity of the water, and conductivity is proportional to salinity. A compass establishes the orientation of the profiler as it twists and turns and rises and falls in the turbulent water.