Oceanography

Shear probes

This extremely brief overview of oceanographic measurement techniques can only cover the essentials of the most important platforms and instruments. Special equipments exists, and new special equipment is being designed every day, to address specific problems. The shear probe may serve as an example. It is desgined to give insight into oceanic turbulence at the centimeter scale. Turbulence is characterized by currents which vary over short distances and short time intervals, so an instrument designed to measure turbulence has to be able to resolve differences in current speed and direction over a vertical distance of not more than a meter or so.

One such shear probe is a cylindrical instrument of less than 1m length with two electromagnetic or acoustic current meters at its two ends. By measuring current speed and direction at two points less than 1m apart it allows the determination of the current shear over that distance. To allow a reliable measurement not influenced by the heaving motion of the ship the probe falls slowly and freely through the ocean. Its maximum diving depth is programmed before the experiment , and the probe returns to the surface when that depth is reached. It is than picked up by the ship, and the internally recorded data are retrieved.

Another type of free-fall instrument used microstructure sensors that measure velocity fluctuations on a special scale of about 10mm. It used a piezo-electric beam that generates small voltages as the turbulent velocity varies the lift and thus the bending force on an aerofoil as it moves through the water.