Acoustic Shadowgraph Mapping of Convective Activity

The Acoustic Shadowgraph is a device that allows internal ocean features to be monitored for long periods of time while remaining submerged and unattended. It transmits acoustic pulses through the ocean features and records the resulting intensity patterns on two vertically separated hydrophones. These are analysed to yield the required information. The apparatus is relatively inexpensive to operate and is proving useful in monitoring sub-surface convection, motion in large-scale chimneys, and thermal vents. This article presents technical details of the device and describes how it has been used to investigate subsurface convection and heat transfer in the North Greenland Sea. Some of the results are shown and briefly analysed.