An autonomously recording inverted echo sounder: ARIES II

A self-contained instrument, the Autonomously Recording Inverted Echo Sounder (ARIES II). carrying two 250-kHz beam side-scan sonars and with the capacity to record sonar data sampled at 3.2 kHz for 168 h with the sonars operating with a pulse repetition rate of 2 Hz, has been constructed and tested in a mooring deployment that lasted for 25 days near the edge of the continental shelf west of Scotland. The mean water depth was 146 m. ARIES II was positioned at a mean transducer depth of 34.6 m with sonars directed upward at 20° to the horizontal to obtain acoustic returns from targets at or near the sea surface. The instrument was preprogrammed to record continuously over periods of 2, 4, and 13 h, the last to cover the M2 tidal cycle dominant in the area. Sonographs are presented to illustrate observations of surface waves and wave groups, internal solilons. rain showers, and Langmuir circulation. An analysis is made of the effects of surface waves, currents, and internal waves on the instrument. The potential use of the instrument is demonstrated in providing estimates of the propagation direction and speed of internal waves, as well as in estimating the drift, orientation, and mean separation of Langmuir bands. The separation is found to increase with wind speed.