Marine Physical Laboratory
Scripps Institution of Oceanography

Turbulent mixing rates in the sea have been found to vary as the fourth power of the fine scale (~10 m) vertical shear. The global distribution of shear is poorly known, associated with phenomena, which are potentially quite localized in space and time. With the construction of the R/V Roger Revelle it was deemed appropriate to develop an exploration-oriented Doppler sonar system for the global-scale mapping of the fine-scale shear and velocity fields. Termed the Hydrographic Doppler Sonar System (HDSS), it consists of nested quartets of 50 and 140 kHz transducers, each oriented in the standard Janus configuration. The System is installed in two 4' x12' wells, one on either side of the Revelle's keel. The 50 kHz transducers each measure 0.5 x 1.0 m. They are fabricated from 1-3 ceramic composite tiles sandwiched between sheet-steel front and back masses. The 140 kHz transducers measure 0.2 m x 0.3 m and are fabricated of diced PZT-8 ceramic potted in urethane.

The investment in narrow-beam transducers is motivated by the quest for precise depth resolution. For long-range sonars, the width of the beam, rather than the length of the transmitted pulse, sets the depth resolution of the measurement.

Transmitting repeat sequence coded pulses through a polyethylene window, the 50 kHz system profiles to depths of order 600-1000 m, with 18 m depth resolution. The 140 kHz system attains depths of 200-350 m with 4 m depth resolution.

The HDSS was installed in Sasebo Japan in late 1999 and has been operational, with brief exceptions, ever since. The System has played a central role in experiments such as ASIAEX, EPIC and HOME, and has provided background information in a variety of other programs. The routine "steaming data" is enabling us to accumulate a picture of the global distribution of shear and the physical phenomena, which cause it.

Submitted on January 22, 2003