Deployment of Pressure Inverted Echo Sounder (PIES)

Pressure inverted echo sounding is a technique that works by transmitting an acoustic pulse from a PIES instrument on the seabed upwards. The pulse is reflected off the water-air boundary at the sea surface and returns back down to the seabed where it is detected by the PIES. This enables an exact measurement of the two-way signal travel time to be calculated.  At the same instant, an accurate measurement of depth is made using a highly precise internal pressure sensor. Average water column velocity can then be calculated directly from the depth (i.e. distance) and travel time data. 

In September, a Sonardyne PIES was free-fall deployed from a small vessel of Liquid Robotics. The design of the instrument ensures that it always lands on the seabed in an upright position making it quick and easy for users to deploy without the involvement of a costly survey vessel and remote operated vehicle (ROV).

Depending on the sampling interval configured, Sonardyne's PIES is capable of remaining on the seabed for up to five years. The unit now in place, in a 950 metre deep water channel between the islands of Hawaii and Maui, is logging an average sound speed reading every seven minutes with an accuracy of approximately one metre per second.

At any time during a Sonardyne PIES deployment, stored data can be retrieved on-demand acoustically at the surface, either from a vessel or using an ASV (Autonomous Surface Vehicle). The unit incorporates Sonardyne's Wideband 2 acoustic communications signal architecture which guarantees data retrieval is both fast and energy efficient. 

On this occasion, PIES data gathered during the Hawaii survey will be recovered at the end of the mission.  Sonardyne engineers will return to the site in a few months time and will acoustically command the PIES unit to disconnect itself from its tripod stand.  It will then return to the surface under its own buoyancy, be recovered onboard and its data downloaded for analysis.