2002 Sound in the Sea:
SIRENA Expedition
Seismic Investigation by REcordingof acoustic waves in the North Atlantic

Deployment of the hydrophone: (top-bottom) Hydrophone being loaded into pressure case; Deploying the float which will come to the surface when the instrument is recovered the next year; Deployment of the hydrophone; Paying out the mooring line between the hydrophone and anchor; Kicking the anchor in; Chris Fox, Sara Bazin and Haru Matsumoto acoustically querying the instrument after deployment to verify its location.

Deploying a hydrophone

Chris Fox, Principal Investigator
Sound in the Sea Project
NOAA Pacific Marine Environmental
Laboratory, Newport, Oregon USA

The procedure to deploy a hydrophone mooring is as follows: We must first identify an area of the seafloor which is relatively flat for 5 - 10 km diameter. This is necessary so that we can be certain of the length of mooring required to suspend the hydrophone into the overlying SOFAR channel. Once a site has been identified, we measure the water properties using an eXpendible BathyThermograph (or XBT) and determine the precise depth of lowest sound speed (the axis of the SOFAR channel). Knowing the water depth and the depth that we want to suspend the hydrophone, we can construct a mooring of just the proper length. Next we position the ship downwind 3-4 km and begin slowly moving toward our desired deployment site as we pay out the mooring into the water. First a large yellow float, with strobe light, flag and floating line to aid in the eventual recovery of the mooring next year. Then the hydrophone instrument, which has been checked out onboard and programmed to begin recording usually the next day. Next 2,500 -4,500 meters of special mooring line design not to stretch or vibrate (strum).

Finally, an acoustic release device that can be commanded to release the mooring and a large anchor composed of surplus aircraft carrier anchor chain. When the ship reaches the desired point, the anchor is released and the mooring in sent to the bottom to record for a year until we return to pick it up. The final procedure is to acoustically “ping” to the release above the anchor from various directions to determine it’s exact location. This effort has been especially difficult this year due again to the high seas which make the ship less maneuverable and generate significant acoustic noise into the environment.