Prior to releasing our OBEM receivers from the seafloor for recovery, we must navigate their position to account for drift due to ocean currents. This is necessary for accurate modeling of the controlled-source electromagnetic data from the SUESI transmitted waveform. We navigate the receivers by pinging on them with a transducer mounted on the hull of the R/V Revelle. The receivers listen for 11.0 kHz and reply with a 12.0 kHz ping.
We drive the ship in a circular tow while logging the two-way travel time and the GPS position of the ship at the moment each ping is sent. After getting several data points for accurate navigation, we send a coded acoustic signal that tells the receiver to release itself from the seafloor.
On an unrelated note, the weather has improved dramatically over the last 24 hours and brought us calm seas and clear skies. After some effort, I managed to get a decent photo with little blur of the starry night sky. Not an easy feat on a constantly swaying ship. Enjoy!