Santa Comes Bearing an OBS
Lamont graduate student Natalie Accardo reports from the Pacific. Blog 3: Jan. 1, 2013.
Christmas found the R/V Melville in the middle of the Pacific Ocean on the last day of a seven-day transit to the NoMelt Project site. In a coincidence that we hoped would be auspicious, we reached our first OBS site late that night. As much as we yearn to be home to do celebrate the holidays with our families, we also realize how fortunate we are to have the chance to do what we do. Many of us began Christmas day with phone calls home to offer holiday greetings to our families and loved ones. Then the entire crew mustered on the upper deck for the requisite group photo, with more than one Santa Claus in attendance. Sunshine abounded as the captain led a crew-wide gift exchange that produced enough chocolate candies to feed an army. The rest of the day was filled with a “coits” (a ring toss) tournament on the main deck, where two young female scientists (that is us!!) came from behind to win the championship and all the pride and glory that come with it. An epic feast topped off with homemade pies and cakes ended the day for most of the crew; for the science party our adventure was just beginning.
We arrived at the first OBS station late into the night of the 25th with apprehension abounding. Recovering OBS instruments from the ocean floor is always a tricky business, especially in our case; these instruments have been sitting beneath more than 3.5 miles of water for over a year. With cold, tired batteries powering the instruments’ acoustic transponders, communicating with them through miles of ocean currents amounts to a whispered conversation on a stormy night.
We initiate communication with an OBS by transmitting audible “chirps” from a communications box in the main science lab to a transducer on the ship’s hull. The transducer acts as a speaker to transmit the chirp through the ocean and down to the instrument. If the OBS is alive and well, it transmits seven chirps in response. Given the distance these signals have to travel, it takes about eight long, stressful seconds to hear the instruments reply. Sometimes there is no reply, and we try again, at different locations, from different angles, with alternate acoustic devices.
Once we know an instrument is up and running, we conduct an acoustic survey by cruising around and sending continuous chirps. We measure the time it takes for the instrument to chirp back to determine the distance to the OBS, providing a precise estimate of the instrument’s actual location on the seafloor. Once we have completed the survey, we are ready to bring the OBS up. We send another series of commands that tells the instrument to release itself from the seafloor and then monitor the distance to it as it rises through ocean. Once on the surface, the captain skillfully steers the ship very close to the OBS so that we can hook lines onto it and pull it safely on board.
Our Christmas Night OBS was successfully recovered, and by New Year’s Day we had retrieved 12 OBS and one magnetotelluric instrument (to be discussed in the next installment). Sadly, two instruments never responded and are assumed lost to the deep; we are likely to never know why. Our success can be seen in the growing army of instruments that stand at attention on the main deck.
We are completing the charge around the perimeter of the deployment, picking up instruments approximately every 10 hours. Soon we will make the turn and head onto the central line of the deployment, where interstation spacing is much shorter and the recoveries come hard and fast. From the Pacific we wish everyone a happy and healthy New Year!