27 October 2013, 18°05'S 62°06'E, Karin Sigloch
The first four days on sea have passed very quickly. We had to get operational without delay: the first recovery of an ocean-bottom seismometer (OBS) happened only six hours after leaving Port Louis. Since then we have recovered eight instruments, and have experienced various weather conditions and technical quirks. Calling an instrument back after 13 months on the seafloor is the hour of truth: was everything done right and did the technology work?
Recovery is straightforward in principle. The ship navigates to the spot where the OBS was deployed last year. We send a short sequence of acoustic pings into the water using the ship's hydrophone. The instrument recognizes its "name" (unique code of pings) and turns a little hook to detach itself and its buoy from the iron anchor that has kept the whole assemblage down. It takes 30-60 minutes to rise to the surface where it is easily noticeable by its flasher, its radio beacon, and its bright orange colour. Only that in practice, things needn't be easy on the open ocean. The acoustic link down to 3000-5000 m is variable, the salt water is corrosive and may have damaged electrical connections or batteries, and high waves or poor visibility can make it difficult to spot the instrument and to hoist it onboard.
We were lucky that during the first few days the weather was sunny and the sea was calm -- good conditions for getting into working mode. For the next few days, a tropical storm will be our companion, travelling westward just as we are, just a few hundred kilometres to the north. Rain, higher waves, and a more physical recovery experience this morning gave us a first taste.
Recovery of our first ocean-bottom seismometer RR17 on a bright day and a calm sea.
Our latest recovery of an OBS as we entered the fringes of a tropical storm system.