Retrieving Instruments from the Deep

Jim Gaherty
December 27, 2011
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OBS bobs on the sea surface as the Langseth approaches.

Over the first 22 days aboard the R/V Marcus G. Langseth, we’ve zigged and zagged our way over a 360×240 mile region of the Pacific plate, first dropping instruments to the seafloor, and then shooting airguns to them (see previous posts). The final step is to recover a subset of the instruments:  34 ocean-bottom seismometers (OBS) that recorded the shooting, and three of the MT instruments.  Excitement abounds – we will see our new data for the first time when we pick up the instruments, and get enticing first peeks that may confirm our ideas about plate structure.  But there is plenty of apprehension as well; many potential obstacles are associated with recovering our instruments from the seafloor, and it is possible that some of them (and the data that they contain) could be lost permanently to the deep.

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Engineer prepares to snag an SIO OBS

Much of the apprehension is stimulated by uncertainty in the communication with the instrument.  As we arrive at each site, we send acoustic “pings” from a transmitter on the ship, asking the instrument to wake up; if the instrument is alive, it will ping back.  A second set of pings instructs the instrument to release its anchor, a process that typically takes several minutes.  Subsequent pings to the instrument allow us to estimate the distance to it and thus whether or not it is rising to the surface.  In shallow water, this system works great. But at 5 km water depth, transmitting acoustic signals is akin to operating a TV remote with a weak battery from across a large crowded room.  Signals are faint, and echoes from the ship and other noise sources mask the instrument signals, such that we were often unsure if the instrument was responding or not.  The uncertainty was maddening.

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WHOI OBS is craned on board.

Despite these uncertainties, the instruments did indeed hear our distant acoustic calls. The first several recoveries went well, and the data look excellent.  However, we then encountered a string of three consecutive instruments that refused to budge from the seafloor.  All responded to our pings, but they could not lift off the bottom.  Stuck in the mud?  Flooded?  After spending several hours attempting to recover them, we sadly moved on.  Forensic analysis of some of the recovered instruments revealed a likely cause: bad AA battery cells in the release systems, such that they have power to communicate, but not quite enough to complete the release process.  Recognizing this, we devised a power-saving routine for the remainder of the recoveries; sending release commands in brief bursts that used minimal power, and then waiting in near-silence until the OBS appeared at the surface.  This proved successful, and in the end, we recovered 30 of 34 OBS, and 2 of 3 MT instruments.  We are frustrated by the losses, but thankful for the data in hand.  Next year we will return to recover the broadband OBS that are recording the earthquake data – in the meantime, we hope to engineer a new plan to coax the four missing OBS back to the surface.