|
||||||
|
|
|||||||
|
||||||||
|
Palos Verdes Shelf Experiment: Whatever Can Go Wrong…
"Murphy was an optimist…" read the poster of Murphy's law that hung in my older brother's room when we were growing up. I believe it should have said, "Murphy was an oceanographer…" because in oceanographic field programs, it seems that anything that can go wrong, will. Of course, in any scientific field program, we do the best we can to have a backup plan, wait to see what Murphy will throw at us, and hope the backup plan works. However, when it comes to putting instrumentation on the ocean bottom in water deep enough that you can't send a diver to rescue your equipment, but shallow enough that the instruments are affected by waves, strong tides, marine growth, and more, you need to have a couple of backup plans. Invariably, things go wrong that are beyond your control, and as a result, there are lots of lost instruments in the ocean. And so we responded with cautious optimism when Carmen White of the U.S. Environmental Protection Agency (EPA) Region 9 office in San Francisco, California, recently approached the U.S. Geological Survey (USGS) Coastal and Marine Geology Program to design and carry out an extensive oceanographic experiment on the continental shelf off the Palos Verdes Peninsula in southern California. The experiment would include measuring such parameters as current velocity, water temperature, salinity, and suspended-sediment concentration in the bottom boundary layer (the water near the sea floor) and the water column (from the bottom boundary layer to the surface) over a whole winter season. The USGS had carried out successful field programs on the Palos Verdes shelf in 1992 and 2004, but certainly not without difficulty. We always love a challenge though, and in early December 2007 a team of USGS scientists and marine technicians from the USGS Western Coastal and Marine Geology Team in Santa Cruz and Menlo Park, California, and the USGS Woods Hole Science Center in Woods Hole, Massachusetts, embarked on another ambitious field effort (see "Palos Verdes Shelf Experiment: What Will Happen to the Contaminated Mud?", this issue).
Oceanographic equipment tends to be large and complex, to strike a balance between gathering as much data as possible and withstanding the rigors of the ocean environment. Three of the tripods we deployed, for example, are over 3 m tall; each weighs more than 1,500 lb and houses 11 waterproof pressure cases just to hold batteries. Two of these tripods are the infamous "Geoprobe" tripods, originally designed in the late 1970s and modified over the years to accept the most state-of-the-art equipment. Geoprobes have been used to monitor sediment-resuspension processes in the bottom boundary layer of coastal waters all around the United States and even in Europe. So, what's the backup plan to recover heavy tripods packed with instruments and deployed in 60-m water depth? First, you try to put two acoustic-release recovery systems on each tripod, so that if one fails, the other will still be able to release your lifeline, the recovery float. (The acoustic release responds to acoustic signals transmitted from a ship at the surface.) You also put an acoustic-ranging device on each tripod that can be interrogated from the surface, in case both of your acoustic releases fail and you need to go searching for the tripod. You also include a tilt sensor so that you know your tripod is sitting upright before you drive away for 4 months (otherwise you must recover and redeploy). If none of these systems works, then the last backup plan comes into play—our secret weapon, the remotely operated vehicle (ROV). For the Palos Verdes shelf experiment, we deployed tripods to collect data in the bottom boundary layer and moorings (instruments on a cable attached to an anchor at one end and a surface or subsurface buoy at the other) to collect data in the water column. Deployment went smoothly in December, and then… Murphy strikes the first time: after surviving 3 months of a particularly intense winter storm season, one Geoprobe had simply had enough; it decided to give up and send up the white flag (the recovery float), begging to be brought home. We saw the recovery float on the surface in late February, more than a month too early, during a mid-deployment coring cruise. Apparently, the wave action at 60-m depth was enough to work the release mechanism loose and let the ball go. It was extraordinarily lucky that we happened to be there and were able to recover the tripod by using the research vessel (R/V) Sea Watch out of the Southern California Marine Institute (SCMI).
Two other tripods deployed at the 60-m-depth sites were small, low-profile tripods designed to measure the current profile in the bottom boundary layer as close to the sea floor as possible. Because these tripods were low profile, they could support only one acoustic release, and so we decided to augment the release system by including a ground line leading from each tripod to a weight and a separate pickup float at the surface. If our acoustic release failed, we could recover the tripod using the pickup line. Murphy strikes the second time: though intended to assist us, the recovery floats merely attracted other vessels. Upon returning to the site in early April, we discovered that one float was missing and the other was hanging on by a thread. It turned out that both had been hit and dragged by passing vessels. One tripod had a broken leg; the other was flipped over. Murphy strikes a third time: one of the subsurface moorings decided not to come to the surface when we asked it to—the acoustic release transmitted a signal indicating that the recovery float had been released, and so by all accounts it should have come up, but it stayed on the bottom. We waited as long as we could, until dark, at which point we had to head off to another site. When we returned the next night, the mooring was still where we had left it. A few hours later, however, at dawn, when we planned to try again for recovery, the mooring was more than 2 km away. So we fired up the ship and went chasing after it. Because the ship had been drifting down the coast during the late night hours, we headed in that direction, stopping every half-kilometer or so to query the acoustic-ranging device to see whether we were getting closer to or farther from the runaway mooring. We finally found it a few hundred meters from the main shipping channel to the Port of Los Angeles, one of the largest ports in the world. A container ship more than 500 ft long was bearing down on our little mooring, but we were able to save it in the nick of time. Murphy's not done with us yet: logistics prevented us from putting a second acoustic release on one of the Geoprobes. Well, as Murphy would have it, that tripod, with just one acoustic release, decided not to talk to us at all. I guess it was feeling left behind, since we had picked up its companion tripod early. It was time for our final backup, the ROV. We returned to the site a few days later on a smaller, more maneuverable vessel, the SCMI R/V Yellowfin, and sent a helping hand down to our lost tripod. Luckily, the acoustic-ranging device, our second backup, worked as planned and enabled us to locate the tripod quickly. We used the ROV to hook a recovery line onto it and brought it back safely with all gear intact.
When you put expensive gear in the ocean for months at a time, problems are to be expected. Fishermen trawl through the site; things grow on and foul the release mechanisms; some instruments just fail. Given its complexity, the 2007-08 Palos Verdes shelf experiment was incredibly successful, thanks partly to the many backup-recovery plans but mainly to an extremely hardworking team, boasting more than 100 people-years of oceanographic experience: Marinna Martini, Jonathan Borden, Rick Rendigs, Chris Sherwood, Christine Sabens, Ellyn Montgomery, Bénédicte Ferré, and Brandy Armstrong of the USGS Woods Hole Science Center and Joanne Ferreria, Marlene Noble, David Gonzales, Hal Williams, Kevin O'Toole, Jamie Grover, and Kurt Rosenberger of the USGS Western Coastal and Marine Geology Team. Thanks are also due to the expert crews and support personnel for the recovery vessels provided by SCMI (R/V Sea Watch and R/V Yellowfin) and Scripps Institution of Oceanography (R/V Robert Gordon Sproul). With their invaluable help, we not only recovered all of our instruments but also recorded more than 15 gigabytes of oceanographic data. Now the fun really begins—trying to decode and analyze such an extensive data set!
|
 |
in this issue:
Palos Verdes Shelf Experiment: Whatever Can Go Wrong Submarine Landslides and Large Earthquakes
USGS Part of Law of the Sea Delegation Carolinas Coastal Change Processes Project
|
|||||||||||||||||||||||||||||||||
U. S. Department of the Interior | U.S. Geological Survey
previous story
