“Knee deep in the water somewhere…”
A large fish reluctantly departs his trawl resistant home before being raised to the surface
The last few days have been filled with action on the deck and in the lab as we shift our efforts to the ‘focused network,’ and the last 8 recoveries of our trip. The focused network contains 13 OBSs in a concentrated region off the Oregon coast between Gray’s Harbor and Willapa Bay. Their close proximity to one another will provide the seismologists with a 2-D map of the subducting plate and hopefully capture the tremor events that are expected at this location. One might think that the shorter amount of time spent in transit between sites, and the fact that they are the shallowest instruments currently deployed, would facilitate swift and quick recoveries. However, there are a few other factors that come into play in the shallow waters of the continental shelf, which have a significant impact on the instruments and their recovery.
The most noticeable phenomenon is the abundance of life at the bottom. The shallow waters allow more sunlight to reach the seafloor creating a much more habitable environment for marine life. Shrimp, crabs, fish, sea stars, and skates seem to be the most common critters to call these nutrient-rich waters home. Furthermore, the Chehalis River, which runs into the Pacific through Gray’s Harbor, floods the region with thick sediments and nutrients. The river has a two-fold effect, firstly it gives the marine life something to eat and secondly, it makes it difficult to see underwater. Seriously. It’s like a blizzard down there. But why, if all we have to do is send an acoustic signal to the OBS, does nutrient and sediment rich water present such a problem? I’m happy to tell you, and show you!
There are but a few moving parts on the TRM, and we have found that they are quite prone to adverse marine interaction. The pop-up buoy canister acts like an upturned bucket collecting sediment as it drifts towards the seafloor. If enough sediment builds up around the device it can become jammed making it difficult for the buoy to release. Alternatively, the release latch itself may become stuck or unresponsive if enough life or crud get jammed in the way. To make matters worse, the high level of sediment in the water can cut visibility to as little as 3ft underwater making it difficult for Jason’s pilots to maneuver around the work sites.
Everyone has been operating at full speed to deal with these complications as they arise. Most notably, Jason has stepped up to the plate and assisted in the recovery of the final 8 TRMs, setting a new record of 4 complete dives in a 24-hour period.