Leg 1 Logbook - Laser Raman Spectroscopy
Day 9 – Here a mound, there a mound, everywhere a mound, mound…
July 15, 2009
0900 hours – On-station at Barkley Canyon, west of Vancouver Island, Canada.
Latitude 48 degrees 18.6 minutes N
Longitude 126 degrees 3.9 minutes W
Before the cruise, we asked Dave Caress to take a look at the high resolution topographic data he generated from the Mapping AUV survey during the 2006 Barkley Canyon Expedition to see if there were any more mounds besides the ones we had already located. At the time, we thought he might come up with maybe 6 or 8 new targets. Much to our surprise, Dave identified 40 suspected hydrate mounds from the topographic data. This is probably not an unreasonable number of targets for a geologist conducting a flyby survey; however, for ocean chemists who cannot resist stopping and poking a sensor into each and every mound we see, we knew that this was way more targets than we could possibly visit in a single dive day. So, before the dive we selected a subset of the targets and made a plan for visiting them in sequence.
In order to help with our navigation locating the mound sites, we took down a homer beacon that we could place in the area and use as a guidepost. Once on the sea-floor, we quickly recognized a few landmarks that allowed us to transit to the Hyberg mound (where Pinkie is located) and placed our beacon on top of the mound. Using this beacon as a guidepost, we then navigated by dead reckoning to the various target mounds. Unlike the day before when we wandered aimlessly for the first hour or so, today we drove straight to our first target. And much to our delight, we found a small hydrate mound there. As it turned out, Dave Caress was quite good at looking at the topographic data and spotting hydrate mounds. Of the 16 sites we visited today, 15 were indeed hydrate mounds. Nice job, Dave!
Homer beacon used for finding the range from and navigating back to a fixed point.
One of the many hydrate mounds found by navigating to small topographic highs on a map prepared by Dave Caress from the mapping AUV data collected in 2006.
Upon reaching our survey mound targets, the first thing we did was use our new “dip stick” probing tool. Before inserting the very valuable and very delicate laser Raman spectrometer pore-water probe, we wanted to “test” the site with an inexpensive probe or stick. Once again Peter Walz came to our rescue with a “dip stick” made from a piece of threaded rod (marked-off in 10 cm increments) and a rubber gripper handle. This way we could probe each mound to see if the sediment was soft enough to insert the pore-water probe, and upon withdrawing see if any oil, gas or hydrate leaked from the hole. We called this procedure “poke and hope”. In most cases, our hopes were rewarded as droplets of oil and gas bubbles leaked out of the hole when the dip stick was retracted.
At one mound, the dip stick could hardly penetrate the sediment at all hitting a solid surface only 5-10 cm down. In this case, we reached into our box of tools on the ROV tool-sled tray and pulled out our benthic hoe. It took only a few shallow scrapes before the ROV pilot was releasing small chunks of oil saturated hydrate from the sea-floor.
“Dip stick” used to poke into the new hydrate mounds to determine how deep we can insert the pore-water probe and to see if they contain gas or oil.
Using the benthic hoe to scrape off the sediment over-burden and expose gas hydrate at one of the hydrate mounds.
We spent the day ranging northeast and south of the known hydrate field finding over a dozen new hydrate mounds. All on-board were impressed with the size of the field and the consistency of the finds in terms of mound size and the presence of oil and gas in every one but one of the targets. And when the sediment cover was thick enough, and no oil seemed to be present, then we inserted the LRS pore-water sampler in order to measure the gradient of methane in the sediment pore-water under in situ conditions and with minimal sediment disturbance.
Pore water probe inserted 24 cm into the sediment on top of a hydrate mound.