The great thing about doing a study which requires seafloor mapping is the instant gratification. As soon as the survey begins, and the sonar starts pinging (or chirping), an image appears on the computer display. If all the planning went well, you get to sit back and watch your data appear, in rainbow colors or stark greyscale lines. Every 30 minutes you click a button on the computer and log time and location. And after however many hours, what you have is a beautiful map of the seafloor.
Of course, it’s not really instant; as I mentioned above, it takes hours to complete a bathymetric (seafloor) survey. The ship is generally moving at only half its max speed (6 knots, or jogging speed), and you have to watch in real-time, waiting with bated breath for the multibeam to sloooooowwwwwwlllllyyyyy display the seafloor, including (hopefully) the feature that you were interested in. And if it doesn’t, you have to either take what you get or recreate your survey on the fly, losing more time.
But in the end, no matter where your survey occurred, you get a fabulous-looking map of the seafloor, or a profile of the sediment. The Thompson has two seafloor-survey instruments: the EM302 multibeam, and the Chirp3260 sub-bottom profiler. They are both sonar systems, meaning that they send out a soundwave (“ping” or “chirp”, respectively) and use the time and intensity of its return to determine the depth of the seafloor. The difference between them is that the multibeam sends out a fan of pings over ~120° of the seafloor, resulting in a “swath” of seafloor data that is about 4x as wide as the depth. The sub-bottom profiler uses different frequencies of sound in order to penetrate the seafloor and send back a profile of sediment layers on the bottom.
I conducted my survey overnight from 8pm on March 20th to 4am March 21st using the multibeam. This was during my normal sleep-time, but there wasn’t anything I could do to change that; it’s just the way the whole schedule worked out. I drafted out my survey lines a month ago, but finalized them only the day before the survey. It was 36 nautical miles long, running up and down the continental slope in about 8 lines. As my normal watch went to bed , I settled back with my caffeine in the computer/electronics lab and let the fabulous crew of the Thompson direct the ship along my survey track.
Six hours, one XBT (expendable bathythermograph; it measures water temperature through the water column in order to calculate sound velocity) and one filled data-hole (due to an unexpectedly shallow region, or poor planning, depending on what/who you want to blame) later, I found myself grinning happily (and slightly sleep-deprived) at my data, a clear image of the continental margin from 1000m, up the slope with a clear shelf-slope break at 290m to the continental shelf at 170-200m. Data density was good enough for 10m resolution, and I had a large enough area to conduct my analysis. Not only did I have all the data I had hoped to collect, I also had data that I hadn’t expected: intensity of return, which basically shows hard and soft sediment on the seafloor, which shows the shape of shelf canyons almost better than the bathymetry alone.
The trick with seafloor surveying is what you DO with the data after you have it, however. Many long hours of data cleaning and analysis await me back in Seattle, or even here on the boat if I care to use my free time to get a head start. Ultimately, the pretty-looking map of the seafloor means almost nothing; we KNOW the seafloor is there, and we KNEW there would be a shelf-slope break, but my project depends entirely on statistical analysis of that data, which can’t be seen at a glance.
But in the meantime, I might just frame my map and wonder about the amazing concept of creating a map based on sound.