A significant part of our cruise has been to "survey" the sea floor. Well, what in the world does that mean, you ask? We are using several instruments to map the sea floor and sub-bottom to learn more about the geologic history and the modern processes shaping the continental margin and influencing the overlying water chemistry.
We are using three primary instruments…an EK80 echo sounder, a multibeam system (EM710), and a Knudsen chirp system. Each of these instruments use sound at different wavelengths to provide data of the water column, seafloor, and shallow sediments below the sea floor.
JP has been leading this effort. Although it was always in our cruise plan, we have been doing a lot more surveying using these instruments than originally planned…at sea, plans change as needed (see previous blog post). Our over the side operations have been stopped for the last two days due to weather (7-12 foot seas) and problems with the vessels bow thrusters (help hold the vessel in one place while instruments are lowered over the side). So, until the weather clears, we are learning a lot about the sea bottom!
The EK80 echo sounder is a VERY impressive and high tech fish finder…that's about as simple as I can state it. It has very high resolution returns from fish, bubbles, essentially most things that have a significant density difference compared to seawater. We, and the USGS and WHOI group on the ship, are using it to provide information on methane seeps along the shelf edge. You can pick up these "flares" of bubbles coming out of the sea floor (see picture). It also provides information on potential turbidity events near the sea floor, or large schools of fish (interesting, but not part of scientific interest).
The multibeam system is essentially a very high tech depth sounder. Anyone that own a boat has a depth sounder so they don't run the boat aground. Well, your depth sounder sends out one ping of sound every 10th of a second or so. The sound leaves the vessel, bounces off the bottom and returns to the boat. Knowing something about the speed of sound in seawater, the instrument can calculate the depth of the water. The EM710 is sending out 500 pings of sound, so rather than hitting one point beneath she ship, you can think of it like a lawn mower, measuring a "swath" of depth as the ship moves over the bottom. That width of that swath is dependent on depth…this one measures a width approximately 4 times the water depth. I should mention that the accuracy of the depth is within a couple centimeters!! So, we can measure the bathymetry very accurately!
Finally, the Knudsen uses a lower frequency sound to penetrate INTO the seafloor. This provides information on sediment deposition with time. In regions of active sediment deposition, you will often see "striping" beneath the seafloor. Sort of like a layered cake…packages of sediment laid on the seafloor over time. This data can help guide our coring operations (assuming we will be able to get back to those later in the cruise).
So, we are doing some really sound science out here…literally and figuratively!