We are officially six days away from dry land at last. Since the main experiment is done and we only need one student on duty at a time, we have switched our watch schedules so the 24 hrs is divided evenly between the 5 of us. My new watch is 9:40 am- 2:30 pm, which is much nicer, and allows me to transition to a relatively normal sleep schedule. I still have to be down in the lab working on data processing for a longer period of time, but I don't have to be around for 8 hours during the middle of the night anymore. All we have to do is check that the instruments are still running occasionally and make a log of our progress along the new survey lines every 30 min. There's not much else to report besides seeing a few squid while we were retrieving OBS's. Hopefully, we'll see some more sea life as we get closer to the much shallower Lau Ridge, which forms the western edge of the Lau Basin.
The new survey area is west of the seismic survey in a relatively unmapped and poorly understood portion of the basin. We are no longer using the airguns, and are just collecting bathymetry, sidescan sonar, gravity, and magnetic data. The methods for each of these is described below:
The airguns sole purpose is for collecting the seismic data: they produce a loud sound and a pressure wave which vibrates the seafloor (similar to an earthquake). These vibrations are picked up by the OBS's on the bottom of the seafloor and after lots of processing, give you an image of the surface of the seafloor and a few km below the seafloor.
We were collecting bathymetry, sidescan, gravity, and magnetic data while we were shooting the airguns as well, but we are now concentrating on an unmapped section west of the original survey area, and are going back and forth on E-W lines to cover the whole area.
The bathymetry data is collected with a multibeam echosounder, which shoots multiple beams of sound down toward the seafloor in an angular "swath," so we can collect data directly below the ship and off to the sides as well. Depending on the depth and the angle of the beams, the swath is usually around 5 miles wide. The beams reflect off of the seafloor and based on the travel time the instrument calculates the distance and comes up with a color-coded (based on depth) image of the surface of the seafloor (the bathymetry).
The sidescan sonar is similar in that it uses sound, but it's most accurate to the sides of the boat (hence "sidescan") and the data directly below is pretty much useless. I'm not sure why this is, but it has something to do with the reflection being to strong. The purpose of the sidescan data is to determine the type of materials that you are looking at. It doesn't give you a nice image of the seafloor like the multibeam, but you end up with a grayscale image based on the intensity of the reflection (aka backscatter). Harder materials like fresh volcanic rock show up as black on the image, and sediments are generally lighter colors. It's useful for determining areas of recent volcanic activity, identifying faults, and determining where sedimented areas are. While the topographic relief of faults is visible on the bathymetry, the sonar is much better for identifying them as faults rather than a ridge or something like that. This is because the majority of the faults around a backarc spreading center are "normal faults" where one block slips down at an angle relative to the underlying block. When this occurs, it exposes the harder rock under the sedimented surface, so the fault shows up as a nice linear black area (most of the time). This data will be very important for my thesis work, which involves identifying and interpreting the structures in the basin, as it is much better for seeing structures than any of the other data.
The magnetometer measures the magnetization of the seafloor. When new basaltic crust (high iron content) is formed, the iron grains are aligned in the direction of the earth's magnetic field at that time and "frozen" in place when the lava hardens. Since the earth's magnetic field changes over time and even flips polarization completely (i.e. the north pole becomes the south pole), crust formed at different times will have iron grains with orientations corresponding to when it was formed. The magnetometer measured these orientations and can give an idea of the relative age of the crust. If the crust was actually formed at a spreading center, you will see "stripes" parallel to the spreading center corresponding to times of different polarization of the earth's magnetic field. If it was formed another way (i.e. arc volcanism) you will see a less organized pattern. The data we are collecting now can potentially resolve a contentious question of how the crust on the west side of the basin was formed, which is pretty cool. This data alone could provide enough information for a thesis to be written. It will likely be the other student under my advisor who will use this, not me.
The gravimeter measures the varying gravitational field of the seafloor. This is mostly controlled by the density of the material (higher density = more mass = stronger gravity). Gravity data is the least intuitive of all of these types, because it doesn't necessarily correspond to anything you can see on the seafloor (i.e. higher topographic relief does not necessarily mean higher gravity). One use of gravity data that I know of is to differentiate between types of volcanoes. Volcanoes formed along a spreading center are composed mostly of basalt, which is a very dense volcanic rock. Volcanoes formed along the volcanic arc due to subduction have a higher proportion of andesite, which is a less dense volcanic rock. Therefore, with gravity data, you can actually determine with reasonable accuracy whether a given volcano on the seafloor is formed at a spreading center or a volcanic arc without having to take any rock samples from it. There actually are a number of volcanoes in the area we are covering whose origins are unknown, and the gravity data should provide some valuable insight on that.
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