We’ve been talking a lot in this blog about the cores we recover, but they’re not the only source of data we use. We also get a lot of information out of the holes those cores leave behind. The holes have the same sediment layers as the cores, and unlike the cores themselves, which can be incomplete or mixed-up, the sediments in the ground can give us a very consistent record, hundreds of metres long. We measure the sediments on the walls of the hole in a process called “downhole logging”.
We don’t log every hole, because the process involves dropping very long, VERY expensive tools down a thin hole in the seafloor, and we don’t want to run the risk of losing those tools without a good reason. But when the hole seems stable, and we hope to get some useful data, there are several logging tools we can use. Most of them are similar to ones used by the physical properties team on the cores.
The Triple Combo is usually the first tool string used, and it covers most of the basic physical properties. It has Calipers (for hole width), a Natural Gamma Radiation sensor (to identify isotope composition), Magnetic Susceptibility (density of certain minerals), and Electrical Resistivity (to look at patterns of composition/minerals).
We also have the option to run the increasingly inaccurately named triple-combo with a small nuclear source, to test the sediments for Gamma Ray Susceptibility. This gives us useful information on the density and porosity of the sediments, but means we have to be even more careful not to lose the tool. Losing millions of dollars worth of specialised sensors is embarrassing enough. Losing a small radioactive sample in a hole at the bottom of the ocean? That’s paperwork we’d rather not deal with. So we only run this on nice stable-looking holes.
Next we use the Formation MicroScanner-Sonic. This string’s main attraction is a Resistivity Sensor, which has four projecting pads that contact the walls of the hole, to get an excellent reading of the electrical resistivity/conductivity of the sediment. This gives us a great picture of the microstructures in the sediment, including faults, veins, fractures, and stresses. The “sonic” part is for the P-wave sensor, which measures the sonic velocity through the sediments, telling us more about its density and structure.
The Versatile Seismic Imager is a very useful bit of kit. We lower the sensor down the hole, and use an airgun to create a seismic wave in the water near the ship. The sound waves travel through the water, through the sediment, and are detected by the sensor. This lets us create an image of the sediment layers, which we can then compare to pre-existing seismic surveys.
One important note is that we only use the airgun when we’re sure there are no marine mammals about, because the sound waves can be dangerous to their sensitive hearing. So whenever we’re using or preparing for the VSI, someone is always on watch for surfacing whales or dolphins, with binoculars and a distance-measuring stick.
Whale watching, lowering the airgun, boom!
Because logging lets us run the same tests on the hole as we have on the cores, it’s incredibly useful for stratigraphic correlation. That is, it helps us figure out how the sediment cores were aligned and spaced before we drilled them out and possibly messed up their spacing. It’s just one of the many tools in our scientific repertoire!