13 May 2013, Friday the 13th huh? How cute.
Ladies and Gentlemen, this is rather embarrassing. These past weeks I have been so enthralled by the differences between life at sea and life on land that I completely forgot to write about the scientific data we will be collecting. Now we are a mere 10 hours away from arriving at The Middle of Nowhere, Ocean and beginning our survey, so I’d better start talking science.
The primary data we are interested in collecting is a map of the seafloor depth in our survey area, which will be measured using a multibeam sonar. This instrument sends out a sound pulse at regular intervals and listens to the reflections of the pulse from the seafloor. We may then infer the seafloor depth by how long the sound pulse takes to return to the ship. At least that’s the quick version.
As a bonus, the sound pulse from the multibeam sounds like a bird chirping. You can hear this chirp throughout the entire ship every ten seconds or so, 24/7. Now I know there are quite a few bird-lovers out there, but I can’t help but ask: Would you still love your birds when they are keeping you up at night?
The data collection feed of the multibeam sonar is shown to the right. In this photo we are passing a seamount on our starboard side. Credit for the photo goes to James Holmes.
Of course, creating such a map is rarely so simple. There are two other types of data we are collecting to refine the multibeam data. The first is position data from a GPS receiver, so that we can precisely locate where the ship was when collecting data. This is incredibly important since we would otherwise place the map in a slightly offset position compared to its true location. The GPS we use is a Trimble NetR9, which is a little bigger than the GPS in your phone. It consists of an antenna and a receiver, which we have set up on the deck above the bridge. The antenna communicates with the GPS satellites and records its position in the receiver. The entire setup would normally be powered by a car battery, but we can luckily just plug the receiver into the ship’s AC power. Why go through all this trouble? Well, your phone can probably tell you where you are to about 4-5 meters (~12-15 ft). This guy will be able to, with a little elbow grease, tell us where we are to about 0.1 meters (~4 inches). If we weren’t continually moving we could do even better, but that’s a story for another day.
Myself and computer technician Daniel Yang inspecting the GPS antenna mounted on the deck of the ship. Credit for the photo goes to James Holmes.
The final instrument we will be using is called an XBT, which is used to measure the speed of sound through water. This directly affects how long it will take the sound pulse from the multibeam to return to the ship, so we really want to know it well so we can properly estimate the depth of the seafloor. The XBT itself is a cannon that launches a torpedo-shaped sensor into the ocean to collect the measurement. The whole thing is a 2.3 caliber instrument and needs to be handled with care, especially when armed. Luckily the ocean is a little hard to miss.
Once we have our map, the fun will begin. See, our goal is not just to map out the seafloor, but also to figure out how well we can measure motion on the seafloor. This could prove very useful when studying earthquakes, volcanoes, and landslides out at sea since these phenomena are otherwise very difficult to observe directly. In order to test this method, we will actually be collecting multiple maps of the seafloor and then comparing them to see how well we can tell them apart. If we can routinely identify differences to within 1 meter (~3 ft) or so, I will be ecstatic.