I thought I would try to do a few posts describing how we measure whale calls on OBSs. Here’s the first one:
I started with the question, “what is a seismometer?” because, well, that’s where it all begins. Our dataset was not collected to measure whales, oh no. It was collected to measure earthquakes. Seismometers on the seafloor have essentially the same function as a seismometer on land, except for some obvious things, like being super-duper water-tight for one. Electronics + water? Not usually a good combo.
You’ll notice that my OBS drawing is a red box. Not so in real life, my friends! I’m just too lazy to draw one of THESE puppies (look for the part drilled into the rock):
Two reasons: (a) Too hard to draw, and (b) too late to change it now. Never mind though. Henceforth, when I draw a seismometer, it will be a red box. OBSs are often just deployed over the side of a ship, and wherever they land is where they’ll stay. But the ones that were used in our study were installed using an ROV (remotely operated vehicle), and were either buried in sediments or drilled into basalt, like in the photo above.
The seismometers that we use were deployed for a year at a time, three years in a row. So they had to have the ability to store a year’s worth of data. That’s 128 samples recorded each second for each of the three axes of the accelerometer. There are about 31.5 million seconds in a year, so that’s in the neighbourhood of 12 billion samples. Holy smokes, that is a lot.