If you're not that confident of your understanding of the Earth's magnetic field, a good place to go before trudging through this page may be Matt Pruis' Magnetics Tutorial. This tutorial wasn't designed for this exact purpose, but you should notice some clear tie-ins to this page.
The Sea Floor Magnetometers were deployed on the 1993 eruption site last September (1995) using the deep-diving submersible Alvin. There are a total of five of them in all. We placed two of them directly on the new lava flow, and one more in this general area, but well off the new flow (this magnetometer will act like a reference magnetometer for the two placed on the new flow). The other two magnetometers were placed by the floc site. One was placed in an area where shimmering water could still be seen, and the second was placed well away from this site (another reference magnetometer).
It cannot be overstated how much of a risk it is to place instruments down on the sea floor for extended periods. The sea floor magnetometers were placed on the sea floor with about 2500 meters water above them at a temperature of about 2 degrees Celcius. They are designed to record the Earth's magnetic field every two hours using a fluxgate magnetometer that is mounted on a tripod-like structure. This tripod structure is made out of titanium, both because of titatium's ability to resist corrosion (a big problem when you deploy instruments in salt water) and because titanium does not have much of a magnetic signature.
