Magnetotellurics uses electromagnetism to image the geologic subsurface through variable resistivity.
Geologist Tom Wright is rolling out a long wire (carried in a heavy and uncomfortable spool on his back). A small current will be run through this wire and it will send an electrical signal into the ground. The return signal will be different depending on the temperature of the sub-surface, and this technique is sometimes used to try and find underground bodies of magma.
This photo was taken in 1987 and you can see how much vegetation has grown since the 1977 eruption.