Mixing electricity with water: The world of marine electromagnetic methods.

The electrical conductivity of crustal and mantle rocks is a transport property determined, in most cases, by the minor fractions of material contained in pore space between nonconducting mineral grains.  Thus conductivity can be used to estimate porosity, tell freshwater from saltwater, oil from water, melt from solid rock, and if the pore fluid has frozen to ice or gas hydrate.  Since conductivity is thermally activated, it can be used to tell hot from cold.  Alone this can tell us a lot about geological processes, but since conductivity is largely independent of seismic velocity, a combined interpretation can be even more valuable.  The magnetotelluric (MT) and controlled-source (CSEM) methods were developed to study conductivity on land in the 1960’s and taken into the marine environment soon after, but remained and academic niche activity.  Commercialization around the turn of the century provided resources to advance instrumentation and interpretation software, pushing marine EM methods into the mainstream.