Electromagnetic Exploration of the Loihi Seamount

The Loihi seamount is an elongated volcano (~15x30km) located on the lower, southeastern flank of the Mauna Loa volcano. It is the newest volcano in the "Loa" locus line running down the Hawaiian volcanic chain and is therefore expected to be near the focus of the Hawaiian hotspot. In July 2006, we conducted an exploration of the Loihi seamount using controlled source electromagnetics (CSEM). We dropped 16 broadband electromagnetic receivers around the seamount at a depth of about 2000m, and 4 receivers along the top at a depth of about 1000m. We then deep-towed a 300 Amp electric field source around and over the seamount, providing a 76,650 A m source dipole for our experiment at three different fundamental frequencies. Attenuation of an electric field in solid materials is a function of the conductivity of the material and the frequency of the source. This relationship is used to derive apparent conductivities from changes in amplitude of the recorded signal. Using the CSEM data, we are constructing a 2D tomographic "slice" through the volcano to reveal the internal conductivity structure. Electrical conductivity is a function of many factors, the most important of which for our study are porosity, melt fraction, and melt volume. We will present conductivity pseudosections of the volcano and initial constraints on the distribution of melt and pore fluids in the seamount edifice.