Application of broadband marine magnetotelluric exploration to a three-dimensional salt structure and a fast-spreading ridge

The recent development of broadband marine magnetotelluric (MT) instrumentation enables resolution of seafloor electrical conductivity structure at shallower depths than was previously available. This dissertation focuses on the application of broadband marine MT to imaging the conductivity structure of a salt intrusion on the continental shelf in the Gulf of Mexico (GOM) and also the magmatic system at the fast-spreading ridge at the East Pacific Rise (EPR).; Gemini Prospect has served as a test bed for the Scripps Institution of Oceanography broadband marine MT instrument. Over 42 sites of MT data have been collected in the period band of 1–3000 s in a two-dimensional (2D) grid over the Gemini salt structure, providing an excellent data set for developing and testing inversion techniques. Top and base-of-salt surfaces provided from a commercial 3D seismic survey allow for verification of the MT results. 2D inversions of the Gemini MT data can recover the salt body remarkably well despite its complex 3D shape, but the results are heavily dependent on which mode of data is inverted. The best agreement is found when inverting the mode with the electric field perpendicular to the dominant structural strike. An improved geological interpretation is found through the joint presentation of both seismic reflection profiles and electrical resistivity models. A thin and shallow resistive feature observed outside the seismic salt volume may indicate a change in porosity or pore fluids associated with a natural trap in the sediments. An overhanging resistive feature correlates with a previously uninterpreted strong reflection and illustrates how MT can constrain structure in regions where the seismic method performs poorly.; At the EPR near 9°50'N, a pilot survey using the broadband instruments shows sensitivity to structure at shallower depths than previous ridge MT experiments. Two-dimensional inversion of data from 4 sites images a high conductivity zone located in the crust and shallow mantle that is associated with the ridge magmatic system and that agrees well with seismic tomography and seafloor compliance results. Resistivities beneath the ridge imply a crustal partial melt fraction of 1–20% and a total melt volume of about 0.75 km3 per axial kilometer of ridge.