Magnetotelluric Constraints on Geologic Structure and Geochemistry at Cotopaxi, Ecuador

Magnetotellurics (MT) is a natural-source electromagnetic geophysical technique that images electrical conductivity. Subsurface electrical conductivity models have been invaluable in exploring the complexity of volcanic systems. Measuring strong contrasts in electrical conductivity in volcanic materials allows for the interpretation and examination of the subsurface geologic structure and geochemistry of the internal system. Used in conjunction, seismic and MT are useful for providing constraints on the chemical composition of fluids as well as inferring the melt fraction of magmas.

In this study, finite difference forward modeling was used to create a sensitivity analysis for the Cotopaxi volcano in Ecuador. Starting model conductor geometry is based on geophysical results from a 2015 Gravity survey. The smallest possible magma system is a 6 cubic km conductor approximately 3 km beneath the subsurface. In order to determine the sensitivity of a magma body to an MT survey with 81 grid stations separated by 10 km and centered over the conductor, nine periods were calculated to hit the smallest target. Impedance tensors as well as apparent phase and resistivity were calculated. In total, 7 starting models were made to consider changes in shape along the x,y plane as well as along the z axis. Analysis of impedance tensor components allowed for the electromagnetic dimensionality of the magma system to be determined for varying subsurface geometries and storage conditions of the reservoir. This sensitivity analysis is in development to inform an magnetotelluric survey in collaboration with IG-EPN in Ecuador.