Electromagnetic monitoring of active volcanic zones: Monitoring of Phase Tensor parameters from Mount St Helens and Stromboli volcanoes

The main goal of volcano monitoring is to understand processes occurring within the magmatic system from there geophysical response signatures that informs on the eruptive behavior of the system. Traditional volcano monitoring approaches have relied primarily on changes in seismicity, and geodetic response which depend on active changes such as magma migration.

The magnetotelluric method, informs the physical property electrical conductivity, which within a magmatic context is sensitive to the presence of fluids, temperature, and melt fraction. High quality magnetotelluric measurements allow identification of conductivity variations related to the physical changes in magmatic systems, making it possible to determine temporal changes in magma content and temperature. Thus, magnetotellurics may be a valuable additional monitoring tool able to detect static phase changes of the magmatic fluids within the system, contributing to our understanding of the dynamics occurring within magmatic systems absent of and/or preceding magma movement within the system.

Mount St Helens as perhaps the most studied volcanic system including a dense 3D MT data set collected during the 2004-2008 dome building eruption provides the opportunity to investigate the difference within the magmatic system in different eruptive states (i.e. eruptive 2004-2008 and current quiescence). This is been achieved by a complete repeat of the initial survey (67 sounding locations) collected in 2005-06 and establishment of 4 ‘continuous’ telemetered systems. Given the current non-eruptive state of Mount St Helens a second system has been selected, Stromboli, currently in an eruptive cycle for installation of 4 continuous telemetered MT systems. Stromboli volcano, maintains a persistent eruptive activity, with near daily minor explosions typically punctuated by 1-2 larger events a year.

Initial evaluation of the streamed measurements has been completed using Phase Tensor parameters, which are immune to galvanic distortions, which can occur from seasonal near-surface conductivity changes (e.g. variations in soil moisture content). The Phase Tensor represents conductivity gradients in the subsurface, and Phase Tensor differences between two measurement times hence reflects the changes in conductivity structure in time. The temporal application of magnetotellurics provides a mechanism to determine short-term and long-term conductivity changes within the magmatic system representative of physical changes within the system, correlatable to active eruptive behavior, properties of the system during periods of quiescence and eruptive phases.