A magnetotelluric study of Mount Ruapehu volcano, New Zealand

Abstract

S U M M A R YMt Ruapehu is an active andesite cone volcano, which marks the southern termination of the Kermadec volcanic arc. Results from 40 broad-band magnetotelluric soundings have been analysed using the phase tensor. This approach provides a way of determining dimensionality, allowing for distortion removal, and visualizing data in a 3-D situation. The phase tensor analysis suggests that the shallow resistivity structure is largely 1-D in character, but that the deeper structure requires a 3-D interpretation. 1-D inversions show that at sites on Ruapehu a shallow conductive layer lies between a high resistivity layer, of a few hundred metres thickness, and higher resistivity layer corresponding to basement greywacke. The low resistivity layer is contiguous with the waters of the highly acidic Crater Lake, and thus is believed to be the hydraulically controlled upper limit of a zone of acid alteration overlain by dry volcanic rock and ash. To the southwest of the volcano the conductive layer merges with a surface conductor associated with Tertiary sediments. Following initial 2-D inversions, the deep resistivity structure has been derived through 3-D inversion of data from 38 sites. This indicates the existence of a dyke-like low resistivity zone that persists to at least 10 km depth and extends from beneath the summit of Ruapehu to the northeast where it appears to connect to a poorly constrained region of high conductivity, which lies outside the network of measurement sites. The low resistivity dyke-like feature may be identified with a volcanic feeder system, which also supplies the other volcanoes of the Tongariro Volcanic Centre and marks the conduit by which hot gases and (occasionally) magma reach the surface.

3-D joint inversion of the magnetotelluric phase tensor and vertical magnetic transfer functions

“…If the lateral model mesh discretization is coarse relative to inter-site distances or to skin depths associated with the highest frequencies used in inversion, it is unclear how successful this approach will be. Heise et al (2008Heise et al ( , 2010, Hill et al (2009), Ingham et al (2009) remove distortion from the transfer functions by a tensor decomposition method described by Bibby et al (2005), arguing that the top-layer cells of the model are too large to properly account for galvanic distortion (Heise et al 2010). Árnason et al (2010) and Cumming & Mackie (2010) attempt to correct static shift of impedances prior to inversion using time-domain electromagnetic measurements.…”

Possible compositional effects on the electrical conductivity of olivine: A preliminary study

“…The versatility of MT has also meant it can be used for a wide range of applications, not only within hydrocarbon and mineral exploration but also within the volcanology field as a tool to investigate magma build up (Ingham et al, 2009). However, for investigations to be successful there must be adequate understanding of the geology, including thermal, compositional and structural information.…”

“…By covering many frequencies (i.e. recording short and long wavelengths) it is possible to see shallow crustal features, such as magma chambers (Ingham et al, 2009) with the high frequency (short wavelength) component. Alternatively MT can be used to investigate very deep (100s of km) features, such as the structure of the upper mantle, with the low frequency (i.e.…”

Three-Dimensional Magnetotelluric Inversion: An Introductory Guide for Developers and Users

“…Although MT profile data often demonstrate the influence of 3-D effects or show deviations from being purely 2-D, 2-D assumptions are usually allowed (e.g., Becken et al 2008b;Ingham et al 2009;Hill et al 2009; among many others). 2-D inversions are then performed to yield 2-D cross-sectional models for profile interpretation.…”

“…3-D MT inversions have been used in various studies such as ore deposit exploration (e.g., Tuncer et al 2006;Farquharson and Craven 2008;Türkoglu et al 2009;Xiao et al 2010;Goldak and Kosteniuk 2010), waste characterization (e.g., Newman et al 2003), tectonic studies (e.g., Uyeshima 2007;Patro and Egbert 2008), volcano studies (e.g., Spichak et al 2007;Heise et al 2008Heise et al , 2010Jones et al 2008;Hill et al 2009;Ingham et al 2009), hydrocarbon exploration (e.g., Mackie and Watts 2007;Hautot and Tarits 2009;Zhanxiang et al 2010) and geothermal studies (e.g., Han et al 2008;Newman et al 2008;Á rnason et al 2010;Cumming and Mackie 2010). Improvements in 3-D inversion are likely to continue as long as MT data are still being acquired.…”