A high frequency magnetotelluric survey of Mount Ruapehu

Abstract

We have made a series of magnetotelluric measurements on the summit plateau of Mount Ruapehu which allow us to determine the electrical properties of the underlying rocks. In volcanic environments, because of the presence of high temperatures and hydrothermal fluids within the volcanic system, it is expected that the structure will generally be highly electrically conductive. In particular, hydrothermal fluids may cause alteration of the local volcanic rocks to yield products which are more electrically conductive than the original rock. Studies, both elsewhere in the world and within New Zealand geothermal systems, suggest that the nature of these alteration products, and how conductive they are, depends upon the temperature at which they are formed. The products of high temperature alteration are more resistive than those formed at lower temperature.

A 3-dimensional model of the electrical structure of Mount Ruapehu, which explains our observational data, shows that there is indeed widespread low electrical resistivity (high conductivity) beneath the summit plateau and that the structure becomes somewhat less conductive with increasing depth. We believe that this low resistivity is probably caused by steam interacting with shallow groundwater to give extensive acidic alteration. Within this general framework two regions of significantly higher resistivity occur. The first of these is at shallow depth (~150 m) beneath the southern part of the summit plateau and the other at a depth of ~1 km beneath the northern part of the plateau. Tests show that these features are not artefacts of the modelling procedure but are required to provide a good fit to the data. We interpret these regions of lower electrical conductivity to represent areas surrounding heat pipes (or volcanic feeders) where the temperature is high enough that more resistive alteration products have been formed. The general low resistivity environment and the distance of the measurement sites from Crater Lake mean that our data are not sensitive to the similar heat pipe that is assumed to feed the lake. It is clear from the extensive hydrothermal alteration detected across the summit, however, that a much more extensive volcano/hydrothermal system exists (or has existed in the past) beneath the summit of Mount Ruapehu.

We infer that there are two possibilities regarding the heat pipe system we have detected. The first is that the two heat pipes beneath the summit plateau are relict features from past volcanism and are no longer active. This is possibly supported by the fact that the northern resistive feature lies at a greater depth. This may indicate that the rate of increase of temperature with depth is lower beneath the northern part of the summit plateau and that the active volcanic feeder system beneath Mount Ruapehu has moved southwards with time. Alternatively, the two heat pipes beneath the summit plateau may remain active, but have no surface features associated with them.

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Technical Abstract

High frequency magnetotelluric soundings were made at 10 sites on the summit plateau of Mount Ruapehu to investigate the structure and extent of the volcanic hydrothermal system. 

Three-dimensional inversion of the data shows that beneath the plateau the electrical resistivity is generally low (1-10Ωm), but that two areas of higher resistivity (~30-100Ωm) exist. One of these lies at shallow (150-500 m) depth beneath the southern part of the plateau, while the other occurs in the depth range of 1000-1500 m beneath the northern part of the plateau. The extensive low resistivity at shallow depth is interpreted as indicating that the entire summit plateau area has been in contact with volcano-geothermal condensate. The higher resistivity regions are inferred to indicate a change within the hydrothermal system from conductive smectite to resistive chlorite alteration products. This change is temperature controlled, suggesting that the higher resistivity regions are zones of elevated temperature (>150˚C). The difference in depth of the two features would thus imply that a larger thermal gradient exists beneath the southern part of the plateau than the northern area. This is consistent with the existence of a near surface layer of ice which covers the northern part of the plateau, but not the southern. Both resistive regions have very limited lateral extent and are interpreted as being either old volcanic feeders or heat pipes which remain active but have no surface manifestation associated with them.