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.

[1] The distribution and connectivity of brine pockets in first year sea ice has a determining influence on the bulk properties of the ice and its interaction with the environment. The structure of the brine network depends upon both temperature and salinity, and a full understanding of the temporal evolution of sea ice physical properties requires measurements that are sensitive to the microstructure and can also be made without disturbing the natural state of the ice. Direct current resistivity techniques are suited to this as the brine fraction is orders of magnitude more conductive than solid ice. However, due to the preferential vertical alignment of brine inclusions, the bulk resistivity of first year sea ice is anisotropic. Although this makes the interpretation of surface resistivity soundings extremely difficult, consideration of the theory of resistivity measurements in an anisotropic medium shows that the anisotropic resistivity structure may be resolved through cross-borehole measurements. Borehole pairs with one current and one potential electrode in each hole allow the determination of the horizontal component of the anisotropic bulk resistivity (r H ). Use of four boreholes allows an estimate of the geometric mean resistivity (r m ) to be derived. Combining these measurements allows calculation of the vertical resistivity (r V ). This is illustrated by measurements made in first year sea ice near Barrow, Alaska in April-June 2008. Over this period significant changes in resistivity are observed which may be shown to be related to both the brine volume fraction and the microstructure of the ice.

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.[1] Cross-borehole DC resistivity tomography has recently been used to monitor the temporal variation of the anisotropic bulk electrical resistivity of first-year Arctic sea ice during the period of spring warming. These measurements cannot be explained by standard models of sea ice microstructure which treat the brine phase as isolated ellipsoidal pores. A simple structural model which does satisfy the observed electrical data shows that the brine phase must be connected both vertically and horizontally. Calculation of the temporal and thermal evolution of the microstructure suggests that although vertical connectivity is through pore tubes and sheets with widths of $100 mm, horizontal connectivity is through much thinner connections which are interpreted as inter-and intragranular brine layers. As the temperature increases the width of vertical channels increases smoothly. In contrast, at temperatures above about À2°C there is a rapid increase in the thickness of horizontal connections which we interpret as a change from conduction through intergranular brine layers to the development of horizontal pores. The electrical data also broadly exhibit a percolation transition predicted by mathematical models. However, the critical brine volume fraction for vertical electrical connection is very small, while that for horizontal electrical connection is derived to be about 0.5%. The difference between these and the critical threshold of 5% for fluid permeability is presumed to arise because of the strong dependence of the latter on brine channel width.

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