Tongariro-Tokaanu Resistivity Structure from Magneto-Telluric Soundings

Abstract

Magneto-telluric resistivity soundings have recently been made at 23 sites between the Tongariro and Tokaanu geothermal fields, in the central Island of New Zealand. The resistivity structure, derived from layered interpretation of the soundings, suggests that these fields are not hydrologically connected. They are separatedby a graben or depression, which has a maximum depth of about 1.2 near Lake Rotoaira, and is filled with Tertiary mudstone, sandstone, siltstone and Quaternary volcanic alluvium. The basement is high resistivity greywacke. Gravity modelling supports this interpretation. Both Tongariro and Tokaanu fields have high-standing,low-resistivity ohm-m) steam condensate aquifers, associated with intense hydrothermal clay alteration. Both fields also have northern structures, and evidence of increasing resistivity at depth where higher temperature alteration may be present