✍ Scribed by George F Risk; Hugh M Bibby; Christopher J Bromley; T.Grant Caldwell; Stewart L Bennie
No coin nor oath required. For personal study only.
Tokaanu-Waihi geothermal field is situated near the southern end of the Taupo Volcanic Zone, New Zealand. Neutral chloride thermal waters discharge at Tokaanu and Waihi in the north of the field on flat land between the andesite volcanoes Tihia and Kakaramea and the shore of Lake Taupo, while steam-heated thermal features occur at Hipaua on the northern flanks of Kakaramea. Electrical resistivity surveys have been made over the field using several different measurement techniques. In the north of the field where roads and tracks allow vehicle access, resistivity profiling using Schlumberger arrays with electrode spacings (AB/2) of 500 m and 1000 m show that Tokaanu, Waihi and Hipaua all lie within a continuous region of low apparent resistivity (5-20 m) and are thus part of the same geothermal system. Along the eastern edge of the system there is a sharp transition to apparent resistivities greater than 100 m in the cold surrounding region. Surveys on Lake Taupo using an equatorial bipole-bipole electrode array towed behind boats (spacing equivalent to AB/2=500 m) found that the low resistivity zone extends offshore by about 1 km. The steep, bush-clad, southern part of the field was surveyed with magnetotelluric (MT) resistivity measurements using both naturally occurring signals and the 50 Hz radiation from the power wires as sources. These measurements found low resistivities over the north-eastern slopes and around the summits of Tihia and Kakaramea, indicating thermal activity. However, the measurements were too widely spaced to allow the field boundary to be clearly delineated. Interpretation of the resistivity and other data suggests that the Tokaanu-Waihi thermal waters rise nearly vertically from a source deep beneath the elevated Geothermics 31 (2002) 45-68 www.elsevier.com/locate/geothermics
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Chemical data obtained in 1983/84 and 1994 from groundwater monitor wells at Wairakei are reviewed and interpreted in conjunction with physical changes in the deeper system. The composition of the waters is controlled by the proportion of deep chloride, shallow meteoric and steamheated components. T