Geophysical techniques for low enthalpy geothermal exploration in New Zealand
Supri Soengkono 1 2 Chris Bromley 1 Robert Reeves 1 Stewart Bennie 1 Duncan Graham 11 GNS Science, Wairakei, Private Bag 2000, Taupo 3352, New Zealand.
2 Corresponding author. Email: s.soengkono@gns.cri.nz
Exploration Geophysics 44(3) 215-227 https://doi.org/10.1071/EG13036
Submitted: 22 April 2013 Accepted: 23 April 2013 Published: 29 May 2013
Abstract
Shallow warm water resources associated with low enthalpy geothermal systems are often difficult to explore using geophysical techniques, mainly because the warm water creates an insufficient physical change from the host rocks to be easily detectable. In addition, often the system also has a limited or narrow size. However, appropriate use of geophysical techniques can still help the exploration and further investigation of low enthalpy geothermal resources. We present case studies on the use of geophysical techniques for shallow warm water explorations over a variety of settings in New Zealand (mostly in the North Island) with variable degrees of success.
A simple and direct method for the exploration of warm water systems is shallow temperature measurements. In some New Zealand examples, measurements of near surface temperatures helped to trace the extent of deeper thermal water.
The gravity method was utilised as a structural technique for the exploration of some warm water systems in New Zealand. Our case studies show the technique can be useful in identifying basement depths and tracing fault systems associated with the occurrence of hot springs.
Direct current (DC) ground resistivity measurements using a variety of electrode arrays have been the most common method for the exploration of low enthalpy geothermal resources in New Zealand. The technique can be used to detect the extent of shallow warm waters that are more electrically conductive than the surrounding cold groundwater. Ground resistivity investigations using the electromagnetic (EM) techniques of audio magnetotellurics (AMT or shallow MT), controlled source audio magnetotellurics (CSAMT) and transient electromagnetic (TEM) methods have also been used. Highly conductive clays of thermal or sedimentary origin often limit the penetration depth of the resistivity techniques and can create some interpretation difficulties. Interpretation of resistivity anomalies needs to be treated in a site specific manner.
Key words: AMT, CSAMT, DC resistivity, electromagnetics, geophysical techniques, gravity, low enthalpy geothermal, MT, shallow temperature, TEM.
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