Gravity, magnetic and resistivity investigations of the Okauia Low Temperature Geothermal System in alluvial sediments of the Hauraki Depression, New Zealand
Supri Soengkono 1 2 Robert Reeves 11 GNS Science, Wairakei, Private Bag 2000, Taupo 3352, New Zealand.
2 Corresponding author. Email: s.soengkono@gns.cri.nz
Exploration Geophysics 48(3) 316-331 https://doi.org/10.1071/EG15052
Submitted: 6 June 2015 Accepted: 18 February 2016 Published: 5 April 2016
Abstract
Gravity, ground magnetics, DC resistivity traversing, and time domain electromagnetic soundings (fixed-loop and in-loop) were conducted to investigate the Okauia Low Temperature Geothermal System near the eastern boundary of the Hauraki Depression in North Island of New Zealand. The gravity study revealed a hidden extension of the Tertiary-age Waiteariki Ignimbrite, underlying plio-Pleistocene alluvium deposits of the Hinuera Formation, which locally host an aquifer of warm geothermal fluids. A 3D magnetic model derived from the ground magnetic measurements helped identify probable paleo-channels within the sedimentary sequence of the Hinuera Formation. These paleo-channels could represent higher permeability at shallow (<50 m) depth, although the exact relationship between the locations of the surface thermal manifestations and identified paleo-channels is still unclear. The results of the resistivity surveys suggest that values less than 25 Ωm are indicative of the presence of warm waters at Okauia. Most warm water occurrences at Okauia take place near the Okauia Fault, particularly at 0 m RL (sea level) elevation. The near-surface extent of the Okauia Low Temperature Geothermal System can be approximated from the results of this study.
Key words: alluvial, geophysical modelling, gravity, low temperature geothermal system, magnetic, resistivity, TDEM.
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