IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS007 Poster presentation 1974
Multi-electrode resistivity surveys around a cyclic hydrothermal system:
the hot lakes of the Waimangu geothermal area, New Zealand
Mrs. Aurlie Legaz
IASPEI
Andre Revil, Jean Vandemeulebrouck, Tony Hurst
We conducted a series of multi-electrode surveys around the Waimangu hydrothermal area (New-
Zealand), using 1.26 km of cable with 20 m electrode spacing. The resistivity survey had two aims,
firstly to obtain a better picture of the structure and thermal state underlying the current thermal
activity, and secondly to see whether we could detect changes in resistivity under Inferno Crater Lake, a
main feature of the valley, which displays a cyclic behaviour in temperature and level (about 30 days).
Direct current electrical resistivity measurements were performed along two profiles; a Wenner
configuration gave the most consistent results, producing a detailed image of the electrical resistivity in
the top 200 metres. Along the first outline, there were very low resistivity zones under recently active
thermal areas. Surface low resistivity areas were globally well correlated with high temperatures and
high CO2 fluxes. Two surveys were conducted on the second outline, past Inferno Crater, at different
stages of the Inferno cycle: we observe resistivity changes near the crater, indicating hydrothermal fluid
movement associated with the cycling of Inferno Crater, which was overflowing during the first survey,
but was a couple of metres lower by the time of the second survey. This work was primarily to further
investigate the cyclic behaviour of Inferno Crater Lake, but also gives an indication of the sensitivity and
repeatability of resistivity techniques in shallow geothermal environments; Inferno Crater can be
considered as a textbook case, which gives us an insight into the displaced volume of fluid, throught the
cycles. Therefore, the interest of the study is to measure the electrical response of observed subsurface
fluid movements, with a view to generalising the method to volcanic and geothermal environments.
Keywords: resistivity, hydrothermal environments, fluid movements