Analysis of Contemporaneous Pseudorandom Data from Two Springs in the Orakei Korako Geothermal Field of New Zealand

Abstract

The Orakei Korako geothermal field in the North Island of New Zealand is an unexploited field consisting of over 30 small active geysers and about 100 hot springs. It provides an excellent opportunity to examine the naturally occurring fluctuations in geothermal surface activity. Contemporaneous measurements of conductivity, temperature, water level, barometric pressure and rainfall were collected at 5 minute intervals over a 2 month period from two springs 150 metres apart. Spectral analysis by both wavelet and Fourier transforms was used to minimise the effects of non-Gaussian signal spikes and examine spectral windows of each set of data. Analysis shows that spring fluctuations are driven predominantly by solar thermal effects with negligible influence from soli-lunar gravity tides. In addition to solar thermal tidal variations, the springs have periodic fluctuations of 11.5 minutes and 24 minutes respectively. A low barometric efficiency indicates that transmissivity in the reservoir is low. System dynamics modelling of one spring suggests that it could be driven by a low vertical flux of geothermal steam interacting with transient groundwater flow.