Fourier analysis of short-period water level variations in the Rotorua geothermal field, New Zealand

The results of a Fourier analysis of water level measurements recorded near the nationally significant Pohutu Geyser in New Zealand are reported. Their analysis clearly identifies periodic pressure variations induced by barometric, gravity, thermal, anthropogenic and subsurface disturbing influences, which can be used as a benchmark for identifying and quantifying detrimental changes in behaviour. A report is given on the performance of Fourier analysis on synthetically generated signals with different levels of white noise and discrete impulses. We were able to attribute periodic variations to various disturbing effects and quantify the magnitude of these changes. Diurnal and semi-diurnal variations for both anthropogenic and barometric disturbing influences are shown to have amplitudes in the range 2.8–5.2 mm-H₂O while earth tidal amplitudes are less than 1 mm-H₂O.