Application of acoustic noise and self-potential localization techniques to a buried hydrothermal vent (Waimangu Old Geyser site, New Zealand)
Résumé
A seismo-acoustic and self-potential survey has been performed in the hydrothermal area of the old Waimangu Geyser (New Zealand), which was violently erupting a century ago. Nowadays, no surface activity is visible there. We set-up an array of 16 geophones and recorded a high and steady acoustic ambient noise. We applied the matched field processing (MFP) approach to the acoustic data to locate the sources responsible for the ambient noise. The white noise constraint processor reveals the presence of a unique and well-focused acoustic source at a depth of 1.5 m below the seismic array. For this very shallow source, the application of MFP enabled the determination of both the source location and the dispersion curve of seismic velocity. The study was completed by self-potential (SP) measurements on several profiles around the acoustic noise source, which displayed a large positive anomaly above it. The results of the SP inversion gave an electric streaming current density source very close to the acoustic one. Both sources likely belong to a shallow hydrothermal structure interpreted as a small convective cell of boiling water beneath an impermeable layer. The joint application of these methods is a promising technique to recognize hydrothermal structures and to study their dynamics.
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