Magnetic Anomalies of the Rotokawa Geothermal Field, Taupo Volcanic Zone, New Zealand

Abstract

The reduction of aeromagnetic data over the Rotokawa geothermal field discussed in this paper is an example of the complexity of reduction of magnetic anomaliesin volcanic region. Second residual magnetic anomalies over the area which have been reduced for the normal geomagnetic field, a first order regional field (interpolated undisturbed magnetic anomalies over outcropsof non-magnetic basement) and magnetic effects of deep (2.5-7km depths) magnetic crustal masses, still contain the effect of large wavelength anomaly with an amplitude of about and a wavelength of 15 Only third order residuals outline the local magnetic anomaliesassociated with of volcanic rocks within the Rotokawa reservoir. Interpretation of these third order residual anomalies by three-dimensional magnetic modelling shows that rocks lying within the low resistivity area either partially or completely demagnetised, at least down to sea level. Details of theresistivity boundary in the northernmost comer of the field be interpreted by shallow, demagnetisedand conductiverocks which extend beyond the resistivityboundary.