Induced Polarization (IP) Anomaly in the Broadlands Geothermal Field

Abstract

An IP anomaly, discovered in 1973 during resistivity surveying, exists over an area of about 300 m coinciding with the resistivity boundary marking the southern edge of the Broadlands geothermal field. Detailed IP, resistivity and magnetic surveys over the site reveal unusual fine structure in the resistivity pattern and a small magnetic anomaly. The cause of the IP anomaly was investigated in 1980 by a 156 m deep drillhole. Low resistivities (5 to 15 and temperatures of u p to measured in the lower half of the hole are consistent with the complex surface resistivity pattern. Downhole logging runs confirmed the existence of the IP effect and from measurements on cores the main IP zone was identified as a layer of sand lying between 25 and 4 5 m deep. date, petrologic examination of the cores has not identified any minerals that seem capable of causing the IP effects. metallic sulphides known to cause such anomalies in other places are absent.