Geophysical Exploration of the Broadlands (Ohaaki) Field-A Review

Abstract

Broadlands Geothermal Field was largely discovered and delimited by geophysical methods in the mid 1960s, and is now one of the most thoroughly investigated fields in the world. It has been used to test many geophysical methods of geothermal exploration and development in New Zealand: several different electrical resistivity techniques were tried, in which more than 260 traversing stations, 700 dipole stations, and 21 km of EM survey line were measured. More than 50 km of seismic refraction line, and nearly 30 km of seismic reflection line, have been recorded. Over 600 gravity stations have been made, and 325 km of airborne magnetic survey line flown. Electrical resistivity traversing was found to be the best reconnaissance tool to locate the field, and roving dipole surveys the best to delimit the boundaries. Seismic refraction measurements combined with gravity anomaly data, gave the most information about geological structures within the field, but were unable to locate normal faults. Near-surface ground temperature, groundnoise, and magnetic measurements mntributed little. Lessons learnt during the tests at Broadlands have helped to develop a geophysical exploration strategy that has been successfully used in New Zealand and overseas.