Abstract: Defining Boundaries and Structure of Broadlands Geothermal Field, New Zealand, by Geophysical Methods

G. F. Risk

Resistivity measurements have outlined the lateral limits of the Broadlands geothermal field of New Zealand to depths of about 1 km. Resistivity experiments with several different electrode arrays have shown some arrays to be far superior to others. The temperatures of more than 30 deep wells at depths of 1 km show a pattern in good agreement with the reservoir boundary outlined by the resistivity measurements.

Locating zones of enhanced permeability has proved a more difficult problem. Measurements of resistivity anisotropy at 0.5-km depth show a good correlation with well flow rates in one area. Attempts are under way to drill into very deep fissured zones at about 2-km depth in the graywacke basement. Sites for wells are being selected in the hottest parts of the field which are assumed to be closest to the upwelling source of hot water.

Gravity measurements have satisfactorily indicated the depth to the graywacke basement rocks (1 to 2 km) and have outlined areas of enhanced density interpreted to coincide with the long-term discharge paths.

Observation of induced polarization effects in several parts of the field suggest that abundant mineral deposition has occurred at these sites.

AAPG Search and Discovery Article #90962©1978 AAPG 2nd Circum-Pacific Energy and Minerals Resource Conference, Honolulu, Hawaii