| Abstract |
During the past decades most of the known hot-water geothermal systems in the Taupo Volcanic Zone (TVZ) have been explored by geophysical techniques. The Broadlands-Ohaaki geothermal field, located on the eastern margin of the TVZ, is one of the thermal systems ("",25 km2) delineated by these surveys. An electric plant (105 MW) is presently being constructed to exploit the reservoir.. The extent of the Broadlands-Ohaaki field was first outlined by resistivity measurements in 1965 (Risk et al., 1977). Between 1968 and 1970 detailed seismic refraction, gravity and magnetic surveys were undertaken to determine detailed structural features within the field (Hochstein ~d Hunt, 1970). Analysis of seismic refraction measurements (about 50 line km) was restricted to short segments «3.0 km) of reversed travel-time distance profiles and only shallow structures «500 m) could be mapped with confidence. Local gravity data corrected for regional effects show residual highs up to 100 JlN/kg (10 mgal) within the geothermal field. These anomalies were qualitatively interpreted by Hochstein and Hunt in terms of densification of porous reservoir rocks caused by thermal alteration and mineral deposition. Since 1970 stratigraphic and rock property data from 25 additional wells have become available and the acquisition of 26 km of common-mid-point seismic reflection data in 1984 has provided better correlation of the volcanic stratigraphy and established a more accurate velocity model for the geothermal field. In addition, the use of efficient r.ay tracing techniques allowed a more comprehensive analysis of refraction data including modelling of unreversed travel-time curves. More detailed knowledge of the geometry of the major stratigraphic horizons was obtained from a reinterpretation of the residual gravity data. A low level airborne magnetic survey was also completed in 1984. The location of seismic and gravity data profiles across the field is shown in Figure 2. Seismic, gravity and magnetic interpretation section for one of the 10 profiles is displayed in Figure 1 (profile B-B'). |