Geophysical Structure Of The Broadlandsohaaki Geothermal Field (New Zealand)

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Journal Article: Geophysical Structure Of The Broadlandsohaaki Geothermal Field (New Zealand)

Abstract
A retrofitting study of the geophysical anomalies of the Broadlands-Ohaaki field was undertaken after production drilling had been completed in 1984. This study uses all relevant information from earlier (1965-1970) geophysical exploration surveys, results from detailed seismic reflection and airborne magnetic surveys made in 1984, and information from 44 exploration and production wells. The seismic reflection survey (12 fold CMP) shows that reflections from the top and the bottom of concealed Quaternary volcanic extrusions and pyroclastic flows can be obtained inside a hot-water system down to about 1 km depth. Processing of the data yields a detailed seismic velocity structure showing significant horizontal and vertical velocity changes inside and outside the field which, in showing significant horizontal and vertical velocity changes inside and outside the field which, in than 1 km are seismically opaque inside the field. The seismic structures down to 1 km depth, together with the density data of all formations and the well data, were used to interpret, in detail, gravity anomalies. The interpretation models define the gross structure of major lithostratigraphic units down to the Mesozoic basement (2.5 km maximum depth) both inside and outside the geothermal field. Seismic structures and gravity models allow the interpretation of magnetic anomalies associated with several concealed, up to 0.6 km thick, Quaternary calc-alkaline lava extrusions. A structural contour map of the Mesozoic basement sediments and isopach maps of major volcanic units, including two productive layers within a sequence of pyroclastic flows, are presented. The results demonstrate the importance of a retrofitting study at the post-drilling phase of an explored hot-water system.

Authors 
S. A. Henrys and M. P. Hochstein








Published Journal 
Geothermics, 1990





DOI 
Not Provided
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Citation

S. A. Henrys,M. P. Hochstein. 1990. Geophysical Structure Of The Broadlandsohaaki Geothermal Field (New Zealand). Geothermics. (!) .