Geophysical investigations of the Ngawha geothermal area

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Macdonald, W.J.P.; Dawson, G.B.; Rayner, H.H.; Hewson, C.A.Y. 1977 Geophysical investigations of the Ngawha geothermal area. [s.l.]: [s.n.]. Report / Geophysics Division 130 23 p.

Abstract: Electrical resistivity traversing within a radius of 10 km of the Ngawha Hot Springs has shown an extensive area of low apparent resistivity within a region of background resistivity of less than 40 ohm m. This background resistivity value appears to be characteristic of a superficial layer about 500 m thick of mudstones of low permeability. The results of resistivity traversing and soundings suggest a simple model of a column of hot water rising from depth and spreading out horizontally below the capping mudstone. It is estimated from the resistivity data that the area of the hot water is between 25 and 50 sq km at a depth of 700 to 1000 m. It is inferred that this area will decrease with increasing depth before increasing again. The geophysical results, together with previous measurements of high temperature gradients, and a maximum temperature of 236deg C at 550 m depth in a borehole at Ngawha Springs, point to the existence of a potentially exploitable geothermal field which should be further explored by a pattern of 12 deep drillholes