Ingham, C.E. 1976 Tongariro power development geophysical surveys. 7, Seismic survey for tailrace tunnel, Rangipo project. : . Report / Geophysics Division 119 13 p. .
Abstract: A geophysical survey comprising five seismic lines and 150 measurements of magnetic anomaly was included in a preliminary investigation which was made to elucidate sub-surface conditions in the vicinity of Rangipo Underground Powerhouse and its tailrace tunnel. This work was reported on by Ingham in 1969. The early worked showed that not only was the bedrock topography steep and irregular but also that the elevation of the bedrock surface often exceeded that of the tunnel soffit by a critically small margin. Clear indication of variability in the quality of the bedrock was also given. A more complete understanding of these problems has been arised at i a step by step way which has led, in the years between 1969 and the present, to a progressive increase in the extent of the investigation. The contribution of geophysics to the widened programme has been the establishment of another six seismic lines bringing the total length of sub-surface section up to 8,113 metres, and the purpose of the present report is to present information obtained along these lines ad summarise the results to date. In the case of the earlier five seismic lines the velocity cross-sections were derived from the seismic field data alone. In much of the latter work however, the refraction surveys were carried out in close conjunction with the drilling programme and the fullest possible use of the drillhole information was made in interpreting the seismic data. The expanded drilling programme now yielding 19 completely cored holes has, in the higher parts of the surveyed areas demonstrated overburden structures sufficiently complicated that one would hardly expect to elucidate them using seismic data alone. In the lower parts of the area, beneath the bed of the Waikato in particular, subsurface conditions which are not amenable to rigorous attack by refraction method have also been chosen to exist. A situation involving errors typical of the higher parts of the area is discussed under "V line" (along the access tunnel alignment) and one typical of lower parts of the area involving velocity reversals is discussed under the heading 'Line z25-z26". It should be appreciated though that the division of error type by elevation is probably none too rigorous (auth)
