Ingham, C.E. 1969 Tongariro power development geophysical surveys: 2. Magnetic measurements near the confluence of Tongariro River and Waihohonu Stream 3. Seismic investigations: Moawhango dam site and Paradise Valley. [s.l.]: [s.n.]. Report / Geophysics Division 56 12 p.
Abstract: (2) Magnetic measurements: Two possible dam sites on the Tongariro River and one on the Waihohonu Stream have been considered by the Ministry of Works. All of these sites are less than one mile upstream from the confluence of the two rivers. They were chosen to take advantage of the hard andesite which outcrops at a number of places in the river beds. In the Tongariro Gorge the andesite outcrops are small. They can be seen over a length of 100-200 feet in the river bed. There is no obvious evidence that their vertical extent is more than about 60 feet. In the Waihohonu Stream the andesite is exposed along the 600 feet of a river bed and can be seen further up the walls of the gorge. It was considered that a brief magnetic survey might outline the underground extension of the andesite. (3) Moawhango Dam: The Moawhango dam is to be built where the Moawhango River passes through a narrow greywacke gorge which is about 200 feet deep. At present this gorge is the only outlet from a basin-like depression which is made up of a system of valleys. The ridges forming the downstream rim of this basin are made up of Upper Tertiary deposits and volcanic ash, beneath which the greywacke disappears at a very short distance from the gorge on both banks. Final lake level may not be more than 20 feet below the crests of the lowest parts of these ridges and it is important to find out if water could leak through. The seismic lines run along the crestal areaa of these ridges and the purposes of the survey were: (a) to check that in the Tertiary there are no deep valleys which may be filled with permeable ash or alluvium; (b) to determine the shape of the greywacke sub-surface and thus obtain information about possible leakage paths along the greywacke contact (auth)
