Whiteford, P.C. 1982 Volcanological observations at Tongariro National Park, 3: magnetometer network 1973-1981. [s.l.]: [s.n.]. Report / Geophysics Division 151 64 p.
Abstract: A differential magnetometer network has been operating in the Tongariro National Park, near the active volcanoes Ruapehu and Ngauruhoe, since 1973 to search for local magnetic field changes that might be produced by volcanic activity. Three magnetometer sensors have been operated since August 1973 and these lie roughly on a line running NNW and at distances at 3, 9 and 14 km from the crater lake of Ruapehu. A fourth sensor operated for only 6 days 0.5 km NW of the crater lake, the sensor being destroyed by an eruption in April 1975. Another sensor has been operating since 1976 at a site 5.5 km NE of the line of the above sensors and 4 km SW of Ngauruhoe's crater. The total magnetic field at each sensor is measured once every five minutes and the differences in magnetic fields between pairs of sensors are recorded together with the total field at one sensor. From the records, hourly means, 4-hour means, and 5-day means are produced. The 5-day means significantly reduce noise with periods of several days and less. Several other methods of reducing the noise level have been tried but have not been successful. Although several significant eruptions of Ruapehu and Ngauruhoe have occurred since August 1975 no local magnetic field changes larger than 5 nT related to volcanic activity have been detected. It appears that any local magnetic field changes occurring during the 1973-1975 eruptions of Ngauruhoe have not been detected because the network was at that time not close enough to Ngauruhoe. There have probably been no local magnetic field changes associated with the eruptions of Ruapehu that have occurred since 1973. These eruptions were not preceded by seismic activity within the volcano or by high lake water temperatures, unlike some eruptions of Ruapehu that occurred prior to 1973
