Assessment of Faults and Fractures at the Mokai Geothermal Field, Taupo Volcanic Zone, New Zealand

Abstract

The Mokai geothermal field is located 25 km north-west of the Wairakei field in the Taupo Volcanic Zone, central North Island of New Zealand. Its main reservoir was defined by an area of low electrical resistivity (about 12-16 km2) which is associated with hydrothermally demagnetised rocks at depths between 500 and 1000 m. Lateral outflow of thermal waters occurs toward the lower topography of the Waikato River valley in the north. An analysis of detailed digital topography data (25 m grid) by contour and shaded relief plots was made to identify lineaments caused by faults and their fractures in the Mokai area. The result confirms the existence of three dominant structural trends at Mokai striking NE, N, and NW. A map of 'fault and fracture density' (FFD), defined as the total length of lineaments per unit area, was constructed for the area. It shows that the Mokai geothermal reservoir and its thermal outflow are associated with a zone of high FFD, suggesting the significant influence of geological structures on the hydrology of the present geothermal system. The result also suggests that a deep feed zone of thermal waters lies outside the southern resistivity boundary of the field.