Ground subsidence of a geothermal field during exploitation

Abstract

Ground subsidence at Wairakei was first measured in 1956 when bench mark levels were compared with those established in 1950. The network of bench marks was extended during the next few years and periodic re-levelling of these marks has indicated that the area affected by subsidence probably exceeds 25 square miles. Further levelling since 1964 has established that the area of maximum subsidence occurs outside the production field. The present maximum subsidence rate in this area is 1.3 feet per annum. Some measurements of horizontal ground movement have been made and a relationship between this surface strain and vertical subsidence has been found. Exploitation of the Wairakei geothermal field has clearly been the major cause of the ground subsidence. At first, a simple relationship between subsidence and aquifer drawdown was evident, indicating elastic ground compression caused by a loss of buoyancy. In more recent years the ground has continued to subside despite a stabilising of aquifer pressure. Although the region of maximum subsidence occurs outside the production field, the steam transmission pipes and the main concrete hot water drain at Wairakei have been affected by the ground movement and modifications to accomodate the surface strain have been necessary. At the Broadlands geothermal field, at present under investigation, an extensive levelling survey has been undertaken. It is hoped that early recognition of ground subsidence will assist the design and sitting of major installations in this field.