Subsidence Surveying

Abstract

Subsidence is measured by comparing levels on benchmarks that are derived by survey levelling techniques fkom surveys taken some time apart. Subsidence is usually expressed as annual rates. In New Zealand the geothermal fields at Wairakei-Tauhara, Ohaaki, Kawerau., Rotokawa, Mokai and Ngawha have extensive benchmark networks that are levelled typically every 2 years. NZ environmental regulatory authorities usually require the geothermal field operator to monitor subsidence as a condition of granting resource consents. With the advent of digital levels and using an adaptation of traditional precise levelling techniques high accuracies are efficiently obtained. Significant potential errors in benchmark elevations can be eliminated or mitigated by using suitable equipment, sound procedures, calibration and equipment checking. Subsidence levelling data is also used for gravity surveys and for designing and setting out geothermal engineering projects. Similar survey techniques are used worldwide for monitoring subsidence due to mining and extraction of water and oil.