Using Time-Series Aerial Thermal Infrared Surveys to Determine Near-Surface Thermal Processes at the Ohaaki Geothermal Field, New Zealand

Abstract

Repeat aerial thermal infrared surveys (TIR) are used to monitor changes in the extent and intensity of surface thermal activity. Aerial TIR surveys conducted in 1989, 1998 and 2013 at the Ohaaki Geothermal Field, New Zealand, are compared and interpreted with respect to such changes. These may be caused by natural variations, or by reservoir pressure, temperature and phase changes associated with operation of the Ohaaki Geothermal Power Station, since its 1988 commissioning. Aerial TIR data shows areas of increased steam heating between 1989 and 1998. These were followed by cooling of some areas of steam-heated ground between 1998 and 2013, resulting in fewer active surface thermal features. Some locations of steam-heated surface thermal activity in 1998 and 2013 are somewhat different compared to locations observed in 1989. This suggests that near-surface permeable pathways, particularly for rising steam, have changed. This could be caused by the development of alternate pathways during the increased steam heating phase between 1989 and 1998, as well as other processes such as tension cracking of the ground surface at the edges of a local subsidence bowl. Thermal seeps into the adjacent Waikato River are identified in the aerial TIR data. Most river-side thermal seeps present in 1989 near bore BR20 (an area affected by subsidence) were no longer visible by 1998, and are inferred to have ceased or become submerged. Only a few small seepage areas were detectable by aerial TIR in 2013. Observations of the increase and then subsequent decrease in surface thermal activity based on the TIR data are consistent with other thermal feature monitoring data in the Ohaaki area.