Ground penetrating radar (GPR) and shallow temperature survey at Orakei Korako

Report: TR 2023/08

Authors: Geothermal Scientific Investigations Ltd

In 2012 a Ground Penetrating Radar (GPR) and downhole temperature measurement survey was undertaken around the boardwalk at Orakei Korako geothermal field, Taupō Volcanic Zone, New Zealand.

In 2023 this survey was repeated to identify if and where changes in the subsurface had taken place over the last 11 years.

Nine 2012 downhole temperature measurement sites and ten surface temperature measurements sites were repeated for a direct comparison with our 2023 data. Downhole temperature measurements were taken at depths of 0.5 m, 1.0 m and 1.5 m where possible, using a K-type digital temperature probe with a quick response tip. Surface temperature measurements were obtained at the start and end points of each GPR transects using Digital Temperature probe with a K-type thermocouple.

Thirteen GPR sites were repeated in 2023, five 2012 GPR sites were not repeated due to inaccessibility to the site. GPR profiles from seven new sites were also collected during the 2023 survey. In 2012 we used a GSSI SIR-2000 GPR unit with a 200 MHz antenna. In 2023 a Leica DS2000 GPR unit fitted with dual frequency antennas of 250 MHz and 700 MHz was used. The 700 MHz antennae collected high resolution data from the shallow subsurface (0-3 m depth), while the 250 MHz antennae captured data from deeper layers (0-7 m) at a coarser resolution.

Our repeat survey identified areas that are heating up and cooling down, mapped migrating subsurface heat pathways, located ascending steam conduits, identified fractures, mapped zones of hydrothermal alteration, as well as imaged the thickness of the siliceous sinter terraces. The combined techniques of GPR and temperature measurements offers an innovative method and new perspective for characterising changes that take place in the shallow subsurface over time.