Testing the tunable diode laser system in extreme environments: Measuring high and low CO2 concentrations in both active volcanic and geothermal settings

Abstract

Measuring CO2 emissions in geothermal and volcanic areas is sometimes difficult because of large areas to cover and sites often inaccessible. Measuring high levels of CO2 concentration can provide information on hidden structure in geothermal areas and recording changes in CO2 concentration on volcanic areas can help monitor the level of volcanic activity. The purpose of this study was to use the Tunable Laser Diode (TDL) absorption spectroscopy method to test levels of CO2 concentrations at two extreme environments: White Island volcano, the most active volcano of New Zealand, with large and concentrated gas fluxes, and Ngapouri geothermal area, a small geothermal area in the Taupo Volcanic Zone, New Zealand, with relatively low and diffuse gas emissions. In 2017, for the first time using TDL at White Island, CO2 concentration measurements performed across the active fumarole fields had the highest CO2 concentrations of 657 ppm. TDL survey measurements were also conducted across fault strands near the Ngapouri geothermal area, and the results complemented CO2 flux results obtained with the accumulation chamber method. Higher CO2 concentrations were measured close to the mapped Ngapouri splays with a maximum of 484 ppm. The maximum CO2 flux measured in the same area was 100 g m−2 day−1 however the highest CO2 fluxes measured along the transects and by the mapped faults were less clear, but the CO2 concentrations increased closer to the fault splays. Advantages and disadvantages of using a TDL system have been described and compared to the accumulation chamber method. The results from the TDL system demonstrated that CO2 concentrations can be used as a tool, with other geophysical tools, for both detecting and highlighting geological structures where no obvious thermal activity is present and for monitoring purposes on active volcanoes.