Looking for Permeability: Mass and Heat Flow Assessment Using High Resolution soil CO₂Flux Surveys within the Taupo Volcanic Zone, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Soil CO2 flux (φCO₂) has increasingly become important as a global exploration and monitoring tool in geothermal and volcanic fields. As CO₂ is the second most abundant gas in magma-hydrothermal systems, its study is vital for the location or management of those systems. Often one of the only surface expressions is the diffuse gas flux streaming through the soil zone. This thesis reports the investigations into heat and mass at the Rotokawa geothermal field’s thermal area, and White Island volcano’s crater floor hydrothermal system. Surface measurements were taken at high spatial resolution across the fields in a large sampling campaign during the summers of 2010/2011 and 2011/2012. A large dataset was built up which allowed for greater accuracy during geospatial modelling. The models are 2d pixel plots of the soil gas flux and temperature and are used to estimate values of heat and mass flow for the respective magma-hydrothermal systems. Both field areas have a large anomalous diffuse gas flux through the soil zone and related conductive heat flow anomaly, which indicates relative permeability from the source to the surface in these areas. That the rising fluids from the deep source can be sampled at the surface simply is a powerful tool for the exploration and management of these systems. Rotokawa has a diffuse gas release of over 600 t d⁻¹ and an associated heat flow through soil of 37 MWt while White Island has a diffuse gas release of 116 t d⁻¹ and 19.5 MWt of heat flow through the soil. Translating these values to total heat and mass flow values: Rotokawa has a mass flow 125 kg s⁻¹ and a heat flow of 314 MWt and White Island’s crater floor has a mass flow of 100 kg s⁻¹ and a heat flow of 22 MWt. Fluid flow pathways are mapped from the surface and show arcuate and hot spot spatiality, controlled by fault related permeability and structure. soil gas and temperature surveying elucidates Shallow structures that otherwise may have been hidden from status quo surface mapping. The method used in this study is applicable to both known thermal areas and blind thermal areas by addressing not only the flux but also the nature of the soil gases. Further study of White Island has found more evidence for the existence of seawater infiltration of the crater magma-hydrothermal system.