Delineation of catchment zones of geothermal systems in large-scale rifted settings

摘要

Hydrothermal convection in the Taupo Volcanic Zone (TVZ), New Zealand, is driven by heat extracted at the brittle-ductile transition, which is in turn supplied by magmatic intrusion of the lower-crust. We present a numerical model that approximates this circulation in a statistical sense, being constrained by TVZ dimensions and mean thermal properties, but incorporating a permeability distribution of arbitrary heterogeneity. A particle tracking methodology that accounts for advective and dispersive transport is introduced, and ensembles of several tens of thousands of flow paths are constructed for each of the modeled geothermal systems. These flow paths reveal the nature of mass recharge in multicell convective systems, and suggest the ages of waters in TVZ geothermal systems vary between 5 and 50 kyr. Flow path ensembles are used to delineate catchments for each of the modeled systems and a method for calculating their areas is introduced. Catchment area is shown to be proportional to system heat output and area, which is consistent with a 1-D analytical model of heat and mass transfer. As an approximation to catchments delineated by particle tracking, we outline a method of Voronoi tessellation based on the positions and heat outputs of geothermal systems. This method is used to delineate catchments for geothermal systems in the TVZ and Iceland.