Geophysical Association of Ireland Seminar on Environmental Geophysics Dublin, 15 th February 2012 GEOPHYSICS (MOSTLY ELECTROMAGNETIC AND THERMAL MODELLING) IN (HIGH- AND LOW-ENTHALPY) GEOTHERMAL ENERGY INVESTIGATIONS Mark Muller 1 , Alan Jones 1 , Javier Fullea 1 , Chris Yeomans 1,3 , Markus Loewer 2 , Laura Ayres 3 , Mohammednur Desissa 4 and Derek Reay 4 1 Dublin Institute for Advanced Studies 2 ETH Zurich, Switzerland 3 University of Birmingham, U.K. 4 Geological Survey of Northern Ireland Abstract High- and low-enthalpy geothermal resources are associated with key geological features and electrical conductivity characteristics that make them particularly amenable to exploration, assessment and monitoring using electromagnetic techniques. Our paper illustrates and discusses the challenges in deriving subsurface electrical conductivity images from electromagnetic surveys that are adequate firstly to detect potential resources and secondly to allow reliable prediction of the physical properties that define an exploitable resource. In the case of low-enthalpy resources we draw on early experience from the SFI-funded IRETHERM project and the results of two recent magnetotelluric surveys over geothermal energy prospects in Northern Ireland. We also present early results from a novel 3-D geophysical-petrophysical thermal modelling approach that is used to determine the origin of Ireland’s subsurface heat and temperature distribution and to better constrain temperature prediction at depth. 1. Introduction Deep geothermal resources that might support regional-scale space-heating and/or electricity generation are to be found in either hot geothermal waters in permeable aquifers or in hot, dry rock in radiogenic heat-producing granites. Permeable aquifers supplying geothermal waters are suitable for direct exploitation for energy, while hot, dry rocks provide a resource requiring the engineering of a porous heat-exchange system at depth through hydraulic fracture stimulation (hydro-fracturing) – the Enhanced Geothermal System (EGS) concept. Geothermal resources are