3D GEOPHYSICAL INVERSION MODELLING OF POTENTIAL FIELD DATA APPLIED TO THE INVESTIGATION OF FOUR GEOTHERMAL RESOURCE AREAS

In recent years, advances in the mineral exploration sector have made 3D geophysical inversion modelling of gravity and magnetic data routine. Options for inversion modelling are many and varied. For example, inversion modelling can be performed with or without geologic boundary constraints and rock property field measurements can be explicitly incorporated into the modelling process. New codes can also enable 3D inversion modelling of magnetic survey data in areas containing remanence. These advances help reduce the problem of non-uniqueness and drive the inversion model result towards a more geologically-realistic prediction. The estimation of 3D density and magnetic susceptibility variations in the subsurface through geophysical inversion modelling can be valuable for investigation of geothermal areas. For example, density and magnetic susceptibility can be used to map subsurface lithology, the margins of which may constitute zones of permeability. Alternatively, magnetic susceptibility variations may reflect changes in subsurface alteration which, if properly identified, could designate clay-rich, low permeability areas to avoid. This paper presents examples of 3D geophysical inversion modelling applied at four geothermal areas: gravity modelling at Akutan Island (Alaska) and Bradys geothermal area (Nevada) as well as magnetic modelling at Ross River (Yukon) and Baker Hot Springs (Washington State). The potential field inversion modelling proved effective at identifying geologic structure and lithologic domains in 3D including one area suspected to contain remanence. In addition, an existing 3D geologic model was quantitatively tested with 3D inversion modelling to identify portions of the 3D geology which are or are not consistent with surface gravity measurements. The relative merits and effectiveness of potential field inversion modelling at each of the four areas will be discussed.