The accurate imaging of geothermal systems from the ground surface down to great depths is an interdisciplinary problem common to geothermal resource exploration and development. Rocks can be characterized mainly in terms of their lithology, mineralogy, fracture distribution, permeability, thermal conductivity and porosity, and similarly the geothermal fluid (and its circulation) by its geochemistry, flow pattern, velocity, temperature and pressure. Some of these data are obtained by well logging and from laboratory tests conducted on drillhole cores. In general, the distribution of geothermal wells is not random, and well data are limited in terms of quantity and depth range. Accordingly, a sophisticated spatial modeling technique is indispensable in the three-dimensional imaging of geothermal systems. We describe a versatile 3-D modeling method that can be used to determine the temperature, flow velocity, and distribution of geological units within a geothermal field based on well log data. The model results for the Hohi geothermal area, Japan, provide plausible estimates of temperature, flow velocity, and geology to a depth of 3000 m. Superimposition of the three spatial models we obtained shows that, at Hohi, two geothermal reservoirs are localized near highly fractured fault zones that provide paths for the ascent of thermal fluids from depth. (C) 2007 CNR. Published by Elsevier Ltd. All rights reserved.