Magmatic geothermal systems, as they exist in Iceland, are complex geological structures. Key features, such as hydrothermal upwelling zones and intrusive heat sources, are embedded in a highly heterogeneous host rock. The latter comprise quasi-horizontally layered basaltic lava flows of variable texture and morphology, repeatedly intruded by magma chambers and numerous intersecting sub-vertical dykes and sub-horizontal sheets. In order to estimate whether seismic techniques can detect the important geothermal features, this study examined the seismic velocity structure of the fossil geothermal system of Geitafell in southeast Iceland. We combined seismic tomography field experiments with ultrasonic measurements in the laboratory to obtain a comprehensive picture of the velocity systematics and to investigate the scale-dependence of experimental velocity determination. We recorded six shallow seismic profiles over outcrops of different parts of the exposed magmatic system and we investigated 10 specimens of basalt, dolerite, and gabbro in the laboratory. Our results demonstrate that in the fossilized geothermal system of Geitafell, seismic velocities can vary over a wide range of around 1500 m/s. Considering this large spread of wavespeeds, velocity anomalies caused by geothermal activity are likely to be masked by the heterogeneity of the host rock. This places stringent demands on acquiring high quality data and an optimal survey design for successful seismic exploration. Moreover, we discovered that ultrasonic velocities measured in the laboratory under comparable pressure (depth) conditions are up to 15% higher than seismic velocities inverted from the field data. This is of great importance and must be taken into account when interpreting exploration-scale field tomograms with the help of laboratory test data. (C) 2015 Elsevier Ltd. All rights reserved.