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Crosshole radar tomography is a useful tool in diverse investigations in geology, hydrogeology, and engineering. Conventional tomograms provided by standard ray-based techniques have limited resolution, primarily because only a fraction of the information contained in the radar data ͑i.e., the first-arrival times and maximum first-cycle amplitudes͒ is included in the inversion. To increase the resolution of radar tomograms, we have developed a versatile full-waveform inversion scheme that is based on a finite-difference time-domain solution of Maxwell's equations. This scheme largely accounts for the 3D nature of radar-wave propagation and includes an efficient method for extracting the source wavelet from the radar data. After demonstrating the potential of the new scheme on two realistic synthetic data sets, we apply it to two crosshole field data sets acquired in very different geologic/hydrogeologic environments. These are the first applications of full-waveform tomography to observed crosshole radar data. The resolution of all full-waveform tomograms is shown to be markedly superior to that of the associated ray tomograms. Small subsurface features a fraction of the dominant radar wavelength and boundaries between distinct geological/hydrological units are sharply imaged in the fullwaveform tomograms.

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to stimulate the reservoir for a "hot dry rock" geothermal project, that was initiated by a private/public consortium in the city of basel, approximately 11500 m 3 of water were injected between December 2nd and 8th, 2006, at high pressures into a 5 km deep well. More than 10500 seismic events were recorded during the injection phase, and minor sporadic seismic activity was still occurring more than two years later. the present article documents the focal mechanisms of the 28 strongest events, with M L between 1.7 and 3.4, that have been obtained by the swiss seismological service (sED) during and after the stimulation. the analysis is based on data that was recorded by a six-station borehole network, operated by the project developers, as well as by several permanent and temporary surface networks. the hypocenters of the events are located inside the stimulated rock volume at depths between 4 and 5 km within the crystalline basement. Of the 28 faultplane solutions two are normal faulting mechanisms and one is a strike-slip mechanism with a strong normal component. All others are typical strike-slip mechanisms with mostly Ns and EW striking nodal planes. As a consequence, the t-axes are all nearly horizontal and oriented in a NE or sW direction (mean azimuth 46 ± 11 degrees) and the P-axes of the strike-slip events point in a NW or sE direction (mean azimuth 138 ± 13 degrees). Overall, the observed focal mechanisms agree with what would be expected from both the stress observations within the well and the stress field derived from the previously known natural seismicity. Introductionto stimulate the reservoir for a "hot dry rock" geothermal project, that was initiated by a private/public consortium in the city of basel, approximately 11500 m 3 of water were injected at high pressures into a 5 km deep well, between December 2nd and 8th, 2006, (Häring et al. 2008). A six-sensor borehole array, installed by the operators of the project at depths between 317 and 2740 meters around the well to monitor the induced seismicity, recorded more than 10500 seismic events during the injection phase. On December 8th, 2006, three events ocurred that exceeded the safety threshold for continued stimulation (including one event with a magnitude M L = 3.4), so that injection was stopped prematurely and the well was vented. In the following days about one third of the injected water volume flowed back out of the well (Häring et al. 2008). the seismic activity declined rapidly thereafter, but even more than two years later, sporadic microseismicity was being detected in the stimulated rock volume by the downhole-instruments.Meanwhile, despite the setback with respect to the original plan, efforts to analyse the large amount of data generated by this project have produced first results, some in the form of internal reports to the sponsor (Geopower basel AG), and others as publications in the open literature. An overview of the project, including a documentation of injection pressures and flow rates, can be found in Häring et...