Since 1981 three wells have been drilled to depths of between 2.0 and 2.6 km in the Carnmenel 1 is granite, Cornwall, Engl and in order to create a HDR geothermal system. These wells are separated by between 150 and 300 m and have been hydraulically connected by massive injections of both water and viscous gel (50 c p o is e l. Passive microseismic monito ring of the hydraulic stimulation and c i r c u - l at i o n experiments has been used since 1982 to determine the size and structure of the reservoir, and m n i to r i t s growth. The active seismic survey techniques of cross-hole sei smics and vertical seismic prof i 1 in g (VSP) have been introduced to complement the passive microseismic monitoring in characteri sing the reservoir. The cross-hole seismic surveys indicate that the microseismicity defines the area of joint dilation. The attenuation of high frequencies in the region of microseismic i t y suggests that there servoir is composed of a complex zone of cracks rather than a single large fracture. VSP surveys also show a good agreement between the the micro seismically defined reservoir and seismic signal attenuation. Recent improvements in hardware, computer processing and interpretation indicate that active seismics will play an increasingly important part in mapping and understanding geothermal reservoirs .

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