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From five months of observation, seismic activity on Milos Island is confirmed to occur in the form of swarms with a clustering in time, space and possibly magnitude of events. This activity is confined to depths greater than at least 4 km, so that its spatial distribution cannot help directly to delineate features of the shallow drilled geothermal reservoir. An array of up to 30 simultaneously-operating three-component tape-recording seismometers are used to detect propagation anomalies experienced by waves on their way to the surface through domains which may contain reservoirs. Clear and strong anisotropy is demonstrated by shear wave splitting which may be attributed tentatively to orientated fluid-filled microcracks or fracture systems. The density of points of observation and different positions of source regions reveal differences in orientation of anisotropy indications, heterogeneity of local stress and inherited deformation systems. A joint inversion of arrival times for the relocation of hypocentres and three-dimensional heterogeneity in P and S velocity structure delineates kilometric scale heterogeneities. In the uppermost 2 km an anomaly indicated as corresponding to a highly fractured, liquid filled domain has been sampled as a reservoir by the productive drillholes. The distribution of seismicity and type of heterogeneity are tentatively discussed with respect to phase transition in geothermal fluids in different states of confinement and with respect to the variation of mechanical behaviour and phase transition towards a possible heat source at depth.

Additional information

Authors: HIRN A, Institut Physique du Globe de Paris (FR);SACHPAZI M, Institut Physique du Globe de Paris (FR);NERCESSIAN A, Institut Physique du Globe de Paris (FR)
Bibliographic Reference: EUR 12304 EN (1990) 62 pp., MF, ECU 4
Availability: (2)
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