Elements For A Model Of The Geothermal Field Of Milos From Seismological Data

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Journal Article: Elements For A Model Of The Geothermal Field Of Milos From Seismological Data

Five months of observation has confirmed that seismic activity on Milos island occurs in the form of swarms, with events clustered in time, space and possibly magnitude. Since the data show this activity to be confined to depths greater than 4 km, their spatial distribution cannot help to delineate directly features of the shallow geothermal reservoir which has been proven by drilling. An array of up to 30 simultaneously operating three-component tape-recording seismometers allowed detection of anomalies in the propagation of waves from these sources on their path to the surface through domains which may contain reservoirs. Clear and strong anisotropy was demonstrated by shear-wave splitting, which may be attributed tentatively to oriented fluid-filled microcracks or fracture systems. The density of observation points and different positions of source regions allowed strong changes to be documented, heterogeneities in the direction of anisotropy which may be related to the particular 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 delineated kilometric scale heterogeneities. In the uppermost 2 km an anomaly indicated as corresponding to a highly fractured, liquid filled domain has been shown to correspond with the reservoir which has been proven by drilling. A change with depth of the type of velocity anomaly may indicate a change in fracture density and liquid saturation. The distribution of seismicity and type of heterogeneity and anisotropy are tentatively discussed in terms of phase transitions in geothermal fluids in different states of confinement and eventually in terms of the variation of mechanical behaviour and phase transition on approaching a possible heat source at depth.

A. Hirn, M. Sachpazi and A. Nercessian

Published Journal 
Geothermics, 1989

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A. Hirn,M. Sachpazi,A. Nercessian. 1989. Elements For A Model Of The Geothermal Field Of Milos From Seismological Data. Geothermics. (!) .