The recovery of geothermal energy for power generation invariably involves the withdrawal of very substantial amounts of heat/water from the subsurface. For example, the commercial operation of a 100 MW power plant in a liquid dominated reservoir situation will require the recovery of about 5x10l6J/yr of heat and a mass flow of water of about 5x1010kg/yr. spicuous phenomena are associated with this rate of production. Usually there are changes in the subsurface temperature, a significant rate of lowering of the local reservoir fluid pressure and an associated lowering of the ground water surface in the production region. Moreover, there are more subtle effects such as subsidence, strain and tilt of the ground surface, changes in the local gravity field and changes in the subsurface electrical conductivity. All these phenomena represent responses of the reservoir to production and convey information on the evolution of the system. The recording of these signals can therefore be helpful in reservoir monitoring during production. Obviously, the evolutionary pressure phenomena are known to everyone engaged in reservoir engineering and the recording of reservoir pressure in available boreholes and of the groundwater level are standard tools in production monitoring. Although ground subsidence and changes in the gravity field have been recorded in a few cases, the elastomechanical phenomena have not received much attention.

The purpose of this brief note is to present a preliminary evaluation of the potential of the elastomechanical methods in practical reservoir engineering and related areas. Assuming simple relevant situations, the strength of the field signals will be estimated and compared to other ground surface data such as gravity and D.C. electrical signals that. are also of interest in reservoir monitoring. Because of greater difficulty in observing surface strain, we will limit our discussion to vertical ground displacement and tilt signals.

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