Anomalous seismic events observed at the CSM HDR project, U.K.

Abstract

Abstract Seismic events detected during fluid injection into a crystalline jointed rock (granite) with an anisotropic stress regime are predominantly due to shear dislocation at the joints. Although seismic signals from shear dislocation are the most commonly recognised, it is becoming evident that there are other processes at work. Data from the Camborne School of Mines (CSM) hot dry rock (HDR) geothermal project site are presented in this paper, to show that there are microseismic events which are not caused by shear dislocation. These events vary from long period events detected during a gas lift test to high-frequency events detected during a massive viscous stimulation at a depth of 2300 m. These events do not show the clear P and S waves associated with shear failure. The long period events look very similar to the events detected during boulder splitting experiments in the laboratory and those detected at the Los Alamos HDR site during hydraulic injection. Tube waves detected durng the circulation of water between two wells indicates that water was squirted in and out of the well. Mechanisms for the generation of these events are not clear, but their presence does suggest that the modelling of rocko joint behaviour during fluid injection will have to take these anomalous events into consideration in order to produce a representative model.