HR: 1340h
AN: NG33B-0173    [Abstracts]
TI: Active seismic monitoring of changes of the reflection response of a crystalline shear zone due to fluid injection in the crust at the Continental Deep Drilling Site, Germany
AU: * Beilecke, T
EM: t.beilecke@gga-hannover.de
AF: Institute for Applied Geosciences, Stilleweg 2, Hannover, 30655 Germany
AU: Kurt, B
EM: k.bram@gga-hannover.de
AF: Institute for Applied Geosciences, Stilleweg 2, Hannover, 30655 Germany
AU: Stefan, B
EM: buske@geophysik.fu-berlin.de
AF: Institute for Geological Sciences, Free University Berlin, Malteserstrasse 74-100 (Haus D), Berlin, 12249 Germany
AB: In theory and in the laboratory variations of the hydraulic pressure can be detected with seismic methods: A lowering of the hydraulic pressure leads to the closure of micro-cracks within the rock (increase of the differential or effective pressure). Subsequently, the seismic velocities increase. An increase of the hydraulic pressure leads to reverse seismic effects. Consequently, seismic impedance contrasts and associated reflection amplitudes vary in the case of a propagating fluid pressure front in a rock matrix with inhomogeneous permeability - as is the case at shear zones. The largest amplitude changes can be expected with vertical ray inclination on the impedance contrast. Generally, the expected effects are small however (Kaselow, 2004). The practical utilization of active seismics for the detection of pressure changes at large scale in hard rock is currently being studied at the Continental Deep Drilling Site (KTB). The injection of water (200 l/min) in a depth of about 4000 m into the so-called SE2 shear zone in the KTB pilot hole was monitored with active seismics between May 2004 and April 2005. The core of the experiment layout is a fixed 5-arm geophone array consisting of 24 3-component geophones, buried at about 70 cm depth. The source signal is a vertical vibrator sweep of 30 s length with the spectrum 30-120 Hz. The signal is sent into the ground 32 times during each cycle, detected with the array and recorded separately for each geophone channel, without prior correlation with the source signal. This allows maximum post-processing with seismic processing and analysis tools and especially permits the use of array properties to increase the signal-to-noise ratio. Critical parameters of the experiment are the repeatability of the source signal as well as the stability of the receiver properties. Another pivot is the hydraulic pressure and its distribution built up within the rock matrix. Estimations based on model calculations show that a change of seismic reflections can be detected above a well head pressure of about 15 MPa (Kaselow, 2004). Additionally, the fluid pressure at depth must be distributed within at least the first Fresnel-Zone of the seismic wavefront. After one year of injection only about 12 MPa were reached at the well head of the KTB pilot hole. One consequence is in particular that sensitive seismic signal processing needs to be applied. The standard deviation of the amplitude spectra of the raw data recorded with the array shows values around 36% if the strong direct waves are integrated in the analysis. The weak reflection signals from the target zone show values around 78%. The latter is due to the relatively low level of the wanted signals with respect to the ambient noise level. Band-pass filters and the application of the so-called diversity stack can reduce the errors. Another improvement can be achieved with selective, time-window based amplitude-dependent signal suppression before correlation (Polom, 1999) in an improved manner. References: KASELOW, A. 2004. The Stress Sensitivity Approach: Theory and Application. Dissertation, Freie Universit„t Berlin. POLOM, U. 1999. Elimination of Noise Caused by Spikes and Bursts in Vibroseis Data. Pure and Applied Geophysics, 156, 319-344.
DE: 0905 Continental structures (8109, 8110)
DE: 0935 Seismic methods (3025, 7294)
DE: 5102 Acoustic properties
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
SC: Nonlinear Geophysics [NG]
MN: Fall Meeting 2005