In autumn 2017 a network of 14 broadband seismic stations was deployed at the Theistareykir high temperature geothermal field (NE Iceland). This experiment was conducted as part of the current efforts to characterize the field's main structures, and possible short and long term stress variations due to the ongoing fluid injection and extraction operations which started in autumn 2017. In this work, we use two years of continuous seismic records (October 2017–October 2019) to compute a 3D shear wave velocity model of the geothermal field and to detect possible crustal stress changes related to the injection and production activities. From phase cross-correlations of the vertical component recordings, we measure the Rayleigh wave group velocity dispersion curves to obtain 2D group velocity maps between 1 and 5 s. Subsequently, we use a neighborhood algorithm to retrieve the 3D shear wave velocity model of Theistareykir. Mainly, two sets of elongated high and low velocity anomalies can be observed oriented in a NW/WNW direction, parallel to the lineaments of the active Tjörnes fracture zone. Velocity reductions west of Ketilfjall and at Baerjafjall could indicate the location of upflow zones of the magmatic reservoir or hydrothermal system. We analyzed the temporal evolution of phase and amplitude of phase auto-correlations using the stretching technique and discuss their behavior in relation to the geothermal field operations. We notice a slightly stronger long-term velocity decrease in the reservoir region compared to outer regions. This could be related to the mass depletion in that area (higher fluid extraction compared to the water reinjection). In summary, our findings show how a monitoring network can be set up to enable a detailed imaging and monitoring of reservoir behavior in general.

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Timing errors are a notorious problem in seismic data acquisition and processing. A technique is presented that allows such timing errors to be recovered in a systematic fashion. The methodology relies on virtual- source responses retrieved through the application of seismic interferometry (SI). In application to recordings of ambient seismic noise, SI involves temporal averaging of time-windowed crosscorrelation measurements. The retrieved interferometric responses are typically dominated by surface waves. Under favorable conditions, these interferometric responses therefore approach the surface-wave part of the medium's Green's function. Additionally, however, its time-reverse is often also retrieved. This implies time-symmetry of the time-averaged receiver-receiver crosscorrelations. In this study, this time-symmetry is exploited: by comparing the arrival time of the direct surface wave at positive time to the arrival time of the direct surface wave at negative time for a large a number of receiver- receiver pairs, relative timing errors can be determined in a weighted least-squared sense. The proposed methodology is validated using synthetic data. The results hold particular promise for large N seismic arrays.@en

Timing errors are a notorious problem in seismic data acquisition and processing. A technique is presented that allows such time shifts to be detected and corrected in a systematic fashion. The methodology relies on virtualsource responses retrieved through the application of seismic interferometry (SI). In application to recordings of ambient seismic noise, SI involves temporal averaging of time-windowed crosscorrelation measurements. Because surface waves dominate the ambient seismic field, the retrieved interferometric responses are typically also dominated by surface waves. Under favorable conditions, these interferometric responses therefore approach the surface-wave part of the medium's Green's function. Additionally, however, its time-reverse is also retrieved under those conditions. This implies time-symmetry of the time-averaged receiver-receiver crosscorrelations. It is this time-symmetry that is exploited in this study. By comparing the arrival time of the interferometric surface waves at positive time to the arrival time of the interferometric surface waves at negative time for a large a number of receiverreceiver pairs, relative timing errors are determined in a least-squared sense. The proposed methodology is validated using both synthetic data and field data. The results hold particular promise for time-lapse (4D) seismic surveys.@en