Manifestations of supercritical water in magmatic environments have so far only been accessible from analogue outcrops of fossil systems and by simulating pressure/temperature conditions in the laboratory. In order to assess the unknown properties of such reservoirs, scientific drilling is used when Earth surface sampled rocks cannot sufficiently explain past geological processes and when geophysical imaging does not sufficiently explain observed phenomena. However, our understanding of structural and dynamic characteristics of geothermal systems can be improved through application of advanced and/or innovative exploration technologies. Unlike resistivity imaging, active and passive seismic techniques have rarely been used in volcanic geothermal areas, because processing techniques were not adapted to geothermal conditions. Recent advances in volcano-seismology have introduced new processing techniques for assessing subsurface structures and controls on fluid flow in geothermal systems. We present here preliminary analyses of seismic records around a geothermal reservoir located both on-land and offshore along the Reykjanes Ridge, SW-Iceland. We deployed on-land stations (20 broad-band and 10 short-period seismometers) and 24 Ocean Bottom Seismometers which are recording since April 2014. Together with existing permanent stations, the complete network comprises 66 stations. The network was designed so that several processing techniques can be used with the data set and address scientific questions concerning geothermal systems and the oceanic ridge. We present the network deployment, our approach and preliminary results from the first months.@en

We present tomographic results obtained around geothermal reservoirs using seismic data recorded both on-land Reykjanes, SW-Iceland and offshore along Reykjanes Ridge. We gathered records from a network of 83 seismic stations (including 21 Ocean Bottom Seismometers) deployed between April 2014 and August 2015. We obtain crustal velocity images from several tomography methods. First, we used local earthquakes to perform travel time tomography. The processing includes first arrival picking of P- and S-phases using an automatic detection and picking technique based on Akaike Information Criteria. We locate earthquakes by using a non-linear localization technique, as a priori information for deriving a 1D velocity model. We then computed a 3D velocity model by joint inversion of each earthquake’s location and velocity lateral anomalies with respect to the 1D model. Our results confirms previous models obtained in the area, with enhanced details. Second, we used ambient noise cross-correlation techniques which involve the computation of cross-correlation between seismic records. Empirical Green’s functions are estimated and analyzed to derive an S-wave velocity model by surface wave tomography. Third, noise correlation theory shows that zero-offset P-wave reflectivity at selected station locations can be approximated by auto-correlating and stacking station data. With few assumptions, single-station autocorrelations provide local 1D high-resolution structural acoustic-contrast versus depth information. We show that the application of ambient noise interferometry for reflection retrieval complement well the results from both classical and noise tomography methods. @en

We present results from the application of ambient noise seismic interferometry (ANSI) to data that were recorded continuously in 2014 and 2015 at Iceland’s peninsula Reykjanes. The objective of this study is the retrieval of reflected body waves (P-waves) that provide high-resolution velocity-versus-depth as well as subsurface structural information. We show, for a subset of the onshore seismometers, that reflection information is present in the frequency bandwidth 3-8 Hz. We have observed both time-lapse variations where we expect them and time-invariant results where we do not expect to see changes. As we lacked availability of active seismic reflection and well-sonic data that could serve as reference, we discovered, in our search to find a truly independent means for checking the reflection information quality of our ANSI results, that the coda of a global seismic P-wave, that is created by scattering in the crust, contains very high frequent (3-8 Hz) reflectivity information as it shows quite good correspondence with the 40-days ANSI results, whereas random noise correlation results, using the same amount of data as the coda response, shows less resemblance. We conclude that these results justify a more detailed investigation of the merits of the ANSI method for this data set. But then, the ANSI-derived reflectivity estimates in turn clearly suggest that indeed P-wave crustal scattering information from global seismic waves is present in the frequency-range of 3–8 Hz, which is extremely high for global seismic waves. The latter aspect may open up a large range of opportunities for detailed crustal research at any location on the globe where broadband, and even shortperiod, seismometers are installed. @en

We report on the results obtained from a dense seismic deployment over a geothermal reservoir located on the tip of the Reykjanes peninsula, SW Iceland. The available seismic stations have continuously recorded the ambient-seismic wave field between April 2014 and August 2015. Using seismic interferometry (SI), seismic responses were generated between these receivers. SI allows one to turn one receiver of a receiver pair into a so-called ‘virtual source’, whose response is then retrieved by the other receiver. The retrieved virtual-source response approximates the medium’s Green’s function between the two receiver locations. For the ambientseismic recordings used in this study, virtual-source retrieval involves simple time-averaging of the crosscorrelations between these recordings. The virtual-source responses were retrieved in a time-lapse manner. The temporal variation of the ‘coda’ of these virtual-source responses has been evaluated, where the term coda refers to the later arriving, multiply scattered waves. The multiple scattering implies that these waves have sampled the subsurface very densely and hence have become highly sensitive to tiny mechanical and structural changes in that subsurface. We did not find a clear relation between the temporal variation of the coda waves and the injection/ extraction rates of the geothermal reservoir. @en

We report on the results obtained from a dense seismic deployment over a geothermal reservoir. The reservoir has been producing continuously for almost a decade and is located on the tip of the Reykjanes peninsula, SW Iceland. The seismic stations on top of the reservoir have continuously recorded the ambient seismic wavefield between April 2014 and September 2015. The density of the seismic network makes the data well suited for time-lapse seismic imaging of the reservoir. To that end we compute time-lapse responses through the application of seismic interferometry. These interferometric lapse responses are obtained by simple crosscorrelation of the seismic noise recorded by the different seismic stations. We subsequently evaluate the temporal variation of the coda of these crosscorrelations. The term coda refers to the later arriving, multiple scattered waves. The multiple scattering implies that these waves have sampled the subsurface very densely and hence become highly sensitive to tiny mechanical and structural changes in that subsurface. This sensitivity allows one, in principle at least, to monitor the geothermal reservoir. Preliminary results indeed suggest a relation between the temporal variation of the coda waves and the reservoir. Ultimately, this method may lead to a means to monitor a geothermal reservoir in both space and time. @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