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Numerical simulations of seismic wave propagation usually rely on a simple source model consisting of an idealized point location and a moment tensor. In general, this is a valid approximation when the source dimensions are small relative to the distance of points at which the seismic wave motions are to be evaluated. Otherwise, a more realistic spatio-temporal source representation is required to accurately calculate ground motions at the position of monitoring stations. Here, we present a generic approach to couple geomechanical simulations to seismic wave propagation models using the concept of the equivalent force field. This approach allows the simulation of seismic wave propagation resulting from the spatio-temporal dependent earthquake nucleation and rupture processes. Within the geomechanical package two separate geomechanics codes are used to simulate both the slow loading stage leading to earthquake nucleation as well as the successive dynamic rupture stage. We demonstrate the approach to a case of induced seismicity, where fault reactivation occurs due to production from a natural gas reservoir.

Item does not contain fulltext, OBJECTIVE: To define the clinical phenotype and natural history of sporadic adult-onset degenerative ataxia and to identify putative disease-causing mutations. METHODS: The primary measure of disease severity was the Scale for the Assessment and Rating of Ataxia (SARA). DNA samples were screened for mutations using a high-coverage ataxia-specific gene panel in combination with next-generation sequencing. RESULTS: The analysis was performed on 249 participants. Among them, 83 met diagnostic criteria of clinically probable multiple system atrophy cerebellar type (MSA-C) at baseline and another 12 during follow-up. Positive MSA-C criteria (4.94 +/- 0.74, p < 0.0001) and disease duration (0.22 +/- 0.06 per additional year, p = 0.0007) were associated with a higher SARA score. Forty-eight participants who did not fulfill MSA-C criteria and had a disease duration of >10 years were designated sporadic adult-onset ataxia of unknown etiology/non-MSA (SAOA/non-MSA). Compared with MSA-C, SAOA/non-MSA patients had lower SARA scores (13.6 +/- 6.0 vs 16.0 +/- 5.8, p = 0.0200) and a slower annual SARA increase (1.1 +/- 2.3 vs 3.3 +/- 3.2, p = 0.0013). In 11 of 194 tested participants (6%), a definitive or probable genetic diagnosis was made. CONCLUSIONS: Our study provides quantitative data on the clinical phenotype and progression of sporadic ataxia with adult onset. Screening for causative mutations with a gene panel approach yielded a genetic diagnosis in 6% of the cohort. CLINICALTRIALSGOV REGISTRATION: NCT02701036.

The Ketzin CO2 storage site is a site that is intensively monitored for nearly a decade. An array, containing 3-C geophones and hydrophones, is located near the surface to monitor the pressure effects of the insertion of CO2 storage. This array has been passively monitored from 2009 onwards. The sandstone saline aquifer located at 650 m depth (Förster et al., 2009) below the K2 high velocity layer proved to be ideal for CO2 storage. The storage of CO2 causes pressure build up and release in the subsurface (Park et al., 2013, Paap et al., 2014). This build up and release results in seismic events. In this study cross-correlation is used to identify these induced events using a waveform template from a pre-determined event. This master event occurred in 2010, originated from the storage aquifer depth. The cross-correlation method identified two extra events in the 2010 data set. These two events were found using the hydrophone receivers which were used because they have a higher signal to noise ratio (SNR) than the 3-C geophones. Two methods were tested to locate these events using; 1. the time difference between P-and S-wave arrivals, 2. the time difference between the P-wave arrival and the earthquake onset time (t0). Method 2 proved to be most successful in locating the events. The main event and the two extra events found originate all three from the same location area, to the north east of the array. Because cross-correlation searches for the same waveform, events that are near identical can only be found. Events originating from a different location will not be detected. More events can still be found using the data sets of the remaining years. However, researching the 2010 data set revealed that even though events are found, they are of small magnitude and small in number.

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., Applied Geophysics and Petrophysics

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., Applied Geophysics and Petrophysics

Ambient-noise seismic interferometry (ANSI) applied to passive body-wave measurements retrieves an estimate of the reflection response as if from a source at a receiver position. Often, the limited compliance with theoretical assumptions causes erroneous absolute amplitudes of the retrieved physical reflections, and additional artefacts. Nevertheless, the retrieved reflection data may be further used for time-lapse interpretation, since the latter exploits relative amplitude differences. Here, we study the feasibility of applying ANSI to time-lapse passive seismic data to extract the time-lapse reflection signal produced by the exploitation of a reservoir. We base our study on the case of the demonstration site for CO2 storage at Ketzin, Germany. With numerical experiments, we apply ANSI to two passive datasets using a base and a repeat scenario (after velocity decreases in the CO2 reservoir) and modelled by random distributions of band-limited noise sources. We show that the retrieval of an unambiguous time-lapse signal is enabled by increased common illumination between the two datasets. Finally, we apply ANSI by auto-correlation to Ketzin field data and show the retrieval of responses consistent with modelled and active field data. We conclude that ANSI applied to field data has the potential for time-lapse differences extraction and interpretation., Geoscience & Engineering, Civil Engineering and Geosciences

Seismic interferometry applied to ambient-noise measurements allows the retrieval of the seismic response between pairs of receivers. We studied ambient-noise seismic interferometry (ANSI) to retrieve time-lapse reflection responses from a reservoir during CO2 geologic sequestration, using the case of the experimental site of Ketzin, Germany. We applied ANSI to numerically modeled data to retrieve base and repeat reflection responses characterizing the impedances occurring at the reservoir both with and without the injection of CO2. The modeled data represented global transmission responses from band-limited noise sources randomly triggered in space and time. We found that strong constraints on the spatial distribution of the passive sources were not required to retrieve the time-lapse signal as long as sufficient source-location repeatability was observed between the base and the repeat passive survey. To illustrate the potential of the technique, ANSI was applied to three days of passive field data recorded in 2012 at Ketzin. Comparison with the modeled results illustrated the potential to retrieve key reflection events using ANSI on field data from Ketzin. This study supports the idea that the geologic setting and characteristics of ambient noise at Ketzin may be opportune to monitor CO2 sequestration., Geoscience & Engineering, Civil Engineering and Geosciences

Geoscience & Engineering, Civil Engineering and Geosciences