TY - JOUR A1 - Karamzadeh, Nasim Toularoud A1 - Kühn, Daniela A1 - Kriegerowski, Marius A1 - López-Comino, José Ángel A1 - Cesca, Simone A1 - Dahm, Torsten T1 - Small-aperture array as a tool to monitor fluid injection- and extraction-induced microseismicity BT - applications and recommendations JF - Acta Geophysica N2 - The monitoring of microseismicity during temporary human activities such as fluid injections for hydrofracturing, hydrothermal stimulations or wastewater disposal is a difficult task. The seismic stations often cannot be installed on hard rock, and at quiet places, noise is strongly increased during the operation itself and the installation of sensors in deep wells is costly and often not feasible. The combination of small-aperture seismic arrays with shallow borehole sensors offers a solution. We tested this monitoring approach at two different sites: (1) accompanying a fracking experiment in sedimentary shale at 4km depth and (2) above a gas field under depletion. The small-aperture arrays were planned according to theoretical wavenumber studies combined with simulations considering the local noise conditions. We compared array recordings with recordings available from shallow borehole sensors and give examples of detection and location performance. Although the high-frequency noise on the 50-m-deep borehole sensors was smaller compared to the surface noise before the injection experiment, the signals were highly contaminated during injection by the pumping activities. Therefore, a set of three small-aperture arrays at different azimuths was more suited to detect small events, since noise recorded on these arrays is uncorrelated with each other. Further, we developed recommendations for the adaptation of the monitoring concept to other sites experiencing induced seismicity. KW - Microseismic monitoring KW - Induced seismicity KW - Array seismology KW - Shallow borehole sensors Y1 - 2019 U6 - https://doi.org/10.1007/s11600-018-0231-1 SN - 1895-6572 SN - 1895-7455 VL - 67 IS - 1 SP - 311 EP - 326 PB - Springer CY - Cham ER - TY - JOUR A1 - Endrun, Brigitte T1 - Love wave contribution to the ambient vibration H/V amplitude peak observed with array measurements JF - Journal of seismology N2 - This study applies array methods to measure the relative proportions of Love and Rayleigh waves in the ambient vibration wavefield. Information on these properties is of special relevance for frequencies around the horizontal-to-vertical (H/V) spectral amplitude ratio peak. The analysis of H/V curves, a popular technique in site characterisation, commonly assumes that the curves represent the frequency-dependent Rayleigh wave ellipticity. For the detailed interpretation of amplitudes or the inversion of the curves, it is therefore necessary to estimate and correct for the contribution of other wave types to the ambient vibration wavefield. I use available ambient vibration array measurements to determine the relative amount of Love and Rayleigh waves on the horizontal components by frequency-dependent analysis of the main propagation and polarisation directions, with a special emphasis on the H/V peak frequency as determined from the same recordings. Tests with synthetic data demonstrate the feasibility of this approach, at least in the presence of dominant source regions. Analysis of the data from 12 measurements at nine European sites, which include shallow as well as deep locations that span a wide range of impedance contrasts at the sediment-bedrock interface, indicates that the relative contribution of Rayleigh waves varies widely with frequency, from close to 0% to more than 70%. While most data sets show relative Rayleigh wave contributions between 40% and 50% around the H/V peak, there are also examples where Love waves clearly dominate the wavefield at the H/V peak, even for a site with a low impedance contrast. Longer-term measurements at one site indicate temporal variations in the relative Rayleigh wave content between day- and nighttime. Results calculated with the method introduced herein generally compare well with results of modified spatial autocorrelation analysis. These two methods might be used in a complimentary fashion, as both rely on different properties of the ambient vibration wavefield. This study illustrates that it is possible to measure the relative Rayleigh wave content of the noise wavefield from array data. Furthermore, the examples presented herein indicate it is important to estimate this property, as the assumption that there are an equal proportion of Love and Rayleigh waves is not always correct. KW - Ambient vibrations KW - Surface waves KW - Array seismology KW - Polarisation analysis KW - H/V spectral ratio KW - Site characterisation Y1 - 2011 U6 - https://doi.org/10.1007/s10950-010-9191-x SN - 1383-4649 VL - 15 IS - 3 SP - 443 EP - 472 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Hiemer, Stefan A1 - Rößler, Dirk A1 - Scherbaum, Frank T1 - Monitoring the West Bohemian earthquake swarm in 2008/2009 by a temporary small-aperture seismic array JF - Journal of seismology N2 - The most recent intense earthquake swarm in West Bohemia lasted from 6 October 2008 to January 2009. Starting 12 days after the onset, the University of Potsdam monitored the swarm by a temporary small-aperture seismic array at 10 km epicentral distance. The purpose of the installation was a complete monitoring of the swarm including micro-earthquakes (M (L) < 0). We identify earthquakes using a conventional short-term average/long-term average trigger combined with sliding-window frequency-wavenumber and polarisation analyses. The resulting earthquake catalogue consists of 14,530 earthquakes between 19 October 2008 and 18 March 2009 with magnitudes in the range of -aEuro parts per thousand 1.2 a parts per thousand currency signaEuro parts per thousand M (L) a parts per thousand currency signaEuro parts per thousand 2.7. The small-aperture seismic array substantially lowers the detection threshold to about M (c) = -aEuro parts per thousand 0.4, when compared to the regional networks operating in West Bohemia (M (c) > 0.0). In the course of this work, the main temporal features (frequency-magnitude distribution, propagation of back azimuth and horizontal slowness, occurrence rate of aftershock sequences and interevent-time distribution) of the recent 2008/2009 earthquake swarm are presented and discussed. Temporal changes of the coefficient of variation (based on interevent times) suggest that the swarm earthquake activity of the 2008/2009 swarm terminates by 12 January 2009. During the main phase in our studied swarm period after 19 October, the b value of the Gutenberg-Richter relation decreases from 1.2 to 0.8. This trend is also reflected in the power-law behavior of the seismic moment release. The corresponding total seismic moment release of 1.02x10(17) Nm is equivalent to M (L,max) = 5.4. KW - Swarm earthquakes KW - West Bohemia KW - Array seismology KW - Frequency-magnitude distribution Y1 - 2012 U6 - https://doi.org/10.1007/s10950-011-9256-5 SN - 1383-4649 VL - 16 IS - 2 SP - 169 EP - 182 PB - Springer CY - Dordrecht ER -