ALBERT

Description: Hydraulic-fracture induced microseismic events are usually small, and noise levels are high at the surface due to the activities associated with a producing oil field. Similarly, local arrays for the detection of local earthquakes will also benefit from reduced noise levels and detect smaller events. We present a frequency-dependent multi-channel Wiener filtering technique with linear constraints, which employs an adaptive least-squares technique to remove coherent noise in seismic array data. The noise records on a number of reference channels is used to predict the noise on a primary channel, which can then be subtracted. We implement and test first an unconstrained version of this filter, where maximal noise suppression can lead to signal distortion. Two methods of imposing constraints are then introduced to achieve signal preservation. We test this technique with two case studies. First, synthetic signals are added to actual noise from a pilot deployment of a hexagonal array (9 three-component seismometers, approximate size 150 m × 150 m) in an oil field; noise levels are suppressed by up to 11 dB (at 1 - 6 Hz). Secondly we use natural seismicity recorded at a dense array (∼10 m spacing) in Italy where the application of the filter reduces the signal-to-noise ratio by more than 20 dB (at 8 - 15 Hz), using 35 stations. In both cases, the performance of the multi-channel Wiener filters is significantly better than stacking, especially at lower frequency where stacking does not help to suppress the coherent noise. The unconstrained version of the filter yielded the best improvement in the signal-to-noise ratio, but the constrained filter is useful when waveform distortion is not acceptable.

Description: Published

Description: v133-v141

Description: 1.11. TTC - Osservazioni e monitoraggio macrosismico del territorio nazionale

Description: JCR Journal

Description: open

Description: Seismic arrays for detection of small earthquakes benefit from array processing aimed at reducing noise levels. We present a frequency-dependent multichannel Wiener filtering (MCWF) technique, which employs an adaptive least-squares method to remove coherent noise in seismic array data. The noise records on a number of reference channels are used to predict the noise on a primary channel, which can then be subtracted from the observed data. A sequence of aftershocks caused by the Mw 6.1 21 February 2008 mainshock in Spitsbergen was recorded by the ARCES array in northern Norway. This aftershock sequence was filtered using the multichannel Wiener filters in both triggered and continuous modes. The Spitsbergen (SPITS) array, at a much closer distance to the source region, provides reliable reference information on the true number of detectable aftershocks. The conventional delay-and-sum beamforming combined with a band-pass filter could detect only 513 aftershocks with 181 false alarms, using a series of constraints comprised of signal-to-noise ratio, back azimuth, and slowness; the multichannel Wiener filtered results found 577 aftershocks with 165 false alarms using the same constraints. A complete automatic multichannel Wiener procedure is developed for event detection on continuous data. An appropriate signal-to-noise ratio threshold for aftershock detection of 2.7 is suggested. Compared to the beamforming method, the MCWF also reduces false alarms when detecting the same number of aftershocks.

Description: We present relatively relocated earthquake hypocentres for 〉1000 microearthquakes ( M L 〈 3) that occurred during the 2 weeks immediately prior to the 2010 March 20 fissure eruption at Fimmvörðuháls on the flank of Eyjafjallajökull volcano in Iceland. Our hypocentre locations lie predominantly in horizontally separated clusters spread over an area of 10 km 2 and approximately 4 km below sea level (5 km below the surface). Seismic activity in the final 4 d preceding the eruption extended to shallower levels 〈2 km below sea level and propagated to the surface at the Fimmvörðuháls eruption site on the day the eruption started. We demonstrate using synthetic data that the observed apparent ~1 km vertical elongation of seismic clusters is predominantly an artefact caused by only small errors (0.01–0.02 s) in arrival time data. Where the signal-to-noise ratio was sufficiently good to make subsample arrival time picks by cross-correlation of both P - and S -wave arrivals, the mean depth of 103 events in an individual cluster were constrained to 3.84 ± 0.06 km. Epicentral locations are significantly less vulnerable to arrival time errors than are depths for the seismic monitoring network we used. Within clusters of typically 100 recorded earthquakes, most of the arrivals exhibit similar waveforms and identical patterns of P -wave first-motion polarities across the entire monitoring network. The clusters of similar events comprise repetitive sources in the same location with the same orientations of failure, probably on the same rupture plane. The epicentral clustering and similarity of source mechanisms suggest that much of the seismicity was generated at approximately static constrictions to magma flow in an inflating sill complex. These constrictions may act as a form of valve in the country rock, which ruptures when the melt pressure exceeds a critical level, then reseals after a pulse of melt has passed through. This would generate recurring similar source mechanisms on the same weak fault plane as the connection between segments of the sill system is repeatedly refractured in the same location. We infer that the magmatic intrusion causing most of the seismicity was likely to be a laterally inflating complex of sills at about 4 km depth, with seismogenic pinch-points occurring between aseismic compartments of the sills, or between adjacent magma lobes as they inflated. During the final 4 d preceding the eruption onset between 22:30 and 23:30 UTC on 2010 March 20, the seismicity suggests that melt progressed upwards to a depth of ~2 km. This seismicity was probably caused by fracturing of the country rock at the margins of the propagating dyke. Subsequently, on the morning of the eruption a dyke propagated eastward from the region of precursory seismic activity to the Fimmvörðuháls eruption site.

Description: Earthquakes are commonly located by linearized inversion of discrete arrival time picks made from signals recorded at a network of seismic stations. If mis-picks are made, these will contribute to the location, therefore causing potential bias. For data recorded by a dense seismic array, direct imaging methods can be applied instead. We describe the ‘coalescence microseismic mapping’ method, which is a bridge between the two approaches and will operate with seismic data recorded continuously on a sparse array. By continuously mapping scalar signals derived from the envelope of seismic arrivals we derive robust estimates of the spatio-temporal coordinates of the origins of seismic events. Noisy data are migrated away from the correct origin, so do not contribute to errors in location. The method is rooted in a Bayesian formulation of event location traveltime inversion, allows imaging of source locations and has the capacity to handle errors in modelled traveltimes. It has the advantage of working with any 3-D velocity model, which therefore may include anisotropy. It also automatically incorporates both P - and S -wave data. A multiresolution grid search leads to an efficient implementation, with a search over a larger domain including joint inversion for location and velocity structure possible where warranted by the data quality. We discuss the theory and implementation of this method and illustrate it with real data from microseismic events in Iceland caused by melt intrusion in the crust.

Description: We suggest that carbon dioxide exsolved from a mid-crustal basaltic dyke intrusion in Iceland migrated upwards and triggered shallow seismicity by allowing failure on pre-existing fractures under the relatively low elastic stresses (100–200 kPa; 1–2 bar) generated by the dyke inflation. Intense swarms of microseismicity accompanied magmatic intrusion into a dyke at depths of 13–19 km in the crust of Iceland's Northern Volcanic Rift Zone during 2007–2008. Contemporaneously, a series of small normal earthquakes, probably triggered by elastic stresses imposed by the dyke intrusion, occurred in the uppermost 4 km of crust: fault plane solutions from these are consistent with failure along the extensional fabric and surface fissure directions mapped in the area, suggesting that the faults failed along existing rift zone fabric even though the mid-crustal dyke is highly oblique to it. Several months after the melt froze in the mid-crust and seismicity associated with the intrusion had ceased, an upsurge in shallow microseismicity began in the updip projection of the dyke near the brittle–ductile transition at 6–7 km depth below sea level. This seismicity is caused by failure on right-lateral strike-slip faults, with fault planes orientated 23 ± 3°, which are identical with the 24 ± 2° orientation in this area of surface fractures and fissures caused by plate spreading and extension of the volcanic rift zone. However, these earthquakes have T -axes approximately aligned with the opening direction of the dyke, and the right-lateral sense of failure is opposite that of regional strike-slip faults. We suggest that the fractures occurred along pre-existing weaknesses generated by the pervasive fabric of the rift zone, but that the dyke opening in the mid-crust beneath it caused right-lateral failure. The seismicity commenced after a temporal delay of several months and has persisted for over 3 yr. We propose that fluids exsolved from the magma in the dyke, primarily carbon dioxide, percolated updip and to shallower depths predominantly along pre-existing fractures. Increased pore pressure from the volatiles reduced the effective normal compressive stress on faults, increasing the likelihood of failure and allowing the modest stress changes generated by the intrusion to cause failure. Propagation of volatiles through the crust would also account for the observed time delay between the intrusion at depth and the shallow earthquake clusters. A further short-lived cluster of earthquakes at 2–4 km depth beneath the surface exhibits left-lateral strike-slip faulting with epicentres well orientated along a lineation which is identical with other subparallel strike-slip faults in the area that transfer motion between two adjacent spreading segments. These shallow earthquakes lie beyond lobes of significant positive Coulomb stress change caused by the intrusion, implying minimal modifications to the stress field in their vicinity; hence, they continue to respond to the regional stress field rather than the local stress field generated by the dyke intrusion.