Application of multichannel Wiener filters to the suppression of ambient seismic noise in passive seismic arrays

We are concerned with the detection and location of small seismic events, such as can be encountered in monitoring hydro-fracturing with surface sensors. Ambient seismic noise is the main problem in detection of weak seismic phases from these events, particularly as the sites of interest are often within or near producing fields. Band-pass filtering and stacking are the most widely used techniques for enhancing the signal-to-noise ratio (SNR) in passive seismic experiments, but they are of limited value when noise and signal share the same frequency band. Seismic arrays can be used to reduce the unwanted noise (e.g., traffic noise, pumping noise, scattering ground roll) by delay-and-sum techniques (also called beamforming) or by frequencywavenumber filtering. Beamforming maximizes the array response for the assumed direction and slowness of the signal. Whereas in some situations it can be highly effective, and the azimuth and slowness of the signal can be determined by a grid search approach, it is vulnerable to contamination by side-lobe energy, particularly for broadband signals and noise (Rost and Thomas, 2002). Frequencywavenumber filtering can be very effective but requires regularly spaced arrays and implicitly assumes plane-wave propagation. Both methods perform poorly when the waveform changes significantly between stations of the array, as might be caused, for example, by differences in site response. In this article, we present a multichannel Wiener filtering technique, which allows the removal of coherent noise from three-component 2D arrays without making a priori assumptions about the mode of propagation (e.g., no planewave assumption is required for the noise field). We test the effectiveness of this filter with two case studies. In the first case, we add synthetic signals of varying strengths to actual noise data recorded with a hexagonal array during hydrofracturing within a producing oil field in Wyoming, USA. Using this test, we are able to provide estimates of the smallest event detectable with the filtered data, and compare the results with conventional techniques, such as stacking. The second test case is a dense, small-aperture 2D seismic array of 95 stations placed within an area of 130 m 56 m on a landslide deposit in the Northern Apennines, Italy. Numerous microearthquakes have been recorded with this array, whose faint P-phases serve as an ideal data set for testing filtering techniques. Using the two case studies, we discuss the effectiveness of the multichannel Wiener filter on SNR improvement, and show that including horizontal components into the analysis increases the SNR improvement more than using only vertical components

Dense seismic network provides new insight into the 2007 Upptyppingar dyke intrusion

Factors such as network geometry, network size and phase-picking accuracy have significant\ud effects on the precision of seismic hypocentre locations. In turn, the precision of the hypocentral locations\ud dictates the degree to which morphological details within seismic swarms may be resolved. The Icelandic\ud national seismic network (SIL) is designed to monitor seismic activity across large expanses of Iceland in realtime\ud using automated earthquake detection and location software. Here we examine the performance of the\ud SIL network relative to a much denser, local network of seismometers deployed around the Askja volcano in\ud the Northern Volcanic Zone. A subset of earthquakes from the 2007–2008 dyke intrusion beneath Mt. Upptyppingar\ud is used to compare single- and multi-event hypocentral locations. Specifically, we highlight 288, high\ud signal-to-noise ratio events that occurred during an intensive sequence of earthquakes from 6–24 July 2007,\ud when the temporary Askja network was active. A careful refinement of phase onsets recorded by our wellconfigured,\ud dense network of receivers reveals hypocentres clustered tightly on a planar structure, interpreted\ud as a dyke dipping at 49. The root-mean-square (RMS) misfit to the plane (114 m) is only slightly greater than\ud the uncertainties in relative locations of the earthquakes themselves, and constitutes a three-fold reduction in\ud RMS misfit over SIL relative locations. The improved precision, facilitated predominantly by a more favourable\ud network size and configuration, permits a more detailed analysis of the intrusion

Imaging Torfajökull's Magmatic Plumbing System With Seismic Interferometry and Phase Velocity Surface Wave Tomography

Torfajökull volcano, Iceland, has not erupted since 1477. However, intense geothermal activity, deformation, and seismicity suggest a long-lasting magmatic system. In this paper, we use ambient noise tomography to image the magmatic system beneath Torfajökull volcano. One hundred days of ambient noise data from 23 broadband seismometers show the consistent presence of double-frequency microseism noise with significant power between ∼0.1 and 0.5 Hz. Beamforming results indicate microseism noise with persistent higher energy propagating from west and SE directions and apparent velocities below 3 km/s. We use ambient noise seismic interferometry to retrieve Rayleigh waves, and we introduce a method to estimate the reliability of the retrieved surface waves. We find stable estimation of surface wave phase velocities between 0.16 and 0.38 Hz. Azimuthal velocity variations show a trend of higher velocities in the NE/SW direction, the strike of the rift zone intersecting Torfajökull, and orientation of erupted lavas on a NE-SW fissure swarm. Tomographic results indicate low-velocity anomalies beneath the volcano caldera (between −5% and −10%) and even lower velocity variations in the southeast and southwest study area (below −10%), outside the volcano caldera. Low anomalies may indicate the existence of hot material, more prominent outside the caldera outskirts. High-velocity variations (between 5% and 10%) outline the volcano caldera between 4- and 5-km depth and more pronounced velocities (between 10% and 15%) up to 5-km depth in the north of the volcano caldera. We interpret the former as possible caldera collapse structure and the latest as solidified intrusive magma from the old preferred magma paths

Strike-slip faulting during the 2014 Bároarbunga-Holuhraun dike intrusion, central Iceland

Over a 13 day period magma propagated laterally from the subglacial Bárðarbunga volcano in the northern rift zone, Iceland. It created > 30,000 earthquakes at 5–7 km depth along a 48 km path before erupting on 29 August 2014. The seismicity, which tracked the dike propagation, advanced in short bursts at 0.3–4.7 km/h separated by pauses of up to 81 h. During each surge forward, seismicity behind the dike tip dropped. Moment tensor solutions from the leading edge show exclusively left-lateral strike-slip faulting subparallel to the advancing dike tip, releasing accumulated strain deficit in the brittle layer of the rift zone. Behind the leading edge, both left- and right-lateral strike-slip earthquakes are observed. The lack of non-double-couple earthquakes implies that the dike opening was aseismic.Seismometers were borrowed from the Natural Environment Research Council (NERC) SEIS-UK (loans 968 and 1022),with funding by research grants from the NERC and the European Community’s Seventh Framework Programme grant 308377 (Project FUTUREVOLC), and graduate studentships from the NERC and Shell. We thank Ágúst Þór Gunnlaugsson and others who assisted with fieldwork in Iceland and Nigel Woodcock for his helpful discussions. M.T. Gudmundsson, H. Reynolds, and Þ. Högnadóttir supplied ice cauldron coordinates. The Icelandic Meteorological Office, Chris Bean (University College Dublin), and the British Geological Survey kindly provided additional data from seismometers in northeast Iceland, data delivery from IMO seismic database 20151001/01. We thank the two anonymous reviewers for their constructive comments. Hypocenter locations in Figure 1 are listed in Tables S2 and S3. (Department of Earth Sciences, Cambridge contribution ESC3539).This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/2015GL06742

Rapid ascent of rhyolitic magma at Chaitén volcano, Chile

International audienceAlthough rhyolite magma has fuelled some of the Earth's largest explosive volcanic eruptions, our understanding of these events is incomplete due to the previous lack of direct observation of these eruptions. On 1 May 2008, Chaitén volcano in Chile erupted rhyolite magma unexpectedly and explosively. Here, petrological and experimental data are presented that indicate that the hydrous rhyolite magma at Chaitén ascended very rapidly from storage depth to near-surface, with velocities of the order of one metre per second

Motion in the north Iceland volcanic rift zone accommodated by bookshelf faulting

Along mid-ocean ridges the extending crust is segmented1 on length scales of 10–1,000 km. Where rift segments are offset from one another, motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis. Where segments overlap, non-transform offsets with a variety of geometries2 accommodate shear motions. Here we use micro-seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north Iceland. Between the rift segments, we identify a series of faults that are aligned sub-parallel to the orientation of the main rift. These faults slip through left-lateral strike-slip motion. Yet, movement between the overlapping rift segments is through right-lateral motion. Together, these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf-faulting mechanism, named after its resemblance to a tilting row of books on a shelf3. The faults probably reactivated existing crustal weaknesses, such as dyke intrusions, that were originally oriented parallel to the main rift and have since rotated about 15° clockwise. Reactivation of pre-existing, rift-parallel weaknesses contrasts with typical mid-ocean ridge transform faults and is an important illustration of a non-transform offset accommodating shear motion between overlapping rift segments

Montserrat, Karibian kadonnut paratiisi (Montserrat, paradise lost in Caribbean)

Geodynamics geophysics and tectonics; Petrology igneous metamorphic and volcanic studies

Dense seismic network provides new insight into the 2007 Upptyppingar dyke intrusion

Factors such as network geometry, network size and phase-picking accuracy have significant effects on the precision of seismic hypocentre locations. In turn, the precision of the hypocentral locations dictates the degree to which morphological details within seismic swarms may be resolved. The Icelandic national seismic network (SIL) is designed to monitor seismic activity across large expanses of Iceland in realtime using automated earthquake detection and location software. Here we examine the performance of the SIL network relative to a much denser, local network of seismometers deployed around the Askja volcano in the Northern Volcanic Zone. A subset of earthquakes from the 2007–2008 dyke intrusion beneath Mt. Upptyppingar is used to compare single- and multi-event hypocentral locations. Specifically, we highlight 288, high signal-to-noise ratio events that occurred during an intensive sequence of earthquakes from 6–24 July 2007, when the temporary Askja network was active. A careful refinement of phase onsets recorded by our wellconfigured, dense network of receivers reveals hypocentres clustered tightly on a planar structure, interpreted as a dyke dipping at 49. The root-mean-square (RMS) misfit to the plane (114 m) is only slightly greater than the uncertainties in relative locations of the earthquakes themselves, and constitutes a three-fold reduction in RMS misfit over SIL relative locations. The improved precision, facilitated predominantly by a more favourable network size and configuration, permits a more detailed analysis of the intrusion

Paradiset som försvann (The lost paradise)

Geodynamics geophysics and tectonics; Petrology igneous metamorphic and volcanic studies