1,721,021 research outputs found
Full waveform seismological advances for microseismic monitoring
No full textThe observation of microseismicity has raised the interest of the seismological and geoengineering communities in the last decades, and a significant effort has been spent to develop or improve methodologies able to perform seismological analysis for weak events at a local scale. This effort was accompanied by the improvement of monitoring systems, resulting in an increasing number of large microseismicity catalogs. The interest in microseismicity, involving a synergy among different scientific communities, is in part due to their occurrence both in consequence of natural processes in active regions, swarm areas, hydrothermal and volcanic environments, but also in relation to human activities, e.g., in proximity of mining areas, geothermal systems, oil and gas fields, and water reservoirs. The analysis of microseismicity is challenging, because of the low amplitude and high-frequency content of recorded seismic signals. Whereas many techniques only rely on a minor information provided by microseismicity data, full waveform recordings contain a broad information on the physical processes at the microseismic source as well as on the properties of the surrounding media. This paper reviews recent methodological developments in seismology, which exploit full waveform recordings, upon the analysis of their amplitude, frequency, duration, and polarization properties or the direct modeling of their waveforms, to infer microseismic source properties. We specifically review recent advances targeting the problem of the detection of weak microseismic signals, the automated location using full waveforms, the determination of source parameters through focal mechanisms, moment tensor, and finite source inversion, and the classification of microseismicity waveforms and their attributes. A particular care is given to the assessment of the automation potential of these methods, which is a requirement toward the analysis of massive microseismicity data sets. The review discusses the potential and limitations of many developed techniques, and highlights recent promising ideas, which can significantly contribute toward a better understanding of microseismic processes in the next future
Automated seismic event location by travel-time stacking: An application to mining induced seismicity
No full textThe sheer approach to shale gas exploration and exploitation associated risks
No full textThe abstract presents the SHEER project that primarily aims at assessing the environmental impacts of shale gas extraction and exploration, and secondly, at developing best practices to reduce its environmental footprint. The description of SHEER database comprising multidisciplinary data concerning the shale gas exploitation test sites, processing procedures, results of data interpretation and recommendation as well as other documents describing the state of the art is included. One of the test sites is the real-time research monitoring field at Wysin, Pomerania, where the hydraulic fracturing took place. The seismic monitoring main aim was to detect microseismic events at the fracturing site and therefore contribute to a better characterization of the induced seismicity related to the shale gas extraction Finally, a collaboration between SHEER and IS-EPOS project is presented within the Thematic Core Services-Anthropogenic Hazard e-Platform
A complex linear least-squares method to derive relative and absolute orientations of seismic sensors
No full textDetermining the relative orientation of the horizontal components of seismic sensors is a common problem that limits data analysis and interpretation for several acquisition setups, including linear arrays of geophones deployed in borehole installations or ocean bottom seismometers deployed at the seafloor. To solve this problem we propose a new inversion method based on a complex linear algebra approach. Relative orientation angles are retrieved by minimizing, in a least-squares sense, the l 2-norm between the complex traces (hodograms) of adjacent pairs of sensors. This methodology can be applied without restrictions only if the wavefield recorded by each pair of sensors is very similar. In most cases, it is possible to satisfy this condition by low-pass filtering the recorded waveforms. The main advantage of our methodology is that, in the complex domain, the relative orientations of seismic sensors can be viewed as a linear inverse problem, which ensures that the preferred solution corresponds to the global minimum of a misfit function. It is also possible to use simultaneously more than one independent data set (other seismic events) to better constrain the solution of the inverse problem. Furthermore, by a computational point of view, our method results faster than the relative orientation methods based on waveform cross-correlation. After several tests on synthetic data sets we applied successfully our methodology to different types of real data. These applications include the alignment of borehole sensors relative to a Vertical Seismic Profiling (VSP) acquisition and the orientation of Ocean Bottom Seismometers (OBS) relative to a neighbouring land station of known orientation. Using land stations, the absolute orientation of OBS can be retrieved. Finally, as a last application, we checked the correct orientation for land stations of a seismological array in Germany. © 2012 The Authors Geophysical Journal International © 2012 RAS
Misalignment angle correction of borehole seismic sensors: The case study of the Collalto seismic Network
No full textOne of the most critical problems affecting seismological data acquisition is related to possible misorientation of three-component seismic sensors. This generally happens when their orientation cannot be measured directly, as in the case of sensors deployed in boreholes. We describe here the sensor reorientation procedure of the Collalto Seismic Network, a microseismic monitoring network located in northeastern Italy that consists of 10 broadband three-component stations deployed in boreholes. We apply a procedure based on the misfit minimization of a complex trace recorded by a given station with respect to a reference station for which orientation is known. The main advantage of this methodology is that the reorientation of seismic sensors can be viewed as a linear inverse problem in the complex domain, which ensures that the preferred solution corresponds to the global minimum of a misfit function. Furthermore, it is also possible to simultaneously use more than one seismic event to better constrain the solution of the inverse problem. In this article, we further compare the orientation results obtained for a seismometer-seismometer sensor pair with those obtained using an accelerometer-seismometer sensor pair. We finally show the reorientation results for all the stations of the network, obtained using eight teleseismic events that occurred between January 2012 and May 2014
Automated seismic event location by waveform coherence analysis
No full textAutomated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring. Since microseismic records are generally characterized by low signal-to-noise ratio, automated locationmethods are requested to be noise robust and sufficiently accurate. Most of the standard automated location routines are based on the automated picking, identification and association of the first arrivals of P and S waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases. Although current methods can accurately pick P onsets, the automatic picking of the S onset is still problematic, especially when the P coda overlaps the S wave onset. In this paper, we propose a picking free earthquake location method based on the use of the short-term-average/long-term-average (STA/LTA) traces at different stations as observed data. For the P phases, we use the STA/LTA traces of the vertical energy function, whereas for the S phases, we use the STA/LTA traces of a second characteristic function, which is obtained using the principal component analysis technique. In order to locate the seismic event, we scan the space of possible hypocentral locations and origin times, and stack the STA/LTA traces along the theoretical arrival time surface for both P and S phases. Iterating this procedure on a 3-D grid, we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial coordinates of the seismic event. A pilot application was performed in the Campania-Lucania region (southern Italy) using a seismic network (Irpinia Seismic Network) with an aperture of about 150 km. We located 196 crustal earthquakes (depth < 20 km) with magnitude range 1.1 < ML < 2.7. A subset of these locations were compared with accurate manual locations refined by using a double-difference technique. Our results indicate a good agreement with manual locations. Moreover, our method is noise robust and performs better than classical location methods based on the automatic picking of the P and S waves first arrivals. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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