1,721,001 research outputs found
Developments in ambient noise analysis for the characterization of dynamic response of slopes to seismic shaking
Full text linkIn the last few decades, we have witnessed a growing awareness of the role of site dynamic response to seismic shaking in slope failures during earthquakes. Considering the time and costs involved in acquiring accelerometer data on landslide prone slopes, the analysis of ambient noise offers a profitable investigative alternative. Standard procedures of ambient noise analysis, according to the technique known as HVNR or Nakamura’s method, were originally devised to interpret data under simple site conditions similar to 1D layering (flat horizontal layering infinitely extended). In such cases, conditions of site amplification, characterized by a strong impedance contrast between a soft surface layer and a stiff bedrock, result in a single pronounced isotropic maximum of spectral ratios between horizontal and vertical component of ambient noise. However, previous studies have shown that the dynamic response of slopes affected by landslides is rather complex, being characterized by multiple resonance peaks with directional variability, thus, the use of standard techniques can encounter difficulties in providing reliable information. A new approach of data analysis has recently been proposed to exploit the potential of information content of Rayleigh waves present in ambient noise, with regard to the identification of frequency and orientation of directional resonance. By exploiting ground motion ellipticity this approach can also provide information on vertical distribution of S-wave velocity, which controls site amplification factors. The method, based on the identification of Rayleigh wave packets from instantaneous polarization properties of ambient noise, was first tested using synthetic signals in order to optimize the data processing system. Then the improved processing scheme is adopted to re-process and re-interpret the ambient noise data acquired on landslide prone slopes around Caramanico Terme (central Italy), at sites monitored also with accelerometer stations. The comparison of ambient noise analysis results with the outcomes of accelerometer monitoring reveals potential and limits of the new method for the investigations on slope dynamic response
Using ambient noise to characterise seismic slope response: The case of Qiaozhuang peri-urban hillslopes (Sichuan, China)
Full text linkThe Mw 7.9 Wenchuan earthquake of May 12th, 2008 shattered and induced failures on hillslopes surrounding the centre of the town of Qiaozhuang, located 250 Km NE of the mainshock epicentre. This motivated investigations on the possible occurrence of site amplification phenomena. The initial efforts involved a temporary accelerometer monitoring carried out between April and October 2009 on Weigan hill and Mount Dong, located SW and NE of the Qiaozhuang centre, respectively. The monitoring results revealed that the local geological setting, characterised by Silurian phyllites with sub-vertical schistosity (at Weigan) and by fractured Precambrian limestones (at Dong), exert major influence on the slopes resonance phenomena, with a secondary contribution due to the topographic setting. To extend the investigation on local slope response, a series of ambient noise recordings were conducted at several sites of the two hills, as well as at Mount Shizi (located NW of the town centre), which is topographically and geologically similar to Mount Dong. The focus was on the sites monitored by accelerometer stations, whose seismic records provided the opportunity to validate the outcomes of ambient noise analysis. Noise data were analysed using two approaches: the standard Nakamura's method and a new technique based on analysis of instantaneous polarization properties, aimed at estimating ellipticity of Rayleigh waves. Data interpretation was hampered by a contingent factor, i.e. environmental conditions characterised by a strong persistent E-W polarized noise at low frequencies (below 1 Hz), and by the complex of geologic and geomorphic conditions. The latter, particularly in the case of Weigan hill, appear responsible for a considerable amplification of vertical ground motions. The repeated recordings and the comparison of the outcomes of the two techniques of noise analysis with the accelerometer data showed that the new technique provides more stable and consistent results. Furthermore, the new technique was able to reveal site resonance properties that Nakamura's method fails to recognise because of transient overlapping of different type waves. Overall, the ambient noise analysis provided evidence of i) low frequency resonance (~1 Hz) acting at the scale of the entire relief (on Weigan Hill), ii) directional amplifications at intermediate frequencies (~4 Hz) affecting ridge crests made of fractured limestone (on Mounts Dong and Shizi), and iii) higher frequency resonances (7–20 Hz), which vary at a very local scale. This information is relevant for inferring the susceptibility of local slopes to earthquake-induced failures, and hence for the collateral seismic hazard assessment in the Qiaozhuan peri-urban areas
Seismic response of steep slopes inferred from ambient noise and accelerometer recordings: the case of Dadu River valley, China
Full text linkSeismic site effects (local topographic / lithologic amplification, directivity phenomena) can have substantial impact on slope dynamic response and susceptibility to earthquake-induced failures. However, the instrumental data needed to quantify the site effects on slopes are still scarce. Here we investigate dynamic response of steep slopes in the Dadu River valley (Sichuan Province), one of the China’s regions most struck by large magnitude earthquakes. We analyse ambient noise recordings acquired at the sites of a local accelerometer network established few years after the 2008 Wenchuan earthquake. The analysis benefits from the application of a new technique that determines the ellipticity of Rayleigh waves present in noise wavefield from the instantaneous polarization analysis of the recorded signals. The technique is adapted for application to steep surface by rotating the noise recordings according to a reference having as axes the slope down-dip direction, the slope surface strike and the normal to the slope surface. This allows obtaining the curve of Rayleigh wave ellipticity as function of frequency, whose maxima provide information on site response characteristics, and leads to a more reliable identification of resonance phenomena on steep slopes (40° inclination). The results of the ambient noise analysis are consistent with the available accelerometer data and provide clear indication of significant resonance effects, with regard to their potential impact on local seismic slope stability. In particular, the slopes around the confluence of a local torrent with the Dadu River near the Lengzhuguan village exhibit strong site amplifications related to the presence of surficial deposits overlying intensely fractured granite bedrock. The frequency, amplitude and direction of resonance phenomena derived from the analysis of instrumental data indicate that site amplification is likely to have lesser impact on the stability of the lower part of the Mt. Dagang, which forms a long steep sub-planar slope facing the Dadu River. Our findings demonstrate that expeditious ambient noise recordings can provide useful information on relative susceptibility of slopes to seismically induced failures
Using multi-temporal interferometry and Sentinel-1 data to monitor ground instability hazards related to open-cast mining operations
No full textSurface mining represents the predominant mineral and coal exploitation method in Europe and worldwide. Different mining methods can be involved, but most often: open-cast (open-pit) mining; terrace mining; contour strip mining; quarrying. Although surface mining is considered safer than underground mining, ground instability hazards represent an ever-present concern and important problem limiting the mining operations (e.g., slope instability in high wall open-cast mines and quarries). However, given the often large extent of areas affected by surface mining and life span of mines (tens of years), long-term monitoring via traditional in-situ methods is typically impractical (economically and technically). Here we focus on the use of an advanced multi-temporal interferomery (MTI) and Sentinel-1 imagery for mapping and monitoring of ground instabilities in open-cast mines and adjacent areas. Open-cast mines represent a good target for MTI, because they are i) often very large (from few to tens of km2); ii) free of or covered by sparse vegetation; iii) require long-term regular monitoring, which can now rely on free of charge Sentinel-1 data from the European Space Agency. However, a cursory review of the recent literature (international journals) suggests that in comparison to applications to underground mines, MTI has been relatively little exploited to investigate ground instabilities related to surface mining. One reason for this is that some portions of open-cast mines can lack measurable radar targets due to rapid changes of ground surface caused by intense mining operations (e.g., overburden stripping, waste material damping). We argue that this limitation can now be mitigated by the higher frequency and regularity of acquisitions provided by Sentinel-1 (nominally every 6 days since October 2016). To illustrate the potential of MTI for detecting and monitoring ground instabilities induced by surface mining, we present case study example of the Belchatow mine (Poland), one of the largest brown coal open-cast operations in Europe (about 12 km long and 3 km wide). The Belchatow open-cast mine reaches the depth of 310 m and has been affected by a number of slope failures in the last few decades. The failures disrupted the mining operations, destroyed in part the mining machinery and resulted in high economic losses. In this study we assess the recent and current stability conditions in the mine area by exploiting several tens of medium resolution (~20 m) images acquired by Sentinel-1 since October 2014. Despite the local lack of information (areas affected by intensive surface disturbance due to ongoing mining operations), the MTI results provided a valuable synoptic overview of the ground instability/stability condition within and outside the active mine. Although it is not simple to provide short-term forewarning of the impeding slope failure on the basis of the surface displacement time series alone, our experience indicate that long-term, high frequency MTI measurements offer the following benefits: i) better anticipation of risk of failure over large areas and more rational design of ground-based monitoring networks; ii) better planning for maintenance and management of open-cast mines
Advanced satellite radar interferometry for deformation monitoring and infrastructure control in open-cast mines and oil/gas fields
No full textWe focus on the use of advanced multi-temporal interferometry (MTI) for mapping and monitoring of ground deformations caused by open-cast mining and hydrocarbon production. We also show how MTI can be exploited to monitor the stability of infrastructure in adjacent areas. Open-cast mines represent a good target for MTI, because they are (1) often very large (from few to tens of km2); (2) free of or covered by sparse vegetation; (3) require long-term regular monitoring. The operational deformation monitoring via MTI can now rely on free of charge medium-resolution Sentinel-1 data, consistently and regularly acquired by the European Space Agency (ESA) since 2014. To illustrate the application potential of MTI based on Sentinel-1 data, we present the case study of the Belchatow mine (Poland), one of the largest open-cast mines in Europe. We stress that thanks to wide-area coverage; space-borne MTI represents a cost-effective approach to monitoring ground/slope instability hazards in large open pits, as well as the stability of the associated engineering structures and facilities. On-land oil and gas fields are also often huge and ground deformations induced by their exploitations can be profitably targeted by MTI. This is illustrated through an example of MTI application from the Middle East that relies on high-resolution (3 m) radar data. The example highlights the possibility of obtaining extremely dense (spatially continuous) information, which is important for monitoring complex ground deformations caused by oil field exploitation
Multitemporal analysis of ambient noise polarization to characterize site response in the town of Amatrice, shattered by the 2016 central Italy earthquake
Full text linkThe Mw 6.0 earthquake that hit central Italy on 24 August 2016 caused an abnormally high level of destruction in the town of Amatrice. In order to clarify the role of site response in causing such a disaster, a series of ambient noise recordings acquired in the aftermath of the event are analysed here to identify site resonance properties from the ratios H/V between horizontal and vertical amplitudes of ground motion. Although the noise data acquisition was limited by the emergency management activities, the use of a new analysis technique, which isolates the contribution of Rayleigh waves to the noise wavefield and averages instantaneous estimates of H/V ratios, provided more stable results compared to the standard Nakamura's technique based on mean spectral ratios. The results demonstrated the occurrence of significant resonance phenomena, but without an obvious correlation with the spatial distribution of damage severity. It is apparent that the damage severity was also influenced by some additional local factors related to building vulnerability. Moreover, the time series analysis revealed seasonal variations in the Rayleigh wave ellipticity curves likely related to the water content changes in the surface deposits and their influence on the Poisson coefficient. Finally, the new method proved capable of recognizing time-varying directions of Rayleigh wave propagation. This capability could be exploited to support other passive seismic methods (e.g. ReMi), whose results reliability is limited by the lack of control on wave direction origin
Passive seismic techniques for the assessment of dynamic slope stability conditions
No full textSlope stability analysis in seismically active areas must take into consideration the influence of site response during earthquakes. While widespread accelerome-ter monitoring of slopes appears impractical, it is possible to derive information relevant for stability conditions from passive seismic techniques based on the ac-quisition and analysis of ambient noise generated by natural and anthropic sources. A useful technique is Nakamura’s method, which consists in analysing ratios between horizontal (H) and vertical (V) component spectra of noise record-ings. An analysis of azimuthal variation of H/V can reveal directional resonance phenomena affecting slope areas, providing the frequency and direction of max-imum ground motion amplification. Determination of site response properties from seismic noise mainly depends on the identification of polarisation direction and ellipticity of Rayleigh waves. However, in ambient noise records only part of acquired signals may show coherent characteristics referable to Rayleigh waves. Thus, it is useful to develop techniques that allow selecting, within noise record-ings, wave trains that can be reliably identified as Rayleigh waves. A new promis-ing technique is based on the identification of instantaneous polarisation proper-ties from analytical signal transformation. The study of slope dynamic response to shaking can also benefit from the analysis of cross-correlation among simulta-neous noise recordings. This type of analysis provides dispersion curves for Ray-leigh waves, which can then be used to constrain S-wave velocity models and, consequently, to infer mechanical properties of slope materials
Remote and Terrestrial Ground Monitoring Techniques Integration for Hazard Assessment and Prediction in Densely Populated Mountain Areas
No full textGoing 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
- …
