Noise Spectral Ratio Research Articles

Soil-building resonance effect in the urban area of Villa d’Agri (Southern Italy)

This study aims to evaluate the possible occurrence during seismic shaking of interaction effects between the shallow subsoil and the overlying buildings of the Villa d’Agri town (Southern Italy). From the geological and building heritage points of view, the investigated area represents a typical town along the Apennines chain and therefore the obtained results could be easily exported to similar urban areas. A total of 239 single-station seismic ambient noise measurements were thus performed, 154 on the urban soils and 85 within buildings. Both subsets of measurements were planned to cover the main outcropping lithologies and building typologies. By means of the Horizontal-to-Vertical Noise Spectral Ratio technique, isofrequency and isoamplitude maps have been reconstructed for the whole area. The results evaluated on the 65 tested buildings allowed to infer the empirical relationship between the first vibrational period, T(s), and their height, H(m). Using this relationship and available information on the height of each building, the first vibrational frequency was estimated for 659 buildings belonging to the urban area of Villa d'Agri. Finally, in order to assess the possible occurrence of the double resonance phenomenon, we compared the main frequency of each building with that interpolated for the underlying foundation soil. The results show that the probability of being affected by the double resonance effect is very high only for a couple of buildings, high for about 16% of the buildings, moderate for the 23% and practically null for most buildings (60%).

Shear wave velocity and attenuation structure of Ischia island using broad band seismic noise records

In volcanic areas, accurate localization of earthquakes requires detailed velocity and, possibly, attenuation models, taking into account wide lithological variability and high geothermal gradients. Ischia island (Campania region, Italy) is a seismically-active volcano recently affected by a Mw 3.9 event (Casamicciola, August 21, 2017, 1 km depth). Due to the lack of a specific velocity model, the earthquakes occurred on the island were localized using the one developed for the nearby Campi Flegrei caldera. The aim of this work is the definition of a mean representative 1D shear-wave velocity (Vs) and attenuation (Q) model of the shallower crust (up to 2 km depth) of Ischia. Seismic noise array and spectral ratios techniques were applied to broad band seismic signals recorded by temporary and permanent networks updated after the August 2017 earthquake. The values of both shear-wave velocity (Vs) and quality factor (Q) are realistic, with Q values comparable with those obtained for Campi Flegrei and Stromboli volcanic areas. By taking into account stratigraphic information from deep wells and ultrasonic measurements of velocity on granite and trachytic lava samples, a geological interpretation of the resulting velocity model is provided. Such a model can have significant implications for understanding the dynamics of a volcano, mainly those leading to seismic activity.

Can broad-band earthquake site responses be predicted by the ambient noise spectral ratio? Insight from observations at two sedimentary basins

Site-effect assessments performed through earthquake-based approaches, such as the standard spectral ratio (SSR), require good quality records of numerous earthquakes. In contrast, the use of ambient noise appears to be an attractive solution for ease and rapid computation of site responses with sufficient spatial resolution (microzonation), especially in low seismicity areas. Two main approaches are tested here: the horizontal-to-vertical spectral ratio (HVSR) and the noise-based SSR (SSRn). The HVSR uses the relative amplitude of the horizontal and vertical components of the ambient noise. Instead, the SSRn defines the spectral ratio between the seismic noise recorded simultaneously at a site and at a rock reference station, similar to earthquake-based SSR. While the HVSR is currently used in hundreds of site-specific studies, the SSRn approach has been gradually abandoned since the 1990s. In this study, we compare the results obtain from these two approaches with those of earthquake-based SSR. This comparison is carried out for two sedimentary basins, in Provence (southeastern France) and in Argostoli (western Greece). In agreement with the literature, the HVSR does not provide more than the fundamental resonance frequency of the site (f0). The SSRn leads to overestimation of the SSR amplification factors for frequencies higher than the minimal f0 of the basin (f0min). This discrepancy between SSRn and SSR is discussed, and appears to be mainly dependent on the local geological configuration. We thus introduce the hybrid standard spectral ratio (SSRh) approach, which aims to improve upon the SSRn by adding an intermediate station inside the basin for which the SSR is known. This station is used in turn as a local reference inside the basin for the SSRn computation. The SSRh provides site transfer functions very similar to those of the SSR, in a broad frequency range. Based on these results, the SSRn (or SSRh) should be further tested and should receive renewed attention for microzonation inside sedimentary basins.

Integration of geological and geophysical data for re-evaluation of local seismic hazard and geological structure: the case study of Rometta, Sicily (Italy)

The village of Rometta, northern-eastern Sicily (Southern Italy), experienced severe damage during the most energetic earthquakes that occurred in eastern Sicily and southern Calabria in the last centuries. Geological maps indicate that Rometta primarily lies on a stiff plate of Upper Pliocene – Lower Pleistocene calcarenites and only to minor extent on Middle Pleistocene overlying clays. Rometta represents an interesting case study for site response investigation because of the apparent mismatch between the currently available geological knowledge and the level of damage caused by historical seismic events. The local seismic response has been investigated through a grid of 64 single-station measurements of ambient seismic noise by the Horizontal to Vertical Noise Spectral Ratio technique (H/V). Also, phase velocity dispersion curves from seismic array through the Extended Spatial Auto-Correlation method were analyzed. The H/V curves obtained show a ubiquitous frequency peak between 0.5 Hz and 0.9 Hz due to the deep interface between the metamorphic substrate and sedimentary sequence, and a secondary peak in the 2.5–15 Hz interval in most of the measurement points that may be related to a larger extent of the clay outcrops with respect to what is already known from geological investigations. Joint inversion of dispersion and H/V curves was also performed in order to obtain the velocity profile. Results acquired through the combined use of geophysical methods furnished useful information for seismic hazard evaluation where surface geology is not clearly visible because of urbanization and vegetation cover, thus suggesting that a wide extent of clays may reasonably be the cause of past earthquake damage distribution.

Assessment of multiple geophysical techniques for the characterization of municipal waste deposit sites

In this study, we tested the ability of geophysical methods to characterize a large technical landfill installed in a former sand quarry. The geophysical surveys specifically aimed at delimitating the deposit site horizontal extension, at estimating its thickness and at characterizing the waste material composition (the moisture content in the present case).The site delimitation was conducted with electromagnetic (in-phase and out-of-phase) and magnetic (vertical gradient and total field) methods that clearly showed the transition between the waste deposit and the host formation.Regarding waste deposit thickness evaluation, electrical resistivity tomography appeared inefficient on this particularly thick deposit site. Thus, we propose a combination of horizontal to vertical noise spectral ratio (HVNSR) and multichannel analysis of the surface waves (MASW), which successfully determined the approximate waste deposit thickness in our test landfill.However, ERT appeared to be an appropriate tool to characterize the moisture content of the waste, which is of prior information for the organic waste biodegradation process.The global multi-scale and multi-method geophysical survey offers precious information for site rehabilitation studies, water content mitigation processes for enhanced biodegradation or landfill mining operation planning.

Directional effects of tectonic fractures on ground motion site amplification from earthquake and ambient noise data: A case study in South Iceland

The geology of SW-Iceland is characterized by alternating basaltic lava units, hyaloclastite formations, postglacial sedimentary filled valleys and alluvial plains, as well as highly fractured bedrock within the Reykjanes Peninsula volcanic rift zone and the South-Iceland transform fault system. Historic earthquakes within this region reach magnitudes 6.5–7. Using earthquake and ambient noise recordings from 15 seismic stations within the rift and transform zones we compared wavefield polarization and seismic site response in order to assess characteristics of local amplification of ground motion. Ambient noise and earthquake ground motion spectral ratios are comparable in frequency and can qualitatively be subdivided into three groups: one with a spectral ratio characterized by a single predominant frequency of horizontal amplification, one with a bi- or multimodal and one characterized by a relatively constant amplitude across the frequency range. Seismic wavefield polarization within the transform zone has a prevailing direction of amplification towards 110°−150°N in the frequency range 1.0–3.0Hz, having a quasi-perpendicular relationship with mapped faults and fractures. Shear wave splitting results show that the wavefield polarization and fast S wave directions tend to be orthogonal, i.e. highly dependent on the anisotropy of the medium.

A microtremor survey to define the subsoil structure in a mud volcanoes area: the case study of Salinelle (Mt. Etna, Italy)

A microtremor survey aiming to obtain information on subsoil structure was performed in the lower southwestern Etnean flank (Sicily, Italy). The area is characterized by emissions of muddy and salty water which create specific pseudo-volcanic edifices. Recording sites were both randomly selected in the area and along two profiles. Horizontal-to-vertical noise spectral ratio, polarization and surface wave prospecting techniques were used. The spectral ratios results set into evidence different frequency ranges that appear interesting to infer information on the subsoil structure. The depth of the hydrocarbon reservoir was inferred through 1D modeling, by using the information coming from the literature and experimental surveys. Moreover, a strongly directional effect suggests the existence of a NNW-SSE-oriented mud feeding dike.

Selection of Time Windows in the Horizontal‐to‐Vertical Noise Spectral Ratio by Means of Cluster Analysis

The selection of the elementary analysis windows in continuous noise recordings for optimal estimation of the mean horizontal‐to‐vertical spectral ratio (HVSR) curve is generally performed by visual inspection of HVSR curves considered as functions of time. Starting from full‐length records, HVSR curves are determined in consecutive time windows of appropriate lengths. Time windows with HVSR curves that are anomalous on the basis of a simple visual inspection are generally ignored in the computation of the average HVSR curve. It is often very difficult to optimize the selection of time windows to be used for the calculation of the HVSR curve representative of a site. The use of nonobjective selection criteria produces results which depend on personal opinions of the operator and for which reliability cannot be assessed with quantitative parameters. We implemented an automatic procedure, based on cluster analysis, for the determination of the optimal window subset for the computation of the average HVSR curve. The procedure is based on the application of the agglomerative hierarchical clustering, using a measure of proximity of the standard correlation between HVSR curves and, as a rule for merging clusters, the average linking criterion. The procedure has been applied to 814 measures of seismic noise, carried out for the first‐level microzonation of 20 municipalities of Eastern Sicily characterized by high seismic hazard. A critical comparison of the results obtained by the clustering procedure implemented with those previously obtained by processing the same recordings with a technique based on the visual comparison of the spectral ratios of all the analysis windows has shown that the automatic clustering procedure seems to be capable of achieving a better partitioning of a set of HVSR curves and thus provides effective help in the process of distinguishing between peaks mainly linked to the site effects and others mainly related to the source effects.

Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna: Observations and Fracture Orientation Modelling

Ambient noise measurements performed on the western flank of Mt. Etna are analyzed to infer the occurrence of directional amplification effects in fault zones. The data were recorded along short (<500 m) profiles crossing the Ragalna Fault System. Ambient noise records were processed to compute the horizontal-to-vertical noise spectral ratio as a function of frequency and direction of motion. Wavefield polarization was investigated in the time–frequency domain as well. Peaks of the spectral ratios generally fall in the frequency band 1.0–6.0 Hz pointing out directional amplifications that are also confirmed by the results of the time–frequency analysis, the largest amplification occurring with high angle to the fault strike. A variation of the frequency of the spectral peak is observed between the two sides of the fault, possibly related to a damage fault asymmetry. Measurements performed several kilometers away from the fault zone do not show behavior that is as systematic as in the fault zone, and this suggests that the observed directional effects can be ascribed to the fault fabric. We relate the polarization effect to compliance anisotropy in the fault zone, where the presence of predominantly oriented fractures makes the normal component of ground motion larger than the transversal one. In order to test the direction and the type of fractures that are expected in the fault zone, we modeled the brittle deformation pattern of the investigated fault. Theoretical results are in good agreement with field observations of the fracture strike.

Statistical correlation of earthquake and ambient noise spectral ratios

The Horizontal-to-Vertical Spectral Ratio from earthquake (HVSR) and from ambient noise (HVN) recordings realistically indicate the fundamental frequency of soil response but, for the majority of the worldwide examined sites, they do not provide reliable amplification curves as calculated by the earthquake standard Spectral Ratio (SSR). Given the fact that HVSR and especially HVN can be easily obtained, it is challenging to search for a meaningful correlation with SSR amplification functions for the entire frequency band and to use the results for the SSR estimate at a further site where only noise measurements are available. To this aim we used recordings from 75 sites worldwide and we applied a multivariate statistical approach (canonical correlation analysis) to investigate and quantify any correlation among spectral ratios. The canonical correlation between SSR and HVN is then used to estimate the expected SSR at each site by a weighted average of the SSR values measured at the other sites; the weights are properly set to account more for sites with similar behaviour in terms of the canonical correlation results between HVN and SSR. This procedure, repeated for all sites in turn, constitutes the basis of a cross validation. The comparison between the inferred and the original SSR highlights the improvements of site response estimation with respect to the use of ambient noise techniques. The goodness and limitations of the reconstruction procedure are explained by specific geological settings.

New developments in ambient noise analysis to characterise the seismic response of landslide-prone slopes

Abstract. We report on new developments in the application of ambient noise analysis applied to investigate the dynamic response of landslide-prone slopes to seismic shaking, with special attention to the directional resonance phenomena recognised in previous studies. These phenomena can be relevant for seismic slope susceptibility, especially when maximum resonance orientation is close to potential sliding directions. Therefore, the implementation of an effective technique for site response directivity detection is of general interest. In this regard methods based on the calculation of horizontal-to-vertical noise spectral ratio (HVNR) are promising. The applicability of such methods is investigated in the area of Caramanico Terme (central Italy), where ongoing accelerometer monitoring of slopes with different characteristics offers the possibility of validation of HVNR analysis. The noise measurements, carried out in different times to test the result repeatability, revealed that sites affected by response directivity persistently show major peaks with a common orientation, consistent with the resonance direction inferred from accelerometer data. In some cases such a directivity turned out parallel to maximum slope direction, but this cannot be considered a systematic feature of slope dynamic response. At sites where directivity is absent, the HVNR peaks do not generally show a preferential orientation, with rare exceptions that could be linked to the presence of temporarily active sources of polarised noise. The observed variations of spectral ratio amplitude can be related to temporal changes in site conditions (e.g. groundwater level/soil water content variations affecting P wave velocity and Poisson's ratio of surficial layer), which can hinder the recognition of main resonance frequencies. Therefore, we recommend conducting simultaneous measurements at nearby sites within the same study area and repeating measurements at different times in order to distinguish significant systematic polarisation caused by site-specific response directivity from polarisation controlled by properties of noise sources. Furthermore, an analysis of persistence in noise recordings of signals with systematic directivity showed that only a portion of recordings contains wave trains having a clear polarisation representative of site directional resonance. Thus a careful selection of signals for HVNR analysis is needed for a correct characterisation of site directional properties.

Seismic property characterization of lithotypes cropping out in the Siracusa urban area, Italy

The town of Siracusa is located in the south-eastern coast of Sicily (southern Italy) in a wide inlet engraved into a limestone formation. This lithotype, which is overlaid by soft sediments such as sandy clays and detritus, can be considered as the local bedrock. Present study aims to investigate on the dynamic properties of main lithotypes outcropping in the study area and their relationships with the local seismic response. Non-invasive seismic prospecting techniques using the vertical component of surface waves (MASW and ReMi) were adopted, as well as ambient noise measurements, processed through the Nakamura technique. The best solution for the experimental dispersion curves was selected using the neighbourhood algorithm and information about the deeper part of the estimated shear wave profiles was obtained by considering the ellipticity of the fundamental peak. Moreover, a cluster analysis was performed to subdivide into homogeneous groups the experimental noise spectral ratios. Results obtained pointed out that the use of combined different methods provides a robust way to characterize the investigated soils and to reduce the problems linked to the non-uniqueness of solutions during the interpretation of geophysical data.

Effects of Surface Geology on Seismic Ground Motion Deduced from Ambient-Noise Measurements in the Town of Avellino, Irpinia Region (Italy)

The effects of surface geology on ground motion provide an important tool in seismic hazard studies. It is well known that the presence of soft sediments can cause amplification of the ground motion at the surface, particularly when there is a sharp impedance contrast at shallow depth. The town of Avellino is located in an area characterised by high seismicity in Italy, about 30 km from the epicentre of the 23 November 1980, Irpinia earthquake (M = 6.9). No earthquake recordings are available in the area. The local geology is characterised by strong heterogeneity, with impedance contrasts at depth. We present the results from seismic noise measurements carried out in the urban area of Avellino to evaluate the effects of local geology on the seismic ground motion. We computed the horizontal-to-vertical (H/V) noise spectral ratios at 16 selected sites in this urban area for which drilling data are available within the first 40 m of depth. A Rayleigh wave inversion technique using the peak frequencies of the noise H/V spectral ratios is then presented for estimating Vs models, assuming that the thicknesses of the shallow soil layers are known. The results show a good correspondence between experimental and theoretical peak frequencies, which are interpreted in terms of sediment resonance. For one site, which is characterised by a broad peak in the horizontal-to-vertical spectral-ratio curve, simple one-dimensional modelling is not representative of the resonance effects. Consistent variations in peak amplitudes are seen among the sites. A site classification based on shear-wave velocity characteristics, in terms of Vs30, cannot explain these data. The differences observed are better correlated to the impedance contrast between the sediments and basement. A more detailed investigation of the physical parameters of the subsoil structure, together with earthquake data, are desirable for future research, to confirm these data in terms of site response.

Correlation between damage distribution and ambient noise H/V spectral ratio: the SESAME project results

In the framework of the SESAME project one of the tasks was the compilation of all available ambient noise measurements within urban environments affected by historical or/and recent strong earthquakes in Europe. The aim of such a task was to give an answer to the question; “How does horizontal-to-vertical ambient noise spectral ratio compare with damage in modern cities?”. For this purpose five European urban areas, namely, Angra do Heroismo (Portugal), Fabriano and Palermo (Italy), Thessaloniki and Kalamata (Greece) were selected for which spatial damage information was available either in terms of modified Mercalli intensity or in EMS98 damage grades. The geological setting of the examined sites as well as the causative earthquakes are satisfactorily known. Ambient noise recordings compiled for all examined sites have been homogeneously processed by a technique developed and agreed upon SESAME project. Using a standard multivariate statistical analysis, namely, factor analysis and canonical correlation, the horizontal-to-vertical ambient noise spectral ratio (HVNSR) is correlated with damage pattern observed within examined urban areas. Results show that, in some cases (Thessaloniki, Palermo), the HVNSR seems to be able to differentiate between areas previously shown to be associated with higher damage. In other cases however (Angra do Heroismo, Fabriano, Kalamata), the correlation is not statistically significant indicating thus the complex character of the parameters involved, implying that currently there is no a straightforward way that a value of HVNSR can correctly predict the extent to which a given region will be associated with increased damage.

Local seismic response in Catania (Italy): A test area in the northern part of the town

Site effects were studied in an area located in the northern part of Catania town using ambient noise measurements. The study zone is particularly suitable for investigating local seismic response both because most of the terrains characterizing the urban area are here outcropping and a considerable amount of field measurements of their elastic properties is available. The recorded ambient noise was processed through the Nakamura H/V spectral ratios technique. The results set into evidence significant differences in the amplification of the horizontal component versus the vertical one in the litho-types outcropping in the study area. No amplifications were observed in the H/V spectral ratios of sites located on the clayey basement, as well as in those located on massive lavas. By contrast, a tendency towards significant H/V spectral peaks, in the frequency band 2.5–7.0 Hz, was observed in terrains formed by sandy clays, gravelly sands, detritus and altered lavas. Results of ambient noise spectral ratios were compared with preliminary findings from earthquake recordings at two stations located in the investigated area. Finally we computed the theoretical transfer functions using 1D numerical modelling. The results pointed out that the observed H/V peaks can be interpreted as the resonance effect due to the impedance contrast between the soft sediments and the basement quaternary clays, found out at a depth ranging from 20 to 50 m.

Vocal function following radiation for non-laryngeal versus laryngeal tumors of the head and neck.

The larynx may receive high doses of radiation even in the absence of disease. Preliminary investigation has provided evidence that significant voice alterations exist in patients who received radiotherapy (RT) for non-laryngeal tumors of the head and neck. This study evaluates subjective and objective parameters of vocal function in this patient population compared with a control group of patients irradiated for early glottic tumors. Retrospective cohort study. Vocal function in patients irradiated for non-laryngeal and early glottic tumors was assessed in a comprehensive manner and compared. Microanalytical and macroanalytical acoustic analyses, aerodynamic measurements, and videostroboscopy were performed on vowel production data. The Voice Handicap Index was administered for self-assessment of voice quality. All subjects were male, smokers, and greater than 12 months post-RT. Seventeen patients with non-laryngeal tumors and 13 patients with early glottic tumors were evaluated. Microanalytical acoustic parameters were worse for 75% (6 of 8) of the acoustic measures of vowel production in the non-laryngeal group. These include jitter, relative amplitude perturbation, amplitude perturbation quotient, normalized noise energy, pitch amplitude, and spectral flatness ratio. Macroanalytical acoustic analyses revealed no difference in fundamental frequency but numerically smaller phonational frequency range in the non-laryngeal group. All aerodynamic measures, including mean phonation time, mean airflow, and vocal fold diadochokinetic rate, were decreased in the non-laryngeal group. Videostroboscopy demonstrated increased supraglottic activity in the non-laryngeal group. Voice handicap was significantly greater in the non-laryngeal group. When compared with patients receiving RT for early glottic tumors, there is objective and subjective evidence of vocal dysfunction in patients treated with wide-field RT for non-laryngeal tumors.

An indication of the soil topmost layer response in Quito (Ecuador) using noise H/V spectral ratio

H/V noise spectral ratio (HVSR), standard spectral ratio (SSR), and receiver functions (RF) techniques have been used in the Quito (Ecuador) urban area to estimate the frequency dependence of soil response. Two amplified frequencies obtained by the HVSR method appear on about 60 sites. Taking into account the most amplified frequency rather than considering only the first amplified frequency, generally associated with the fundamental frequency, we find that iso-frequency curves tightly fit the surface geology. The second amplified frequency is interpreted as the fundamental frequency of the soft thin topmost layer, which in some cases amplifies the surface ground motion more than the rest of the soil column. This hypothesis is further supported by the results provided from the SSR and RF studies at a station located on top of a solid waste landfill, and by a study of known thickness of a waste landfill, using the HVSR method.

Signal-to-noise ratio definition for magnetic recording channels with transition noise

This paper proposes a new signal-to-noise ratio (SNR) definition for magnetic recording channels with both additive and medium noise components. The proposed SNR is a generalized version of E/sub b//M/sub 0/, the information bit energy to noise spectral height ratio, widely used in average-power-constrained communication channels with additive white noise. The goal is to facilitate comparison of efficiencies of read channels that may operate at different symbol densities because of varying code rates.

The theoretical response of sedimentary layers to ambient seismic noise

For over thirty years, attempts have been made to gain information about sediment amplification during earthquakes from observations of ambient seismic noise. While the results of several feasibility studies have been encouraging, theoretical support for the technique is scant. We present a model for the response of sedimentary layers to ambient seismic noise. The noise sources are modeled as a random distribution (in time and space) of point forces located on the Earth's free surface. This model is applied to a site where observed noise spectral ratios, relative to a rock site, have previously been shown to reveal the fundamental resonant frequency of a soft clay layer. Approximating the sediment site as a single layer over a half‐space, the horizontal noise spectrum predicted by our model reveals the fundamental resonance and first harmonic of the layer. We also examine an estimate of site response proposed by Nakamura (1989), which is formed by dividing the horizontal‐component noise spectrum by that of the vertical component. Nakamura's estimate applied to both observed and predicted noise‐spectra was also successful in identifying the fundamental resonance, with a slight (&lt;10%) shift toward lower frequencies. Future work is needed to determine the generality of our results, and to elucidate the influence of the simplifying assumptions.

On the Coding Gain of Linear Binary Block Codes

The coding gain of linear binary block codes on the AWGN channel is studied. Antipodal signaling and hard decision demodulation are assumed. Simple asymptotic expressions for the gain at high and low energy to noise spectral density ratios are derived. Monotonicity of the coding gain is also discussed.