1,721,079 research outputs found
Soil and structure damping from single station measurements
No full textIn seismology and seismic engineering soils and structures are modeled as oscillators characterized by modal (resonance) frequencies, shapes and damping. In 1973 Cole proposed the RandomDec technique to estimate both the damping and the fundamental mode of structures from the recorded time series at a single point, with no need for spectral analyses. Here we propose a number of modifications to the original RandomDec approach, that we group under the name DECÓ, which allow to determine the damping as a function of the frequency and therefore the damping of all the vibration modes. However, the motion of structures is so amplified at the resonance frequencies that detecting the characteristic parameters by recording ambient vibrations is relatively easy. More interesting is to apply the DECÓ approach to the soil in the attempt to estimate the mode damping from single station measurements. On soils, the resonance frequencies are normally identified as peaks in the horizontal to vertical spectral ratios of microtremors. However, at these frequencies what is observed is a local minimum in the vertical spectral component, sometimes associated to local maxima in the horizontal components, whose visibility depend on the specific amount of SH and Love waves at the site. The determination of soil damping is therefore a much less trivial task on soils than on structures. By using microtremor and earthquake recordings we estimate the soil damping as a function of shear strain and observe that this is one order of magnitude larger than what is measured in the laboratory on small scale samples, at least at low-intermediate strain levels. This has severe consequences on the numerical seismic site response analyses and on soil dynamic modeling
NELLE NTC LE STRUTTURE RISUONANO. E I TERRENI?
No full textNelle Norme Tecniche sulle Costruzioni, dal punto di vista dinamico le strutture sono trattate come oscillatori armonici di cui è chiesta la conoscenza del periodo proprio, al fine di stimare l’accelerazione che dovranno sostenere in caso di terremoto. Anche i terreni sono oscillatori armonici che non vibrano casualmente ma amplificano specifiche frequenze, in funzione delle proprie caratteristiche meccaniche e geometriche. Le Norme Tecniche sulle Costruzioni non richiedono però una conoscenza delle frequenze proprie dei terreni e si limitano a trattali in modo più superficiale, per ragioni storiche che però potrebbero oggi essere superate agevolmente. In questo scritto vediamo – in estrema sintesi – come sarebbe possibile trattare i terreni per quello che in effetti sono e quali sono i pericoli principali dell’ignorare il vero comportamento dinamico del sottosuolo.According to the present Italian Building Code, structures are modelled as harmonic oscillators whose resonance frequency must be known in order to establish the maximum acceleration that they have to withstand for the design earthquake. Subsoils, too, are harmonic oscillators: they do not vibrate randomly but they amplify specific frequencies, that are function of their mechanical and geometrical properties. The Italian Building Code (as well as the building codes of several other countries) does not require the knowledge of the resonance frequencies of the subsoils and deals with them in more a superficial way, that could be improved by taking into account a few modern seismological techniques for subsoil characterization. In this paper we describe the main risks connected to the ignorance of the real dynamic behaviour of the subsoils and shortly describe a way to avoid them
The complementarity of H/V and dispersion curves
No full textNoninvasive geophysical techniques based on the dispersion of surface waves in layered media are commonly used approaches for measuring shear-wave velocity profiles of the subsoil. Acquiring surface waves is a simple task, but the interpretation of their dispersion curves poses a number of challenges. In an increasing number of cases, shear-wave velocity profiles are derived from the inversion of dispersion curves of surface waves and single-station passive horizontal-to-vertical (H/V) spectral ratios, mostly using a blind joint fit of the two sets of curves. Here we emphasize the benefits of carrying out H/V surveys prior to any array acquisition. We propose to start by collecting at least two H/V recordings at a site to verify the 1D plane-parallel soil condition, as this is essential in dispersion curve inversion/modeling. Then, we look for the diagnostic features of velocity inversions in the H/V curves: when they occur, the interpretation of dispersion curves is made difficult by mode splitting/superposition and Love wave arrays will not be effective. Then we inspect the shape of the H/V curves: flat curves acquired on rock usually imply poor dispersion curves. Large receiver spacings are recommended in the arrays and Love wave arrays will not be efficient. Flat curves on soft material sites represent gently increasing VS gradients and Rayleigh wave arrays should be preferred. H/V curves with high frequency peaks indicate shallow impedance contrasts: this makes Love wave arrays efficient for the soft layer characterization, but provide little information at depth. H/V curves with low frequency peaks indicate deep bedrock and their inversion can provide approximate VS profiles down to greater depths than from an array. Equipped with the information coming from accurate H/V observations, practitioners could make better-informed decisions about array acquisition geometries, source/surface wave types, and inversion strategies
A seismic passive imaging step beyond SPAC and ReMi
No full textThe basic property of passive imaging is that, given any
two points, one of them can be taken as the source of the
waves and the other as the recording station. This property
can be derived from the statistical self-alignment of the rays
along the vector joining the two points, and applies also to
nondiffuse wavefields like seismic tremor. It provides a statistical
basis for the use of the stationary phase integral,
allowing passive interferometry under the mild constraint
of mechanical homogeneity at a local scale. Combined with
the tremor’s large spectral bandwidth, it allows one to recover
the local Green’s function from spatial correlation.
Furthermore, combining this property also with the azimuthal
isotropy of either the wavefield or the array, and using
the statistical mode as the estimator, provides a new technique
to measure the local velocity dispersion in the subsoil.
This technique exploits the potential of spatial autocorrelation
(SPAC) and refraction microtremor (ReMi), allowing
one (1) to use sparse small-aperture arrays with simple
geometry, (2) to dispense with the fitting of Bessel functions,
and (3) to measure, in a few minutes, the local (phase and
group) wave velocity as a function of frequency of potentially
all the wave-propagation modes — body and surface
— and not just of the one prevailing at each frequency
Combining single-station microtremor and gravity surveys for deep stratigraphic mapping
No full textAny stratigraphic reconstruction by means of surface geophysical
methods is affected by the nonuniqueness of data
inversion and by the resolution-depth trade-off. The combination
of different geophysical techniques can reduce the
number of degrees of freedom of the problem. We have focused
on two low-impact single-station geophysical techniques:
microtremor and gravity. These have been used
by previous authors for stratigraphic mapping only by comparing
the results independently. We suggest a procedure to
combine microtremor and gravity data into a unique subsoil
model and explore to what extent their combined use can
overcome their individual weaknesses and constrain the final
result. We apply the procedure to the Bolzano sedimentary
basin, Northern Italy, to derive a 3D bedrock model of the
basin. We use microtremor data to map the ground resonance
frequencies and derive an initial 3D bedrock depth
model by assuming a VS profile for the sediment fill. Then,
we define a density model for rock and sediments and perform
3D gravity forward modeling. We then perturb the VS
and density models and find the parameters that best fit the
observed gravity anomalies. Data uncertainties are examined
to explore the significance of the results. Joint use of the two
techniques successfully helps interpret the stratigraphic
model: Ground resonance frequencies guarantee the spatial
resolution of the bedrock geometry model, whereas gravity
data help constrain the frequency to depth conversion
A surface seismic approach to liquefaction
No full textThe liquefaction potential of soils is traditionally assessed through geotechnical approaches based on the calculation of the cyclical stress ratio (CSR) induced by the expected earthquake and the 'resistance' provided by the soil, which is quantified through standard penetration (SPT), cone penetration (CPT), or similar tests. In more recent years, attempts to assess the liquefaction potential have also been made through measurement of shear wave velocity (VS) in boreholes or from the surface. The latter approach has the advantage of being non-invasive and low cost and of surveying lines rather than single points. However, the resolution of seismic surface techniques is lower than that of borehole techniques and it is still debated whether it is sufficient to assess the liquefaction potential. In this paper we focus our attention on surface seismic techniques (specifically the popular passive and active seismic techniques based on the correlation of surface waves such as ReMiTM, MASW, ESAC, SSAP, etc.) and explore their performance in assessing the liquefaction susceptibility of soils. The experimental dataset is provided by the two main seismic events of ML = 5.9 and 5.8 (MW = 6.1, MW = 6.0) that struck the Emilia-Romagna region (Northern Italy) on May 20 and 29, 2012, after which extensive liquefaction phenomena were documented in an area of 1200 km2. We found that they appear not to have sufficient resolution to address the seismic liquefaction issue. However, it also emerged that the pure observation of the surface wave dispersion curves at their simplest level (i.e. in the frequency domain, with no inversion) is still potentially informative and can be used to identify the sites where more detailed surveys to assess the liquefaction potential are recommended
Jerusalem. The holy sepulcher. Research and investigation
No full textnon c'è abstract per un capitolo di libr
Measuring shear wave velocity, Vs, of a hidden layer: an application to soil improvement under roads
No full textIn a number of practical cases, a typical one being the investigation of the subsoil properties below roads or
foundations, one faces the problem of measuring the elastic properties of a geological layer (here called “hidden layer”)
underlying a more compact and rigid surface layer. In such cases, the effectiveness of common surface seismic methods is poor
for different reasons, but mostly linked to the reflection–transmission properties of the waves at a stiff-to-soft interface. Borehole
methods are more efficient, but expensive and only provide vertical information at certain points. Attempts carried out in the
past to characterize the hidden layer properties through surface seismic techniques consisted in placing the seismic source on
the surface alongside, but off the stiff artificial layer (road or foundation). An alternative approach is presented based on placing
the seismic source just below the stiff artificial layer. In cases where soil improvement–compaction are carried out through
injection of expanding resins, then the hidden layer can be easily reached via the injecting tools and in some cases (e.g., urban
settings characterized by laterally continuous artificial layers or roads constructed on embankments) this can be the only viable
option. The results obtained from this approach using a number of practical cases where roads affected by differential sinking
have later been compacted will be presented. The average soil improvement that can be achieved with the specific kind of
expanding resin used in this study is then quantified
HVSR deep mapping tested down to ∼1.8 km in Po Plane Valley, Italy
No full textThe Horizontal to Vertical Spectral Ratio – HVSR – of seismic noise is extensively used in seismic microzonation for its capability to provide a good approximation to the subsoil main resonance frequencies of geotechnical and seismic engineering interest. This implies, in turn, that it has also an approximate passive subsoil mapping capability independent of the level of noise illumination, albeit limited to relatively shallow depths, since tilt sensitivity makes HVSR unreliable below ∼0.1 Hz. However, we have experimentally verified that HVSR subsoil mapping capability extends to depths in the kilometer range by applying it to the largest sedimentary basin of Italy, the Po Plain Valley. There, we were able to resolve the major stratigraphic discontinuities down to the sediment-bedrock interface, for which we estimated a depth of ∼1.6 km with a 25% uncertainty, while the surface mapped from oil exploration indicates a depth of ∼1.8 km. Quarter of an hour recordings gave always stable signals that, fitted to synthetic curves using as a constraint the parameters of the shallow subsoil, provided a stratigraphic map consistent with the independent survey. This candidates HVSR as a fast and inexpensive, first-order subsoil mapping tool down to depths of geological and exploration interest
Passive single-station techniques applied for dynamic characterization of reinforced concrete buildings
Full text linkThis work aims to better understand and improve the dynamic characterization of concrete frame buildings through the combined use of finite element modeling and applied seismology. The behavior of the FEM model is compared with values obtained directly in situ through non-invasive tests based on a sensor capable of detecting the seismic microtremor and provide direct information in terms of oscillation periods and displacements. The case study structure was measured using a seismometer, and, at the same time, modeled using SAP2000. By starting from extremely different initial data, multiple variations were made to the model to produce an increase in frequency, aligning it with the one detected instrumentally
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