1,721,205 research outputs found
Fast ERT to estimate pollutants and solid transport in water flow: a laboratory experiment
No full textThe understanding and evaluation of the phenomena related to pollutants diffusion, solid transport or lithic material deposition within a river have a prominent role in hydraulics and fluvial geomorphology. In this respect, the direct measurement of the amount of transported material is certainly one of the most difficult tasks in the field of river hydraulics due to the heterogeneity and representativeness of the samples. In this paper, we investigate the applicability of a very fast (about ten seconds or less), non-invasive electrical resistivity tomography to identify and estimate pollutant and solid transport variations in a fast water flow. To do this, three experimental tests, in a laboratory channel with a flow rate of 0.5 m/s, are presented using both a pollutant flow and different types of solid transport (fine and coarse sands). Data are interpreted by means of classical models that relate the variation in electric conductivity to variations in concentration or in volume fraction of suspended material. Whereas the identification and positioning of the pollutant or solid transport is successful, the estimation of governing parameters still remains mainly qualitativ
P- and S-wave velocity models from surface wave dispersion curves data transform
No full textThe inversion of surface wave dispersion curves is a well-established investigation approach to provide S-wave velocity models. Recent works have shown the possibility of exploiting the relation between surface wave wavelength and investigation depth to estimate directly the time-average S-wave velocity along a line or over an area. Moreover, the same relation is sensitive to Poisson's ratio and can be therefore used to retrieve also the time-average P-wave velocity model. Here we show that, starting from the estimated time average VS and VP models and using a Dix-like formula, we can estimate local VS and VP models. We use a synthetic 1D example to outline the method and then we show the results on a field dataset
UTILIZZO DI PROVE PER ONDE SUPERFICIALI PER IL PROGETTO DI INTERVENTI DI MITIGAZIONE DELLE VIBRAZIONI
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Session report: Report and discussion-technical session geophysical surveys using mechanical waves, and/or electromagnetic techniques
No full textThe role of geophysical tests in geotechnical and geo-environmental engineering is constantly increasing. Contributions in this session are mainly focused on applications even though some contain methodological aspects. Most contributions make use of Electric Resistivity Tomography (ERT) as in situ method and of a combination of methodologies (seismic and non seismic methods) to improve the characterization on the same test site. Some attempts to use geophysical techniques at the laboratory scale are also reported. A comparison of results from different complementary seismic techniques to infer the soil shear wave velocity profile at the same site is also reported. In the paper, after a general overview of the articles presented in the session, the potentiality of geophysical methods in laboratory will be discussed together with an analysis and discussion of the results obtainable with different seismic methods (particularly for what surface wave tests are concerned) for the determination of the subsoil shear wave velocity profile
Time-average velocity estimation through surface-wave analysis: Part 2 - P-wave velocity
Full text linkSurface waves (SWs) in seismic records can be used to extract local dispersion curves (DCs) along a seismic line. These curves can be used to estimate near-surface S-wave velocity models. If the velocity models are used to compute S-wave static corrections, the required information consists of S-wave time-average velocities that define the one-way time for a given datum plan depth. However, given the wider use of P-wave reflection seismic with respect to S-wave surveys, the estimate of P-wave time-average velocity would be more useful. We therefore focus on the possibility of also extracting time-average P-wave velocity models from SW dispersion data.We start from a known 1D S-wave velocity model along the line, with its relevant DC, and we estimate a wavelength/depth relationship for SWs.We found that this relationship is sensitive to Poisson's ratio, and we develop a simple method for estimating an "apparent" Poisson's ratio profile, defined as the Poisson's ratio value that relates the time-average S-wave velocity to the time-average P-wave velocity. Hence, we transform the time-average S-wave velocity models estimated from the DCs into the time-average P-wave velocity models along the seismic line. We tested the method on synthetic and field data and found that it is possible to retrieve time-average P-wave velocity models with uncertainties mostly less than 10% in laterally varying sites and one-way traveltime for P-waves with less than 5 ms uncertainty with respect to P-wave tomography data. To our knowledge, this is the first method for reliable estimation of P-wave velocity from SW data without any a priori information or additional data
Imaging near-surface sharp lateral variations with surface-wave methods — Part 1: Detection and location
Full text linkNear-surface sharp lateral variations can be either a target of investigation or an issue for the reconstruction of reliable subsurface models in surface-wave (SW) prospecting. Effective and computationally fast methods are consequently required for detection and location of these shallow heterogeneities. Four SWbased techniques, chosen between available literature methods, are tested for detection and location purposes. All of the techniques are updated for multifold data and then systematically applied on new synthetic and field data. The selected methods are based on computation of the energy, energy decay exponent, attenuation coefficient, and autospectrum. The multifold upgrade is based on the stacking of the computed parameters for single-shot or single-offset records and improves readability and interpretation of the final results. Detection and location capabilities are extensively evaluated on a variety of 2D synthetic models, simulating different target geometries, embedment conditions, and impedance contrasts with respect to the background. The methods are then validated on two field cases: a shallow low-velocity body in a sedimentary sequence and a hard-rock site with two embedded subvertical open fractures. For a quantitative comparison, the horizontal gradients of the four parameters are analyzed to establish uniform criteria for location estimation. All of the methods indicate ability in detecting and locating lateral variations having lower acoustic impedance than the surrounding material, with errors generally comparable or lower than the geophone spacing. More difficulties are encountered in locating targets with higher acoustic impedance than the background, especially in the presence of weak lateral contrasts, high embedment depths, and small dimensions of the object
Approximate Direct Estimate of S-wave Velocity Model from Surface Wave Dispersion Curves
No full textSurface wave dispersion curves are inverted to obtain 1D local VS model. The inversion is based on a layered model and the result is non-unique. A linear relationship can be found between weighted average S-wave velocity at different depths and surface-wave phase velocity at different wavelengths. This relationship, if known for a category of models, can be used to directly estimate an approximated velocity profile from the dispersion curve without the need for inverting. We show on a synthetic case that the error between estimated and true velocity is of the same order of that obtained by inversion
Evaluation of different methods for deriving geotechnical parameters from electric and seismic streamer data
Full text linkGeotechnical parameters of linear earth structures, such as embankments and earth dams, are usually obtained from point-wise investigations through drilling or penetration tests, commonly time and cost consuming. Non-invasive geophysical investigations can be considered alternative for a preliminary screening of earth structures physical properties, given their surveying speed and their depth and length of investigation. Seismic and electrical methods can be also used, through specific correlations, for the estimation of geotechnical soil characteristics. Several methodologies have been developed over the years combining two or more geophysical techniques for the estimation of geotechnical parameters. In this paper, data from geophysical surveys were used for the comparison of geotechnical parameters forecasted with three different methods (with theoretical, statistical, and field based approaches respectively). Their strongpoints and limitations were also evaluated by comparison with available direct geotechnical investigations. Integrated seismic and electrical data from extensive surveying performed over seven retaining structures located in Piedmont Region (NW Italy) were used to forecast their fine content and hydraulic conductivity distributions. Geophysical data were acquired using seismic and electric streamers, useful for the simultaneous execution of the surveys in motion along the earth structures. The results of this study show the effectiveness of the proposed data acquisition approach and elaboration procedures as a first screening tool for earth retaining structure safety assessment. The increased capability of the theoretical method to better predict geotechnical parameters with respect to the other methodologies is also reported
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