124 research outputs found
Shear wave profiles from surface wave inversion: the impact of uncertainty on seismic site response analysis
Inversion is a critical step in all geophysical techniques, and is generally fraught with ill-posedness. In the case of seismic surface wave studies, the inverse problem can lead to different equivalent subsoil models and consequently to different local seismic response analyses. This can have a large impact on an earthquake engineering design. In this paper, we discuss the consequences of non-uniqueness of surface wave inversion on seismic responses, with both numerical and experimental data. Our goal is to evaluate the consequences on common seismic response analysis in the case of different impedance contrast conditions. We verify the implications of inversion uncertainty, and consequently of data information content, on realistic local site responses. A stochastic process is used to generate a set of 1D shear wave velocity profiles from several specific subsurface models. All these profiles are characterized as being equivalent, i.e. their responses, in terms of a dispersion curve, are compatible with the uncertainty in the same surface wave data. The generated 1D shear velocity models are then subjected to a conventional one-dimensional seismic ground response analysis using a realistic input motion. While recent analyses claim that the consequences of surface wave inversion uncertainties are very limited, our test points out that a relationship exists between inversion confidence and seismic responses in different subsoils. In the case of regular and relatively smooth increase of shear wave velocities with depth, as is usual in sedimentary plains, our results show that the choice of a specific model among equivalent solutions strongly influences the seismic response. On the other hand, when the shallow subsoil is characterized by a strong impedance contrast (thus revealing a characteristic soil resonance period), as is common in the presence of a shallow bedrock, equivalent solutions provide practically the same seismic amplification, especially in the frequency range of engineering interest
Tackling Lateral Variability Using Surface Waves: A Tomography-Like Approach
Lateral velocity variations in the near-surface reflect the presence of buried geological or anthropic structures, and their identification is of interest for many fields of application. Surface wave tomography (SWT) is a powerful technique for detecting both smooth and sharp lateral velocity variations at very different scales. A surface-wave inversion scheme derived from SWT is here applied to a 2-D active seismic dataset to characterize the shape of an urban waste deposit in an old landfill, located 15 km South of Vienna (Austria). First, the tomography-derived inverse problem for the 2-D case is defined: under the assumption of straight rays at the surface connecting sources and receivers, the forward problem for one frequency reduces to a linear relationship between observed phase differences at adjacent receivers and wavenumbers (from which phase velocities are straightforwardly derived). A norm damping regularization constraint is applied to ensure a smooth solution in space: the choice of the damping parameter is made through a minimization process, by which only phase variations of the order of the average wavelength are modelled. The inverse problem is solved for each frequency with a weighted least-squares approach, to take into account the data error variances. An independent multi-offset phase analysis (MOPA) is performed using the same dataset, for comparison: pseudo-sections from the tomography-derived linear inversion and MOPA are very consistent, with the former giving a more continuous result both in space and frequency and less artefacts. Local dispersion curves are finally depth inverted and a quasi-2-D shear wave velocity section is retrieved: we identify a well-defined low velocity zone and interpret it as the urban waste deposit body. Results are consistent with both electrical and electromagnetic measurements acquired on the same line
Time-lapse monitoring of root water uptake using electrical resistivity tomography and mise-à-la-masse: a vineyard infiltration experiment
Abstract. This paper presents a time-lapse application of
electrical methods (electrical resistivity tomography, ERT; and
mise-à-la-masse, MALM) for monitoring plant roots and their activity
(root water uptake) during a controlled infiltration experiment. The use of
non-invasive geophysical monitoring is of increasing interest as these
techniques provide time-lapse imaging of processes that otherwise can only
be measured at few specific spatial locations. The experiment here described was conducted in a vineyard in Bordeaux (France) and was focused on the
behaviour of two neighbouring grapevines. The joint application of ERT and
MALM has several advantages. While ERT in time-lapse mode is sensitive to
changes in soil electrical resistivity and thus to the factors controlling
it (mainly soil water content, in this context), MALM uses DC current
injected into a tree stem to image where the plant root system is in effective
electrical contact with the soil at locations that are likely to be the same
where root water uptake (RWU) takes place. Thus, ERT and MALM provide
complementary information about the root structure and activity. The
experiment shows that the region of likely electrical current sources
produced by MALM does not change significantly during the infiltration time
in spite of the strong changes of electrical resistivity caused by changes
in soil water content. Ultimately, the interpretation of the current source
distribution strengthened the hypothesis of using current as a proxy for
root detection. This fact, together with the evidence that current injection
in the soil and in the stem produces totally different voltage patterns,
corroborates the idea that this application of MALM highlights the active
root density in the soil. When considering the electrical resistivity
changes (as measured by ERT) inside the stationary volume of active roots
delineated by MALM, the overall tendency is towards a resistivity increase
during irrigation time, which can be linked to a decrease in soil water
content caused by root water uptake. On the contrary, when considering the
soil volume outside the MALM-derived root water uptake region, the
electrical resistivity tends to decrease as an effect of soil water content
increase caused by the infiltration. The use of a simplified infiltration
model confirms at least qualitatively this behaviour. The monitoring results
are particularly promising, and the method can be applied to a variety of
scales including the laboratory scale where direct evidence of root
structure and root water uptake can help corroborate the approach. Once
fully validated, the joint use of MALM and ERT can be used as a valuable
tool to study the activity of roots under a wide variety of field
conditions
Seismic noise and controlled source surveys: tools for seismic hazard deterministic approach (field measurements in Venice Plain, Italy)
The work concerns the study of surface wave dispersion in order to infer shear wave structural model of a Venice Plain area, Italy. Wave dispersion is studied using controlled source survays and seismic noise cross correlation. The study involves also local earthquakes monitoring and HVSR technique. The structural model obtained is used to compute a detrministic hazard seismic scenario of the studied area
L'impiego di sismi sintetici nella costruzione di scenari di scuotimento del suolo nella regione Veneto.
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