Surface waves are widely used in near-surface geophysics and provide a noninvasive way to determine near-surface structures. By extracting and inverting dispersion curves to obtain local 1D S-wave velocity profiles, multichannel analysis of surface waves (MASW) has been proven as an efficient way to analyze shallow-seismic surface waves. By directly inverting the observed waveforms, full-waveform inversion (FWI) provides another feasible way to use surface waves in reconstructing near-surface structures. This paper provides a state of the art review of MASW and shallow-seismic FWI and a comparison of both methods. A two-parameter numerical test is performed to analyze the nonlinearity of MASW and FWI, including the classical, the multiscale, the envelope-based, and the amplitude-spectrum-based FWI approaches. A checkerboard model is used to compare the resolution of MASW and FWI. These numerical examples show that classical FWI has the highest nonlinearity and resolution among these methods, while MASW has the lowest nonlinearity and resolution. The modified FWI approaches have an intermediate nonlinearity and resolution between classical FWI and MASW. These features suggest that a sequential application of MASW and FWI could provide an efficient hierarchical way to delineate near-surface structures. We apply the sequential-inversion strategy to two field data sets acquired in Olathe, Kansas, USA, and Rheinstetten, Germany, respectively. We build a 1D initial model by using MASW and then apply the multiscale FWI to the data. High-resolution 2D S-wave velocity images are obtained in both cases, whose reliabilities are proven by borehole data and a GPR profile, respectively. It demonstrates the effectiveness of combining MASW and FWI for high-resolution imaging of near-surface structures.

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High-Resolution Characterization of Near-Surface Structures by Surface-Wave Inversions: From Dispersion Curve to Full Waveform

Numerical solutions have been obtained for the vertical uplift capacity of strip plate anchors embedded adjacent to sloping ground in fully cohesive soil under undrained condition. The analysis was performed using finite element lower bound limit analysis with second-order conic optimization technique. The effect of anchor edge distance from the crest of slope, angle and height of slope, normalized overburden pressure due to soil self-weight, and embedded depth of anchor on the uplift capacity has been examined. A nondimensional uplift factor defined as Fcγ owing to the combined contribution of soil cohesion (cu), and soil unit weight (γ) is used for expressing the uplift capacity. For an anchor buried near to a sloping ground, the ultimate uplift capacity is dependent on either pullout failure of anchor or overall slope failure. The magnitude of Fcγ has been found to increase with an increase in the normalized overburden pressure up to a certain maximum value, beyond which either the behavior of ...

Lower bound solutions for uplift capacity of strip anchors adjacent to sloping ground in clay

The vertical uplift resistance of two closely spaced horizontal strip plate anchors has been investigated by using lower and upper bound theorems of the limit analysis in combination with finite elements and linear optimization. The interference effect on uplift resistance of the two anchors is evaluated in terms of a nondimensional efficiency factor (eta(c)). The variation of eta(c) with changes in the clear spacing (S) between the two anchors has been established for different combinations of embedment ratio (H/B) and angle of internal friction of the soil (phi). An interference of the anchors leads to a continuous reduction in uplift resistance with a decrease in spacing between the anchors. The uplift resistance becomes a minimum when the two anchors are placed next to each other without any gap. The critical spacing (S-cr) between the two anchors required to eliminate the interference effect increases with an increase in the values of both H/B and phi. The value of S-cr was found to lie approximately in the range 0.65B-1.5B with H/B = 1 and 11B-14B with H/B = 7 for phi varying from 0 degrees to 30 degrees.

Vertical uplift resistance of two closely spaced horizontal strip anchors embedded in cohesive-frictional weightless medium

In this paper, numerical and analytical methods are used to evaluate the ultimate pullout capacity of a group of square anchor plates in row or square configurations, installed horizontally in dense sand. The elasto-plastic numerical study of square anchor plates is carried out using three-dimensional finite-element analysis. The soil is modeled by an elasto-plastic model with a Mohr-Coulomb yield criterion. An analytical method based on a simplified three-dimensional failure mechanism is developed in this study. The interference effect is evaluated by group efficiency η, defined as the ratio of the ultimate pullout capacity of group of N anchor plates to that of a single isolated plate multiplied by number of plates. The variation of the group efficiency η was computed with respect to change in the spacing between plates.

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Three-dimensional numerical and analytical study of horizontal group of square anchor plates in sand

The complexity involved with the phase unwrapping procedure, while performing the existing spectral analysis of surface waves (SASW) on the basis of two sensors, makes it difficult to automate and requires frequent manual judgment. As a result, this approach generally becomes tedious and may yield erroneous results. The multichannel analysis of surface waves (MASW) technique can resolve the problem of phase wrapping. However, the MASW technique normally requires a large number of closely spaced sensors, typically 24–48 or even more. We have developed a new method that is fast, accurate, and generally resolves the unwrapping of phase with the use of just two sensors, provided the signal-to-noise ratio remains high. In this approach, the unwrapping of the phase can be performed without any manual intervention and an automation of the process becomes feasible. A few examples, involving synthetic test data and surface-wave tests, have been tested to determine the efficacy of our approach. Comparisons ...

Resolving phase wrapping by using sliding transform for generation of dispersion curves

Effects of site stiffness and source to receiver distance on surface wave tests' results

The vertical uplift resistance of a group of two horizontal coaxial strip anchors, embedded in a general c–ϕ soil (where c is the unit cohesion and ϕ is the soil friction angle), has been determine...

Vertical uplift capacity of a group of two coaxial anchors in a general c–ϕ soil

A fast and accurate method to compute dispersion spectra for layered media using a modified Kausel-Roësset stiffness matrix approach

In case of irregular dispersive media, a proper analysis of higher modes existing in a dispersion plot becomes essential for predicting the shear wave velocity profile of ground on the basis of surface wave tests. In such cases, an establishment of the predominant mode becomes quite important. In the current investigation for Rayleigh wave propagation, the predominant modes have been evaluated by maximizing the normalized vertical displacements along the free surface. Eigenvectors computed from the dynamic stiffness matrix (DSM) approach are analyzed to find the predominant mode. The results obtained are then compared with those reported in the literature. By varying the displacement amplitude ratios of the predominant mode to the other modes, dispersion plots have also been generated from the multichannel analysis of surface waves (MASW) method. The establishment of the predominant mode becomes especially significant, where usually only two to six sensors are employed and the governing (predominant) modal dispersion curve is usually observed rather than several multiple modes, which can be otherwise identified by using around 24 to 48 sensors.

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Tarun Naskar

Papers

Effects of site stiffness and source to receiver distance on surface wave tests' results

Vertical uplift capacity of a group of two coaxial anchors in a general c–ϕ soil

A fast and accurate method to compute dispersion spectra for layered media using a modified Kausel-Roësset stiffness matrix approach

Resolving phase wrapping by using sliding transform for generation of dispersion curves

Predominant modes for Rayleigh wave propagation using the dynamic stiffness matrix approach