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Surface wave methods have been developed as a commonly used non-invasive tool for shear-wave velocity (Vs) profiling of near-surface soil and rock formations. Inversion is a key step to back-calculate the Vs profile from dispersion curves; however, the non-linear and ill-posed nature of the problem sometimes yields non-unique Vs profiles. To overcome these challenges, a new method was developed to circumvent inversion and directly estimate Vs profiles from the fundamental mode dispersion trend for sites with Vs gradually increasing with depth. This method is developed based on the relationship between the phase velocity of surface waves at a given frequency with the time-averaged shear-wave velocity within a depth of one-half wavelength. Three field sites and three simulated sites covering shallow, medium and deep locations with gradually increasing Vs with depth are employed to verify the feasibility of the proposed methodology. Results indicate that the method can efficiently give a reasonable and realistic estimation of Vs profiles. The goal of this method is to provide an alternative to obtain Vs profiles with high certainty and low computational cost for surface wave testing, but not necessarily to find a ‘better’ profile than inversion or to locate each layer interface.

Surface wave methods have been extensively employed to estimate time-averaged shear-wave velocity (Vs) to 30 m (Vs,30) for seismic site classification. However, the inversion of surface wa...

Determination of the depth of surface-opening (visible) cracks is of critical importance for evaluating the structural safety of concrete elements. Accurately determining the crack depth wi...

Surface waves propagating in layered media inherently possess multimodal dispersion characteristics. However, traditional surface wave testing methods employing measurements at the free surface usually capture only a single apparent dispersion curve, especially when using short geophone arrays common to near surface and geotechnical-scale investigations. Such single-mode or fragmented multimode apparent dispersion curves contain only a fraction of the possible dispersion information, thus limiting the accuracy of inverted profiles. To enable more robust measurement of higher Rayleigh-wave modes, a recently developed hybrid minimally invasive multimodal surface wave method is combined herein with the widely used geotechnical standard penetration test (SPT), which is employed as a practical and ubiquitous downhole source. Upon superimposing surface wave dispersion data for a range of SPT impact depths within the soil, higher modes can be measured more consistently and reliably relative to traditional non-invasive testing methods. As a result, misidentification of multiple dispersion modes can be practically eliminated, significantly improving the accuracy and certainty of inversion results.

A Minimally-invasive Multimodal Surface Wave (MMSW) geophysical testing method, which is a hybrid of surface and borehole seismic methods, was developed recently by the authors to measure more extensive multi-mode dispersion data and thus improve the accuracy of inversion profiles. The new MMSW method employs a borehole geophone at selected depths to record seismic waves from different source offsets on the soil surface. Presented in this paper is a procedure for estimating a range of optimum geophone depths to capture a given higher mode by the MMSW method. Stiffness matrix and finite-element-based numerical simulations of the MMSW method are performed to identify the relationships between critical geophone depths and apparent cutoff frequencies of higher modes. Specifically, it is shown for increasing velocity profiles that 1) at a given borehole sensor measurement depth, the apparent cutoff frequencies of higher modes increase with mode number, 2) at a given frequency, the critical geophone depth at which a higher mode will first become dominant increases with mode number, and 3) for a given higher mode, the apparent cutoff frequency decreases as measurement depth increases. A preliminary field test is conducted using a vertical geophone placed at five different depths while impacts are applied to the soil surface from 3.66 to 43.89 m from the borehole, with an impact spacing of 3.66 m. Dispersion images from the five geophone depths are superimposed to produce a dispersion image having three modes with improved clarity relative to the surface-only Multichannel Analysis of Surface Waves (MASW) method. A comparison of the experimental and theoretical apparent cutoff frequencies for higher modes is used to validate the theoretical prediction of critical depths by the stiffness matrix method. Matching of experimental and theoretical apparent cutoff frequencies could provide additional optimization constraints to reduce the uncertainty of final inversion profiles.

On April 14, 2010, a devastating Ms 7.1 earthquake occurred in Yushu, China. In the most severely struck area of Jiegu Town, approximately 94 % of the structures were damaged. A seismic intensity map was obtained based on field investigation data of structural damage in 63 residential areas, and the differences in structural damage in four intensity regions were quantitatively compared using a seismic capacity index and a damage index. The typical damage for five types of structures in the epicentral area of Jiegu Town was described and summarized in detail. Some recommendations for improving seismic capacity of buildings are provided; they include avoiding mountainous topography, non-uniform materials, and irregular layouts, as well as considering an appropriate arrangement of structural columns and belt courses for hollow concrete block and brick masonry structures. This paper provides valuable information for structural seismic design, post-earthquake reconstruction, and related work.

This paper presents a computational and experimental study on seismic stiffness profiling of layered pavement systems using the multi-channel analysis of surface waves (MASW) and multi-channel simulation with one receiver (MSOR) testing procedures. Development of a new custom-programmed data acquisition system for MASW and MSOR testing using MATLAB software and National Instruments hardware are detailed. A receiver coupling method is presented, which was found to perform the best among several methods examined by the authors. The cross-correlation function is employed to statistically quantify the repeatability of impacts, which is critical for MSOR tests in which multi-channel records are simulated by performing multiple impacts over a range of incremental offsets from a single fixed receiver. Experimental dispersion data from MASW and MSOR tests performed at the same asphalt pavement site with the same testing system are compared, and MASW is found to enable measurement of dispersion data to much higher frequencies using the authors’ methods than MSOR. Inversion results from the MASW and MSOR data at the same asphalt pavement site also are compared, and it is found that MASW is able to provide measurements of the stiffness of the pavement surface layer with reduced variability.

To measure more complete multi-mode dispersion data and thus improve the accuracy of inversion profiles, the authors recently developed a minimally-invasive hybrid surface-and-borehole method. The new method employs a borehole geophone at selected depths to record seismic waves from a moving source on the soil surface. The goal of this paper is to develop a procedure for estimating the ranges of optimum geophone depths for capturing the higher modes. Stiffness matrix and finite element-based numerical simulations of the hybrid testing method are performed to identify the relationships between critical geophone depths and cut-off frequencies. A preliminary field test is conducted using a vertical geophone placed at five depths while impacts are applied to the soil surface over a range of offsets. Dispersion images from the five geophone depths were superimposed to produce a dispersion image having three modes with improved clarity relative to the surface-only MASW method. A comparison of the experimental and theoretical cut-off frequencies of higher modes is used to validate the prediction of critical depths by the stiffness matrix method. Matching of such experimental and theoretical cut-off frequencies is proposed as additional optimization constraints to reduce the uncertainty of final inversion profiles.

A study on the differences between testing soils and pavements using surface wave methods is presented. The differences in theoretical dispersion curves are illustrated using the transfer matrix method and the stiffness matrix method for soils and pavements, respectively. The Levenberg-Marquardt and simulated annealing methods are applied for inversion of experimental data on soils and a concrete foundation slab, and the relative merits and differences of the two inversion methods are discussed.