Author: Seyed Mohammad Ali Zomorodian Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Spectral analysis of surface waves (SASW) method is an in-situ seismic method used for determining the thickness and elastic properties of soil and pavement. The SASW method is fast and economical to perform since no boreholes are required. The method is suitable for sites where the use of large equipment is difficult or where sublayer conditions make it difficult to perform other seismic tests. The SASW method is also ideal for preliminary field investigations to be conducted prior to more detailed site investigation, and for quality control and monitoring of ground improvement. The purpose of this research was to improve the SASW method by incorporating multi-mode propagation in the backcalculation procedure. In order to facilitate the investigation carried out in this study, two computer programs were developed to simulate SASW tests (and also Steady-State surface wave tests) and to calculate theoretical dispersion curves. The program for calculating theoretical dispersion curves was based on the root-searching procedure used in existing backcalculation methods. The computer programs developed in this study were used in a case study to demonstrate difficulties encountered by existing methods in dealing with multi-mode situations. It was shown that: (i) wavelength filtering criteria used by existing methods yield inconsistent (i.e. erroneous) dispersion curves when more than one propagation mode participate in the wave field, and (ii) backcalculation procedures based on root-searching cannot identify predominant propagation modes and hence fail to yield accurate results in the case of multi-mode propagation. Two developments were made in the present study to overcome the above difficulties. First, a new wavelength filtering criterion was adopted. In this criterion, the dispersion data point for a particular frequency is rejected (i.e. filtered out) if the values of phase velocity obtained from two different receiver-to-receiver spacings are not in close agreement. In this manner, inconsistencies that might result in the dispersion due to multi-mode propagation are avoided. Second, a new procedure to calculate the theoretical dispersion curve was developed. This procedure is based on the maximum vertical flexibility coefficient (at each frequency) of the theoretical layered model. Unlike root-searching methods, the maximum vertical flexibility coefficient method easily identifies predominant propagation modes. A computer program was developed in this study for backcalculation of SASW data based on the flexibility coefficient method. Least-squares optimization using the down-hill simplex method was also implemented in this program to automate the backcalculation process. The accuracy of the above proposed procedures was demonstrated using SASW field tests. The shear wave velocity profiles obtained using the procedures developed in this study are in good agreement with those obtained from other in-situ seismic tests. (Abstract shortened by UMI.).
Author: S. Nazarian Publisher: ISBN: Category : Automation Languages : en Pages : 13
Book Description
With minimal simplifying assumptions and approximations, an automated version of the spectral analysis of surface waves (SASW) method has been developed to determine in situ shear wave velocity profiles of soil deposits. Testing time for each site, including collection of data, construction and inversion of dispersion curve, should not be more than thirty minutes. To accelerate field testing, a multi-sensor geophone array and a PC-based data-acquisition, signal-analysis package have been developed. The two greatest challenges have been the development of a fully-automated procedure to construct the dispersion curve from field data and a fast yet, theoretically-sound, inversion process to determine the shear wave velocity profile from a given dispersion curve. In this paper, the major theoretical and practical aspects of these two steps are described and discussed with examples.
Author: Jeffrey D. Bertel Publisher: ISBN: Category : Electronic dissertations Languages : en Pages :
Book Description
The Spectral-Analysis-of-Surface-Waves (SASW) method is an accepted means of measuring shear wave velocity (VS). In some settings, SASW measurements have produced results that were inconsistent with results from other methods of measurement. The effectiveness of the SASW methodology at complex geotechnical sites was investigated to identify site conditions where the SASW approach may produce erroneous results. Analytical simulations of surface wave measurements were performed. A traditional SASW methodology (global analysis), and a more rigorous approach (array analysis) were used to generate experimental dispersion curves. The effectiveness of these approaches was evaluated by comparing the experimental results to the true dispersion curve. The global analysis yielded dispersion curves that tend to underestimate surface wave velocities at long wavelengths. The array approach worked well for both simple, gradually increasing VS profiles as well as for some complex profiles with large VS contrasts. In some cases both the global and array analysis produce an experimental dispersion curve that is not consistent with the theoretical dispersion curve, especially for soft-over-stiff profiles, a common profile encountered in the field. These results have implications for earthquake site response analysis.
Author: Jianhua Li Publisher: ISBN: Category : Electronic dissertations Languages : en Pages : 220
Book Description
Surface wave methods have become an important tool for non-intrusively and inexpensively determining shear wave velocity (V [subscript-s]) profiles for many geotechnical earthquake engineering applications. The primary objectives of this study are to (1) compare active-source and passive (ambient vibration) surface wave methods for developing V [subscript-s] profiles to depths of 200 to 300 m at deep soil sites, and (2) identify the primary factors affecting the reliability and consistency of surface wave methods. This comparative study became possible with the advent of a unique low- frequency field vibrator developed as part of the National Science Foundation's (NSF) Network for Earthquake Engineering Simulation (NEES) program. This vibrator is able to actively excite surface wave energy down to frequencies of less than 1 Hz. Four surface wave methods (two active-source methods and two passive-source methods) were applied in this study, namely: (1) the Spectral-Analysis-of-Surface-Waves (SASW) method, (2) the active-source frequency- wavenumber ([function]-k) method, (3) the passive-source frequency-wavenumber ([function]-k) method and (4) the refraction microtremor (ReMi) method. The focus of this study is on two critical aspects of surface wave methods: (1) development of a reliable surface wave dispersion curve from field measurements, and (2) compatibility between the experimental dispersion curve and the theoretical model used in the inversion procedure to develop the final V [subscript-s] profile. Measurements were performed at eleven sites distributed over a distance of about 180 km in the upper Mississippi Embayment in the central United States, where soil deposits are hundreds of meters deep. Limitations associated with each of the four methods were identified in this study. With respect to the SASW method it was found that potential phase unwrapping problems could cause an erroneous estimate of the dispersion curve. These errors were found to be associated with an abrupt mode transition caused by a strong velocity contrast at a shallow depth. With respect to the active-source [function]-k approach, it was demonstrated that near-field effects caused by a short near- source offset produced an underprediction of the surface wave dispersion curve at long wavelengths. Recommendations for acceptable source offset distances were developed based on the results from this study. The performance of the passive approaches (passive [function]-k method and ReMi method) was shown to be strongly dependent on the local ambient wavefield characteristics. Results from a study of the ambient wavefield characteristics at the 11 sites showed high ambient vibration levels at all sites in the frequency range of 1 to 4 Hz. Passive measurements using a circular array provided good comparisons with the active-source methods out to wavelengths of 500 m (2.5 times the array aperture) in most cases. Poor performance at one site was shown to be due to a multi-source wavefield at low frequencies. An improved comparison at this site was achieved by applying high-resolution processing methods. The ReMi method was found to provide good results down to frequencies of 3 to 4 Hz (wavelengths of 100 to 150 m) but very poor performance at lower frequencies (
Author: Dr. Neil Anderson Publisher: ISBN: Category : Bridges Languages : en Pages : 52
Book Description
Surface wave (Rayleigh wave) seismic data were acquired at six separate bridge sites in southeast Missouri. Each acquired surface wave data set was processed [spectral analysis of surface waves (SASW)] and transformed into a site-specific vertical shear-wave velocity profile (SASW shear-wave velocity profile). The SASW shear-wave velocity profiles generated for each bridge site were compared to other geotechnical data including seismic cone penetrometer shear-wave velocity profiles, cross-borehole shear-wave velocity profiles, and borehole lithology logs. The geotechnical data presented herein indicate the SASW shear-wave velocity profiles correlate well with subsurface lithology logs and available cross-borehole shear-wave velocity control. More specifically, clays, silts and sands exhibit relatively characteristic SASW shear-wave velocities, which increase incrementally with increasing depth of burial. The authors believe these correlations demonstrate that SASW shear-wave velocities are reliable.
Author: Maria Catalina Orozco Publisher: ISBN: Category : Rayleigh waves Languages : en Pages : 574
Book Description
This research focuses on estimating the shear wave velocity (Vs) profile based on the dispersion curve obtained from SASW field test data (i.e., inversion of SASW data). It is common for the person performing the inversion to assume the prior information required to constrain the problem based on his/her own judgment. Additionally, the Vs profile is usually shown as unique without giving a range of possible solutions. For these reasons, this work focuses on: (i) studying the non-uniqueness of the solution to the inverse problem; (ii) implementing an inversion procedure that presents the estimated model parameters in a way that reflects their uncertainties; and (iii) evaluating tools that help choose the appropriate prior information. One global and one local search procedures were chosen to accomplish these purposes: a pure Monte Carlo method and the maximum likelihood method, respectively. The pure Monte Carlo method was chosen to study the non-uniqueness by looking at the range of acceptable solutions (i.e., Vs profiles) obtained with as few constraints as possible. The maximum likelihood method was chosen because it is a statistical approach, which enables us to estimate the uncertainties of the resulting model parameters and to apply tools such as the Bayesian criterion to help select the prior information objectively. The above inversion methods were implemented for synthetic data, which was produced with the same forward algorithm used during inversion. This implies that all uncertainties were caused by the nature of the SASW inversion problem (i.e., there were no uncertainties added by experimental errors in data collection, analysis of the data to create the dispersion curve, layered model to represent a real 3-D soil stratification, or wave propagation theory). At the end of the research, the maximum likelihood method of inversion and the tools for the selection of prior information were successfully used with real experimental data obtained in Memphis, Tennessee.
Author: Seyed Mohammad Ali Zomorodian
Author: S. Nazarian
Author: Jeffrey D. Bertel
Author: 
Author: 
Author: Richard D. Woods
Author: Jianhua Li
Author: Barbara Aileen Luke
Author: Dr. Neil Anderson
Author: Maria Catalina Orozco