Date of Graduation

7-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Civil Engineering

Advisor

Clinton M. Wood

Committee Member

Liam Wotherspoon

Second Committee Member

Sarah V. Hernandez

Third Committee Member

Christopher L. Liner

Fourth Committee Member

Michelle L. Barry

Keywords

Horizontal to Vertical Spectral Ratio (HVSR), Infrastructure Health Monitoring, Multichannel Analysis of Surface Wave (MASW), Near-field Effects, Surface Wave Methods, Transformation Technique

Abstract

This dissertation is aimed at understanding two aspects of active surface wave methods to improve the accuracy and reliability of this method. These include (1) the performance of four common wavefield transformation methods for the multichannel analysis of surface wave (MASW) data processing and (2) near-field effects. Toward this end, extensive field measurements were conducted considering different factors affecting these two topics. The MASW and microtremor horizontal to vertical spectral ratio (MHVSR) were then employed to examine their efficiency for infrastructure health monitoring.

Regarding the performance of the four common transformation techniques, it was observed that for sites with a very shallow and highly variable bedrock topography with a high-frequency point of curvature (>20 Hz), the Phase Shift (PS) method leads to a very poor-resolution dispersion image compared to other transformation methods. For sites with a velocity reversal, the Slant Stack (p) method fails to resolve the dispersion image for frequencies associated with layers located below the velocity reversal layer. Overall, the cylindrical frequency domain beamformer (FDBF-cylindrical) method was determined to be the best method under most site conditions. This method allows for a stable, high-resolution dispersion image for different sites and noise conditions over a wide range of frequencies, and it mitigates the near-field effects by modeling a cylindrical wavefield. However, the FDBF-cylindrical was observed to be dominated by higher modes at complex sites. Therefore, the best practice is to use more than one transformation method (FDBF-cylindrical and FK methods) to enhance the data quality.

Regarding the near-field effects for active surface wave methods, it was observed that near-field effects are independent of surface wave type (Rayleigh and Love) and depth to impedance contrast. For sites with a very shallow impedance contrast, the FDBF-cylindrical transformation technique outperforms others in terms of dispersion resolution by significantly mitigating near-field effects. It is also revealed that source type is an important parameter, influencing the normalized array center distance criteria required to mitigate near-field effects. The best practical criteria for near-field mitigation include a normalized array center distance of 1.0 or greater for low-output impulsive sources such as a sledgehammer and a normalized array center distance of 0.5 for high-output harmonic sources such as a vibroseis. These criteria should not be violated when using a limited number of source offsets (≤2). But, if the multiple source offset approach (≥3 source offsets) is used where some of the source offsets meet the criteria, the near-field criteria can be violated for other source offsets. Additionally, it is recommended to use the multiple source offset approach along with the FDBF-cylindrical for data processing to mitigate near-field effects.

For health monitoring of earthen hydraulic infrastructures, MASW was determined to be effective for detecting weak zones of such structures. In this regard, it is very important to use the reference shear wave velocity profiles to avoid misinterpretation of the results. Additionally, the grid pattern MHVSR method was determined to very effective for landslide evaluations for sites with shallow and complex bedrock topography, where bedrock is a key feature in the slope stability model.

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