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Surface wave methods have increased in popularity as a means to acquire rapid and accurate shear wave velocity ( VS) profiles for engineering applications. Use of the multichannel analysis of surface waves (MASW) method, in particular, has proliferated due to multiple factors, including the ease with which strong signal-to-noise ratio can be achieved and the increased dispersion resolution offered by multichannel acquisitions. However, typical MASW processing to extract dispersion information assumes the surface waves propagate through a layered model. Errors can arise when significant lateral variability is present in the underlying stratigraphy as encountered in certain geologic settings such as residual deposits. This study investigated the effects of such variability on the dispersion information acquired with MASW. In particular, a spatially-correlated Gaussian random field was used to model the natural fluctuations in stiffness introduced by depositional processes, which differs from the approach in other studies where more specific anomalous features have been explored. Numerical modeling was subsequently performed to simulate surface wave propagation in the representative geotechnical site condition. The recovered surface waves were used to develop a subsurface stiffness profile using a dispersion-based pseudo-2D MASW approach and a tomographic approach using full waveform inversion (FWI). The results demonstrate that considerable natural spatial variability significantly complicates interpretation of dispersion information in two primary ways: (1) uncertainty can arise regarding what the dispersion curve exactly quantifies since none of the underlying VSprofiles nor the average VSprofile are obtained; and (2) the dispersion images exhibit evidence of false depth-related dispersion information indicative of multiple “fundamental” modes from the superposition of multiple stratigraphic units. The FWI procedure that bypasses extraction of dispersion information was found to better recover the underlying subsurface conditions when compared to the pseudo-2D MASW results at the cost of additional computational efforts.

Rayleigh waves employed in engineering geophysical applications are typically generated by vertical strikes on a metallic plate that serves as a coupler between an active input source (e.g., a sledgehammer) and the ground surface. Seismic surface waves can also be generated with horizontal impacts (i.e., Love waves) using specialized coupling plates. Survey design, including the source characteristics and data acquisition parameters, can influence the acquired signals and the information derived from their analysis. Despite a growing body of research on Love wave testing, much of the basic information regarding optimal Love wave field data acquisition including source effects, near-field limitations, and receiver spacing remains unquantified. The objective of this study was to improve understanding of Love survey parameters and their effect on resolution. Rayleigh and Love waveforms were collected with multiple active seismic sources at three sites and a systematic comparison was made between the two types of waves. Also, seismic wave propagation was simulated using the code SPECFEM2D to further investigate their differences. The results revealed essential information about the role of different source and survey parameters on surface wave data. For example, the minimum source offset to avoid near-field effects was comparable for Rayleigh and Love waves (0.3–0.4 of the maximum wavelength). At closer source offset locations, Rayleigh waves were more affected by near-field effects and showed an additional 10% underestimation of phase velocities. The results from this case study provide new insights about survey design and its effects on surface wave testing.

Changes in soil fabric following liquefaction have been studied using various in situ methods that often return inconclusive or conflicting observations. The time–rate variation of stiffness, when observed, is usually not evaluated over significant periods of time, limiting investigations about aging in post-liquefaction regain of stiffness. Even more uncommon is the application of geophysical techniques to evaluate changes in shear wave velocity (V S) as a proxy for small-strain stiffness. This study uses controlled blasting to examine long-term post-liquefaction regain of stiffness following multiple blast events. The multi-channel analysis of surface waves (MASW) technique was used to observe changes in V S of aged deposits at a test site in South Carolina. Blast-induced liquefaction of the target liquefiable layer resulted in significant reduction to its initial small-strain stiffness owing to the destruction of the aged soil fabric. The time-rate variation in V S indicated that the initial small-strain stiffness was not re-established over many months following liquefaction. Following a second blast event, the small-strain stiffness reduced again, but recovered more quickly, similar to previously reported observations of young sand deposits. This study provides a significant basis for interpreting in situ body and surface wave measurements of aged and young sand deposits densified using blast liquefaction.

Major advancements in surface wave testing over the past 2 decades have led researchers to revisit and re-analyze archived seismic records, particularly those involving measurements on the Moon. The goal of such recent efforts with lunar seismic measurements has been to gain further insights into lunar geology. We examined the active seismic data from the Apollo 16 mission for their surface wave information using a multichannel approach. The inversion of Rayleigh surface waves provided a subsurface estimate for the uppermost 8 m of the lunar subsurface with the shear wave velocities varying from 40 to 50 m/s at the surface to velocities in the range of 95–145 m/s with an average of 120 m/s at a depth of about 7 m. Generally, the results from this inversion demonstrated good agreement with previous studies. Also, we carried out numerical modeling of wave propagation in a highly-heterogeneous domain to examine the effects of such anomalous features on the acquired seismograms. Results confirmed that a sharp-contrast bi-material domain can indeed produce significant coda wave as reflected on the lunar seismic traces.

The use of surface wave testing for near‐surface engineering applications has increased in recent years. Typical surface wave analysis is based on the dispersion of surface waves in one‐dimensional layered models. One‐dimensional models are inappropriate for measurements at sites with appreciable lateral variability, a likely scenario in many engineering applications. Use of such models can subsequently undermine the reliability and accuracy of the surface wave results. Full waveform inversion (FWI) is a high‐resolution imaging technique that is proven to outperform the conventional dispersion‐based analysis of surface waves. Much of the near‐surface literature has focused on full waveform inversion of Rayleigh waves developed by the interaction of primary‐ and vertically polarized shear waves (P‐SV), and the capabilities of surface waves generated by horizontally polarized shear waves (Love waves) in mapping near‐surface spatial variabilities have not been fully explored. In this numerical study, full waveform inversion of Rayleigh and Love waves was performed on two different spatially correlated Gaussian random fields (mean VS of 200 and 500 m/s) to mimic the natural spatial variability of geologic materials. Each soil structure was produced at a low and high level of stiffness variability. Two sources with different frequency contents, 25 and 50 Hz, were used to evaluate the effects of source characteristics on the resolution of Rayleigh and Love waveform inversions. The inverted results from the high‐velocity domain demonstrated that Love waveform inversion using high‐frequency seismic sources outperforms Rayleigh full waveform inversion in detecting the shape and the velocities of horizontally deposited geologic materials. Results from the low‐velocity domain also confirmed that Love full waveform inversion was comparable or superior to Rayleigh full waveform inversion, though the performance difference was less significant. However, the 25‐Hz frequency inversions yielded superior results than the 50‐Hz frequency inversions for the low‐velocity domain because the dominant wavelength of the high‐frequency signals becomes so small that it offers an impractically small investigation depth.

Source characteristics significantly affect the signals generated during seismic testing. Proper source selection plays a major role in data quality and can potentially improve investigation outcomes. This is particularly true for surface wave testing where the goal is to establish the frequency variation of phase velocity. Little research has compared the input energy caused by different base plates when impact sources are used. Consequently, data were collected using the Multichannel Analysis of Surface Waves (MASW) method with three base plate configurations (aluminum, aluminum over a rubber mat, and polyethylene) and two different sledgehammers. The variations in signal frequency content and amplitude spectra, energy, and dispersion images were systematically analyzed. There were appreciable differences in the energy introduced by different base plate configurations. Both the aluminum/rubber and the polyethylene base plates led to power spectra increases relative to the aluminum base plate. Subsequently, the aluminum/rubber and polyethylene base plates typically transferred more energy into the subsurface. This was not necessarily the case when the base plates were used on soft surficial soils. The variations in predominant frequency content were also less substantial, though the aluminum/rubber and polyethylene base plates developed slightly lower-frequency content at the expense of higher-frequency components in the dispersion curves. Despite the noticeable differences in energy transfer and frequency content, the base plate materials did not appreciable alter interpretation of the dispersion behavior at the sites given the uncertainty present in the dispersion images. This highlights that the selection of MASW base plate materials can be correspondingly driven by practical considerations such as noise, portability, and durability. Consequently, base plate materials with viscoelastic characteristics are a promising alternative to conventional metallic plates for coupling impact sources in surface wave testing, though care should be exercised when employing them at sites with soft ground conditions.

There are a significant number of bridges for which information regarding the foundation is missing or incomplete. It is extremely challenging to evaluate the performance of such unknown foundation bridges (UFB), particularly against scour or when their foundations are reused. For critical UFB, it is often necessary to estimate performance by developing an appropriate working model of subsurface foundation conditions. Typically, nondestructive testing (NDT) has served as the most viable alternative due to the costs and risk associated with excavation, coring, and probing. NDT can be quite difficult to perform and their results can contain significant uncertainty in highly urban settings. The primary objective of this study was therefore to compare performance of borehole NDT methods when evaluating the depth of two in-service unknown foundations (concrete-filled steel pipe piles and H-piles) in highly urban settings. The borehole magnetic, parallel seismic, borehole sonic, and borehole radar methods were implemented to determine the foundation bottom locations. Though uncertainty was present in all measurements, the borehole magnetometer and radar results proved the most conclusive. Parallel seismic testing did not yield any evidence of foundations due to issues with background noise and lack of direct access to the foundation. Likewise, borehole sonic testing was generally inconclusive due to issues with sensor directivity and attenuation. Borehole magnetometer estimated the depth to the foundation bottom as 8.6 m and 9.2 m at the two sites. Borehole radar estimates for the depth to foundation bottom compared favorably at 9.8 m and 8.0 m for the two sites. Given the borehole construction and depth to competent rock at the sites, these results for borehole magnetometer and radar were likely a minimum estimate for the location of the foundation bottoms. Such information can help evaluate long-term performance of this system as part of rehabilitation and reuse efforts.

Construction of drilled shafts for transportation infrastructure presents quality assurance challenges related to structural anomalies, including voids and degraded concrete. Efforts to evaluate anomalies often use stress-wave non-destructive testing (NDT) techniques such as the crosshole sonic logging (CSL) method. CSL relies on the propagation of stress waves between access tubes inserted alongside the reinforcement cage. Differences in travel time and amplitudes can be used to estimate concrete quality within the source-receiver ray paths. Crosshole tomography (CT) advances this approach further and models ray paths to solve an inverse problem based on first arrival times. However, both stress-wave methods cannot typically evaluate anomalies smaller than 10% to 15% of the shaft cross sectional area. Recent advances in wave propagation modeling have led to the development of full waveform inversion (FWI) approaches. FWI attempts to generate a more detailed tomographic model that matches the entire waveform recordings. This paper presents findings from a numerical study where full waveform tomography was performed on a portion of a drilled shaft with a structural anomaly. Crosshole stress-wave propagation was simulated using a spectral element method (SEM) forward modeler. The resulting waveforms were then used as inputs into a CT travel-time inversion algorithm using ray-path modeling and into a FWI algorithm using the entire recorded signals. The results highlighted that FWI estimated concrete velocities and the geometry of the anomaly more accurately than CT in the idealized numerical model. This was the case even though information from more sources and receiver pairs was used for the CT inversion.

Stress-wave based nondestructive testing (NDT) systems such as cross-hole sonic logging (CSL), low strain integrity testing, and ultrasonic echo devices (e.g. Sonicaliper) are routinely deployed fo...

Foundation length is an important parameter when assessing bridge vulnerability to failures related to scour hazards. As a result, multiple non-destructive testing methods have been developed to evaluate the geometry of unknown foundations, each with their own advantages and disadvantages. Subsurface methods that rely on measurements from boreholes alongside the foundation are often the most robust when evaluating foundation length, particularly for complex foundations (e.g. footings supported on piles). In this study, such a system was developed to evaluate the length of foundations at two bridge sites in Philadelphia using ultrasound acoustic waves (i.e. P-waves). Characteristics of the foundations and the site conditions are summarised as well as the ultrasound system components. The system was lowered in a borehole alongside each foundation as 100 kHz P-waves were generated to develop a reflection image of the pile–soil interface. Foundation length was evaluated based on discontinuity of the reflected...

An ultrasonic P-wave reflection imaging probe is developed and utilized to noninvasively image the geometry of a deep foundation supporting a bridge pier. The source ultrasonic transducer emits compressive waves into saturated soil that subsequently transmits to and reflects back from an embedded object, and the receiver transducer measures the reflections that are used to construct an image. The components of the system, including the custom transducer probe and data acquisition hardware, were integrated with the nees@UCLA cone penetration testing truck. Some fundamentals of propagation of ultrasonic waves in soil are presented first, including transducer directivity, effects of unsaturation, the size of the internal scale of the soil, and transducer coupling. The system components are then presented, followed by field testing. The probe successfully imaged a pile foundation in very soft saturated clay but not in stiffer and/or unsaturated soils. The probe could be very useful for nondestructive quality assurance of structural elements constructed in situ in soft saturated soils.

An ultrasonic p -wave reflection imaging system is used to noninvasively image submerged soil models with embedded anomalies and complex geometric layer contacts. The ultrasonic transducers emit compressive waves into water that subsequently transmit into the underlying soil, and measurements of the reflections are used to construct the images. The properties of the transducers and data acquisition hardware and software are explained. Fast signal stacking is used to improve signal-to-noise ratio and provide clearer images. Transducer directivity is explained as a wave passage effect, and transfer functions are derived for square and circular transducers to quantify directivity. The transfer functions agree reasonably with measured amplitude data. The cause of errors in the imaged position of dipping reflectors is explained, and a Kirchhoff migration algorithm is implemented to correct these errors. A soil model consisting of embedded high- and low-impedance anomalies, dipping soil layer contacts, and an u...

Ground strains were estimated from vibrations measured during pile driving operations at a buried, prehistoric archeological site to monitor potential construction impacts. Subsurface characteristics of the site were investigated using multiple cone penetration test (CPT) soundings and the shear wave velocity profile was measured using the seismic CPT method. Embedded geophones and surface accelerometers were then used to measure ground vibrations during pile driving. Displacement gradients were estimated from the vibrations using the following three methods: (1) the difference between adjacent displacements divided by sensor spacing; (2) peak particle velocity divided by depth-dependent wave velocity (i.e., at the depth where the sensor was placed); and (3) peak particle velocity divided by frequency-dependent wave velocity from a measured dispersion curve. Methods (1) and (3) agreed well, while method (2) caused errors that depended on depth of embedment of the sensors and distance from pile driving. Errors in (2) were attributed to a mismatch between the depth-dependent wave velocity and the wave velocity on the frequency band that carried the largest velocity pulse through the dispersive soil profile. Ground strains were related to displacement gradients based on theoretical solutions of harmonic body waves and Rayleigh waves in dispersive elastic media. The peak estimated ground strains were smaller than the threshold volumetric shear strain, but a few centimeters of settlement were nevertheless observed at the site. The spatial extent of the settlement is characterized using attenuation rules fit to the vibration data, and by calibration with a settlement gauge. Ground cracking and vertical offsets that could potentially mask the archaeological history of the site were neither observed nor predicted from the observed vibration amplitudes. Estimated impact on archeological interpretation of artifacts in their stratigraphic context was likely insignificant except in the immediate region where the piles were driven. This insight will assist in future planning at sites with similar subsurface stratigraphy.

It has long been understood that the near-surface structure of the earth affects several aspects of civil engineering practice, particularly in seismic applications. Therefore, the implementation of a competent subsurface exploration program not only addresses the need for comprehensive site characterization but also reduces the devastating consequences of seismic activities. Typical geotechnical practice for subsurface exploration has often relied on a combination of drilling, in situ testing such as standard penetration testing and cone penetration testing, and laboratory testing of field samples. However, geophysical techniques have been developed over the years to evaluate near-surface velocity models as a proxy for subsurface stiffness in seismic applications. In this regard, seismic geophysical methods such as refraction, reflection, downhole and crosshole testing, spectral analysis of surface waves (SASW), and multichannel analysis of surface waves (MASW) have continued to receive increased attention due to their advantages over traditional geotechnical subsurface exploration techniques. This has particularly been the case in the United States and in more developed countries across Europe and Asia. However, other countries have been slower to adopt such methods, and a comprehensive review of their domestic research literature can highlight potential improvements in their standards of practice for site characterization. Considering Iran, the domestic literature on seismic near-surface geophysical methods was investigated by examining the temporal variation of several parameters of interest, including the number of published studies, methods used (e.g., active versus passive), depth of investigation, geometry of subsurface profiles developed, and the desired application. Based on this review, it was noted that more than half of the seismic geophysical studies in Iran are exclusively relying on active approaches. Of these active-based methods, more than half rely on conventional seismic refraction. Moreover, approximately 75% of studies target depths equal to 30 m or shallower, and the overwhelming majority of publications (more than 80%) develop 1D profiles. Finally, it was observed that a lag time of multiple years was often required for state-of-the-art methods to be used in Iran, particularly in the case of refraction microtremor, SASW, and MASW. This lag time also implies that these methods have not been used domestically in certain applications where other countries have grown more accustomed to using these methods. Based on results of the increased rate of usage in recent years, it is expected that seismic geophysical testing, especially surface-wave methods, will become more common in Iran and that researchers as well as practitioners will adopt these methods for near-surface site characterization and other applications.