Seismic surface-wave analysis

The multi-channel analysis of surface waves (MASW) is a widely employed surface wave method. The fundamental mode inversion is the usual scheme in MASW, because separating the fundamental mode from other high modes is feasible with an adequately long array. However, the extracted fundamental mode represents only part of the dynamic characteristics, it may deviate from the theoretical fundamental mode due to truncation effect, and part of it may correspond to another higher mode or leaky mode, resulting in mode misidentification. Conversely, the spectral analysis of surface waves (SASW) method features a more rigorous inversion scheme by matching the effective mode, but it may suffer from the tricky phase-unwrapping in the dispersion data reduction. This study introduces an improved dynamic response solution for elastic-layered media subjected to vertical loads. The proposed dynamic response solution is fast and accurate, facilitating the full wavefield inversion in terms of the frequency-velocity spectrum. The MASW frequencyvelocity spectrum inversion considers testing configuration and comprehensively models all wave phenomena, including near-field effect, truncation effect, and leaky waves. The MASW frequency-velocity spectrum inversion merges the convenience of MASW dispersion analysis with the rigorous inversion scheme adopted by SASW. The new MASW frequency-velocity spectrum inversion is compared with the SASW effective mode inversion and MASW fundamental mode inversion. The results show that both SASW effective mode inversion and MASW frequency-velocity spectrum inversion produce better inverted results than the MASW fundamental mode inversion, while the process of MASW frequency-velocity spectrum inversion is more convenient and robust. Finally, a field example is used to demonstrate the applicability of frequency-velocity spectrum inversion.

SUMMARY A 2-D orthogonal distributed acoustic sensing (DAS) array designed for seismic experiments was buried horizontally beneath the Kafadar Commons Geophysical Laboratory on the Colorado School of Mines campus at Golden, Colorado. The DAS system using straight fibre-optic cables is a cost-efficient technology that enables dense seismic array deployment for long-term seismic monitoring, favouring both earthquake-based and ambient-noise-based surface wave analysis for subsurface characterization. In our study, the horizontally orthogonal DAS array records ambient noise data for a period of about two months from November 2018 to January 2019. During this time, the array also detected seismic signals from an ML3.6 earthquake at Glenwood Springs, Colorado, which exhibit opposite signal polarities in the orthogonal DAS section recordings. We derive the transformation matrix for DAS strain measurements in horizontally orthogonal cables to retrieve both Rayleigh and Love wave dispersion information from the single-component DAS signals using the 2-D multichannel analysis of surface waves method. In addition, ambient noise interferometry is applied to long-term DAS noise recordings. Our theoretical derivation demonstrates that Rayleigh and Love wave Green's functions are coupled in the noise cross-correlation functions (NCFs) of DAS receiver pairs. Stacking NCFs over the horizontally orthogonal DAS array can constructively recover the radial Rayleigh wave component but destructively suppress the Love wave component. The multimodal Monte Carlo inversion of the earthquake-based Rayleigh wave and Love wave dispersion measurements and the noise-based Rayleigh wave measurement reveals a 1-D layered structure that agrees qualitatively with geological surveys of the site. Our study demonstrates that although straight fibre-optic cables lack broadside sensitivity, using appropriate DAS array configuration and seismic array methods can extend the seismic acquisition ability of DAS and enable its application to a broad range of scenarios.

We use the multi‐channel analysis of surface waves (MASW) method to analyze synthetic seismic data calculated using models with high shear‐wave velocity (Vs) contrast. The MASW dispersion‐curve images of the Rayleigh wave are obtained using various sets of source‐offset and spread‐size configurations from the synthetic seismic data and compared with the theoretically calculated fundamental‐ and higher‐mode dispersion‐curves. Such tests showed that most of the dispersion‐curve images are dominated by higher‐mode energy at the low frequencies, especially when analyzing data from long receiver offsets and thus significantly divert from numerically expected dispersion‐curve trends, which can lead to significant Vs overestimation. Further analysis showed that using data with relatively short spread lengths and source offsets can image the desired fundamental‐mode of the Rayleigh wave that matches the numerically expected dispersion‐curve pattern. As a result, it was concluded that it might be possible to avoid higher‐mode contamination at low frequencies at sites with high (Vs) contrast by appropriate selection of spread size and seismic source offset.

\nResearch in the important parameters for modeling the dynamic behavior of soils has led to rapid development of the small strain stiffness and damping ratio for use in the seismic method. It is because, the experimental determination of the damping ratio is problematic, especially for hard soils sample. Many researchers have proved that the surface wave method is a reliable tool to determine shear wave velocity and damping ratio profiles at a site with very small strains level. Surface wave methods based on Rayleigh waves propagation and the resulting attenuation curve can become erroneous when higher modes contribute to the soil’s response. In this study, two approaches has been used to determine the shear strain amplitude and damping ratio of residual soils at small strain level using Spectral Analysis of Surface Wave (SASW) method. One is to derive shear strain amplitude from the frequency-response curve and the other is to derive damping ratio from travel-time data. Then, the results are compared to the conventional method.\n

This study focuses on advancements in three broad aspects of active surface wave methods: modeling, testing, and inversion. Transfer matrix, global matrix, and stiffness matrix methods were employed to comprehensively model layered structures with half-space boundary conditions for soil structures with increasing/anomalous stiffness profiles, or pavement structures with decreasing stiffness profiles and leaky waves. All three methods were programmed in MATLAB as forward algorithms. The finite element method was adopted to simulate surface wave testing for various half-space site structures with absorbing layers using increasing damping technique. An improved experimental dispersion analysis scheme was developed by scanning phase-velocity and intercept-time followed by a power-spectrum analysis to minimize side lobes and increase sharpness of dispersion images. The multichannel simulation with one receiver (MSOR) method was applied to capture the dispersion characteristics of soil sites. The reciprocity principle for surface Rayleigh waves was verified by comparing dispersion images from MSOR and multichannel analysis of surface waves (MASW) testing at the same site with the same testing system. A multichannel surface wave acquisition system was developed to improve the accuracy of measuring high-frequency and high-velocity dispersion data on pavement sites. A minimally-invasive multimodal surface wave (MMSW) method was proposed to measure multi-mode dispersion data of Rayleigh waves by using either embedded receivers at various depths to record surface wave motions generated from moving impacts on the ground surface or using a multichannel seismograph with an array of geophones on the soil surface for measuring surface wave motions caused by Standard Penetration Test hammer blows at various depths in a borehole. Stiffness matrix and finite element simulations of the MMSW method were employed to identify the critical geophone depths for optimum measurement of higher-mode motions. A hybrid genetic-simulated-annealing (GSA) algorithm was applied to solve multiple minimization and non-linear optimization problems to match the theoretical dispersion curves against their experimental counterparts. Results from simulation and real-world studies demonstrate that the advancements made in the three aspects of surface wave methods can improve the accuracy of surface wave testing results with higher resolution of experimental dispersion data, more complete multi-modal dispersion data, and higher certainty of inversion.

Time-averaged shear wave velocity of the soil layers to a depth of 30 m (Vs30) is primarily estimated by different geophysical and geotechnical site investigation techniques, such as downhole and crosshole seismics (DS and CS), spectral analysis of surface waves (SASW), multichannel analysis of surface waves (MASW), and the standard penetration test (SPT). Sufficiently dense field measurements of the Vs30 using these techniques are not feasible for VS30 mapping at a regional scale. Therefore, secondary information, such as geological and topographical constrains are used as proxies to predict the Vs30 for mapping purposes. In the present study, a Vs30 map for Sylhet City has been prepared based on the geological and topographical constrains. The map has five Vs30 units having a distinct mean Vs30 value for each unit. This Vs30 map can be used as an effective proxy to estimate site response of the soil layers in Sylhet City.

Determining Q of near-surface materials from Rayleigh waves

Estimation of near-surface shear-wave velocities and quality factors using multichannel analysis of surface-wave methods

Surface cracks directly influence the integrity of asphalt pavement structures, the durability of the pavement, and driving safety. Assessment of cracking distress is of vital importance for pavement maintenance and rehabilitation. Compared with other methods, the spectral analysis of surface waves (SASW) method, a seismic wave-based nondestructive method, has advantages in estimating the deterioration of modulus and quantitatively evaluating the depth and severity of surface cracks by means of the Rayleigh wave propagation characteristic. The objectives of this paper are to monitor long-term attenuation characteristics of in-situ modulus of a semi-rigid asphalt pavement, to determine when the surface-opening cracks will occur as associated to the degree of modulus reduction, and to assess the depth of surface-opening crack through the dispersion characteristics of Rayleigh wave propagation using the SASW method. First, a general trend of modulus deterioration of asphalt layer was developed in an accelerated pavement testing (APT). Then the factors affecting the dispersion characteristics of the Rayleigh wave were determined through theoretical derivation. Finally, a series of experimental tests on a pavement segment with well-controlled surface-opening cracks was performed to explore how a surface-opening crack in asphalt pavement would vertically influence the propagation of Rayleigh waves at different crack widths. It was found that the general trend of modulus deterioration could be divided into four stages, and the surface-opening cracks occurred in the fourth stage with 40–50% modulus reduction rate. In addition, the relationship between crack depth and the shortest wavelength in the Rayleigh wave dispersion curve was developed according to different crack widths.

We performed active and passive surface wave methods at four sites in the Cherry Creek fault zone (CCFZ) in Lake Chaplain, Washington to detect the depth to bedrock below Quaternary deposits. The surface wave method was performed at four sites (PS-3, PS-1, PS-4 a nd PS-5) from east to west across the CCFZ with approximately one kilometer separations. We used a multichannel analysis of surface waves (MASW), a passive surface wave measurement using 4.5Hz geophones on a linear array (Linear-MAM) and a passive surface wave measurement using broadband accelerometers (SPAC) to obtain dispersion curves. Dispersion curves obtained by the three different methods are in excellent agreement in ranges of overlap. Maximum wavelengths obtained using the 2ST-SPAC, Linear-MAM and MASW were about 850, 200 and 50 m, respectively. As a rule of thumb, 1/2 to 1/3 of the maximum Rayleigh wave wavelength is indicative of the penetration depth of the surface wave method. The extremely deep penetration capability of the SPAC is obvious when compared to conventional surface wave methods, such as MASW or ReMi™. Phase velocities obtained from three methods were combined to produce a single dispersion curve for each site. The inversion scheme based on Genetic Algorithm was applied to the observed dispersion curves, resulting in the S-wave velocity site profiles. Results from the 2ST-SPAC survey near Sultan, Washington, near the Skykomish River (Figure 1a) provide estimates for the depth to bedrock below Quaternary sedimentary deposits as a companion study to the “Geologic map of the Lake Chaplin 7.5-minute quadrangle, King and Snohomish County, Washington” (Dragovich and others, 2014a). Four seismic survey sites marked PS-1, PS-3, PS-4 and PS-5 (Figure 1a) straddle the main strand of the Cherry Creek fault zone (CCFZ) as shown on Plate 1 of Dragovich and others (2013). Earlier study sites of Sultan B and Sultan C (Hayashi and others, 2014) are also shown for comparison (Figure 1a).

Automatic passive data selection in time domain for imaging near-surface surface waves

Combining seismic and electric methods for predicting bedrock depth along a Mediterranean soil toposequence

The use of MASW method in the assessment of soil liquefaction potential

The Multi-channel Analysis of Surface Wave (MASW) method is a shallow seismic method which may complement the traditional seismic refraction method used in engineering applications. Introducing this newly-developed method, this paper presents a case history of an MASW survey in a tailings dam, where the original topography has been covered with tailings (red mud) from an aluminium refinery over the years.An eight-line 3.6 kilometre MASW survey was carried out in the tailings dam of Queensland Alumina Limited in Boyne Island near Gladstone, Queensland to estimate the depth of the competent layer for construction of a new tailings dam wall.The field procedure used was similar to the off-end shooting method used for seismic reflection surveys. This was further simplified by the use of a purpose-built 24-channel land streamer. The seismic source was a weight dropping system mounted on a vehicle, which drops a 50kg steel weight from a height of 0.8 metres. The recording system used was a Seistronix RAS-24 seismograph. The data were analysed and inverted using the SurfSeis software by Kansas Geological Survey.The depths of the original surface and underlying clay are profiled in terms of their S-wave velocities. The result shows the effectiveness of the MASW method even where velocity reversals are present.

The primary outcome of this work was detection and delineating voids and mines in the cities of Baxter Springs, Galena, and Treece in southeastern Kansas that represented a potential risk to infrastructure and the safety of residents. We used the multi-channel analysis of surface waves (MASW) and two versions of the backscatter analysis of surface wave (BASW) seismic methods to interrogate the shallow subsurface. Each approach was to various degrees successful in providing unique characterizations and complementing the other with useful information about possible void/mine and/or subsurface roof failure locations. The combination of multiple processing techniques for analyzing surface-wave data provided more complete structural information about the sites and allowed high-confidence interpretations of the surface-wave results.