Jianghai Xia

Advantages of Using Multichannel Analysis of Love Waves (MALW) to Estimate Near-Surface Shear-Wave Velocity

As theory dictates, for a series of horizontal layers, a pure, plane, horizontally polarized shear (SH) wave refracts and reflects only SH waves and does not undergo wave-type conversion as do incident P or Sv waves. This is one reason the shallow SH-wave refraction method is popular. SH-wave refraction method usually works well defining near-surface shear-wave velocities. Only first arrival information is used in the SH-wave refraction method. Most SH-wave data contain a strong component of Love-wave energy. Love waves are surface waves that are formed from the constructive interference of multiple reflections of SH waves in the shallow subsurface. Unlike Rayleigh waves, the dispersive nature of Love waves is independent of P-wave velocity. Love-wave phase velocities of a layered earth model are a function of frequency and three groups of earth properties: SH-wave velocity, density, and thickness of layers. In theory, a fewer parameters make the inversion of Love waves more stable and reduce the degree of nonuniqueness. Approximating SH-wave velocity using Love-wave inversion for near-surface applications may become more appealing than Rayleigh-wave inversion because it possesses the following three advantages. (1) Numerical modeling results suggest the independence of P-wave velocity makes Love-wave dispersion curves simpler than Rayleigh waves. A complication of “Mode kissing” is an undesired and frequently occurring phenomenon in Rayleigh-wave analysis that causes mode misidentification. This phenomenon is less common in dispersion images of Love-wave energy. (2) Real-world examples demonstrated that dispersion images of Love-wave energy have a higher signal-to-noise ratio and more focus than those generated from Rayleigh waves. This advantage is related to the long geophone spreads commonly used for SH-wave refraction surveys, images of Love-wave energy from longer offsets are much cleaner and sharper than for closer offsets, which makes picking phase velocities of Love waves easier and more accurate. (3) Real-world examples demonstrated that inversion of Love-wave dispersion curves is less dependent on initial models and more stable than Rayleigh waves. This is due to Love-wave’s independence of P-wave velocity, which results in fewer unknowns in the MALW method compared to inversion methods of Rayleigh waves. This characteristic not only makes Love-wave dispersion curves simpler but also reduces the degree of nonuniqueness leading to more stable inversion of Love-wave dispersion curves.

Mapping Poisson's Ratio of Unconsolidated Materials from a Joint Analysis of Surface‐Wave and Refraction Events

Seismically, σ can be determined if P(Vp) and S-wave (Vs) velocities are known. This would indicate that two separate (Pand S-wave) surveys should be performed in order to obtain the separate maps for Vp and Vs. Running both types of survey for one project will be expensive in terms of equipment, data processing, and overall time. In addition, S-wave survey is generally known as being much more difficult to acquire good quality data than the P-wave survey.

Evaluation of the MASW technique in unconsolidated sediments

Shear (S) wave velocities derived from the MASW (multi-channel analysis of surface wave) technique and borehole measurements at seven well locations in unconsolidated sediments of the Fraser River Delta are compared. The overall difference between these two sets of S-wave velocities is about 15%. S-wave velocities from the MASW technique at an additional location are also obtained and await comparison with borehole measurements.

Configuration of Near‐Surface Shear‐Wave Velocity by Inverting Surface Wave

The shear (S)-wave velocity of near-surface materials (such as soil, rocks, and pavement) and its effect on seismic wave propagation are of fundamental interest in many groundwater, engineering, and environmental studies. Ground roll is a Rayleigh-type surface wave that travels along or near the surface of the ground. Rayleigh wave phase velocity of a layered earth model is a function of frequency and four earth parameters: Swave velocity, P-wave velocity, density, and thickness of layers. Analysis of the Jacobian matrix in a high frequency range (5-30 Hz) provides a measure of sensitivity of dispersion curves to earth model parameters. S-wave velocities are the dominant influence of the four earth model parameters. With the lack of sensitivity of the Rayleigh wave to Pwave velocities and densities, estimations of near-surface S-wave velocities can be made from high frequency Rayleigh wave for a layered earth model. An iterative technique applied to a weighted equation proved very effective when using the LevenbergMarquardt method and singular value decomposition techniques. The convergence of the weighted damping solution is guaranteed through selection of the damping factor of the Levenberg-Marquardt method. Three real world examples are presented in this paper. The first and second examples demonstrate the sensitivity of inverted S-wave velocities to their initial values, the stability of the inversion procedure, and/or accuracy of the inverted results. The third example illustrates the combination of a standard CDP (common depth point) roll-along acquisition format with inverting surface waves one shot gather by one shot gather to generate a cross section of S-wave velocity. The inverted S-wave velocities are confirmed by borehole data.

Practical Aspects of MASW Inversion Using Varying Density

We use multi-channel analysis of surface waves (MASW) on seismic data to assess the practical impact of the assumed compressional-velocity and density parameters on the inverted shear-wave velocity results. The practical secondary-parameter impact evaluation was performed on data acquired in the arctic. The ice-sheet compressional-, shear-wave velocity and density variations with depth were available from laboratory and field measurement and served as reference for analyzing the MASW inversion. Presented MASW results demonstrate that the a-priori knowledge of density variation with depth can reduce the final inverted shear-wave velocity results typically by 6-7 % for the most of the middle layers from the velocity model and as much as 15% for some individual layers. After comparing the optimal parameter selection approach with the general theoretical recommendations it was concluded that using constant density leads to overestimation of shear-wave velocities by 7-8 %. Such an overestimation can lead to liquefaction underestimation at earthquake sites. Therefore, these findings can be of value for the engineering community interested in accurate shear-modulus site evaluation. Optimal density parameter selection can improve the reliability and quality of the final 2D Vs section. However, optimal processing MASW parameter selection, such as density variation with depth, can be best achieved using field tests and is the preferred approach over theoretical recommendations.

Reason and condition for mode kissing in MASW method

Identifying correct modes of surface waves and picking accurate phase velocities are critical for obtaining an accurate S-wave velocity in MASW method. In most cases, inversion is easily conducted by picking the dispersion curves corresponding to different surface-wave modes individually. Neighboring surface-wave modes, however, will nearly meet (kiss) at some frequencies for some models. Around the frequencies, they have very close roots and energy peak shifts from one mode to another. At current dispersion image resolution, it is difficult to distinguish different modes when mode-kissing occurs, which is commonly seen in near-surface earth models. It will cause mode misidentification, and as a result, lead to a larger overestimation of S-wave velocity and error on depth. We newly defined two mode types based on the characteristics of the vertical eigendisplacements calculated by generalized reflection and transmission coefficient method. Rayleigh-wave mode near the kissing points (osculation points) change its type, that is to say, one Rayleigh-wave mode will contain different mode types. This mode type conversion will cause the mode-kissing phenomenon in dispersion images. Numerical tests indicate that the mode-kissing phenomenon is model dependent and that the existence of strong S-wave velocity contrasts increases the possibility of mode-kissing. The real-world data shows mode misidentification caused by mode-kissing phenomenon will result in higher S-wave velocity of bedrock. It reminds us to pay attention to this phenomenon when some of the underground information is known.

Using Surface Wave Method to Define a Sinkhole Impact Area in a Noisy Environment

A sinkhole developed at Calvert Cliffs Nuclear Power Plant, Maryland in early 2001. To prevent damage to nearby structures, the sinkhole was quickly filled with dirt (approximately 40 tons). However, the plant had an immediate need to determine if more underground voids existed. The location of the sinkhole was over a groundwater drainage system pipe buried at an elevation of +3 feet (reference is to Chesapeake Bay level). Grade in the sinkhole area is +45 feet. The subsurface drain system is designed to lower the local water table from approximately +20 feet above Bay level to +10 feet. The subsurface drain system is connected to the top of the condenser cooling water discharge conduit at an elevation of -4 feet. The cause of the sinkhole was a subsurface drain pipe that collapsed due to saltwater corrosion of the corrugated metal pipe. The inflow/outflow of sea water and ground water flow caused dirt to be removed from the area where the pipe collapsed. A high-frequency surface-wave survey was conducted to define the sinkhole impact area. Five surface-wave lines were acquired with limited resources: a 24-channel seismograph with a hammer and an aluminum plate as a source. Although the surface-wave survey at Calvert Cliffs Nuclear Power Plant was conducted at a noise level 50-100 times higher than the normal environment for a shallow seismic survey, the shear (S)-wave velocity field calculated from surface-wave data delineated a possible sinkhole impact area. The S-wave velocity field showed chimney-shaped low-velocity anomalies that were directly related to the sinkhole. Based on S-wave velocity field maps, a potential sinkhole impact area was tentatively defined. S-wave velocity field maps also revealed, depending on the acquisition geometry, one side of the water tunnel of the power plant.

Seismic Reflection: Upstream, Downstream, And On Earthen Dams And Dikes

High-resolution seismic reflection has been used successfully to characterize material and investigate a variety of problems associated with earthen dams and abutments. Limitations and challenges of seismic reflection when interrogating these structures and lithologies are nontrivial and require very critical thinking. Seismic reflection has proved an effective tool for mapping confining units for integration into the cutoff wall at the Keechelus Dam in Cle Elum, Washington; mapping lithologies and bedrock structures for earthquake retrofitting at Bend, Oregon; delineating karst in bedrock beneath the dam core responsible for subsidence on the upstream side of a major flood control structure at Clearwater Dam, Missouri; and detecting high permeability zones within a glacial outwash embankment of a water retention dam near Enumclaw, Washington. Extreme geometries and material variability associated with any man-made structure are the most formidable challenge to seismically imaging. Inconsistent source wavelets, out-of-the-plane energy, extreme statics (topography and velocity based), and source noise (disproportionately high percentage of surface waves) are all problems that are not unique to earthen dams, dikes, and levees, but they are certainly more prevalent with those types of structures. Success of the technique in these settings is source characteristics and spatial oversampling.

Near-surface shear wave reflection surveys in the Fraser River delta, B.C., Canada

Summary Shallow shear wave reflection surveys using high frequency vibroseis techniques provides information about the Tertiary bedrock surface and Pleistocene sediments on the Fraser River delta, in B.C., Canada. Besides the clear advantage of shear wave profiling in this shallow gas environment, the added resolution potential and ability to measure the shear wave velocity field enhances the fusion of this surface seismic data into earthquake site response estimations. Surface materials ranging from undisturbed, native delta sediments to clay/ rubble fill used in dike construction seemed amenable to the generation and recording of shear waves when using a small (6,000 kg) vibrator and engineering recording systems. The thickness of Pleistocene sediments and depth to Tertiary bedrock were mapped at three locations within the delta where gaps existed in the geologic record. These three sites provided cultural and near-surface settings that uniquely tested this shallow imaging technique.

Detection Of Higher Mode Surface Waves Over Unconsolidated Sediments By The Mx4 Wmethod

In engineering application of surface waves it is critically important to accurately extract the fundamental mode dispersion curve. Among several factors that may adversely affect the extraction is the existence of higher modes with significant amount of energy. A calculated phase velocity can be an average of the fundamental and the higher-modes phase velocities or it can be the phase velocity of a specific higher mode, depending upon the specific method used for the application, unless the higher modes are properly handled during the data acquisition and processing steps. Therefore, it will have a practical value to observe the higher mode generation through field experiments and examine for any parameter that can be controlled during data acquisition.