Multichannel analysis of surface waves

Abstract

The frequency-dependent properties of Rayleigh-type surface waves can be utilized for imaging and characterizing the shallow subsurface. Most surface-wave analysis relies on the accurate calculation of phase velocities for the horizontally traveling fundamental-mode Rayleigh wave acquired by stepping out a pair of receivers at intervals based on calculated ground roll wavelengths. Interference by coherent source-generated noise inhibits the reliability of shear-wave velocities determined through inversion of the whole wave field. Among these nonplanar, nonfundamental-mode Rayleigh waves (noise) are body waves, scattered and nonsource-generated surface waves, and higher-mode surface waves. The degree to which each of these types of noise contaminates the dispersion curve and, ultimately, the inverted shear-wave velocity profile is dependent on frequency as well as distance from the source.Multichannel recording permits effective identification and isolation of noise according to distinctive traceto-trace coherency in arrival time and amplitude. An added advantage is the speed and redundancy of the measurement process. Decomposition of a multichannel record into a time variable-frequency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency component in a unique and continuous format. Coherent noise contamination can then be examined and its effects appraised in both frequency and offset space. Separation of frequency components permits real-time maximization of the S/N ratio during acquisition and subsequent processing steps.Linear separation of each ground roll frequency component allows calculation of phase velocities by simply measuring the linear slope of each frequency component. Breaks in coherent surface-wave arrivals, observable on the decomposed record, can be compensated for during acquisition and processing. Multichannel recording permits single-measurement surveying of a broad depth range, high levels of redundancy with a single field configuration, and the ability to adjust the offset, effectively reducing random or nonlinear noise introduced during recording.A multichannel shot gather decomposed into a sweptfrequency record allows the fast generation of an accurate dispersion curve. The accuracy of dispersion curves determined using this method is proven through field comparisons of the inverted shear-wave velocity (v s ) profile with a downhole v s profile.

A 3D algorithm based on the combined inversion of Rayleigh and Love waves for imaging and monitoring of shallow structures

“…To extract surface wave dispersion curves, Aki (1957) proposed the spatial autocorrelation (SPAC) method, which has been generalized to the extended spatial autocorrelation method by Ohori et al (2002). Further methods use different transforms and they obtain similar results, for example, the slant-slack transform (McMechan & Yedlin 1981), the frequency-wavenumber analysis (maximum likelihood method, Capon et al 1967;Capon 1969;Horike 1985) and the multichannel analysis of surface waves Park et al 1999).…”

Assessment of soil compaction properties based on surface wave techniques

“…The inversion of dispersion curve was applied using a non-linear square technique to generate the one-dimensional shear wave velocity profile, as illustrated in Fig. 3(d) [12]. Table 1 shows the results of dry density from MASW survey and sand replacement test.…”

“…However, a new method called Multichannel Analysis of Surface Waves (MASW) has introduced, to develop an efficient and accurate method to estimate near-surface V S from ground roll. MASW technique has developed to overcome the weaknesses of the SASW, which used the dispersion characteristics of Rayleigh waves to determine the variation of the V S in layered systems with depth [12]. MASW technique is an ideal tool to speed up the control process in the evaluation of ground modifications without the requirement to include geogrid reinforcement [2].…”

Development of Continuous Surface Wave System for Stiffness Testing

“…The receiver array was then moved after each measurement in order to obtain the data for each range between the source and the receiver. By using the constraint from the empirical study for the near offset constraint of the distance between the source and the first receiver, d min is recommended in the paper [8][11] [12] as a function of the surface wave wavelength. Therefore, by assuming the Rayleigh wave phase velocity as 300 m/s to 400 m/s, the applicable frequency range of the piezoelectric transducer was between the 500 Hz to 10 KHz.…”