Surface Wave Method for Shallow Soil Exploration
Zhiqu Lu - zhiqulu@olemiss.edu
James M. Sabatier - sabatier@olemiss.edu
National Center for Physical Acoustics, The University of Mississippi
1 Coliseum Dr. University, MS 38677, USA
Popular version of paper 1pPA2
Presented Monday afternoon, May 18, 2009
157th ASA Meeting, Portland, OR
The multi-channel analysis of surface waves (MASW) method is a geophysical technique developed in 1999 [1-2] and used for near-surface soil exploration. One of the remarkable advantages of the MASW method is its non-invasive nature. All the measurements are made on the surface of the ground without the need to drill holes and put sensors at the depth explored. It is rapid, cost-effective, and environmentally friendly.
The MASW exploits the characteristics of Rayleigh waves (named in honor of John Strutt, 3rd Baron Rayleigh) to obtain shear wave velocities of soils at different depths, i.e. the soil profile. The shear wave velocity is an important parameter related to soil physical properties and is widely applied for soil characterization. Rayleigh waves travel along a surface such as an open ground and act like a “ground roll.” Rayleigh waves decay rapidly with depth, and the propagation is confined to a limited depth of soils, typically about one wavelength. Therefore, Rayleigh waves with longer wavelengths (corresponding to lower frequency) penetrate to greater depth and are affected by deeper layer soil properties. This characteristic enables extraction of soil properties of different depths by analyzing Rayleigh waves signals measured at the surface.
A MASW method consists of several parts: (1) surface wave generation, (2) multi-channel surface wave recording, (3) the dispersive curve determination, and (4) inversion.
Depending on the depth explored, Rayleigh waves or surface waves can be actively generated by impacting or vibrating the ground vertically with devices such as sledgehammers, electromechanical shakers, heavy weight drops, or track-mounted vibrators, called vibroseis.
A number of geophones are placed on the ground along a straight line with identical separation to measure vertical surface vibrations. Recently, a device called land streamer, a geophone array connected by a cable and towed by a vehicle, is being used more often to speed up testing. In this study a laser Doppler vibrometer driven by a stepper motor was used as a non-contact sensor to detect surface vibrations.
The waveforms of detected surface waves are transferred into a frequency-domain through Fast Fourier Transform (FFT). The dispersive curve, i.e. the phase velocity of Rayleigh waves as a function of frequency, is determined from the trace of maximum amplitude of the surface waves.
Using wave propagation theory and an initial guess of a soil profile, an iterative algorithm performs the inversion of the dispersive curve to obtain the shear wave velocity as a function of depth, the shear wave velocity profile.
Repositioning the source and geophone array and repeating the MASW procedure, a series of soil profiles can be obtained, which can be used to construct a two-dimensional shear wave velocity image.
Applications of the MASW method may include many areas such as pavement quality evaluations, foundation studies, soil compaction control, agricultural land management, subsurface layer imaging, levee and dam embankment integrity assessment, and detection of geological anomalies (voids, pipes, tunnels, unexploded ordnances, and landmines).
[1] Park, C.B., R.D. Miller, and J. Xia. 1999. Multichannel analysis of surface waves. Geophysics, Vol. 64. 800-808.
[2] Xia, J., R.D. Miller, and C.B. Park. 1999. Estimation of near-surface shear-wave velocity by inversion of Rayleigh waves. Geophysics Vol. 64. 691-700.