Additional Information
Applications
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Multichannel Analysis of Surface Waves
The Multichannel Analysis of Surface Waves (MASW) is a relatively new approach for obtaining near surface shear wave velocity information, and determining layer depth and thickness. It can also be used to locate and roughly delineate subsurface anomalies such as voids.
A key advatage to this innovative seismic method is that it measures ambient seismic noise, therefore it can be used in noisy areas where seismic reflection and refraction cannot be used, such as construction zones and urban environments. The source – in this case noise – are Rayleigh waves produced by passing vehicles, industrial plants, and such, in the area of the survey. Rayleigh waves have velocities that depend on their wavelength, a phenomenon called dispersion. Short wavelengths sample to shallow depths, with the longer wavelengths sampling to greater depths. Using a dispersion curve, and applying the ratio of Raleigh wave velocity to shear wave velocity (app. 0.9:1), the shear wave velocity can be estimated. The shear stiffness (G) of the ground can thus be calculated from the shear velocity when material density is known.
MASW has a relatively wide range of applications which include:
- rippability studies
- void detection
- basement mapping
- determine layer depth
- fault mapping
- provide shear wave velocities
- NEHRP soil classification
When conducting MASW investigations, if ambient noise is not present at the site, other sources may be employed. The heavier the source (low frequency), the deeper the penetration is. A combination of both high frequency sources and low are often used to measure dispersion over a broader bandwidth to explore from just below grade to depths as great as 100 feet. Typical sources range from sledgehammers and dropped weights to bulldozer and vibroseis trucks.
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Shear-wave dispersion curve(top) derived from ambient seismic noise, shown on the bottom, is picked and modeled. |
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