What is MASW?
MASW stands for Multichannel Analysis of Surface Waves and it can be used to determine depth to bedrock, shear strength or soil stability, liquefaction studies, sinkhole mapping, fault mapping and assessing earthquake resilience. The most common application is the determination of the shear strength, in other words the stiffness of the ground.
As the name suggests, MASW uses surface waves to determine the shear wave velocity, VS. This can be presented in 1D, 2D or 3D depending upon the project requirements.
The shear wave velocity is an elastic condition and defines ground stiffness under load, an important measure of ground stability, especially during earthquake events or for construction projects close to railways or heavily trafficked roads. In fact, it can be an extremely useful site investigation method ahead of construction of said same rail and road networks.
It is possible to determine shear wave velocity with other seismic methods that rely directly upon the shear wave motion, for example, shear wave reflection and refraction surveys. However, utilizing MASW, or one of the related surface wave methods such as SASW and ReMi (Spectral Analysis of Surface Waves and Refraction Microtremor) can be easier. This is due to the fact that surface waves are stronger than shear waves thus are less affected by ambient noise, and that ambient noise can even be used as a source.
Surface waves are the slowest seismic wave but the strongest. They travel in complex motion, where particles are both deformed and displaced. In earthquakes these are the waves that do the most damage.
Most common are “active” MASW investigations, which are carried out with a user generated seismic source (such as a hammer or weight drop) and are typically limited to 30-meter depth. “Passive” MASW survey is when ambient, low-frequency, noise acts as the source, and this can produce velocity soundings in excess of a 100m deep, albeit at reduced resolution compared to the “active” method.
To investigate surface waves, with the “active” method, you use a similar field procedure and layout as for refraction surveys and often data are gathered for both at the same time. The big differences come in the processing of the shot records, which is markedly different for surface wave analysis and requires dedicated software.
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