Multichannel Analysis of Surface Waves

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Multichannel Analysis of Surface Waves (MASW)

MASW is a seismic geophysical technique for estimating near surface soil properties.  MASW is carried out from ground surface with no disruption to underground utilities, is relatively easy and fast, and can be carried out at locations that are difficult to access for drill rigs or other investigative equipment.

Applications

• Seismic site characterization according to building codes
• Soil parameter estimation for vibrating machine foundations, wind turbines, towers, and high rise buildings
• Non-destructive evaluation of subsurface soil condition including hard-to-sample soil deposits, pre- and post-construction soil conditions
• Delineation of soft soil and/or contaminated soil
• Estimation of shear modulus of soil with depth
• Estimation of dynamic soil properties, such as shear wave velocity and low strain shear modulus
• Detection of underground cavities, utilities, and objects
• Assessment of soil condition for slope stability analysis
• Condition assessments for pavement and concrete structures

 

Field Setup

To carry out a MASW test, several transducers (i.e. geophones) are deployed in a line at constant spacing from a source (Figure 1).  A mechanical source is used to generate vibration in the ground that is recorded by the geophones. The vibration source is selected to produce enough energy to reach the design penetration depth.  A common source is a manual sledgehammer, though heavier or more complicated sources can be used depending on the required penetration depth and site condition.  Typical design investigation depths are 15 to 30 m below ground surface.

Theory

MASW uses the physical property that the penetration depth of Rayleigh waves is inversely proportional to frequency.  Thus, higher frequencies travel through shallower strata, and lower frequencies propagate mostly in the deeper layers.  Each frequency carries the information associated to a specific depth of the medium that it is traversing.  The recorded signals in the field (Figure 2), which are in time domain, constitute the basis of the calculation of phase velocity profile (dispersion curve) of the site. 

Subsequently, inversion of the constructed dispersion curve leads to the estimation of the shear wave velocity profile at the site.  Figure 3 shows the variation of shear wave velocity with depth and distance at a typical site.