The Multichannel Analysis of Surface Waves (or MASW) is a seismic survey technique used for evaluating the elastic properties and stiffness of the subsurface soils for geotechnical purposes. The propagation of the surface waves in the sub-surface soils depends on the shear-wave velocity (Vs) distribution and on the elastic constants on the materials such as the Young’s modulus. The shear wave velocity is directly related to the stiffness of the soils. The velocity information and estimated parameters using MASW can be presented to the geotechnical in profiles, cross-sections, plan maps and 3D earth models.
The data acquisition for MASW is done similar to Refraction Seismic using the same survey spread, energy source and instrumentation. Data processing of MASW consists of four basic steps:
Recording and processing the surface waves
Constructing the dispersion models
Calculating the dispersion curves
Calculating the velocity variation with depth
The Multichannel Analysis of the Surface Waves (MASW) is an active source seismic methodology which record shear waves that propagate at the surface form the excitation source. The analysis is based in the Raleigh wave dispersion phenomena. The shear wave velocity estimated with MASW is directly related to the soil elastic moduli used for estimating the soil bearing load capacity and rock quality. The MASW interpretation results are presented in one dimensional Velocity vs Depth-profiles calculated by the inversion techniques. The shear wave velocities of the subsurface rocks and soils are estimated and assigned to the center of the acquisition spread. The shear wave velocity calculated the top 30 meters of the stratigraphic section is referred to as Vs30 and it is used for seismic site classification. According to the National Building Code of Canada the soils and rocks can be classified in six groups “NBCC2005 Site Classification for Seismic Site Response”.
Advantages
When compared to other surface geophysical methodologies such as GPR, MAG and EM, the refraction seismic is more expensive and time consuming; although this technique is irreplaceable in geological and other geotechnical applications considering the high degree of resolution and accuracy obtained in the estimation of the physical properties of the rock and soils and the details of the stratigraphic sections, bedrock profile and earth velocity models. The main advantages of the system are:
Good lateral and vertical resolution of stratigraphic layers
Estimation of lateral and vertical variations of the elastic properties and density of the soils and rocks
Detailed cross sections of stratigraphic layers, bedrock, water table and underground features such as voids and sinkholes
Simple array and field deployment
Small number of energy sources and geophones compared to reflection seismic
Geophysics HM
