What is Multi-channel Analysis of Surface Waves (MASW)
Multichannel Analysis of Surface Waves (MASW) is a geophysical technique used for subsurface exploration. It focuses on measuring surface waves (specifically Rayleigh waves) that travel along the Earth’s surface to determine the shear wave velocity profile of the subsurface.
MASW is most often undertaken using a vehicle towed geophone array coupled with a seismograph for recording.
How Does MASW Work?
Multi-channel Analysis of Surface Waves uses dispersion of surface waves (Rayleigh waves) to determine shear wave velocity (Vs) with depth.
Dispersion is where the different wavelengths travel at different velocities and as a result, they separate from one another as they travel. Usually with the longer wavelengths travelling faster and the shorter wavelengths trailing behind. Dispersion of Rayleigh waves occurs because the longer wavelengths penetrate deeper, they usually travel faster than shorter wavelengths as they (usually) access stiffer materials with higher seismic velocities at depth.
A common field setup includes a vehicle towed land streamer (which consists of a geophones mounted on skid plates with seismic cable and reinforced webbing) coupled to a seismograph. The seismic source is usually a sledgehammer or weight drop.
Each shot record is recorded by stacking a number of individual hammer blows to improve the signal to noise ratio. During data processing, each shot record generates a single 1D shear wave velocity profile which is located at the array centre. In order to generate 2D profiles, many shot records must be taken along a survey line. In contrast to seismic refraction, where the geophone array is stationary, 2D MASW profiling requires that the entire array is moved before another shot record is taken. This process in repeated many times along the survey line.
Common Applications of Seismic Refraction
MASW can provide information about the stiffness and layering of underground materials, making it useful for a variety of applications. Geotechnical applications include: assessing soil stiffness, mapping depth to bedrock and other layers, providing input parameters (Vs) for calculating dynamic moduli/ Poisson’s ratio and seismic site classification.
Data Processing
MASW data processing involves conversion of each raw seismic record to a dispersion image using a modified Fourier transform (essentially converting the data from the time domain to the frequency/velocity domain). A dispersion image shows the distribution of wave energy on a plot of phase velocity vs. frequency. A dispersion curve is then picked through the energy peaks identified as corresponding to a particular Rayleigh wave mode.
Example of a dispersion image with an interpreted dispersion curve (small white squares).
The extracted dispersion curve is then run through an inversion algorithm to calculate a single 1D shear wave velocity (Vs) profile with depth. This process is repeated for each overlapping seismic record. The many individual 1D Vs profiles are then gridded to generate a continuous 2D Vs profile.
Example of an interpreted 2D Vs profile. Interpreted bedrock level is shown with a dashed magenta line.
Advantages of MASW
The main advantages of MASW include:
- Rapid data acquisition: using a vehicle towed array production rates can be significantly faster than other similar methods.
- Relatively high lateral and vertical resolution: depending on survey geometry.
- Velocity Reversals/Inversions: MASW is capable of resolving velocity inversions/reversals. Making it suitable for investigations where a stiff or hard layer overlies a softer layer.
- Relatively unaffected by groundwater: shear wave velocity is relatively unaffected by the presence of groundwater. This is not the case for the P-waves measured using Seismic Refraction.
- Non-destructive: The method does not require drilling or disturbing the subsurface, making it non-invasive and suitable for sensitive environments or areas where drilling might be impractical.
Challenges and Limitations of MASW
While MASW has several advantages, it also has its limitations. Some of the key limitations include:
- Seismic Noise: Seismic refraction relies on a source of energy (such as a hammer strike or explosive charge) to generate seismic waves which have a significantly stronger signal compared to background seismic noise. Ambient seismic noise can be a significant problem for seismic refraction in urban environments, in adverse weather or in areas where plant or machinery are operating nearby.
- Interference from higher Rayleigh Wave modes: The fundamental mode is usually preferred for interpretation of the dispersion curve. However, it is relatively common to have energy from higher modes present on the dispersion images. In some cases the higher modes may dominate the images and/or overlap with the fundamental mode making correct interpretation difficult.
- Usually limited to Shallow Depths when using an active source: MASW undertaken using a vehicle towed setup is primarily effective for shallow subsurface investigations (up to 30 m depth). For deeper investigations passive seismic investigations may be considered.
- Low Resolution for Small-Scale Features: MASW may not effectively detect small-scale subsurface features, such as narrow faults.
- Difficulty in Identifying Layer Boundaries: In some cases, especially where layers have similar seismic velocities, MASW may have difficulty distinguishing between different layers or determining the exact boundaries between them.
