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Multi-channel Analysis of Surface Waves (MASW)

MASW is an active (surface waves generated by impact source) or passive (surface waves generated by cultural or natural activities) geophysical method that exploit the patterns and distributions of surface seismic waves. The sampling depth of a particular frequency component of surface waves is in direct proportion to its wavelength, and this property makes the surface wave velocity frequency dependent, i.e., dispersive. The multichannel analysis of surface waves (MASW) method utilises this dispersion property of surface waves for the purpose of VS profiling in 1D or 2D format. This provides very important geotechnical parameters and permits the reconstruction of geometries and thicknesses of the subsurface, depths of substrata, and soil profiles, via the study of seismic waves generated on the surface.

Surface wave (Rayleigh wave) energy, generated using an acoustic source, is recorded at predetermined receiver locations (R1, R2, etc.). A dispersion curve (phase velocity versus frequency) generated from the acquired field data is inverted and used to generate a 1-D shear wave velocity sounding. If additional MASW data sets are acquired at adjacent locations, 2- D or 3-D shear-wave velocity models can be created.

Modified from “Introduction to Geotechnical Geophysics – N. Anderson and N. Croxton, Circular – Number E-C130, October 2008”.

MASW (S-waves) can be deployed using the same equipment as for SRT with only changes in geometries and sometimes geophone frequency. Alternatively (commonly used), the geophones and seismic cable can be mounted on a “landstreamer”, which consists of a number of heavy metal plates linked by a Kevlar belt. The entire system is towed from a vehicle that houses the seismograph. This system is the most used deployment for the acquisition of 1D soundings for the creation of a 2D profile. There is also an option to deploy the array as a towed streamer and spiked geophones (as for SRT). This is a compromise between the acquisition speed of the “landstreamer” and the high signal/noise ration of the SRT.

Example 1: Sample of MASW 2D profile from a series of 1D soundings, showing the highly variable depth to rock and the lateral extent of layer inversion (i.e., Stiff Clay over Soft Clay), which was missed by the BH.

Example 2: Sample of MASW 2D profile from a series of 1D soundings, showing the presence of voids/soft fill.

Example 3: Sample of MASW for Pre and Post compaction and normalised difference. The investigation was carried out to evaluate the efficacy of dynamic compaction and establish the ideal parameters (i.e., weight, height and spacing).

Example 4: Sample of tabulated geotechnical parameters: Poisson’s Ratio, Young’s Modulus, and Shear’s Modulus. This is possible when both MASW and LSR (i.e., P and S waves) are concurrently acquired.

What are the most common uses of Multi-channel Analysis of Surface Waves (MASW)?

Rippability assessment
Depth to rock and soil profile
Rock and soil characterisation
Fill delineation
Geological structures/features detection/delineation, e.g., voids/cavity, boulder floaters, faulting and fault zone, dykes, palaeochannels
Land/road compaction assessment (pre and post)
Concrete slab subsurface condition (e.g., voids, slipping)
Vs30 (the time-averaged shear-wave velocity (Vs) to a depth of 30 meters, is a key index adopted by the earthquake engineering community to account for seismic site conditions)

MASW FAQs

What are surface waves?

Surface waves are a type of seismic waves that travel just below the surface of the ground consisting mostly of Rayleigh waves and Love waves. Rayleigh waves, the most fundamental of the surface waves, also known as ground roll, spread through the ground as ripples, with a longitudinal and a transverse component. As it rolls, it moves the ground up and down, and forward and backward in the direction of the moving wave.

How MASW uses the acquired waves records?

Surface waves are dispersive, meaning the phase velocity of the waves is frequency-dependent in a vertically heterogenous medium. Using the data collected from the generation of Rayleigh or Love waves, a dispersion image is created. Dispersion curves are extracted from this image and an inversion process converts the dispersion curve into a shear velocity (VS) profile that is used to estimate soil stiffness.

Why MASW is important for geotechnical engineering?

Shear modulus, elasticity modulus, bulk modulus and Poisson ratio, are fundamental characteristics of soil materials and are closely related to the shear wave (S) velocity. Consequently, the determination of S-wave velocity variations of the subsurface is important in geotechnical engineering.

How long does it take to complete a MASW survey?

MASW survey time depends on size and complexity of the site, the required resolution and density of the data to achieve the scope of the project, e.g., MASW application can vary from single 1D soundings for Vs30 or Elastic moduli to many line kilometres for pavement compaction.

How accurate is MASW?

MASW can be very accurate. This is dependent on the geometry of the array, e.g., total length, geophone and shot spacings, and the seismic source frequency. MASW is very frequency dependent, and this will determine coverage and resolution in the shallow or deep part of the subsurface.

How much does a MASW survey cost?

The costs for a MASW survey are determined by the size and complexity of the site, and the project requirements, e.g., pre/post compaction, voids detection, Vs30, Elastic moduli etc. Therefore, it is difficult, and misleading, to provide an exact dollar value. A very indicative cost for the fieldwork component can vary from $2,500 to $4,500 per day. This excludes mobilisation and demobilisation, which can vary by 10s of thousands of dollars, Australia is big! Also, it excludes processing and reporting, which depends on the amount of data acquired.

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No matter how simple or complex is your project, we can assist in many ways…a technical explanation? A practical field implementation? A budget discussion? Or a full project planning and execution? Just ask away.