Seismic Multi-channel Analysis of Surfact Waves-MASW
The multi-channel analysis of surface wave (MASW) technique is commonly used to investigate the elastic properties (stiffness) of the ground for geotechnical engineering and seismic hazard determination. HGI has performed numerous MASW surveys in support of geotechnical studies including mapping variations in shear-wave velocity with depth, void detection mapping, and seismic site assessment for infrastructure and building construction.
The MASW method is commonly used to investigate the elastic properties (stiffness) of the ground for geotechnical engineering and seismic hazard determination.
Dispersion, or change in phase velocity with frequency, is the fundamental property utilized in surface-wave methods. Phase velocity of surface-wave is sensitive to the Shear wave velocity (Vs); phase velocity of surface-wave is typically 90-95% that of the Shear wave velocity. Surface wave dispersion can be significant in the presence of velocity layering, which is common in the near-surface environment. There are other types of surface waves, or waves that travel along a surface, but in this application we are concerned with the Rayleigh wave, which is also called “ground roll” since the Rayleigh wave is the dominant component of ground roll.
“Active source” surface-wave surveying means that seismic energy is intentionally generated at a specific location relative to the geophone spread and recording begins when the source energy is imparted into the ground. This is in contrast to “passive source” surveying, also called “microtremor” surveying, or sometimes referred to as “refraction microtremor” (or the commercial term “ReMi”) surveying. In this type of surveying there is no time break, and motion, from ambient energy generated by cultural noise, wind, wave motion, etc. at various, and usually unknown, locations relative to the geophone spread, is recorded.
The seismic source for a MASW survey can be a sledgehammer or a weight drop, as shown in the figure above. The weight can be mounted on the back of a truck or on a UTV.
MASW surveys are conducted using the same source and seismograph equipment as the more common P-wave seismic refraction surveys, requiring only a change to lower frequency geophones (typically 4.5Hz). They are much easier to conduct than shear wave surveys, and benefit from increasing source power efficiency (for each sledgehammer blow 67% of the energy produced is in the form of surface-waves, 26% shear waves, and 7% P-waves) and consequently improved signal to noise. The technique works best in soft rock geology conditions with minimal or constant topography change across the spread.
MASW TECHNIQUE
MASW first measures seismic surface waves generated from various types of seismic sources (such as a sledgehammer or weight drop), analyzes the propagation velocities of the surface waves, then finally deduces shear-wave velocity (Vs) variation below the survey array that is most responsible for the analyzed propagation velocity pattern of surface waves.
Shear-wave velocity (Vs) is one of the elastic constants and is closely related to Young’s and shear moduli. Typically, Vs. is a direct indicator of the ground strength (stiffness) and, therefore, commonly used to derive load-bearing capacity.
The overall procedure to produce a typical two-dimensional (2D) shear-wave velocity cross-section consists of collecting multichannel seismic records at multiple locations. Each record generates a one-dimensional (1D) velocity (Vs) profile (depth sounding), which are then combined to produce the final 2D cross-section.
A seismic survey consists of geophones with uniform spacing, a source, a seismograph to record data, a field laptop, and a power source
EXAMPLE
The 2D shear-wave velocity cross-section below is from a previous survey in support of a geotechnical investigation at a tailings storage facility in Nevada to identify and characterize a weak clay layer in the subsurface ahead of a planned raise of the tailings. A number of survey lines were collected at both the toe of and on the tailings dam to map the spatial extents and depth of this weak clay layer, in order to optimize the concurrent drilling program and provide a cost-efficient mapping of the survey area.
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DRILLING CORRELATION
The concurrent drilling program collected a suite of CPT test data to characterize the subsurface materials. The plot to the right displays a section of a MASW profile and the coincident drilling data, with a good correlation noted between the N60 blow count log (left most log) and the shear-wave velocity – with higher blow counts correlating to higher shear-wave velocities (green shades) and the weak layer (blue shades) in the MASW profile correlating to low blow counts.
