SurfSeis - Sigma Multichannel Analysis of Surface Wave (MASW)

An Introduction

MASW a seismic method for near-surface (< 30 m) characterization of shear-wave velocity (Vs). It utilizes the Rayleigh-type surface waves (normally called "ground roll") recorded by multiple receivers (geophones) deployed on an even spacing and connected to a common recording device (seismograph). Surface waves recorded as they propagate along the receiver line are analyzed through powerful and diverse multichannel processing techniques similar to a pattern-recognition approach.

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The entire procedure for MASW usually consists of three steps:
  1. acquiring multichannel records (or shot gathers),
  2. extracting the fundamental-mode dispersion curves (one curve from each record), and
  3. inverting these curves to obtain 1-D (depth) Vs profiles (one profile from one curve).

SurfSeis - Sigma

SurfSeis - Sigma

SurfSeis - Sigma

Then, by placing each 1-D Vs profile at a surface location corresponding to the middle of the receiver line, a 2-D (surface and depth) Vs map is constructed.

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The Power of the Multichannel Approach

When seismic waves are generated using an impact source such as a sledgehammer both surface and body waves are generated propagating in all directions. Some of these waves are reflected and scattered as they encounter shallow and surface objects (for example, building foundations, culverts, ditches, boulders, and so forth) and become noise. Furthermore, there are always ambient noise vibrations from traffic and human activities. The main advantage of the multichannel approach is in its capability to distinguish all of these noise waves from the signal wave (the fundamental mode of Rayleigh waves) through diverse seismic attribute analysis. Identification of signal and noise waves based on one of the attributes (the arrival-time pattern) is illustrated at right using a multichannel field record.

SurfSeis - Sigma

SurfSeis - Sigma

Dispersion Analysis--Multichannel Approach

(2-D Wavefield Transformation)

Dispersion properties of all types of waves (both body and surface waves) are imaged through a wavefield-transformation method that directly converts the multichannel record into an image where a dispersion pattern is recognized in the transformed energy distribution, as illustrated at left. Then, the necessary dispersion property (like that of the fundamental mode) is extracted from a specific pattern. All other reflected/scattered waves and ambient noise are automatically removed during the transformation.

SurfSeis - Sigma

SurfSeis - Sigma

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Kansas Geological Survey
Updated May 1, 2003
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