As effectiveness of the MASW method was acknowledged among the practitioners and researchers worldwide, its application to pavement investigation began (Ryden et al.,
2003; 2004; Forbriger, 2003; Ryden and Lowe, 2004) and the complicated nature of the guided wave phenomenon over highly inverse-velocity structure of pavement started to
be unveiled (Fig. 3). The more accurate nature of surface waves in this case, which was first speculated by Martincek (1994), was clearly proven by Ryden et al. (2003) as being
dominated by seismic Lamb waves (Fig. 4). Higher modes and leaky waves (Gucunski and Woods, 1992) previously reported could be studied and confirmed in a much more
detailed manner by following the multichannel approach (Ryden et al., 2003; 2004; Ryden and Lowe, 2004).
It was in the mid-90s when KGS started a project to utilize surface
waves. Knowing the advantages with the multichannel method
proven throughout almost half-century of its history for exploration of
natural resources, their goal was a multichannel method to utilize
surface waves mainly for the purpose of geotechnical engineering
projects. From the extensive studies performed by SASW
investigators, they acknowledged that surface wave properties must
be more complex than previously assumed or speculated, and that
the two-receiver approach had clearly reached its limitation to handle
the complexity. Based on the normal notion that the number of
channels used in seismic exploration can directly determine
resolving power of the method, they utilized diverse techniques
already available after a long history of seismic data analysis
(Telford et al., 1976; Robinson and Treitel, 1980; Yilmaz, 1987) and
also developed new strategies in field and data processing to detail
surface wave propagation properties and characterized key issues to
bring out a routinely-useable seismic method.
The first documented multichannel approach for surface-wave
analysis goes back to early 80s when investigators in Netherlands
used a 24-channel acquisition system to deduce shear-wave velocity
structure of tidal flats by analyzing recorded surface waves (Gabriels
et al., 1987) (Fig. 2). It first showed the scientific validity of the
multichannel approach in surface wave dispersion analysis and, in
this regard, the study can be regarded as a feasibility test of the
approach for routine use in the future. Then, using uncorrelated
Vibroseis data, Park et al. (1999) highlighted the effectiveness of the
approach by detailing advantages with multichannel acquisition and
processing concepts most appropriate for the geotechnical
engineering applications. A subsequent boom in surface wave
applications using the MASW method for various types of
geotechnical engineering projects has been observed worldwide
since that time. There were a few other earlier other applications of
multichannel approach to aid oil-exploration reflection surveys
(Al-Husseini et al., 1981; Mari, 1984).
|Fig. 1. A field record showing reflections signals hidden by strong surface waves in raw data are seen after filtering.
|Fig. 2. Summary of work performed by Gabriels et al. (1987)
Fig. 3. Dispersion images from a set of multichannel pavement data matched
against theoretical Lamb-wave dispersion curves for the listed model.
|Fig. 4. Generation mechanism of Lamb waves from vibration of a free plate and corresponding dispersion curves.
Multichannel Approach (MASW)
In early 2000s, the MASW (Multichannel Analysis of Surface Waves) method came into popular use among the geotechnical engineers. The term “MASW” originated from the
publication made on Geophysics by Park et al. (1999). The project actually started in mid-90s at the Kansas Geological Survey (KGS) by geophysicists who had been utilizing
the seismic reflection method—long used in the oil industry to image the interior of the earth for depths of several kilometers. Called the high-resolution reflection method, it
was used to image very shallow depths of engineering interest (e.g., 100 m or less). Ironically enough, surface waves at the time were regarded as most-troublesome noise to
be attenuated at any expense (Fig. 1).