Passive Surface Wave Method

As the surface-wave method gained in credibility and more diverse applications were attempted, the demand for deeper investigation was also growing, motivating the
utilization of those passively generated surface waves by natural (e.g., tide, thunder, etc.) and cultural (e.g., traffic) activities (Louie, 2001; Suzki and Hayashi, 2003; Park et al.,
2004; Yoon and Rix, 2004).  While an active survey provides a dispersion curve in a relatively high (short) frequency (wavelength) range (for example, 15 50 Hz and 3 30 m), a
passive survey can fill the dispersion trend at lower (longer) frequencies (wavelengths) (for example, 8 18 Hz and 30 160 m) (
Fig. 1).  
The passive method has been drawing significant attention since early 2000s in the USA (Louie, 2001; Suzki and Hayashi, 2003; Park et al., 2004; Yoon and Rix, 2004; Asten,
2006).  The passive method is a multichannel approach from its birth (e.g., MSM with 4 or more channels).  SPAC was invented as a result of excellent mathematical
manipulation to cancel out one variable (incoming angle, the azimuth) of passive surface waves so that the subsequent data processing can become less dubious and it was
perhaps the most optimal approach at a time when only a limited number of channels (e.g., six or less) were available.  On the other hand, today’s multichannel approach (e.
g., 24 or more channels) should benefit more from the wavefield transformation method.      

More recently, the usefulness of combining active and passive dispersion data is being acknowledged (Park et al., 2005).  Doing so can increase not only the investigation
depth by broadening useable bandwidth of dispersion data, but also increases overall accuracy of the analysis by enhancing the ability of modal identification for observed
dispersion trends (
Fig. 4).  
Because the incoming direction (azimuth) of passive surface waves could not be known ahead of time, the receivers had to be laid out in a two-dimensional (2D) array such
as a triangle or circle (Okada, 2003) (
Fig. 2).  Although this type of surface wave application had come under study a half-century ago in Japan under the name of
Microtremor Survey Method (MSM) (Aki, 1957), it was not well known among relevant com¬munities in Western countries until quite recently, with the exception of a few
study groups (Asten, 1978; Asten and Henstridge, 1984).  Spatial Autocorrelation (SPAC) (Aki, 1957) and f-k methods were commonly used to process passive surface
waves for dispersion analysis.  More recently, an imaging method similar to the one used in the active method was developed (Park et al., 2004; 2006; Park, 2008).  The
main advantage of the
passive MASW method is an increased investigation depth (e.g., > 30 m) (Fig. 3).   
Fig. 1. Comparison of dispersion images from passive (left) and active field data.
Fig. 2.  Different types of 2-D receiver arrays for passive remote
MASW surveys.
Fig. 3.  A 2-D Vs map generated from a passive roadside
survey showing an investigation depth of about 80 m.
Fig. 4.  Utility of combining passive and active data sets together illustrated by using field data sets.  It can increase usable
bandwidth of dispersion analysis (left column), whereas it can also increase credibility of modal identification (right column).  
Data Courtesy of the Kansas Geological Survey