Passive MASW is divided into two different types based on field logistics and type of shear-velocity (Vs) profiles (1-D or 2-D) usually sought: passive remote (Park et al., 2004)
passive roadside (Park and Miller, 2008) MASW surveys (Fig. 2).  The former seeks a 1-D Vs profile of bulk materials ranging up to a hundred meters along the surface
and in depth.  On the other hand, the latter type can generate a 2 D Vs profile covering up to a hundred meters in depth and as much surface distance as the survey is
continuously conducted.  These two passive MASW methods utilize those surface waves generated passively from ambient cultural activities such as traffic.
Fig. 1.  Typical field layout for microtremor survey method (MSM).
Fig. 2.  Typical field layout for passive remote (left) and passive roadside (above)
MASW surveys.  
Types of the Passive MASW Method

As the surface-wave method gains in popularity among engineers and geophysicists, demand for
increased investigation depth is also growing. However, the amount of active-source energy necessary to
gain a few more hertz at the low-frequency end of a dispersion curve (e.g., 5-7 Hz)—and thereby to
increase investigation depth by several tens of meters—often rises by
several orders of magnitude,
rendering efforts with an active source impractical and uneconomical. On the other hand, passive surface
waves generated from natural (e.g., tidal motion) or cultural (e.g., traffic) sources are usually of a low-
frequency nature (e.g., 0.01-20 Hz) with wavelengths ranging from a few kilometers (natural sources) to a
few tens (or hundreds) of meters (cultural sources), providing a wide range of penetration depths and
therefore a strong motivation to utilize them.  This type of application originated almost half century ago in
Japan under the name Microtremor Survey Method (MSM) (
Fig. 1).  This method in its original form
adopted a limited number of receivers (usually less than ten) for data acquisition.  It originated by
employing a dispersion analysis scheme called spatial autocorrelation (SPAC) method.  Although SPAC
was, perhaps, the most effective scheme at the time when only a limited number of channels were
available, its use nowadays for multichannel surveys with twenty-four or more channels tends to degrade
resolution, sensitivity, and multimodal delineation capabilities in comparison to the 2-D wavefield
transformation methods, which are more appropriate for the higher-definition multichannel recording.    

The passive MASW method, on the other hand, usually uses more (twenty-four or more) channels than
MSM and aims at a full exploitation of advantages with multichannel recording and processing
approaches.  It therefore has a greater flexibility in field logistics and an enhanced robustness in data
processing with an increased resolution in the analysis of both modal identity and azimuth properties of
surface waves.