Fig. 1.  Schematic of the active MASW field survey.
Fig. 2.  Schematic of the passive remote MASW field survey.
Fig. 3.  Schematic of the passive roadside MASW field survey.
MASW - Types

Depending on the way surface waves are generated, there are basically two types:  active and
preset plan with a proper impact source such as a sledge hammer or weight drop, whereas
passive surface waves are generated by sources (cultural and natural) unrelated to the survey,
such as traffic and tidal motion.  

The active (
Fig. 1) MASW method was first introduced in GEOPHYSICS (Park et al., 1999).  It is the
conventional mode of survey, collecting data in a roll-along mode using an active seismic source
(e.g., a sledge hammer) and a linear receiver array.  The two
passive methods utilize surface
waves generated passively from ambient cultural (and natural) activities such as traffic (and
thunder, tidal motion, atmospheric pressure change, etc.).  The
passive remote (Fig. 2) method
(Park et al., 2004; 2005) employs a two-dimensional (2-D) receiver array such as a cross or
circular layout to record passive surface waves.  This results in the most accurate evaluation of
shear-wave velocity (Vs) but is a more intensive field operation and requires a wide open space
for the array.  This can be a good choice if a relatively regional one-dimensional (1-D) Vs profiling
is needed.  The
passive roadside MASW method (Fig. 3) (Park and Miller, 2006) adopts the
conventional linear receiver array and tries mainly to utilize those surface waves generated from
local traffic.  It tries to overcome limitations with the passive remote method—such as securing a
spacious area and inconvenience in field operations—by sacrificing some accuracy (usually less
than 10%) of the Vs evaluation.  With the passive roadside method, the array can be set along a
sidewalk or on the shoulder of a road and the survey can continue in a roll-along mode for the
purpose of 2-D Vs profiling.  Using a land streamer can improve survey speed by as much as a
few orders of magnitude.  In addition, an active impact (e.g., by using a sledge hammer) can be
applied at one end of the array to trigger a long (e.g., 30 sec) recording of data.  This can result in
the active-passive
combined analysis of surface waves for the purpose of obtaining both shallow
(e.g., 1-20 m) and deep (e.g., 20-100 m) Vs information simultaneously.