The passive roadside (Fig. 1) MASW method adopts the conventional linear receiver array and tries mainly to utilize surface waves generated from local traffic.  It tries to
overcome limitations with the passive remote method in securing a spacious area and inconvenience in field operations by sacrificing the accuracy (usually less than 10%) of
the Vs evaluation.  With this method, the array can be set along a sidewalk, median, or 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 for the array 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 recording (e.g., 10 sec) (Fig. 1).  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 (Fig. 2).  Although it can result in a certain degree of overestimated
Vs values (usually less than 10%) in comparison to the remote method that employs a 2-D receiver array, this survey mode can be useful and convenient because of the
significant advantage in field operations.  
The 2-D Vs profile in Fig. 2 was obtained from a
roadside survey that used a linear receiver array of 5
m spacing and repeated recording at ten different
surface locations by moving the array by four stations
(20 m).  A 20-lb sledgehammer was used to deliver
an active impact at the beginning of the 30 sec
recording.  Each record was then split into active (0-2
sec) and passive (2-30 sec) portions, respectively, to
go through different data processing schemes,
generating two different dispersion images that were
combined (vertical stacking) for the purpose of
enlarging bandwidth of dispersion patterns to be
recognized.   

Data Acquisition

Dispersion Analysis
Fig. 1.  A field layout for the passive roadside MASW survey.
Fig. 2.  A 2-D Vs map obtained from a roadside passive MASW survey that also employed the sledge hammer impact at the
beginning of 30-sec recording.
Passive Roadside MASW Survey

As the active survey mode often does not achieve sufficient depth of
investigation, the passive surface wave method seems an alternative.  In
addition, as the necessity of surveys inside urban areas grows, utilizing those
surface waves generated by local traffic is deemed to be a fascinating choice
recently (Asten, 1978; Louie, 2001; Okada, 2003; Suzuki and Hayashi, 2003;
Yoon and Rix, 2004; Park et al., 2004).  Although a 2-D receiver array—such as
a circle or cross layout—must be used for the most accurate analysis of
passive surface waves of any origin, a more convenient method that could be
implemented with a conventional 1-D linear receiver array deployed along a
roadside has been sought among researchers and practitioners.  This is
because the 2-D array method requires an open spacious area for receiver
deployment, which is not always easily available in an urban area populated
with buildings and other structures.  This type of roadside passive method is
now being introduced into relevant engineering communities, with its
effectiveness and theories continuously scrutinized by practitioners and
investigators (Louie, 2001; Park et al., 2006).