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.