The Passive MASW method utilizes surface waves generated from ambient cultural activities (e.g., traffic from vehicles and trains, industrial noise, etc.) and natural (e.g., earthquakes, thunder, tidal motions, atmospheric pressure changes, etc.).
The Passive Remote method (Park et al., 2004; Park et al., 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 at the expense of more intensive field operations and the burden of securing a wide open space for the array. This can be a good choice when relatively regional one-dimensional (1-D) Vs profiling is needed.
The Passive Roadside MASW method (Park and Miller, 2008) adopts the conventional linear (i.e., horizontal 1-D) 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 difficulty 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 the sidewalk 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 the 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) record of data. This can result in the combined active-passive 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.
Very often one of the advantages of the passive surface-wave method is the increased depth of investigation. This can be the case when the available passive sources provide frequencies lower than those from using conventional active source (e.g., sledgehammer, weight drop, etc.)(Ivanov et al., 2013). Lower frequencies of the Rayleigh wave relate to longer wavelengths and greater depths of investigation.
However, frequently passive data can be dominated by higher mode energy only, which can mistakenly be interpreted as fundamental mode of the Rayleigh wave. Such misinterpretation can lead to overestimated shear-wave velocity estimates The combined use of active and passive MASW survey can help interpret the mode of the observed passive surface wave energy and avoid possible overestimations(Park et al., 2007).
Ivanov, J., B. Leitner, W. T. Shefchik, T. J. Schwenk, and S. L. Peterie, 2013, Evaluating hazards at salt cavern sites using multichannel analysis of surface waves: The Leading Edge, 32, 289-305.
Park, C. B., and R. D. Miller, 2008, Roadside passive multichannel analysis of surface waves (MASW): Journal of Environmental and Engineering Geophysics, 13, 1-11.
Park, C. B., R. D. Miller, D. Laflen, C. Neb, J. Ivanov, B. Bennett, and R. Huggins, 2004, Imaging dispersion curves of passive surface waves: 74th Annual International Meeting, SEG, Expanded Abstracts, 23, 1357-1360.
Park, C. B., R. D. Miller, N. Ryden, J. Xia, and J. Ivanov, 2005, Combined use of active and passive surface waves: Journal of Environmental and Engineering Geophysics, 10, 323-334.
Park, C. B., R. D. Miller, J. H. Xia, and J. Ivanov, 2007, Multichannel analysis of surface waves (MASW)--active and passive methods: The Leading Edge, 26, 60-64.