Multichannel Analysis of Surface Waves
Depending on the nature of the seismic source the multichannel analysis of surface waves (MASW) method can be categorized as active or passive. The Active MASW uses active seismic sources (e.g., a sledge hammers, weight drop, charges, etc.) and a linear receiver array, and data collected in a roll-along manner. 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.). Depending on the receiver configuration there are two possibilities. The Passive Remote MASW (Park et al., 2004; Park et al., 2005) employs a two-dimensional (2-D) receiver array and the Passive Roadside MASW(Park and Miller, 2008) uses linear (i.e., horizontal 1-D) receiver array
An important acquisition distinction between the active and passive methods is that with the active method the distance of the source is known, while with the passive method it is unknown. Optimized source distance can be beneficial for the quality of dispersion curve imaging and interpretation. The ability to control the source distance to the receiver spread is one of the advantages of the active method over the passive methods.
The Active MASW method (Figure 1) was first introduced in The Leading Edge(Miller et al., 1999). It is the conventional mode of survey using an active seismic source (e.g., a sledge hammer) and a linear receiver array, collecting data in a roll-along mode. 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.).
Figure 1--Active MASW method. More information is available on "Active MASW."
The Passive Remote MASW 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 (Figure 2). This results in the most accurate evaluation of shear-wave velocity (Vs) at the expense of more intensive field operation and the burden of securing an open-wide space for the array. This can be a good choice if a relatively regional one-dimensional (1-D) Vs profiling is needed.
Figure 2--Passive Remote MASW method. More information is available on "Passive Remote MASW."
The Passive Roadside MASW method (Park and Miller, 2006) adopts the conventional linear receiver array and tries mainly to utilize those surface waves generated from local traffic (Figure 3). 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 the passive roadside 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 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 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.
Figure 3--Passive Roadside MASW method. More information is available on "Passive Roadside MASW."
Four Steps of MASW Process
The entire procedure for MASW usually consists of four steps (Miller et al., 1999):
- Acquiring multichannel records (or shot gathers),
- Estimating the fundamental-mode dispersion curves (one curve from each record)
- Inverting these curves to obtain 1-D (depth) Vs profiles (one profile from one curve), and
- Assembling multiple 1-D results into 2-D or 3-D images.
Procedure for 2-D Shear-Velocity (Vs) Profiling
By placing each 1-D Vs profile at a surface location corresponding to the middle of the receiver line, a 2-D (surface and depth) Vs map is constructed through an appropriate interpolation scheme.
The Power of the Multichannel Approach
When seismic waves are generated using an impact source such as a sledge hammer both surface and body waves are generated propagating in all directions. Some of these waves are reflected and scattered as they encounter shallow and surface objects (for example, building foundations, culverts, ditches, boulders, and so forth) and become noise. Furthermore, there are always ambient noise vibrations from traffic and human activities. The main advantage of the multichannel approach is in its capability to distinguish all of these noise waves from the signal wave (the fundamental mode of Rayleigh waves) through diverse seismic attribute analysis. Identification of signal and noise waves based on one of the attributes (the arrival-time pattern) is illustrated at using a multichannel field record.
Dispersion Analysis--Multichannel Approach
(2-D Wavefield Transformation)
Dispersion properties of all types of waves (both body and surface waves) are imaged through a wavefield-transformation method (Park et al., 1998; Luo et al., 2008) that directly converts the multichannel record into a dispersion-curve (a.k.a. Overtone) image, on which different dispersion patterns can be recognized in the transformed energy distribution, as illustrated below. Then, the necessary dispersion feature (e.g., the fundamental mode of the Rayleigh wave, the first higher mode, etc.) is interpreted and estimated following a specific trend.
Luo, Y. H., J. H. Xia, R. D. Miller, Y. X. Xu, J. P. Liu, and Q. S. Liu, 2008, Rayleigh-wave dispersive energy imaging using a high-resolution linear Radon transform: Pure and Applied Geophysics, 165, 903-922.
Miller, R. D., J. Xia, C. B. Park, and J. M. Ivanov, 1999, Multichannel analysis of surface waves to map bedrock: The Leading Edge, 18, 1392-1396.
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. [PDF available online, 1.1 MB]
Park, C. B., R. D. Miller, and J. Xia, 1998, Imaging dispersion curves of surface waves on multi-channel record 68th Annual International Meeting, SEG, Expanded Abstracts, 1377-1380.