See user guide.
The most common purpose for running a passive MASW survey is to increase the investigation depth (Zmax) beyond the limit of most active surveys (e.g., 30 m). There are two conditions that have to be met to increase Zmax; a more powerful source that can generate "strong" low frequencies (long wave-lengths) of surface waves, and a longer receiver array that can effectively record such long wavelengths.
The impact power (E) of an active source like a sledge hammer can be increased by adding more weight (m) and/or increasing the impact speed (v) because it is the kinetic energy [E = (1/2)mv2] that is transformed into the elastic (seismic) energy upon impact. In this way, an accelerated weight drop source can generate a greater E than a sledge hammer. In reality, however, this artificial increase of E can be limited because it eventually faces the obstacle of operational and economical cost. That's why the passive MASW surveys utilizing ambient vibrations (usually generated from traffic) have become popular. Figure 1 illustrates the comparison of E from typical active and passive sources. It indicates the common passive E is greater than the active E by a few orders of magnitude. These surface waves from of traffic origin are generated from irregular places on the road when vehicles are travelling over them (Figure 2). The usual frequency range is from a few to a few tens of hertz (e.g., 1-30 Hz) and the low-end frequencies (e.g., 1-10 Hz) are most useful because they are not easily generated with sufficient energy using typical active sources, but are often critical to increase Zmax beyond the common range (e.g., ≥ 30 m). It is speculated that the combination of the large mass and shock-absorbing mechanism (tire and suspension spring) of the vehicle facilitates the generation of such low frequency surface waves.
The array length (L) has to be in proportion to the Zmax; the longer array is needed for a deeper investigation. The most commonly used relationship between the two is KminL ≤ Zmax ≤ KmaxL, with Kmin = 0.5 and Kmax = 1.0. Although the actual relationship can be influenced by other acquisition and processing factors such as signal-to-noise (SN) ratio of acquired data and the specific algorithm used during the dispersion analysis, the most common rule of thumb is L = 2Zmax (Figure 3). In this case, it is usually assumed that the optimum source offset (X1) is about 1/4 of the array length; i.e., X1 = (1/4)L (Park et al., 1999; Park et al., 2002; Park and Carnevale, 2010). It is also the source offset (X1) that can contribute positively in increasing Zmax (Park and Carnevale, 2010; Yoon and Rix, 2009). Therefore, some studies indicate it should be X1 + L = 2Zmax (Yoon and Rix, 2009). In this case, X1 should remain within a reasonable range to avoid the use of an excessively short (or long) array; for example, X1 + L = 2Zmax with X1 ≤ L. However, it seems that the significance of X1 can vary with site geology (i.e., velocity structure) and it can be out of the equation as far as the array length (L) becomes sufficiently long for Zmax (for example, L ≥ 2Zmax). With a passive survey, the dimension (D) of the array is set approximately twice Zmax (Figure 4); D ≈ 2Zmax. This also applies to the active/passive combined survey that is essentially identical to the active survey using a long recording time (e.g., T ≥ 30 sec) as further explained below.
Although it is always recommended to use a 2-D array for a passive survey such as a circle or an L-shaped array for the maximized accuracy in dispersion analysis, it is not always convenient to secure such spacious areas, especially during an urban survey. Deployment of the conventional linear (1-D) array is often the only option available. The most convenient way of utilizing passive surface waves during an active survey for the 2-D velocity (Vs) profiling is the active/passive combined survey. This is the same as an ordinary active survey that uses the linear array and continues to make measurements at successive locations by moving the source/receiver configuration (i.e., a roll-along survey). The only difference is its recording time (e.g., T ≥ 30-sec) significantly longer than the usual time used in an active survey (e.g., T=2-sec). The first 1-2 seconds of the record will contain most of the active surface waves, while the rest of the record contains ambient vibrations of passive surface waves. In this way, it is a combination of the conventional active survey and the passive survey using the 1-D (linear) receiver array. Although the dimension of the passive array in this case (i.e., D=L) will be shorter than the length usually recommended for a passive survey (e.g., D≈2L), the recorded strong low-frequency surface waves will increase the chance of imaging dispersion patterns at the lowest frequencies with the highest definition that the given array can ever provide. This is the way the investigation depth can exceed the range that can be achieved by an active survey alone; for example, Zmax ≥ L.