Common-Mid-Point (CMP) Gathers

Common MASW surveys use a 1D linear receiver array (RA) to generate a 2D shear-wave velocity (Vs) cross section through a roll-along approach. Although surface waves spread into all azimuths, only the propagation along the path of the RA is used for subsequent analysis that generates a 1D velocity (Vs) profile to be placed at the RA center. Multiple such surveys can ultimately accomplish an equivalent 3D survey; for example, by running parallel 2D lines. The most effective 3D survey, however, can take place only by using a 2D RA accompanied with a proper 2D shot pattern, similar to the 3D reflection survey. In this way, surface waves generated from one shot point and propagating into multiple azimuths can be used to generate velocity (Vs) profiles at multiple locations. Efficiency of surface wave energy is therefore maximized.

In addition, if acquired field records are sorted into common-mid-point (CMP) gathers, the surficial sampling size (bin size) can be controlled not only by the pattern of receivers but also by the shot pattern. Controlling the shot pattern is a lot more convenient than controlling the pattern of whole receivers. This allows the bin size to become smaller than a receiver spacing used in the field. As a consequence, a relatively small number of acquisition channels (e.g., 24 channels) can still be used for a dedicated 3D survey by using a stationary 2D RA of a long receiver spacing (e.g., 5 m) to cover a relatively large area (e.g., 50 m x 50 m) with a relatively small (high) bin size (spatial resolution) (e.g., 1 m x 1 m).

The general scheme of a dedicated 3D MASW data acquisition is illustrated in Figure 1a.  A 24-channel 2D RA is laid with a 20-m longitudinal (West-East) and transverse (South-North) spacing. A pattern of multiple shots, among many other possibilities, is also displayed. To illustrate how a CMP gather is created, eight (8) different shot-to-receiver raypaths are marked in red lines, which have different shot-receiver offsets while sharing the same (common) midpoint (CMP). This 8-trace CMP gather can go through the normal sequence of MASW analysis to generate a 1D velocity (Vs) profile to be placed at the corresponding CMP location.  Figure 1b shows all such CMP's in red dots that are created from the given shot-receiver configuration displayed in Figure 1a and have six (6) or more shot-receiver pairs (traces) sharing the same CMP. Each CMP point will have its own 1D velocity (Vs) profile.  It is noted that the CMP interval (10 m) is only half the shot interval (20 m) and more CMP's (96) are created than the number of shots applied (55). This example illustrates that a 3D survey can be simpler than  multiple 2D surveys.  In addition, a CMP gather contains surface wave propagations from multiple azimuths, yielding a more realistic subsurface sampling than the conventional 2D/1D survey.

In order to sort all these field shot gathers into 3D CMP gathers in ParkSEIS-3D, all shot-to-receiver mid points are first mapped on a fictitious chart for all possible combinations.  Then, those points within a given bin size (e.g., 1m x 1m) are grouped  together as the same CMP. Next, only those CMP gathers are selected for subsequent analysis that meet certain conditions such as an aperture size (e.g., 30 m), minimum number of traces (e.g., 6), and source-receiver offset distributions. The aperture size (D) is determined by the maximum investigation depth being sought (Zmax); i.e., D = 2Zmax. These selected CMP gathers then go through the normal MASW analysis sequence of dispersion and inversion to generate 1D velocity (Vs) profiles to be placed at the corresponding CMP locations.  Finally, a 3D grid data set is created from these profiles by using an appropriate interpolation scheme.

Figure 1. (a) Illustration of a dedicated 3D MASW survey with a stationary 24-channel 2D (X-Y) receiver array and a pattern of multiple shots. A common-mid-point (CMP) is marked that has multiple (8) shot-receiver raypaths sharing the same point. (b) CMP's (red dots) that have six (6) or more traces created from the shot/receiver (SR) pattern in (a).