Beam steering

Having seen some coherent energy in the data, we now would like to determine precisely where it is coming from. Beam steering is a process that determines the plane wavefield incident on the array as a function of arrival direction. Beam steering is equivalent to a 3-D slant stack. We stack the data (or compute semblance) along many different planar trajectories, and display the results, either as a function of time or averaged over time.

Figure  illustrates one way in which the results of beam steering analysis can be presented. In this figure, the semblance value for plane wave energy is displayed as a function of the apparent slowness of the plane wave and the azimuth direction (or equivalently, as in slant stacking, as a function of the ray parameters p_x and p_y). A vertically propagating plane wave has zero apparent slowness, thus the center of the plot gives the semblance value for vertical incidence. The extreme edge of the plot corresponds to an apparent velocity of 2 kilometers per second. If the medium velocity at the surface is also two kilometers per second, then this corresponds to energy propagating horizontally. Thus in this one plot we get an idea of the plane wavefield incident on the array in all directions.

Figure 2 shows semblance values for plane waves incident on the array for two of the records from the Iceland dataset. Each record in the dataset was 60 seconds long. We compute semblance as a function of azimuth, apparent velocity, and time, and then average semblance over the time axis to produce a composite picture for the entire record. There is clearly agreement between the two records. Energy arrives from the upper left at an apparent velocity of 2 kilometers per second. Since we know that the near-surface velocity is about 2 kilometers per second, this energy is most likely propagating horizontally from a surface source (most likely a river known to lie in this direction). More interesting is the energy that arrives at higher velocities. It could represent endogenous sources within the earth or scatterers.

 

beam1
Figure 2 Semblance measure of the plane wavefield incident on the array for two different records of the Iceland dataset. Semblance has been averaged over time to produce a single image for each record. White denotes a large semblance coefficient. The largest semblance values have been clipped and are shown in black. There is considerable agreement between the two. The outer edge of the circle corresponds to an apparent velocity of 2 kilometers per second. Energy arriving at this apparent velocity is most likely from a source on the surface. A river located in the direction to the upper left is suspected to be a strong surface source.