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.

Figure 2

1/13/1998