The stacked images in Figures 4.21–4.23 appear “grainier’’ at source–receiver distances near 0◦ and 180◦ than at 90◦, owing to a smaller number of seismograms available at these distances. Why might...

The stacked images in Figures 4.21–4.23 appear “grainier’’ at source–receiver

distances near 0◦ and 180◦ than at 90◦, owing to a smaller number of seismograms

available at these distances. Why might one expect the number of seismograms to

diminish at small and long ranges?