Figure 11. (a) Comparison of P-wave images. All events are time aligned at the tie point, showing that, at this location, P-wave velocity is invariant perpendicular and parallel to fractures. (b) Comparison of SH images. Reflections on Line 1 occur at later times than do corresponding reflections on Line 2 because of the wave-propagation physics of fast-S and slow-S modes through the dominant, younger fracture set that extends across most of the overburden above the fracture target. Modified from Alford and others (1989).