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Figure 2. S-wave propagation through a fracture system observed when the source and
receiver are aligned in the same azimuth, as is done when processing actual S-wave seismic
data. S1 is the fast-S mode. S2 is the slow-S mode. The azimuth angle in this graphic defines
the orientation of the positive-polarity end of both the source and receiver relative to the
fracture planes, whereas the angle in previous figures defines the orientation of the receiver
relative to the source. Only S1 propagates parallel to fractures, and only S2 propagates
perpendicular to fractures. Modified from Sondergeld and Rai (1992).