Figure 1 – End-on view of a fractured test sample from the source end. The source vector is polarized at an angle ? relative to the azimuth of the fracture planes. As the source remains fixed at one end of the test sample, the receiver at the opposite end is rotated 360 degrees at angular increments of 10 degrees relative to the orientation of the positive-polarity of the S-wave source vector. These test data show that only a fast-S (or S1) mode propagates parallel to fracture planes (responses A’ and C’), and only a slow-S (or S2) mode propagates perpendicular to the fracture planes (responses B’ and D’). A mixture of S1 and S2 is observed at all other azimuth orientations. Amplitude behavior is affected by the continually changing angle between the vector orientations of the positive-polarity ends of the source and receiver elements. Dashed circle T0 defines the travel time origin T = 0 for the wavelets. Modified from Sondergeld and Rai (1992).