f Secondary shear waves generated from source boreholes

In cross weIl tomography experiments there are two types of sources commonly used. The most common are point sources, such as the airgun or dynamite, and axial sources. In general, the point sources generate tube waves which travel up and down the borehole and these tube waves contain more than 99% of the energy generated by the source (Winbow, 1989). AdditionaIly, point and radial sources also generate shear waves (Heelan, 1953, Lee and Balch, 1982). Axial sources do not generate tube waves (Winbow, 1989; Meredith, 1990) and therefore will not be discussed in this abstract.