Since the source is a variable pressure source, it was taken to have a vertical symmetry axis. The same receiver was used throughout the data recording process, so it seemed reasonable to assume that it did not move up and down, or rotate during the experiment. This proved to be a good assumption. There were two rotations involved, one about a vertical axis to point the receiver at the other borehole, and the other in the vertical plane joining the two boreholes. This was likely to be more complicated, since it would depend on both the wave type and the vertical angle between the receiver and the various source locations..