Interpretation

The lack of sharp velocity contrasts mean there are no clearly observable reflections in the impulse response. The visible events are diving waves (refractions) of increasing order. The first arrival is the the direct wave that would be characterized as the `P' arrival in terrestrial geophysics. The second arrival is equivalent to a `PP' event (or first order multiple) having bounced once on the solar surface. About six distinct arrivals are visible in Figure 3, corresponding to multiples up to fifth-order.

Since there are no distinct velocity interfaces, only a smooth velocity gradient, the head waves continually curve upward. The contrast between this behavior and head waves on terrestrial exploration seismograms is illustrated in Figure 5.