SUBTRACT ANY TWO SHOT GATHERS

We will soon be doing some data-guided theory. Carefully examine Figure 1. I have only this one shot gather (a photo image) but I'd like you to imagine two of them. Imagine the earth really is a one-dimensional layered medium with surface scatterers. Then the two shot gathers would look identical EXCEPT for the diffraction at 250m offset. This diffraction would be differently positioned on each shot gather depending on the distance of the shot from the surface scatterer. Let us subtract these two gathers. Now the layered media reflectors all go away and we have a gather containing only two copies of the surface diffraction.

To study the surface scatter events, we need to be rid of the layered media primary events. These could be gotten rid of by the simple subtraction or they could be gotten rid of by a spatial lowpass filter or a spatial PEF, or in the event of gentle dip, by various kinds of "steering filters". We can return to the practical issue of separating the simple reflections from the surface scatter after we have covered some matters of principle.