* Jon Claerbout * and * Joe Dellinger *

This paper appeared in pages 34 to 37 of the October, 1987, copy of ``The Leading Edge''.

*© 1987 Jon Claerbout, Joe Dellinger and the SEG*

(Click here to get quick access to all the figures and their captions.)

The principle of reciprocity says that when a vertical source (a vibrator) and a vertical receiver (a geophone) are interchanged, the same seismogram will be recorded in each case. In a field study, Fenati and Rocca [1984] found that the reciprocal principle also applies surprisingly well even when the required conditions are technically violated, such as when an isotropic source (dynamite) is used with a vertical receiver (again a geophone).

In a split-spread survey (such as in the top part of Figure 1 ) waves can be recorded along the same ray path in both a forward and reversed direction. Since both directions should give the same result, why pay the cost of recording twice? Recording costs money.

Perhaps the reason is the minor sampling advantage that can be gained by using a slightly different split-spread geometry such as that in the bottom part of Figure 1 . This shooting geometry has an advantage over an off-end geometry with the same number of geophones because for the off-end case CDP gather geometries vary with midpoint. This advantage of split spread over off end must be balanced against the additional cost of having twice as much wire out on the ground. Note that in the split-spread geometry shown here the reciprocity principle is implicitly invoked, because traces shot in opposite directions are interleaved in the gather.

``Eisner's paradox'' is a clever attack on the principle of reciprocity from a theoretical point of view, but the paradox can be resolved by elementary (nonmathematical) physics. Understanding this paradox might help you decide whether it makes sense to shoot split spreads.

Eisner's paradox was first presented at the 1980 SEG Friday workshop on the topic of ``Seismic Reciprocity''. Reciprocity is counter to intuition, and just when everyone in the audience was starting to believe in reciprocity, Elmer Eisner (like a magician) presented the following example purporting to show that reciprocity contradicts energy conservation! He presented the paradox in a simple, direct way, much as we shall here, with no mathematics. I (JFC) was there that day and heard much discussion of Elmer's talk but I do not recall anyone able to resolve his paradox, nor could I, so I urged Elmer to publish it. The paper appeared as a short note in Geophysics [1983] . Unlike the talk, the paper included a lot of mathematics --- just the kind of distraction a clever magician might conjure up to distract an audience of theoreticians. Eisner states that half of the Geophysicists and Mathematicians he has presented this paradox to consider it to be a valid counterexample.

There are two experiments to consider: (A) source at the focus on the enclosed end, and (B) source at the focus on the cut-off end. These two experiments are shown together in Figure 2 , with experiment A on the top and B on the bottom. In each case the source (marked by an S) emits rays evenly in all directions. Each ray can either escape through the hole in the cut-off end, or can reflect from the ellipse and arrive at the receiver (marked by an R).

We can now restate the problem.
Reciprocity says that
the integral of the received wave over the cut sphere in experiment A
equals
the integral of the received wave over the uncut sphere in experiment B.
Observe that
the integrand over the uncut sphere is nonzero over almost all angles,
whereas
the integrand over the cut sphere is nonzero over only half of all angles.
This does not necessarily mean that one integral is twice that of the other,
because the arriving energy varies with angle. In fact, we know that
it is the * cut * sphere that receives * more * energy.

Each ray carries a local plane wave and the square root of the energy of each plane wave is its amplitude. Graphing the energy and the amplitude versus angle and numerically integrating the areas shows that the amplitudes in the two cases are the same and the energy in one case is indeed almost twice that in the other.

You saw a paradox only if you confused energy integrals with amplitude integrals.

Mathematically, it seems that
``A'' squared approximately equals * half * ``B'' squared,
yet simultaneously ``A'' equals ``B''.
This happens if A and B are both zero.
Eisner suggested perhaps it can also be true if both A and B are infinite.
Neither zero nor infinity seems a satisfactory answer.
Dahlen and Odom suggested it is wrong to analyze
the problem with geometrical optics
and that we should use diffraction theory.
But that drags in the limitations of diffraction theory too.
Now you know everything in the published record.
Has the paradox been resolved or has the mystery grown?

We claim the resolution of the paradox requires another ingredient, not yet mentioned. It is a simple fact that you probably learned in sophomore physics, but may have since forgotten. Please study the diagrams and contemplate the paradox a while before you continue on to the resolution .