THE AVO MYTH - In case you haven't heard, a large percentage of exploration drilling is based on software that looks for "amplitude vs. offset" anomalies, on the assumption that these variations point to the presence of  hydrocarbons. 

My problem with this oversimplified logic is that it deflects attention from effective inversion.  I do not say that such amplitude differences do not exist, I just say they will be dwarfed by other factors, and that the undeniable success of such discoveries is better explained by what I say below.  

I might suggest that you should consider the processing potential that will appear when the industry finds out I am right, and that new discoveries are possible where AVO missed. 

But then who am I? (Just a stubborn iconoclast who won't give up.) In any case, the discussion below gets into the tuning phenomenon in depth. As I have said, tuning is the hidden enemy of seismic resolution, and every person dependent on seismic results should understand it.

When  coherent noise is strongly present, the noise pattern at any  trace offset is generally predictable, and  AVO observations become somewhat of a joke.  This observation is based on looking at many thousands of shots.  

However, tuning is the monster in the closet.  Almost every visible event on a seismic record is a tuned composite of overlapping primaries, as we again see to the right. If the "nodes" are "in phase" we get strong amplitudes. If they are not, we may see nothing. Amplitude vs. offset results are no exception.  

The question - Can we explain offset oriented amplitude changes using the tuning phenomenon only?

The answer - Certainly. Of course it is a little complicated, but all the elements are basic. 

These primary events must be separated before surfaces can be trusted, or any difference in reflection amplitude can be taken seriously.

My answer was "everything you describe is tuning, not AVO". 

The frustrating thing was that I had tried to convince him of the tuning phenomenon a number of times before. Of course his PHD is in geological engineering, so he probably can be excused (but how about all the geophysicists he was dealing with.) So excuse me for repeating myself. 

One reason I was so sure was that I had just finished a report where I had simulated an almost identical  gathers pattern just by varying the gap between two primary events (the top and the bottom of a coal seam). The common element between the two situations was the very high reflection coefficients involved. 

This simulation is shown middle of the following picture composite, to the left. It provides the character match that you should study. What remains is the explanation of what caused the apparent change in reflector spacing. The answer is that the (outside trace) energy spends a bigger percentage of its time in the low velocity bed, effectively increasing the reflector spacing. All of this is shown below. I have to say that these are just the facts. 

For emphasis I will restate the main point.

"The travel path of the outside trace spends a higher percent of its time in the very low velocity bed, thus distorting the normal moveout picture. This modifies the effective time gap between events, changing the character of the tuning."