Thoughts on AVO.

How do we find out about the subsurface? Seems like a simple problem. There is rocks down there. For some professions that might be good enough, but to find the precious hydrocarbons our planet has locked away it is not nearly enough.

Actually finding out what is 3km or more below the surface is a significant problem.

One method is to drill a hole. Simple, but fairly effective for a small area. We have developed many tools to find out what is down there. Measuring everything from background radiation in rocks to how resistive a rock is.

We can use several holes drilled over a large distance to infer what is down there, but in terms of finding new places to drill that will have hydrocarbons, maybe not the most effective.

Seismic can give us a good idea of what is down there. Essentially we introduce a wave pattern into the earth and then record how it comes back to the surface at various locations and then sum the results to enhance the signal and remove the noise. Once we process these "stacked" waves we can get a good idea of what the subsurface looks like. Figure 1 below shows 2 sources being reflected at a common point and being recorded. Since they have a common reflection point the received wave contains information about the same point.

fig 1

It is important to remeber that it is not a photo, but a mathematical model based on our understanding of rock and wave physics. This means it is not perfect.



The model may exhbit a "bright" spot where the signal returned from the sub surface changes significantly on the displayed section. These can often be caused by a gas reservoir. The gas in the rocks strongly effects how the waves travel and how they return to surface. In figure 2 below we see the amplituides at 1.4 and 1.6 get stronger as we move right to left and then die off again. This may indicate a reservoir. Each second on the diagram correspods to 5000'.




fig 2

Unfortunately often might not even be 50% of the time and drilling every bright spot means a lot of dry holes being drilled. Obviously this is not what we want, drilling a hole 3km into the surface is an expensive prospect.

In figure 1 we would see a difference in amplitude between the two rays despite the fact they are reflecting from the same point, this is caused by the different angle the rays strike the reflector at. As mentioned earlier, summing these or stacking them removes this information but produces a good signal of that point. The formulas for this relationship are complex and have been simplified over time. A simplifictaion by Shuey is the most common way to express this relationship.

Analyzing how the relationship of how the amplitudes change with incidence angle can answer some important questions. Do we see the amplitudes getting weaker as we move further from the source? Or do we see them getting stronger?

In general the responses can be classified in 4 ways and will tell us what type of rock and what type of fluid we have in the pores.

This is the basic idea behind AVO and a formation charged with gas will exhbit a different AVO response than one charged with water. And more importantly it can be used to differeniate a gas charged bright spot from another anomaly.

This gives us a basic understanding of AVO and a place to move forward from.