Presentation on theme: "Near traces or Middle traces or Far traces The A mplitude V ersus O ffset mythology. If ever a single picture could prove a negative, this one might be."— Presentation transcript:
Near traces or Middle traces or Far traces The A mplitude V ersus O ffset mythology. If ever a single picture could prove a negative, this one might be the one, so please take the time to study it! You are looking at a raw gather set of some 200 traces, running from left to right. The wild things you see happening are real, and typical of the whole in-line. To suggest we can choose which third to use seems a little naïve. Unfortunately much time has been spent (and is still being spent) on such processing dead ends. Every continuous pattern that does not follow normal moveout is coherent noise. When these lineups are not what we expect, we often look right past them. The number of errant events on this slide is so great, that one almost has to go into a staring trance to see them all. I had suspected this type of noise would be present, and feared the application of pre-stack migration would confuse the issue. To check this I requested this migration free data. The careful study that follows does not show any strong multiple evidence, yet, after migration the noise certainly has that appearance. What we see on this pure data is real however. This slide is with my own simple minded NMO applied. The studies I show on slides 5 & 6 show the pure input. There is no question but what we are looking at horizontally traveling events beginning fairly early in the spread. Basic physics tells us that the vertical energy is waylaid at the cloning point, blocking the normal paths to the outside traces. These truths are plainly visible on these pictures. I detail them on the next few slides. You might have to stare for a little while. See how many patterns you can pick out.
An early wave trap occurrence. Note the expansion ending in some sort of explosion above marker 2; 2.A trend line to be drawn on the next slide. 3.The over-corrected leading edge of the wave trap starting at point 4. 4.Here is where the chalk the cloning begins. The expansion is obvious, but of great interest is the backward dipping event just below. 5.Once again, these backward sloping events are of great interest. This blast of energy shows the oscillation of the horizontally traveling wave guide, producing point sources of energy that radiate in all directions. The important thing to note is the backward sloping series that is moving towards the inside traces. As I will note below, careful visual analysis finds remnants of this pattern moving inward with time. The void left by the blockage of vertical energy lets these events stand out. 6.These well defined but weaker emanations are still moving toward the inside. 7.I have no current explanation of this strong energy, except that it seems obviously connected to the cloning at 4. We have no idea whether the strength is natural, or if it is an interference phenomenon. The possibility of an internal multiple between 1 and 2 is something I study in the removal work. 8.If you look very closely here, you will see a series of X’s. These define mute points. On the next slide I draw that deep mute in, and discuss the matter. Review of Important observations - By the way, the trace at the immediate left is the stack. The raw power of the stack to bring out the common denominator continually impresses me. Of course that common property is the correct NMO. A number of strongly dipping (reverse) events can be seen in the deep section, if you stare at the picture long enough. The system challenge is to separate these obvious noise components from the real reflections. Take some time here.
Notes: The yellow line to the left is the deep mute that was used for the results below. The white is a possible upper limit. The red is an interesting trend we don’t yet understand. The next 3 slides round out the pre-migration discussion. The following 6 contain examples from different areas. They show the horizontal wave trap where the critical angle chances with trapping conditions. A final result example.
The multiple? paradox: At the time the initial Nexen summary show was submitted, I was going with the presence of long multiples. But now, after Looking at the pre-migration evidence I have changed my mind. What has not changed is the perception of a lot of sometimes confusing noise (see the first 3 slides once again). The top graphic is snapped as the NMO corrected gather is fed to the noise removal. At this depth, the moveout error we see looked too great to fit the long repeat picture. It is vital to note that the difference is increasing with time. The last plot on the top half is of the stack of the input data. Because comparing the stacks is vital to this discussion, this stacked vector is saved by the logic to be repeated on the second display. The lower display is called just before sending the stack to the inversion logic. On this pass, the treated set is displayed, followed by the saved input stack and the final “noise removed” stack. My removal logic does not assume the noise events are multiples. Instead it looks for contiguous patterns outside the norm. In this case it did a great job removing rather similar noise events. I have looked at many of these before and after examples during the development. Often the noise is highly variable, and the logic has to make many passes trying to get it all. The answers are always more reasonable, but there is a lot left to do. For example, many patterns are obviously straight refractions and the logic needs to be more flexible in terms of the pattern searching The red lines show the deep mute used for this run (all data below can be ignored).
This is a portion of the first raw gather from a pre migration in-line. The oval shows where a strong event (chalk?) clones into a horizontally traveling wave guide. There is a preponderance of roughly 50cps energy going quite deep. There is little believable deep continuity, and no strong evidence of multiples. To understand what this view is telling us takes some effort.. Please toggle with the next slide where I lift off the 50 cycle stuff. I’ve spent a ton of time just staring at this picture. Each time I come back I see more crossing patterns. For example, look at the events that are crossing the chalk at the top of the oval. Obviously these are not long multiples bouncing from that really strong interface, and neither are they simple water bottoms. Going shallower, we see straight line refractions (at least we can understand them). Now let’s consider the critical angle thing we are seeing inside the oval. Once it is hit, no more energy passes through the chalk, literally making the outside traces useless for deep energy. Furthermore this causes a void in the wave front. As we are taught, nature abhors such things and fills in with whatever it can dream up. Our simple minded answer is that the energy that is trapped in the horizontal mode is reverberating, becoming a series of point sources. Whether you believe this theory or not, there is no arguing with what we can see physically. Notice that the deep patterns are higher in frequency than the the believable strong shallow events. This implies they have not traveled very far and that is important.
Here are the results from the lift-off filter. Note once more the data within the matching oval. Note that the path beyond follows a straight line, showing the energy to be traveling horizontally (refraction). Note also the expanding refraction pattern following the cloning point. Because downward traveling energy has been interrupted at the critical angle, these events fill the gap. All of this is happening in the critical target zone. The next slide shows other examples.
To the right is our final inverted and integrated result on an in-line from another part of the world. To get these results around 1/2 of the outside traces were thrown out by our optimized stack logic. Below are the first five gather sets from that inline. Later we follow the transition from here to the extreme shown below. This occurs near the probable reservoir we point to with the red arrow. Amplitude v.s. Offset – Of course the point we start cloning into a horizontally traveling wave trap depends on the location of the shot. From the many cases I have studied it is clear that this evolution is gradual (not an all or nothing break). We can see that above and in the following slides. Here, when we get to the extreme velocity change caused by hydrocarbon content, the change is more abrupt, and extreme things tend to happen (as at the left). I think the extreme amplitudes we see are a function of reverberations within the wave trap. AVO logic would have spotted the anomaly, because of the impossibly high amplitudes. However we doubt they (the amplitudes) had anything to do with AVO mathematical theory, I was fairly proud of the ADAPS results shown at the right. A wave trap from elsewhere
So we start through the selected data sets (every 25 th ) At the rignt you see the mute used by the optimized stack. The arrows point to an upper event we called a probable reservoir. We say the events start as reflections on the very inside, then clone to wave trap patterns as we move out. It’s important to note this thickening pattern does not apply to all events, so it has to be tied to the wave trap. Notice that while details change, the expansion pattern is consistent.
A mistake we first made, trying to describe the phenomenon, was to assume it started at the critical angle. It may be that the really crazy stuff we are beginning to see starts there, but the cloning obviously is well under way early on. Think of the velocity errors being created.
As we approach the probable reservoir “hot spot”, the cloning and associated expansion gets more extreme.
It is our considered opinion that all of the crap shown at the far right is coming from a single wave trap, and we think that trap is generated by sand we pointed to.
To show this trapping is not that unusual we include a slide from a another project. We have run ADAPS on dozens of gather sets, and this discussion fits the majority. The problem is made evident by the ridiculously long spreads designed for AVO purposes. We say the target pattern (bordered by the green thickness bars) is a horizontally traveling phenomenon. For simplicity we have call it a refraction, but as we show on the next slides, the problem is a lot more complicated than that.. Because time differences between traces on these events are dependent on the one-way velocity of the horizon, and because these amplitudes dominate the energy continuum, velocity calculations are often quite distorted. This is is a huge problem brought about by the (misguided) use of ultra-long spreads. All inversion approaches depend on the consistency of wavelet information derived from the trace statistics. On the inside traces here one can see the distortion caused by early attempts to remove the coherent noise we describe at the end of the series. We have a combination of tough problems here so we were pleasantly surprised that our ADAPS logic always works so well. Imagine using such stacked traces to determine bed thickness (or whatever). I use both the thickening and highly elevated amplitude evidence as proof that these are horizontally traveling events (even if we do not guarantee we understand the ray paths that must exist). There has been a persistent myth in the industry that such thickening was only due to the normal horizontal travel component. Of course if this were true all events would be affected all the time, and they obviously are not. Click on the image to communicate with me, on the blue arrow to go back to the intro, or on the red one to go to the series home.