Does bright mean thick — or does it mean gas?

Why a bright rim appears at all

The sand is thickest out in the basin and thins to nothing against the dome. Somewhere on that flank it passes through tuning thickness, where the reflections from its top and base line up and reinforce. That happens at one particular thickness, and because thickness is laid out in rings around the dome, the bright spot is a ring.

The rock and the fluid are the same all the way round the dome. The rim is an interference effect.

Try this — and what to watch for

1. Look at the three maps before touching anything. The point: the amplitude map on its own supports the wrong conclusion. There is no hydrocarbon anywhere in the model — the sand is full of water from edge to edge — and no lines have been drawn on it yet. There is still a bright rim around the dome on the amplitude map. Check the bar beneath it: that really is the loud end of the scale. If you had been handed only this map, the rim would look like the most promising thing on it, and it is a thickness effect and nothing else.
2. Check where the rim sits. The point: it proves the rim is thickness, not fluid, before any gas exists in the model. Turn on the tuning ring and compare it with the bright rim. They are the same line. The rim traces the contour where the sand passes through tuning thickness, which is a thickness contour, so it closes in a ring around the dome regardless of which way the structure dips.
3. Now put gas in it. The point: now there are two bright things on one map, and you have to tell them apart. Switch to gas updip and watch panel 3 change. A second bright area appears, and this one is cut off along a straight edge — the fluid contact. Because there is regional dip, that edge runs across the thickness rings rather than following them.
4. Learn the test. The point: this is the check behind the phrase "conformable to structure", and it is the most useful thing on this page. Compare the shape of the bright rim with panel 1 and then with panel 2. Tuning follows a thickness contour. A fluid follows a structural contour and stops at a flat, level edge — the flat spot. When somebody says an amplitude anomaly is "conformable to structure", this is the check they have done.
5. Take the dip out. The point: it shows you the case the map genuinely cannot settle, so you know when to stop arguing and go and get more data. Set regional dip to zero with gas still in. The structure is now a perfect dome, so structural and thickness contours are both circles and the two effects become indistinguishable on the map. This is the genuinely hard case, and it is why the check needs dip, well control, or an independent attribute rather than the amplitude map alone.
6. Move tuning instead. The point: it is a real test you can run on real data by reprocessing at a different bandwidth. Back to brine, and sweep frequency from 12 Hz to 50 Hz. The bright ring migrates up the flank as tuning thickness falls. A fluid contact would not move an inch, because it is set by the rocks and the pressure regime, not by your bandwidth. Reprocessing at a different bandwidth and watching whether an anomaly moves is a genuinely useful test.
7. Thin the sand until the whole thing tunes. The point: it is the one regime where reading amplitude as thickness is defensible. Take the basinal thickness below the tuning value. Now the ring stops being a ring: the entire sand is below tuning, so amplitude rises steadily with thickness everywhere and the map looks like a thickness map. That is the one regime where reading amplitude as thickness is safe, and you have to establish you are in it rather than assume it.