stretch phenomena after NMO application on seismic data

Unveiling the Stretch: Why Your Seismic Data Gets a Bad Case of the “Wiggles” After NMO (and What to Do About It)

So, you’ve just run NMO correction on your seismic data. Great! You’re one step closer to seeing what’s hiding beneath the surface. But wait… what’s with all the… stretch marks? Yep, that’s NMO stretch, and it’s a common headache in seismic processing.

NMO, or Normal Moveout correction, is basically the magic trick that lines up reflections from different source-receiver distances, making them look like they all came from the same spot. This is super important for getting a clear picture of what’s going on down there. But, like any magic trick, it has its downsides.

Think of it like this: you’re trying to squeeze a slinky into a smaller box. Something’s gotta give, right? In this case, it’s the seismic wavelet. NMO correction compresses the time it takes for reflections to arrive, especially on those far-offset traces. The result? The wavelet gets stretched out, like taffy being pulled. This “stretch” makes the wavelet longer, lower in frequency, and weaker in amplitude. It’s like your seismic signal just hit a low note and lost its oomph.

Now, how bad is this stretch going to be? Well, it depends:

• The farther away your receivers are, the more stretch you’ll see. Those far-offset traces are the ones getting the biggest time squeeze.
• Shallow reflections get hit harder. Near-surface events with short travel times are more prone to stretching.
• Slower velocities make it worse. Areas with slow subsurface velocities? Get ready for some serious stretching.

Imagine a stretch factor of 1.15. That’s like taking a second and turning it into 1.15 seconds. Sounds small, but it adds up! It means a 15% increase in wavelet length and a 15% decrease in frequency. Not ideal.

So, why should you care about a little wavelet stretching? Because it can really mess with your interpretation.

• Resolution goes down the drain. Losing those high frequencies means you can’t see the fine details anymore. It’s like trying to read a map with blurry glasses.
• Amplitudes get wonky. NMO stretch messes with reflection amplitudes, which are crucial for AVO analysis. AVO helps us figure out what kind of rocks and fluids are down there, but if your amplitudes are distorted, you might as well be reading tea leaves.
• Stacking becomes a blurry mess. Stacking stretched traces together just makes the problem worse, especially for those shallow events. The stretched wavelets clash, and your final image looks like it was taken with a potato.

Okay, so stretch is bad. What can we do about it? Luckily, we have a few tricks up our sleeves:

• Muting is your friend. Just chop off those far-offset traces that are causing the most trouble. It’s like trimming the ends off a frayed rope. The trick is to mute aggressively enough to get rid of the stretch, but not so much that you lose valuable data. It’s a balancing act. There are even automatic muting techniques that can help you find the sweet spot.
• Get fancy with your NMO. Some advanced NMO algorithms try to minimize stretch right from the start. They use clever math to preserve the frequency content of your data.
• Give your spectrum a boost. Spectral balancing techniques can help restore the frequencies that were lost during NMO. It’s like giving your seismic data a shot of caffeine.
• Pre-emptive frequency boost: There is a technique that uses Gabor transforms to anticipate and counteract the frequency loss caused by NMO stretch. It’s like giving your seismic data a pre-workout boost, sample by sample, before the stretching even happens!

NMO stretch is just a fact of life in seismic processing. But by understanding what causes it and how to mitigate it, you can avoid a lot of headaches and get a much clearer picture of what’s going on beneath your feet. And that, after all, is the whole point. The industry is always coming up with new ways to tackle this problem, which is good news for anyone trying to get the most out of their seismic data. Keep experimenting!