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The Showroom - Data for my thesis

I’m working on a field data set from the Mediterranean Sea. There are many intellectual reasons for working this data set. However, there are some motivate my inner child as well, let me explain.

The Specs

The data set was recorded by TGS and is of brilliant quality. Almost 8km (5mi) in offset over the entire length of a salt sheet with 11s recording time. The frequency content is quite nice for conventional acquisition. It was recorded in the Eastern Mediterranean to be more exact in the Levantine basin.

The fun reasons

When I hear Eastern Mediterranean this will immediately trigger images as the following:

Kardamos Beach. CC-BY-SA GGia

Blue sea, nice beaches and interesting geology on wonderful islands. This may be a bit simple, but then again I’m looking at white/gray/black wiggles all day long, let it slide. It’s a happy place in my head. Another fun one to think about is that Levante is the Italian word for sunrise.

Sailing vessel at sunrise CC-BY-SA Valerii Tkachenko

La dolce vita.

The intellectual reasons

Enough of the pretty pictures. Why would I use exactly this data set in this geological setting.

The salt sheet in the Levantine Basin is young. Now let the geology jokes roll in, as in geology we usually say something is young when it’s only “a couple million years old”. So they were deposited around five and a half million years ago. Since then there has been comparatively little time for tectonic overprinting and the like. However, basin-ward there are some saltrollers. So in one and the same data set we can compare very simple “fresh” salt geology with some overprinted more complex setting.

Trying subsalt imaging in this rather controlled setting will provide a sandbox1 for algorithms and processing steps to test.

Salt tectonics

The salt itself is very interesting as well. The study of salt tectonics in general is pretty young (not on a geologists scale, on a “normal” human scale). In the beginning of last century salt and its overburden were considered to be fluids (on geological time scales) and buoyancy drove salt to rise through the upper sediment. At some point people started realizing that buoyancy alone could never explain the behavior of salt in our Earth. Instead of considering salt to have some sort of intrinsic motivation to rise and start some salt diapirism, nowadays, evaporites are considered to stay where they are unless there is some driving force that will cause it be displaced. Don’t get me wrong, especially here in Northern Germany, we do have the rare case that salt diapirism is actually buoyancy driven, but that is one process out of five. This paradigm change is considered to be the change from the “fluid era” to the “brittle era”

However, salt is an excellent decoupling mechanism for tectonic stress regimes. Normally, when you look at stress fields you have a superposition of local, regional and global stress fields that act upon your area of interest.

Superposition of local (blue) and regional (red) stress fields combine their influence an a fault (purple).
When we introduce salt, we can have a look if the salt was actually decoupling our stress regimes. Usually, you can say that the regional stress acts deeper than the local stress.
Decoupling of local (blue) and regional (red) stress fields. Only regional stress field acts on fault (now red).
More will follow on salt tectonics but this topic especially in regard to the Levantine basin is so huge, it deserves it’s on article.

The challenges

I started a little on the challenges of subsalt imaging in the initial blog post about my master thesis, but let me repeat that part.

Salt Body with two features marked. One has a complex salt geometry. One is fairly simple.
Simple salt geometry still defocus the beam, lowering the illuminating energy. For simplicity only uppermost reflection shown. (Opacity shows total energy.)

This is not an accurate model of Levantine basin but it represents the general setup. You also have the two zooms with the seismic beam in both settings. As we can see the complex structure has some basic optic effects on the beam. However, the simple structure also defocuses the beam because micro-undulations in the interface are present.

 

Complex salt structures can cause lens effects, spreading the energy. Reflection not shown. (Opacity shows total energy.)

So it seems that the simple salt geometry would be much easier to image, as there are no strong dips and lenses. There’s only a little defocusing. That’s at least what I thought. Oh how wrong I was. In salt the degree of deformation correlates negatively with the degree of anisotropy. So when there is less deformation present in the salt sheets, the anisotropy gets turned up a notch. And as Helbig (one big author on anisotropy) once said:

Anisotropy is a nuisance!

You got to know my data a little bit. I for one am quite excited to try and get all the juicy details from this one. Let’s see how this one turns out.

The end is near! - My Master Thesis

I just started the end of my journey in university. I registered my Master’s thesis. This feels like quite the step and I’m very proud I’ve come so far.

I’m 25 now and I feel it’s time to move on. There’s so much to explore and new things to discover. However the coming 5 months will be a tough one. I tend to think “easy is for the lazy ones”, so I looked for the challenge and yes I found it.

My Bachelor

For my bachelor thesis I have been working on trace interpolation using an algorithm that is known as the Partial Common Reflection Surface. Working with this has opened quite a few doors for me. The spin-off extended abstract got published at the EAGE and I got to present a poster in Vienna. I’m also fairly sure my two internships with Fugro FSI (now CGG) and Western Geco (part of Schlumberger) were only possible due to my prior knowledge of the Common Reflection Surface stack. So taking a challenge has paid off, why not take another one.

The Challenge

When you work in seismics you will assuredly come across a couple white whales, problems that will haunt you or even the seismic community throughout their entire career. One of these topics is subsalt imaging. Follow me into the depths of this little abyss.

What is Subsalt

Under very special circumstances it may happen that salty water will be evaporated to the point that only the salt is left. A forgetful cook might encounter this a bit more often, however this also happens on a bigger scale on earth.

Salt Body with two features marked. One has a complex salt geometry. One is fairly simple.
One example is the Mediterranean Sea. When the straight of Gibraltar closed, it was basically becoming a big lake in an arid environment. The water leaving this place, left a lot of salt on the bottom (please dear geologists forgive my oversimplification). Salt is quite an interesting thing to work with. On geological scales it behaves more or less like a fluid. Through enough pressure it will also form a perfect seal to upper layers of sediment, therefore serving as an ideal top of a reservoir for carbohydrates. This is the main reason people want to be able to look under the salt. That’s where the term Subsalt comes from.

The challenge in Subsalt imaging

Simple salt geometry still defocus the beam, lowering the illumating energy. For simplicity only uppermost reflection shown. (Opacity shows total energy.)

As you can imagine, salt is quite different from the surrounding rock. In seismics this difference means a strong reflection so a salt body will give us even two strong reflections. Usually this is nice, as the salt body can be imaged quite well. However, if you want to look below this salt body, you are in the tight spot of having only a fraction of the energy left to illuminate the subsalt region and then this energy has to travel through the salt body with the two strong reflectors again.

But this is not the only obstacle. Due to the crystaline structure of salt if may behave anisotropically. Additionally, the following happens when salt is deformed.

 

Complex salt structures can cause lens effects, spreading the energy. Reflection not shown. (Opacity shows total energy.)
Salt deforms under pressure and will adjust to the rigid surround rock. Even the slightest deformation will leave a very non-uniform layer in the worst cases it will form a salt body that has lens shapes as reflecting layer. The non-uniform layer will defocus our seismic beam like a laser pointer with a badly crafted lens that will under no circumstances focus the laser beam. Thus, spreading out the left-over energy even more leaving us with a puzzle of reflection patterns to solve. The lens-like structures are just adding to this problem. Pointing a flashlight at a curved mirror will illuminate a lot of places of the room as the light is spread out along the reflection trajectories.

 

My approach

There are a couple of ways to tackle this problem. Since I have been working with the Common Reflection Surface stack in the prestack domain before, this will be one approach to do it. I will compare it to conventional CMP processing. Then there is this new method we developed in our workgroup. Sergius Dell worked on the Partial Time Migration process, which is a process based on Common Scatter Points that will give a prestack time migrated gather. So for comparability all of these gathers have to be time migrated and processed with great care2.

However, I’ve decided to let you be a part of this process as I’m taking up to much information to put into this thesis, I might as well have an outlet for it.

This will be an interesting journey. Not only are these three completely different processing approaches. It’s dealing with Subsalt imaging, which is one of the big problems in seismic imaging. Additionally, at least two salt layers show anisotropic behavior. Anyone else feel this tingling sensation?

Want to know more?
Let me know in the comments. I’m happy to share!

Losing a friend you did not know you had.

Monsters are real, and ghosts are real too. They live inside us, and sometimes, they win. ~ Stephen King

There are words that do not come easily. Yesterday the world has lost a great person. The actor Robin Williams died.

You may have seen his funny movies. You may know him as Peter Pan. (Bangarang, Robin!) or as ever-inspiring teacher of the club of dead poets. Maybe the clown who cares about terminal patients has gotten to you. He was the funny guy. I once saw him do a stand-up-like performance about playing Call of Duty, it was hilarious. He knew how to draw in his audience. Maybe you know his more serious roles from Good Will Hunting. It really doesn’t matter.

You see this person. You see he has everything. He is funny, has a family, friends, wealth, fame, inspiration. And he has depression. Imagine how many people have told him to “get over it”. “You’ve got everything, what more do you want?” “Just try some Yoga, no one can feel bad doing Yoga!” Unfortunately it’s not about what you have. Depression is the state you’re in when you perfectly know what to do but can’t. It’s the state where your body just says “Nope!” and you’re overwhelmed. It’s the state when the demons in your head break through and whisper terrible things.

You may or may not know these demons. Everyone has their own ones. They tell you you’re ugly, lonely, unloved, unwanted and a bother. It’s fine to be ugly, unloved unwanted but what’s for the worst is being a bother. All the people trying to help you, but no one can really do anything to help. No smart hint or Jedi mind trick has yet helped. The only thing that will lead you out the other side is a friend saying “Don’t worry I know what it’s like, I’ll just be waiting here and I’m still your friend. You’re not alone.”

Today Robin Williams, a great inspiration and a friend who would help you through a tight spot with his movies and his humor, listened to his demons and decided not to bother anyone anymore. In his mind it was a service to us, in my mind it will be a great loss.

I don’t know if anyone else liked him, but losing someone this famous to depression will have consequences. Be there for someone who is facing this struggle, this is not a good time. Tell them their demons are lying and that you’ll still be there when they get out of their bed. I know a lot of people will need that now.

The last curtain falls.
May you find Neverland.

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