Help! Seismic definition question?

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RaSaaLa
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Help! Seismic definition question?

Post by RaSaaLa »

Hello guys,
What is a flip-flop source and when is this in use?

also,
What does it mean when "finite-difference modelling is used to perform a 2D modelling"?
What is finite-difference modelling in seismic?


Thanks :thumbup:

RaSaaLa
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Re: Help! Seismic definition question?

Post by RaSaaLa »

And also, what is a point scatterer?

GuyM
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Re: Help! Seismic definition question?

Post by GuyM »

You should probably check out the SEG Wiki, that has a lot of definitions that can be helpful : https://wiki.seg.org/wiki/Main_Page

Flip-flop shooting is a marine technique for collecting 3D data. You have two sources and fire one (and record) then the next. This allows you to collect twice as many inlines with a single pass of the vessel. It's commonly used with interpolation methods to "fill in" any missing data,

Finite difference methods use an approximation to a derivative rather than the actual derivative to save time.Wikipedia has a mathematical definition : https://en.wikipedia.org/wiki/Finite_difference_method They are often used in forward modelling or the migration of seismic data

A point scatterer is an object that scatters the seismic data; as such, all you get from it is a diffraction pattern rather than a reflected image. If a surface is very rough it may act more like a series of point scatterers rather than a reflective layer, or you could just have an object where there is a high acoustic impedance contrast but its lateral size is too small to produce a clear reflection.

RaSaaLa
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Re: Help! Seismic definition question?

Post by RaSaaLa »

Thank you so much. You have helped me alot.

Can flip-flop shooting be used in 2D? Or is it just for 3D data? Or have i misunderstood the explanation?

Lets say thati have to create a model with finite-difference modelling in a software. I need to create a 2D model with reflections and diffractions.
How will it work when the modelling itself contain finite diffrence modelling? With the definition of finite diffrence, will the calculations take less time because it is finite diffrence modelling? How can I define what this finite diffrence modelling is?
It is hard to understand finite difference method.

And point scatterer is like defined as a diffraction? Is it a diffraction in a specific point since it is called point scatterer?


Sorry that I ask too many questions. It is hard to keep up with the lectures.

RaSaaLa
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Re: Help! Seismic definition question?

Post by RaSaaLa »

Hello again, can i also you one more thing?
Why is the dipping layer associated with water when the model has stacking velcoty?

GuyM
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Re: Help! Seismic definition question?

Post by GuyM »

Okay -

There would be no advantage to shooting 2D flip/flop; you may as well have one airgun array and just use that on a regular cycle. You could choose to process a single cable from a 3D "sail line"as a 2D proifle and use both sources if you wanted to, but the flip-flop part was to facilitate the 3D surveying.

If you are using software for finite difference modelling or migration then to some extent you don't really need to understand the underpinning mathematics in great detail, just the limitations of that specific approach. With (say) migration there are many different algorithms that can handle different situations better or worse. You build the model, and press the "go" button usually with some parameters related to time/depth steps and so on.

With difrations and point scatter you are really back to Huygens Principle; every point on a surface acts as a source of secondary wavelets, and these wavelets constructively and destructively interfering creates the on going wavefield, whether that's a reflection or a refraction. With a plane reflector you get perfect cancellation. The more rugged that reflector is, the more there will be places where those secondary wavelets sum in a different way, giving rise to diffraction tails - energy that is not cancelled out. So for example, at a fault you will get diffractions, because the fault breaks up the linear continuity of the reflector and the secondary wavelets don't fully cancel at the fault. Keep on shrinking the reflector and you get to a something that only produces a single diffraction. Good example might be a well head on the sea floor.

"Why is the dipping layer associated with water when the model has stacking velcoty"?"

Um sorry, don't understand what you are trying to describe there, but in general "stacking velocities" and picked on normal-ray CMP gather data, which has not been corrected for dip. This means on a dipping interface (like the seafloor) the CMP is smeared and laterally mispositioned.

There's an image on this blog post that shows this : http://seismicreflections.globeclaritas ... ation.html

Actually the blog runs through the concepts in processing of a 2D seismic line, which you might find useful, even if its a bit old.

RaSaaLa
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Re: Help! Seismic definition question?

Post by RaSaaLa »

Thank you for the link and your help. It was really useful.

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