Seismic aperture and taper

Anything about geophysics and geoscience: jobs, events, technical or general discussions ...
Post Reply
Tingpingbing
Member
Member
Posts: 5
Joined: Fri May 15, 2020 2:02 pm

Seismic aperture and taper

Post by Tingpingbing »

Hello guys,
I hope everyone is doing fine in these hard times!
It is something that I do not understand.
What are a seismic aperture and a taper?
There are a midpoint and an offset aperture, but I do
not understand how one can define a seismic aperture?
And what is for example a migration taper?

Thank you in advance for your help!

GuyM
VIP Member
VIP Member
Posts: 558
Joined: Sat Mar 24, 2012 11:35 pm

Re: Seismic aperture and taper

Post by GuyM »

These are two different but linked ideas.

The seismic aperture is the difference between the survey area and the target area you want to illuminate fully with seismic energy. You need an aperture because the ray-paths are not vertical; we have an offset between source and receiver for a start, and then we have the refraction effects as the ray passes through the rocks. When we apply imaging steps that take us to a zero-offset-equivalent normal, image or vertical ray (NMO correction, time migration, depth imaging respectively) the overall effect is to focus the wavefield. When we focus an image to make it sharper, it gets smaller.

That's a hand-waving version. You might do some kind of modelling work or survey planning to determine the acquisition aperture you need.

When it comes to processing, aperture also plays a role. We can restrict the migration by controlling the data that goes in to forming the image or the maximum extent of the migration operator can "swing" away from the output trace. This can be important because the migration operator, velocity model and data sampling are all not perfect; that means you might not get perfect destructive and constructive interference when traces are migrated, leading to noise.

Testing different apertures in migration allows you to offset cost (as a large aperture is slower to run) against image quality (how sharp the image is) against signal-to-noise (depending on data and velocity model quality)

Tapering is done at the edge of a survey; you have not enough data to constructively or destructively combine at the edge of the survey, when you head into the low-fold areas at the survey edge (in that acquisition aperture); to control noise - especially on kinematic migrations - you would suually scale down the amplitudes at the edge of the survey, using a taper, to limit this effect.

So -

- you have an aquisition aperture based on the survey design
- you have a processing aperture based on the data, velocity model and algorithm quality
- you have a taper at the edge of the survey to prevent migration artefacts that is the same as the aperture.

Hope this helps?

There's a bit of a discussion from a processing standpoint here:

http://seismicreflections.globeclaritas ... ation.html

http://seismicreflections.globeclaritas ... ation.html

Michael_Seman
Silver Member
Silver Member
Posts: 23
Joined: Fri Nov 24, 2017 4:36 am

Re: Seismic aperture and taper

Post by Michael_Seman »

Not wishing to complicate matters, but you could also have a taper by angle as well as horizontal distance. An algorithm might not be accurate over all angles, so for example you might use angles up to 60 degrees, and taper down to nothing at 65 degrees. But at depth, the horizontal aperture becomes the practical limit, e.g. aperture = 1.73 x depth if you wanted 60 degrees => huge distance.

GuyM
VIP Member
VIP Member
Posts: 558
Joined: Sat Mar 24, 2012 11:35 pm

Re: Seismic aperture and taper

Post by GuyM »

When it comes to shaping the operator it can be very complex.

We used to have a "bottle" shape, with near-offset protection in the near surface so vertical "sides" until it intersected the desired "angle", which was followed until it hit the maximum aperture (offset) desired. (Or maybe a "bottle kiln" is more accurate - these were all over my home area of "The Potteries" in the UK.

On top of that there were anti-alias functions that also feathered the operator shape at steep dip angle based on frequency and velocity.

This does give a lot of controls to play with and shape when it comes to limiting the migration aperture....

Tingpingbing
Member
Member
Posts: 5
Joined: Fri May 15, 2020 2:02 pm

Re: Seismic aperture and taper

Post by Tingpingbing »

Thanks to both of you for your help.

Your explanation was so easy to understand, GuyM.
Thank you so much!

Post Reply
  • Similar Topics
    Replies
    Views
    Last post