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Seismic refraction survey and seismic refraction tomography? 
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Joined: Mon Sep 20, 2010 10:23 pm
Posts: 4
Post Seismic refraction survey and seismic refraction tomography?
What's the main difference between a normal seismic refraction survey and seismic refraction tomography? Are there any differences at all?

Can't seem to find the answer online...

...input very much appreciated!


Mon Sep 20, 2010 10:35 pm

Joined: Thu Jun 17, 2010 6:22 pm
Posts: 13
Post Re: Seismic refraction survey and seismic refraction tomogra
Lary,

Although most data acquisition equipment can be used for both methods, the models, data processing and field procedures are quite different.

Standard seismic refraction method assumes subhorizontal layered model. Within each layer, there are no seismic velocity variations in the vertical direction, although lateral variations are allowed. In seismic refraction tomography (SRT) the velocity can change gradually within each layer both vertically and horizontally, especially for the overburden soils on top of bedrocks. So the velocity model is more flexible.

The methods for interpretation are also different for both types of seismic surveys. Although the inversion process are different, they both need to pick the first arrivals of the traces. SRT involves more complex mathematical algorithms to fit more flexible models.

In the field procedure, SRT generally needs more shots than standard seismic refraction survey to obtain high resolution profiles. In this sense, marine seismic surveys are more cost effective for SRT than land surveys.

hope this helped!


Tue Sep 21, 2010 12:34 am

Joined: Mon Sep 20, 2010 10:23 pm
Posts: 4
Post Re: Seismic refraction survey and seismic refraction tomogra
Thanks!

Is there any commercial software for SRT data processing?

...and, is SRT going to replace conventional seismic refraction? It sounds like a great geophysical method from what you are describing. What's the difference (cons and pros) between SRT and multichannel analysis of surface waves?

It seems to me they both can do velocity tomography.


Tue Sep 21, 2010 12:44 am

Joined: Thu Jun 17, 2010 6:22 pm
Posts: 13
Post Re: Seismic refraction survey and seismic refraction tomogra
SRT is used for different purposes than 'normal' conventional seismics....

Conventional seismic refraction is more cost effective for a lot of simple problems, typically the depth to bedrock, etc.
SRT can provide more comprehensive interpretation, the cost is apparently the key component of most projects using geophysical consulting services, so I don't think SRT can replace conventional surveys, or vice versa.

The data processing using generalized reciprocal and delay time methods for conventional seismic refraction analysis is very simple. The idea is mainly based on 1D models with limited abilities for 2D variations. In the other hand, complicated inversion has to be used in SRT for the real 2D or 3D models similar to those used in DC electrical resistivity profiling or tomography.

FOr software I use: Geometrics SeisImager 2D

Another one is Rayfract http://www.rayfract.com/


Tue Sep 21, 2010 1:04 am

Joined: Mon Sep 20, 2010 10:23 pm
Posts: 4
Post Re: Seismic refraction survey and seismic refraction tomogra
Thanks!

It's amazing to see how far subsurface imaging technologies have developing. I used to know only GPR and a little bit about seismic refraction and reflection.
BTW, what is MASW??

I feel like to take some trainings to be a geophysicist, or just a technician, it so interesting.


Tue Sep 21, 2010 7:03 pm

Joined: Thu Jun 17, 2010 6:22 pm
Posts: 13
Post Re: Seismic refraction survey and seismic refraction tomogra
Glad to see that you are interested in subsurface imaging. It takes some training and experiences to be a fine geophysicist or a technician. You need hands on experiences on geophones, seismographs, cables and data processing, etc. Even for the same method you have worked on for years, a totally new application could give you a hard time since the data processing and the data interpretation could be totally different.

Multichannel analysis of surface waves (MASW) is a completely different seismic method using surface waves instead of body waves as in refraction methods. Currently MASW is mainly used to map the shear wave velocity while the mapping for the P wave velocity is still in development. On contrary, seismic refraction methods are primarily used for mapping P wave velocity although it can also be used for the shear wave velocity mapping. MASW is more cost effective in general, but the investigation depths and the horizontal resolutions are typically less than those of refraction methods....

...so again it's about what you can spend and what you want to achieve.


Tue Sep 21, 2010 7:49 pm

Joined: Mon Sep 20, 2010 10:23 pm
Posts: 4
Post Re: Seismic refraction survey and seismic refraction tomogra
Thanks for all this.
It really cleared stuff up for me. I now have some solid ground to read into it further.


Tue Sep 21, 2010 8:13 pm

Joined: Thu Jun 17, 2010 6:22 pm
Posts: 13
Post Re: Seismic refraction survey and seismic refraction tomogra
no problem! and have fun!


Tue Sep 21, 2010 11:15 pm
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