Difference between seismic refraction survey and tomography

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techzoo
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Difference between seismic refraction survey and tomography

Post by techzoo »

What's the difference between a normal seismic refraction survey and seismic refraction tomography? Are there any differences at all? Thanks!

Preston09
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Re: Difference between seismic refraction survey and tomography

Post by Preston09 »

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

Standard seismic refraction method assumes layers 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 model is more flexible. The velocity can change gradually within each layer both vertically and horizontally, especially for the overburden soils on top of bedrocks.

The interpretation methods are also different for these two kinds of surveys. Although they both need to pick the first arrivals of the traces, the inversion process are different. 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.

techzoo
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Re: Difference between seismic refraction survey and tomography

Post by techzoo »

Preston09, Thanks! Are there any commercial software for SRT data processing?

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Re: Difference between seismic refraction survey and tomography

Post by Preston09 »

I use Geometrics SeisImager 2D. I am sure there are other software available, too.

Tyler
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Re: Difference between seismic refraction survey and tomography

Post by Tyler »

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.

eric2009
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Re: Difference between seismic refraction survey and tomography

Post by eric2009 »

Is SRT going to replace conventional seismic refraction? It sounds like a great geophysical method from what you guys 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.

Preston09
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Re: Difference between seismic refraction survey and tomography

Post by Preston09 »

Conventional seismic refraction is more cost effective for a lot of simple problems, typically the depth to bedrock, etc. Although 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 refraction surveys, or vice versa.

MASW is a completely different seismic method utilizing 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

eric2009
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Re: Difference between seismic refraction survey and tomography

Post by eric2009 »

It's so amazing to see so many subsurface imaging technologies are developing. I used to know only GPR and a little bit about seismic refraction and reflection. I feel like to take some trainings to be a geophysicist, or just a technician, it so interesting.

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Re: Difference between seismic refraction survey and tomography

Post by Preston09 »

eric2009 wrote:It's so amazing to see so many subsurface imaging technologies are developing. I used to know only GPR and a little bit about seismic refraction and reflection. I feel like to take some trainings to be a geophysicist, or just a technician, it so interesting.
Glad to see that you are interested in subsurface imaging. It takes some training and experiences to be a fine geophysicist or a technician. Every geophysical method has its limitations and advantages, the hands on skills with real cases on each method are valuable since you cannot get them from the books. Knowing it and doing it are totally different things. 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 and the data interpretation could be totally different.

chrisvecan
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Re: Difference between seismic refraction survey and tomography

Post by chrisvecan »

To me, the hardest part is the geophysical interpretation. Geophysical survey designs present some challenges, too. Especially for some geotechnical investigations, the investigation depth and resolution need to be planed well within limited/minimal budgets. It all comes down to experiences. It's not hard to be a geophysicist, but hard to be a good one.

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xyz
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Re: Difference between seismic refraction survey and tomography

Post by xyz »

chrisvecan wrote:To me, the hardest part is the geophysical interpretation. Geophysical survey designs present some challenges, too. Especially for some geotechnical investigations, the investigation depths and resolution need to be planed well within limited/minimal budgets. It all comes down to experiences. It's not hard to be a geophysicist, but hard to be a good one.
The interpretation is the fun part, too. Isn't it? After you spent all these time and efforts to design, prepare, travel, and collect data, finally you can sit down and take a look at the data. This is my favorite part of doing geophysical surveys. Archaeology geophysics might be the easiest one in terms of interpretation. You do the geophysical mapping, hand the maps over to archaeologists and let them to figure out what could be buried there. :lol:

chrisvecan
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Re: Difference between seismic refraction survey and tomography

Post by chrisvecan »

Yeah, the interpretation is hard and interesting. Even for 1D problems like magnetotelluric and gravity surveys, sometimes it's really difficult to interpret because of all these corrections and adjustments you have to do. But all these make you a geophysicist instead of just an instrument operator working on idiot-proof equipments.

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Re: Difference between seismic refraction survey and tomography

Post by orzel720 »

chrisvecan wrote: It's not hard to be a geophysicist, but hard to be a good one.
I'm afraid you are wrong. I graduated in 2000 at University of Mining and Metallurgy (the best in Poland), the Faculty of Geology, Geophysics and Environment Protection, Engineering Geophysics Division http://www.agh.edu.pl/en/university/fac ... ction.html and may people resigned during study. To image the difficulty in studying the geophysics I will say that total no of students at the first year were 300 on the Faculty but geophysics division was chosen by ~30 but graduated only ~17.

Best regards
Jerzy

chrisvecan
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Re: Difference between seismic refraction survey and tomography

Post by chrisvecan »

What happened to the rest of the students, around 13 out of 30? They just failed or transferred to other majors? When I was in college, many of my classmates transferred to other divisions studying something else, not because they couldn't make it, just because they didn't like it after a while.

orzel720
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Re: Difference between seismic refraction survey and tomography

Post by orzel720 »

Yes Exactly it happened with them - they changed division, faculty or even university because of different reasons including these you wrote.

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Re: Difference between seismic refraction survey and tomography

Post by orzel720 »

techzoo wrote:Preston09, Thanks! Are there any commercial software for SRT data processing?
Rayfract http://www.rayfract.com/

Preston09
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Re: Difference between seismic refraction survey and tomography

Post by Preston09 »

Yeah, Rayfract is a good option. Although I did not get a chance to use it, I know it's a very competitive program.

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Re: Difference between seismic refraction survey and tomography

Post by john09 »

Usually how often do you place your shots within a cable spread for SRT?

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Re: Difference between seismic refraction survey and tomography

Post by Preston09 »

john09 wrote:Usually how often do you place your shots within a cable spread for SRT?
Ideally you should do the shots between every pair of adjunct geophones to get the maximum resolution. However, you can do less shots depending how the geology is. I usually do more shots when the topography is rough or there are a lot of different ground surface materials. If your survey line go across different zones of soil, pavement, concrete, etc, try at least one shot at each of these zones.

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Re: Difference between seismic refraction survey and tomography

Post by john09 »

OK. I see. There is no universal shot spacing.

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Re: Difference between seismic refraction survey and tomography

Post by Preston09 »

Here is an example showing what conventional seismic refraction survey and SRT can give you:

Seismic refraction, 2 layers:
seismic refraction.jpg
Seismic refraction tomography with same data:
seismic refraction tomography.jpg

techzoo
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Re: Difference between seismic refraction survey and tomography

Post by techzoo »

Thanks for the example! So SRT can help you to find bedrock fracture zones where the wave speed is slower than the rigid zones, right?

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Re: Difference between seismic refraction survey and tomography

Post by Preston09 »

techzoo wrote:Thanks for the example! So SRT can help you to find bedrock fracture zones where the wave speed is slower than the rigid zones, right?
Right. Besides determining the depth to the bedrcok, it helps to find fracture zones, or differentiate rigid bedrocks from weathered bedrocks.

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Re: Difference between seismic refraction survey and tomography

Post by hasnabouftou »

Preston09 wrote:
Tue Jan 11, 2011 10:41 pm
john09 wrote:Usually how often do you place your shots within a cable spread for SRT?
Ideally you should do the shots between every pair of adjunct geophones to get the maximum resolution. However, you can do less shots depending how the geology is. I usually do more shots when the topography is rough or there are a lot of different ground surface materials. If your survey line go across different zones of soil, pavement, concrete, etc, try at least one shot at each of these zones.
hello.
please can you help me, I am using Seisimager2D and in the field wa have done several profiel with 12 geophones and 15 shots and we have done in overlapping between the profiles but sense I am only using seisimager2D lite it provide only inversion with 12 shotsand i and I need a software that can correlate betwen the profiles

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Re: Difference between seismic refraction survey and tomography

Post by pitta »

In my experience the seismic refraction, like GRM, works better if there is a sharp refractor like bedrock under a soil cover. Instead the seismic refraction tomography give better results where the velocity changes are more smooth like on large landslides bodies.

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