I am doing MASW survey, some where I am observing hard concrete surface and some places exposed limestone. In both case I am getting reverse dispersion curve i.e. phase velocity is increasing with frequency. Is it possible?
Is it due to the underground geology or something else?
It is possible. You can verify if first arrivals in your data exibit only direct waves without refraction?
Thank you all. :)
Roberto Sir,
Please elaborate your answer. If no refraction exhibit then first break will show dispersion nature?
Yudi Sir,
I am working on Surfseis for processing and inversion for all dispersion curve. Is there any possibility to get wrong inversion result for reverse dispersion curve with this software? If yes, please suggest somthing more fruitful.
Dear Pranjal
I think that a best practice in MASW interpretation is to integrate the inversion of dispersed wavefield (properly the inversion of picked dispersion curve) with the that of refraction, this is an advantage of MASW acquisition that adopts the same layout of that of refraction. Obviously the integration is performable in the range of depths where the two inversions give a model reconstruction and in the hypothesis that the P interfaces are the same of S interface. With this premise my advice is to analyze the P wavefield that you recorded in order to see if there is coherence in the inversions. In particular if you have a decrease in velocity you should expect the absence of refraction and a direct P phase (first arrivals in P fase) characterized by hygh velocity compatible with the estimated Vs value from MASW, and possibly guided.
Unless you have not already read, I suggest you to read the paper of Park at al. that treats about the seismic investigation of pavements:
http://www.kgs.ku.edu/Geophysics2/Expo%20Update/Pubs/Pubs/PAR-01-03.pdf
In this paper the relationship with refracted wavefield is exposed and it is highlighted the importance of the inversion of higher modes in the MASW approach.
Note: MASW method is a powerfull and cheaper technique to explore the near surface structures without the intrinsic limit of refraction method. A general problem of MASW inversion, in my opinion, is that the reconstructed model is "a local average" of the "real model". This problem is similar to that of model reconstructions obtainable with seismic tomography. So also when we deal with "genuine interfaces" the reconstructed model will show a "high gradient velocity zone" which should be the "reconstructed interface".
Surface wave is the wave that travels through the air-ground interface but we consider approx. 30 meter data in MASW method (for 48 channel streamer & geophone spacing 1m) using only surface wave. How is it possible? Is there any other wave component which gets added to the data to extrapolate the depth up to 30m?
Of course, you can get dispersion curve from first break depended on seismic source i.e. sledge hammer, explosive.
Hello,
May be you should try SFK transform for MASW.
The SFK transform is the enhanced method for f-k analysis of MASW data.
See the matlab code with the SFK transform implementation (dispersion curves are plotted): Code SFK-transform
it is very easy. You run an example: it draws the dispersion curves - you see how it works.
also you can get it on mathworks:
https://www.mathworks.com/matlabcentral/fileexchange/71845-masw-dispersion-curve-with-slant-f-k-transform
Best Regards!
- Badges
- Science topic