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Combining MASW & ReMi with a linear array

Posted: Fri Dec 23, 2011 7:15 pm
by esaez
Dear Marc,

we are doing a lot of active and passive measurements using a linear array of 5m spaced geophones (12 channel array).
After computing dispersions curves using geopsy and seg2curve softwares, we obtain systematically a vertical gap between both curves. ReMi's dispersion curve shows always larger phase velocities.Is that matter directionality of ambient noise ??? Can you suggest me a procedure to combine both dispersion curves ?? Perhaps a phase velocity shift of the ReMi dispersion curve ??

Thank very much

PS: mery christhmas !!

Re: Combining MASW & ReMi with a linear array

Posted: Wed Jan 18, 2012 2:05 pm
by admin
Did you process passive data recorded on a linear array with Geopsy or only with Remi?
If you observe a systematic gap, I suggest to record active source on the same site (if not already done, from your message it is unclear for me whether or not you used active recordings). With Remi or any passive processing of a linear array you cannot be sure if you pick the right velocity level. If your ambient noise contains energy traveling at specific azimuth not aligned with your line, you can easily obtain biased results. Without hesitating too much, if the difference you observe is between active and passive along a line, there is no doubt, you pick the wrong curve with Remi. Being above with Remi is "normal" for wave traveling slightly oblique to the line.

At ISTerre we almost never use linear arrays for ambient vibration. 2D arrays are far better. They can be achieved also with refraction equipments but cables are a little annoying and prevent from achieving large aperture arrays. Positioning of stations is also a little more difficult than for a line. We are working together with Potsdam university (Daniel Vollmer and Matthias Ohrnberger) for a portable, open-source, auto-localized (internal differential GPS), real-time (wireless mesh routing between stations) processing array for ambient vibrations called WARAN, which globally cost the same price per channel as conventional refraction equipments.