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I've testing with data from refraction experiments with active linear array. I want to compare them with passive linear array option (supposing that is equivalent to REMI process). However, its result only gives the .max file with dipersion curve and apparently there is no way to see the f-k spectra. All my results shows curves with velocity increasing with frequency, so I think that they are result of aliasing effect. It would be very interesting see the fk-spectra to control the results of passive linear process.

Thanks.

Hi,

It is not clear to me reading your message if you managed to plot the dispersion statistics out of passive experiments. The produced .max file must be post-processed with max2curve. It is the same processing as usual ambient vibration 2D arrays. I don't know whether you are familiar with it.

The FK spectrum is not displayed in geopsy from passive linear data because it might be confusing. Picking the minimum of the dispersion curve may differ from one person to the other, according to the color scale used for the plot. Instead, a spectrum is computed for each time window (we suppose that you recorded a sufficient time length and that you can cut the signals into various windows to estimate the stationarity with time), and the dispersion is automatically picked from the highest value of the spectrum for each frequency. All results are summarized in .max file. The minimum of all velocities should tend to the correct dispersion curve. To delineate the minimum drawing the velocity histograms is mandatory (see max2curve). Contrary to the interpretation of the raw spectrum, the color scale of histogram plays a minor role.

In all cases, I would not rely only on passive REMI interpretation. In several case, we clearly observed that the help of active measurement is required to clearly identify the correct velocity level. Moreover, if there is a dominant direction in the wavefield, your results can be totally biased. Passive lines offer a good extrapolation of the curve measured by active sources but they can never replace 2D arrays.

Cheers,

Marc

If you really want to have a look at the FK spectra, you can trick the active tool. Set a pseudo source at one side of the line. Then process it like an active source with only one time window over the whole record.

If you want to stack over positive and negative directions, duplicate your signals: make a 'Cut' from menu 'Waveform' with the time range being the total signal length. Now you get your record twice. Set source positions at both end of the profile. Compute these pseudo shots with Active Linear FK. Finally, stack the two obtained plots by selecting them and choosing menu 'Stack' from 'Tools' in the result sheet.