more on auto option using Spac

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luigiV
Posts: 82
Joined: Fri Jul 17, 2020 9:28 am

more on auto option using Spac

Post by luigiV »

Hi Marc,
I submit this new problem that always concerns the auto option with linear array and Spac.
The array is made up of 18 stations spaced by 5m, acquisition time 25 minutes (station 16 was cut because of bad quality).
Entering the coordinates in the table with increasing x (y = 0) and applying Spac does not activate the auto option (see x_coord.jpg).
If on the contrary the coordinates are for increasing y (x = 0) the auto option works correctly (see y_coord.jpg).
Is there an explanation for this?
Finally, is a setting with a 5m rings thickness correct compared to the automatic one (0m)(rings_setting.jpg) ? The target file with 5m rings thickness seem to me more defined than 0m thickness one (comparison.jpg) Do you agree?
Thanks and best regards

luigi
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admin
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Re: more on auto option using Spac

Post by admin »

Hi Luigi,

I checked the "auto rings" along X or Y on a sample data file: it works equally in both directions with the latest release (3.5.0-preview). The problem you encounter is probably linked to a bug on the current release. Thanks for your vigilance.

About the thickness of rings, I have no particular opinion. The zero thickness is closer to Aki's original SPAC. Having a look at your plot, I could see very little difference. If you prefer the 5 m in this case, maybe that for another case the 0 m will be better.

Best regards,

Marc

luigiV
Posts: 82
Joined: Fri Jul 17, 2020 9:28 am

Re: more on auto option using Spac

Post by luigiV »

Hi Marc,
I would be really grateful if you could give me your opinion on the reason of the different spectral response of the "Spac" data and ReMi data refer to the same array (same number of geophones, same spacing, different acquisition time - 30 'for "Spac", 15 records 20s long for ReMi, different sampling frequency - 0.008 for "Spac", 0.002 for ReMi). What could be the reason? insufficient sampling frequency for "Spac" or other ...?

Thanks

Luigi
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admin
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Re: more on auto option using Spac

Post by admin »

Can you swap acquisitions and processing methods?
SPAC can be easily applied on the 15 twenty seconds records. For the opposite, I do not know.
Your SPAC picking could have been slightly higher in terms of velocity, resulting in a curve higher than REMI at higher frequency. You are probably within the range of uncertainties for such kind of sensor layout. At low frequency, SPAC looks like definitively higher than REMI.
Source distribution might be part of the problem. The only way to avoid such questions is to do 2D arrays (at least L-shape array).

Best regards,

Marc

luigiV
Posts: 82
Joined: Fri Jul 17, 2020 9:28 am

Re: more on auto option using Spac

Post by luigiV »

Hi Marc,
I tried, if I understood your answer correctly, to apply Spac to a 20s ReMi record. I hope I got the parameters wrong, but I didn't get any results. From the message window it is clear that there are not enough time windows to have 34 non overlapping blocks.
The opposite is not possible because the ReMi software does not admit files longer than 20s.

In another site I compared the Spac result of an L array (2m spacing) with the ReMi result of the long side of the L array. The Spac seems to provide a dispersion curve with too high velocities based on the frequency of the H / V peak and the depth of the bedrock (greater than 15m, drill hole data).
In my little experience I have often verified how the dispersion curves obtained by FK and Spac are characterized by higher phase velocities than those sampled on ReMi spectra (when these are reliable, of course).

Best Regards

Luigi
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