Drift in position

03-Unexpected behaviour/errors
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moizhyd
Posts: 9
Joined: 22 Jun 2018, 15:42

Drift in position

Post by moizhyd » 18 Jul 2018, 11:58

Hello,

I am trying to scale an accelerogram with a custom spectrum.
After the scaling (which goes ok) I see that velocity and acceleration look reasonable
but the position shows a large drift with respect to time.

I am wondering why this happens and how I could fix it.
Please let me know if you have any insights regarding this.

Best Regards,
Moiz

huffte
Posts: 840
Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
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Re: Drift in position

Post by huffte » 18 Jul 2018, 13:37

Hi moizhyd. First of all, does the displacement drift occur in the unmatched record? If so, then you should likely baseline correct and filter the original record, using SeismoSignal for example, to eliminate the drift. If not, then you might wish to baseline correct and filter the matched record to eliminate the drift. Baseline correction and filtering, when done properly, typically do not alter the response spectrum significantly over a wide range of periods, but there is always the chance that they will.

Are you using SeismoMatch 2018? If so, I believe it already includes baseline correction and filtering options. I'm not certain that SeismoMatch 2016 does. So one approach might be:

1. Perform matching on the record in SeismoMatch
2. Save the matched accelerograms
3. Load the matched accelerograms in SeismoSignal
4. Perform baseline correction and filtering in SeismoSignal
5. Save the adjusted accelerograms
6. Load the adjusted accelerograms into SeismoMatch and perform matching on it.

All this being said, your best option might be to try other records for matching to your target spectrum. If indeed the original record does not have the drift but the matched record does, this would seem to indicate that the alteration through wavelet addition to achieve matching to the target was so dramatic that the original accelerogram was not the best choice for the matching process.

Best of luck moizhyd.
Tim Huff

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