Correlation analysis

[hflores]

Hello,
I'm working on correlating noise. We measured noise with a 22 geophone line with 1.5 m distance between geophones and we "caused" the noise walking from and to the first and to the last geophones in order to correlate the noise to obtain the Green Function.

When I look at my records I can see the steps going and coming either to channel 1or to channel 22. However I'm a little bit concerned on the possibility that Geopsy "changes" the time axis as for both correlations (respect to channel 1 and respect to channel 22) I obtain results as if the waves are traveling from channel 2 to channel 1 or from channel 22 to channel 21 (this last one is actually what I would expect) Or it might be that actually the sources are causing a "stronger" noise from this direction.

Thank you.
Best regards,
Hortencia

[hflores]

Even more, I think my question is even more simple. How does Geopsy calculate correlations "with a reference signal"? For example if I want to use channel 1 as a reference signal does Geopsy calculate channel 1 respect to channel 2 or channel 2 respect to channel 1?

The problem is that I obtain correlations that look quite similar independently from the reference signal I use.

Thank you,
Hortencia

[khorshid92]

Hi I have a question about correlation process at Geopsy.

I used 2 files as a input data in Geopsy and then using Correlation button from Waveform tab. As you can see in the attached file, in the Messages part, correlation is maximum delay set to 0.04995.
Also, Maximum time delay controls the size of the output window. The value m is in seconds. The correlations are displayed between -m and +m.

So, how can I get the time delay between two signals. I know that the maximum in time gives the time delay between two signals. But should I check it in the Figure? I mean how can I extract the exact value as a time delay between two signals?

Thanks.