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Message ID: 6692     Entry time: Sat May 26 20:40:48 2012
Author: Den 
Type: Update 
Category: PEM 
Subject: experiments with seismic noise 

I measured relative motion of 2 seismometers separated by 4, 18 and 38 feet.

DSC_4307.JPG         DSC_4308.JPG

Relative motion for difference distances between seismometers is presented below.

    psd_distance.png        ratio.png


Al frequencies f < fcrit relative motion becomes smaller then motion of each point. At these frequencies, the ratio relative / absolute motion can be approximated as ratio = C fa . The following table summarizes fcrit, C and a for different length between seismometers.


L, ft fcrit, Hz
C a
4 45 0.0074 1.30
18 30 0.0057 1.52
38 15 0.0208 1.43

Using this approximation we can estimate the desired noise floor of the seismometer to subtract seismic noise from MC_F


Desired seismometer noise should be 100 times less then Guralp's. ADC noise is still less then this level, so this will not be a problem.

Note: for longer cavities condition for seismometer noise becomes more week as fcrit decreases with length.

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