I measured relative motion of 2 seismometers separated by 4, 18 and 38 feet.
Relative motion for difference distances between seismometers is presented below.
Al frequencies f < f_{crit} relative motion becomes smaller then motion of each point. At these frequencies, the ratio relative / absolute motion can be approximated as ratio = C f^{a} . The following table summarizes f_{crit, C and a for different length between seismometers.}
L, ft |
f_{crit, 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 f_{crit decreases with length.
} |