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.}

_{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.}