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 < 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.
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.