  In this paper the sensor noise of two geophone configurations (L-22D and L-4C
geophones from Sercel with custom built amplifiers) was measured by performing
two huddle tests. It is shown that the accuracy of the results can be
significantly improved by performing the huddle test in a seismically quiet
environment and by using a large number of reference sensors to remove the
seismic foreground signal from the data. Using these two techniques, the
measured sensor noise of the two geophone configurations matched calculated
predictions remarkably well in the bandwidth of interest (0.01 Hz to 100 Hz).
Low noise operational amplifiers OPA188 were utilized to amplify the L-4C
geophone to give a sensor that was characterized to be near Johnson noise
limited in the bandwidth of interest with a noise value of $10^{-11}
\text{m}/\sqrt{\text{Hz}}$ at 1 Hz.
