  We present a modified commercial L-4C geophone with interferometric readout
that demonstrated a resolution 60 times lower than the included coil-magnet
readout at low frequencies. The intended application for the modified sensor is
in vibration isolation platforms that require improved performance at
frequencies lower than 1 Hz. A controls and noise-model of an Advanced LIGO
'HAM-ISI' vibration isolation system was developed, and it shows that our
sensor can reduce the residual vibration by a factor of 70 at 0.1 Hz
