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 isola
tion 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
