Sensors to quantify movements

Hello,

I'm currently involved in an experimental project that requires quantifying the movement of an object (1 cm x 1 cm) buried in a gravel box. The measurement accuracy has to be around 2-3 mm and frequency around 0.1 Hz. I'm pretty sure that this is comparatively very primitive sensor, but the issue is to find the right commercial product and the reasonable sensor size.

Thanks!