Gravity sensors (gravimeters) can be small, like a breadbox and man-portable. Or they can be mounted on a platform like an aircraft like an Orion flown by NASA.
The most common use a high-quality spring out of something like quartz, but some use superconductance, etc.
The fixed ones you're thinking of are for gravity waves, mainly extraterrestrial. This mapping uses terrestrial views.
The man portable unit I used (mapping igneous dikes) decades ago had quartz knives for balancing edges which we had to be very careful of, otherwise, the unit would get wrecked (the knives were hideously expensive). We handled that baby like a case of eggs.
