[BobS]

"I don't believe gravity shows up on instruments either, but that is real."

Gravity is very measureable. Put a weight on a spring. A change in gravity changes the length of the spring.

[Axenolith]

Maybe with a spring that can detect the gravatational anomalies created by denser areas of the crust, otherwise, it's a change in the mass on the spring that changes the spring length...