I am looking for some technique or machine developed to estimate how deep is a hard rock starting around 100 meter below earth surface. If somebody wants to estimate how deep is that hard rock, how can they know is there any method implemented and used around the world?
A small scale seismic survey as well.
I remember as students we found the water table using a 100m refraction line and a sledge hammer sound source. Scale that up from ~5m to 100m you're going to need a longer line and a larger sound source. Reflection might also become more convenient. Still a lot smaller than a typical 2d survey line though!
Sonar (passive or active) or ground penetrating radar with both work, it can also be estimated from a published geologic profile of the area which are often built from both in addition to drill data.