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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?

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  • $\begingroup$ The best answer would not come from estimating, but directly by drilling. Depends on your resources and what you're actually trying to find out. Also, "hard rock" is relative. Hard rock relative to what? Soft sand? A chunk of steel? $\endgroup$ – Gimelist Mar 22 '17 at 6:26
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    $\begingroup$ Passive seismic tomography. $\endgroup$ – norman_h Mar 22 '17 at 9:27
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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!

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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.

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    $\begingroup$ isn't GPR for few meters rather than hundred meters? $\endgroup$ – rajesh kumar Mar 21 '17 at 16:17
  • $\begingroup$ it depends a lot on what the overburden is made of, 60-70 ft is not unheard of in dry sand. probably not good for your depth but It could be used in other areas so I wanted to include it. Sonar will work just fine, I remember our university lab had an 8 gauge shotgun with footplates I seem to remeber getting signal for several hundred meters. $\endgroup$ – John Mar 21 '17 at 17:06

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