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How deep can I explore with seismic refraction?
Fundamentally, there is no limit. Seismic refraction has been used to image the MOHO. The depth of investigation is governed by the size of the source you use and the length of the seismic spread, which needs to be 4-5 times the depth of investigation.
In practice, the most common use of seismic refraction is in shallow geotechnical, engineering, and groundwater investigations. Most of this is focused on the upper 20 meters or so. It is no coincidence that this happens to be around the upper limit of what can be accomplished with a hammer and plate. Every site is different – the specific geology and amount of cultural noise are important variables that ultimately govern depth of investigation for any given source – but 20 meters for a hammer and plate is a good rule of thumb.
In general, deeper work requires larger sources such as accelerated weight drops or light explosives. The only way to know for sure is to lay out your spread and see if you can get quality first breaks at the far geophones with 8-10 good swings with a sledgehammer.