40km strike length geological question

does anybody know how the geologist may have ascertained a 40km strike length from the drilling program over a 60km distance...

I am certainly not doubting this as I am not a geologist, but how would he come to this conclusion. Is it by visually looking at the core samples or is there some other method as at the time, the laboratory analysis had not started.

40km is very significant indeed if in fact this is true...