As a child I was once given two bent pieces of wire, each with a bend of 90 degrees in it, and told that if I walk over our garden irrigation pipe with the wires initially pointing forwards that they would cross in front of me when I crossed the water pipe because of the presence of the underground water. And so being a compliant child I did it and they being compliant wires crossed just as I was told they would. I was amazed and spent the next half hour crossing over and over the pipe, each time watching as the wires swung in front of me as I crossed the pipe.
That folks is NOT geophysics... again... NOT geophysics!
I sometimes wonder what would have happened if I was instead told that the wires should swing away from each other when you go over the pipe. I bet they would have... and that is kind of the point, there was nothing special about the wires, heck they were from a coat hanger, and if they had instead swung outwards then the "force" acting on them due to the water in the pipe would have been in the opposite direction than for the person who was told that they should swing inwards, that's not science.
Years later I was in the field camp where a driller told me he knew how to find water beneath the surface. Him being a driller I assumed, incorrectly, that he knew the geological setting, maybe a different shade to the dry soil or somehow identifying a water bearing fault zone from dense vegetation growth. But alas no, he reached up into the nearest tree, and broke a Y-shaped branch off. "You see you hold these ends of the stick" (the two upper ends of the "Y") "and when you go over water the third end will rotate downwards". This sounded like the two wires again... nevertheless.... he said "I can show you how it works, grab onto this" (words you don't want to hear at night in a drillers camp) and we both walked forward each holding an end of the stick. Then a remarkable thing happened it began to rotate downwards. Funnily enough he seemed to be holding it a lot tighter than I was, but when I tensed by grip he said "no, no, don't hold it too tight"... mmm
What the driller didn't know was I had recently completed my degree in geophysics and so started asking more probing questions than I would have back as a child with my tow pieces of wire. "Does it depend on how much water is down there? "What happens if it has just rained?" "Does the length of the stick matter"? "What happens if it is a wet stick?" "Have you tried with a dry stick?"... Nothing seemed to upset the system, he would always be able to find water with one small caveat, "I can't tell you how deep the water is". I smelt a fish, and it wasn't that nights camp cooking.
So what is geophysics?
In a nutshell, geophysics uses physical property contrasts (density, magnetic susceptibility, electrical resistivity etc) to map something (ore body, cavity, gas pipe.. practically anything really) in the subsurface, and unlike the art of laylines and water divining ,it does not require the "touch" as it is based on verified physical laws, with experiments/ surveys being predictable, repeatable, and quantifiable.
