Video from DJI Mini 2 drone - obviously spotted by the guys on the ground as they gave it a wave 
Sound from my iPhone…270 metres away in my back garden ( hence not exactly synced !
Inside my house you can hear a double thump - one from sound transmitted through the air, and another transmitted through the ground 
Fascinating to watch how it’s done and just how far the posts go in.
I just wonder who decides how deep, how many, and what spacing is required?
Geologists and structural engineers
Thank you. Do they use some sort of ground radar to achieve their aims or core samples?
I suspect core samples would be taken
I was interested because my building started to sink into the ground. Well, a corner of it did. Long story short after a 7 year battle got paid out by insurance company who then sued Builders. I paid £11,000 to have it underpinned and made safe that was 21 years ago now. No movement since.
Core samples had been taken and found voids, old pieces of wood and other demolition debris before underpinning. Hence builders at fault.
Reminds me of when I was working in Stafford, they built a new multi storey car park next to the Civic Centre and we had to put up with weeks of this, the whole building shook at times, the ground was soft so they put hundreds of those things in, sometimes one followed by another as you could just see them disappear.