Ground Penetrating Radar

On a sunny day (Tue, 19 Apr 2022 18:19:09 -0700) it happened Don Y snipped-for-privacy@foo.invalid wrote in <t3nn2q$mub$ snipped-for-privacy@dont-email.me:

Down under it is used to locate gold veins

I used to work for a geophysics company which occasionally used a hired GPR, usually for detecting buried services on a potential building site.

The resulting reports were so vague and non-committal it was hard to see what value they had. My overall impression was that GPR probably works well in a limited set of ground conditions which only exist elsewhere.

This was about 40 years ago though.

Corpse or ghost-hunting?

I supplied the impulse generators for a wideband GPR that was to be used to de-mine Viet Nam or somewhere. It wasn't successful. Rocks, roots, lumps of junk confused the images.

You can trace wires from their E or H fields. There's cheap stuff to do that.

Could they not deduce (from GPR results) where the services went based on knowledge of where they *entered* the property? I.e., use the GPR and some deductive interpretation to suss out *this* is the path it takes?

Here, you'd be foolish to do your own "survey" as YOU would be liable for any damages to those services that were the result of your survey errors. (periodically, some idiot takes out a back hoe and rips up a gas line or electric service that he didn't know was "there")

We have a (free) service that comes out and marks all of the below grade services (phone, CATV, gas, water, sewer, electric). If you rely on THEIR markings and end up damaging a service, *they* bear the responsibility.

I've no idea who was using these -- nor how. But, there were several units available for purchase so I have to assume they were used, regularly.

But, as I couldn't come up with an "excuse" (as SWMBO would say) to own one, I had to discipline myself to take a pass on the opportunity. (they are large enough to be "noticeable" and consume a fair bit of storage space -- esp as you can't just put them UP on a shelf!)

I have used the service several times. The did not mark the cable TV one time and it was cut by construction. No come back to me for the repair.

Here, the service is really just a sort of "notification bureau". They appear to dispatch the request to each of the separate utilities and each of *them* decides how to address it.

E.g., the municipal water department sends out a guy to mark the water main; the phone company sends out a guy to mark the phone line; the electric company sends out a guy to mark the power; etc.

Several of these "hire out" that activity to a third party "skilled" in that activity. Often, *one* person from such a company will come out and mark multiple services -- no doubt because they are under contract to those multiple services (and are likely smart enough to know that a request from service A at address X will likely see a request from service B at that same address within days/hours -- why dispatch a guy twice if you can combine the trips?)

It behooves you to learn the various marking colors so you can assure yourself that they've ALL been marked before starting any work excavating (regardless of who foots the bill for a f*ckup, YOU will be the party that is without phone/CATV/electric/gas/etc. if someone screws up!)

Many years ago a new development was about to be constructed next to a busy railway station and signal box. A buried cable was found. The power company insisted that it wasn't theirs. The railway company insisted it wasn't theirs. It couldn't be because it was not on their land... So the cable was cut. There was a major signalling failure. It was many hours before anyone realised the connection between the two events and restored train services.

John

The magnetometer in your cell phone should be good enough for tracing wires in walls

My son's company uses GPR in a suite of other tools for finding reinforcing steel inside corroding concrete structures. Their business is remediating structures (wharves, bridges, tall buildings, mine hoppers, etc) that would otherwise have to be torn down.

I can't vouch for how useful the GPR is compared to other methods.

CH

You would (naively) *think* that there were records of all of these pipes, cables, etc. Of course, there are only *general* documents but lots of missing "detail".

When I was a kid, they built a new school in my neighborhood. So, I spent a lot of time watching all phases of its construction.

Some years later, a crew came along to dig up the roadway (essentially, the "driveway" to the school). I told the back hoe operator to be careful of the *pipe* buried where he was digging (having SEEN it being laid there a few years earlier).

"Go away, kid" (no record of the pipe on HIS paperwork).

When he *tore* it out of the ground, we ended up with a week's unscheduled vacation as the school was shutdown for its repair!

THIS neighborhood is considerably newer. Yet, they don't have accurate "maps" of the buried power lines that daisy-chain each of the transformers together. And, that's just *two* connections (A & C) per group of 4 homes!

How would that work, with a DC responding magnetometer reading, and AC through the copper (or Alu) wires? Or are they laid in iron pipes? Even with an AC responding magnetometer the radiating field from two closely laid wires is very low.

In modern homes here (NL), wires are in plastic pipes. I normally pick up the E field with a simple detector, works even in dry concrete walls. It fails on 3-phase wiring, the total E field intensity is very low.

Arie

They make great props for many cable TV "reality" shows, re hunting treasure, etc. Lots of minutes burned showing somebody walking around in a field :-)

Double-D coils can detect various metals by their reflectance and the phase shift of their response. Here's a rough overview:

I knew some metal detector freaks once. We experimented with various coil configs. A single coil oscillator, audio heterodyned, was about best. Human ears are very frequency sensitive and the FM effect is very intuitive when you're waving a coil around.