@paden-cash Now I know why I keep stepping on rebar on US412.?ÿ Just kidding.
This is very interesting to a guy that started using a Shonstedt in 1972. I have never encountered an iron pipe or rebar that returned no signal to either of that manufacturer's models over the years. I have never used a "metal detector", only magnetic locators.?ÿ
I agree that there are techniques specific to working in particular situations, e.g., chain link and wire fences, masonry rebar, railroad slag, electrical lines and others that must be learned and applied to be effective.
I must admit that, in all this time, I have yet to pull the GA-52 out to test the return from an iron pipe driven into particularly difficult ground. I'm usually so glad to be done with the hammer that I can't wait to leave that location. I will have to test that. I would like to try that magnet thing too to reset the magnetic field. Never tried that.
So many ways to skin a cat.?ÿ
JA, PLS, SoCal
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This problem occurs too frequently in wide open areas where the odds of other iron in the area is minimal, especially in the center of county roads.?ÿ We have had cases where both Shonstedt and Subsurface locators are used and both agree that nothing is there when we know there is.?ÿ Dig down in the spot and find the bar, check again and a weak signal is provided.?ÿ Another case was a 3/4" iron pipe about six inches deep near the white stripe on the edge of a highway.?ÿ Everything assured us it was present or removed for some crazy reason within the past five years.?ÿ Started digging to a depth of about three inches, began to get a weak signal, dug the rest of the way to the pipe and still had a relatively weak signal.
Typically, we will dig on a "null" or silent spot when the background makes some signal but it quiets over THE SPOT.?ÿ Have found many monuments in that fashion.
Alloys in the iron or even coatings on the iron can effect the magnetic signal. A normal 10" dock spike sings much louder than a galvanized?ÿ 10" one. that may be caused by the heat in the galvanizing process. The pounding it in can demagnetize it, but pounding works both ways. If you are looking for a buried spike under pavement, pounding the pavement while listening to the detector you will hear a change in the signal while pounding. Surveyors who really want other people to find their irons often set a magnail or a magnet touching the bar before final cover up.
Paul in PA
Here is an update:
We found an iron that the detector could barely notice and pulled it out and laid it on its side. When we touched the detector to the top end it barely made a noise. Then when we moved it to the bottom end we got a loud whistle. It seems to me that a lot of the energy from the hammer pounding it into the ground was absorbed in the top end and threw the poles in the iron out of alignment. Since most of the energy was absorbed in the top end only a small amount made it to the bottom in order to drive it into the ground, the poles in that end of the iron stayed in magnetic alignment. It would be interesting to test this out more.
The steel mill near in my area produced small hills of slag. Somebody decided that they could save / make a little money by incorporating the slag into roadway asphalt mix. Good for them but bad for us surveyors with metal detectors, the whole roadway sings.
You know right away from all of the rust stains on the road that it was paved with slag in the mix.
GTR (ground tire rubber) has been an additive to asphaltic binder for some time now.?ÿ The advocates will tell you the tire grinding process removes the steel from the tire's belts.?ÿ But surveyors around here know that is not true.?ÿ There are thousands of tiny pieces still in the mix.?ÿ And yes, the whole roadbed sings like you're standing on top of a buried locomotive.
@paden-cash?ÿ Not to mention the problem of roadways catching on fire when GTR is used for fill:
"What at first seemed like a brilliant way of getting rid of mountains of old tires has now given new meaning to the old saying about what paves the road to hell.?ÿ Two highways repaired with chunks of rubber are smoking and oozing a toxic, oily goo that is threatening nearby marshes on the Columbia River.?ÿ The state used the rubber from a million recycled tires in place of rock or gravel to provide 7,000 cubic feet of fill when it rebuilt a 150-foot stretch of state Route 100 here in October.?ÿ There are three theories about the cause: that rusting steel fibers from radial tires act like miniature furnaces; that rubber-eating bacteria produce acid and heat; and that the shreds have an insulating effect and allow the mixture to heat up to combustion levels."
Another issue on highways we have encountered is when the yellow striping has something in it for reflectance, I guess, that sets off the magnetic locator.?ÿ Where it really bites is when they have added a new layer of asphalt and not removed those old stripes.?ÿ You dig through the new layer then discover it was the striping fooling you.
Normally the reflectance is from glass beads added after the paint has been placed. Temporary stripping tape does have a metallic backing that resembles foil.
Think of all the bars never recovered becsuse they don't have magnetic properties.
I've found at least three without a signal. I shudder to think of all the ones I may have missed. I'm in a non-recording state, so finding out if a corner has ever been monumented is a crapshoot. I try to always search diligently, but probing with a shovel in a 50+' radius for every corner (that may never have been set) is not very financially viable.
"quiet" rods aren't an everyday occurrence, but I discovered enough of them to be wary.?ÿ
Normally the reflectance is from glass beads added after the paint has been placed. Temporary stripping tape does have a metallic backing that resembles foil.
Glass beads are nonmagnetic. Whether applied in the paint mix gun or broadcast by hand when the paint is still sticky has no effect.?ÿ At one County my "office" was in the maintenance shop (surveyors = maintenance) so for winter fun we stripped the room and my bud' in the sign department pressure sprayed it with white stripe paint with the glass bead nozzle setup.?ÿ We had to sweep off?ÿ the excess beads and vacuum them up but when cleaned up the office was spectacular.?ÿ We got called on the carpet by the big boss but denied knowledge of?ÿ how and why it was done.
OTOH temp striping tape does have a metallic backing but it's usually aluminum and also nonmagnetic.?ÿ I'll stand on the notion that paint striping on roadways has no effect on magnetic searches for subterranean monument rebar.
It most definitely has caused problems, especially when you are expecting a rather weak signal to begin with due to probable depth.?ÿ It falls into the same category as the shallow-buried pipe running horizontally that has ONE SPOT along its length that is many times the signal strength as anywhere else.?ÿ Why that happens is usually an unknown but it is extremely frustrating to encounter.
Alleys and that area near where do-it-yourself mechanics do it themselves can be equally exasperating.?ÿ Found a lovely 3/4"--7/8" open end wrench a couple of days ago within a few inches of the sought-after monument.?ÿ It was down about four inches and did a great job of confusing us.
Found a lovely 3/4"--7/8" open end wrench a couple of days ago within a few inches of the sought-after monument.
Did you consider whether it might BE the monument?
Magnetic locators detect disturbances in the magnetic field of the earth. While a magnet creates a great disturbance, the object does not need to be magnetic to generate a signal from the detector.
All ferrous metals will generate tone, the more regular the shape the stronger the signal. A straight, vertical bar will generate a strong consistent signal. Pounding a bar and causing deformation deforms the magnetic field of the object resulting in a reduced signal.
Any residual magnetism in a bar could be removed by beating on it, but that is not the mechanism that makes it 'guiet'.
On other notes, one manufacturer was touting a locator that worked to a depth of 18 feet on a 1/2 by 18 inch bar. I suspect the developer and his family are in some form of witness protection program now....?ÿ