Coin depth accuracy

[Choc-Minelab]

I notice that lower conductive numbers, below 30, tend to be very shallow. The depth gage, most of the time, is way off and mis-leading. Most of these targets are under the sod when the gage showed 4-6".

 

[Johnnyanglo]

If the target depth says 5" and you dig a coin at 2" it is because the coin did not appear in a standard way that is normally expected. That is, the coin was on edge, near other targets that were deeper, in the vicinity of iron or rust flakes. It was non-standard soil, non-standard target position, or non-standard target character (cut coin, corroded, bonded with silica).

For instance, if the non-ferrous target has a lot of iron mineralization between the coil and itself - the iron will weaken the non-ferrous (coin) signal and make it appear weaker, thus deeper. The meter will say 5" but the coin is really only 2". Again, the target is a non-ferrous soda can at 13", the non-ferrous signal appears at the coil to be only moderately weak, considering the greater size and eddy current generation on the can. The meter assumes it is a coin-sized object, with that much voltage driven in the receive coil it is assumed to be shallow, perhaps only 2-3". You dig, and dig, and dig - no coin, until you get to 13" and find the buried soda can. The meter worked - but the underlying assumption built in to the algorithm it operates led to an incorrect assessment.

Another example. As the operator you have to use discretion when the meter says 12" deep and the TID says 22-44, a penny that deep? Can your E-Trac find a penny at 12". Probably not - not in mineralized soil. So what in the ground could give such as strong response? Probably a bent nail. The magnetic properties of iron (temporarily the iron domains align when interacting with the detector's magnetic field) drive the signal to stronger levels than a mere non-ferrous coin could ever hope. That nail appears as if it were a coin-sized object (an assumption the meter always makes) at 12", when, in fact, it is nail sized object at 8". A little more investigation would proably reveal the true nature of the iron, as the TID would likely be very unstable and bounce around quite a lot.

In the end, the depth meter does what it is supposed to do. We just have to realize that it is working on some basic assumptions that do not apply all the time.

 

[gfatizzi]

Most depth gauges are only a loose estimation anyway. In bad soil, you are lucky to get down 6 inches or less in really bad ground. In that scenario The Etrac still finds them where other VLF detectors just will not! Sometimes depth is highly overated. JMHO. HH George

 

[gfatizzi]

If the target is SMALLER than a coin, say for instance a 22 shell casing which is a low conductor AND small in size, the detector again sees it as a weak signal that, if larger, would read as deeper. Just something more to think about.