18 posts in this topic
No, I'm not talking about politics and being a Moveon.org trainer. I'm talking about resistivity detecting.
Electrical resistivity tomography
From Wikipedia, the free encyclopedia Electrical resistivity tomography (ERT) or electrical resistivity imaging (ERI) is a geophysical technique for imaging sub-surface structures from electrical resistivity measurements made at the surface, or by electrodes in one or more boreholes. If the electrodes are suspended in the boreholes, deeper sections can be investigated. It is closely related to the medical imaging technique electrical impedance tomography (EIT), and mathematically is the same inverse problem. In contrast to medical EIT however ERT is essentially a direct current method.
A related geophysical method, induced polarization, measures the transient response. The technique evolved from techniques of electrical prospecting that predate digital computers, where layers or anomalies were sought rather than images. Early work on the mathematical problem in the 1930s assumed a layered medium (see for example Langer, Slichter). Andrey Nikolayevich Tikhonov who is best known for his work on regularization of inverse problems also worked on this problem. He explains in detail how to solve the ERT problem in a simple case of 2-layered medium. During the 1940s he collaborated with geophysicists and without the aid of computers they discovered large deposits of copper. As a result, they were awarded a State Prize of Soviet Union.
Andrey Nikolayevich Tikhonov, the "father of ERT" When adequate computers became widely available the inverse problem of ERT could be solved numerically, and the work of Loke and Barker at Birmingham University was among the first such solution, and their approach is still widely used.
With the advancement in the field of Electrical Resistivity Tomography (ERT) from 1D to 2D and now-a- days 3D, ERT has explored many fields. The applications of ERT include fault investigation, ground water table investigation, soil moisture content determination and many others. In industrial process imaging ERT can be used in a similar fashion to medical EIT, to image the distribution of conductivity in mixing vessels and pipes. In this context it is usually called Electrical Resistance Tomography, emphasising the quantity that is measured rather than imaged.
Here is one unit being offered by Kellyco.
I know this topic has appeared off and on over the years, but I'd like to better understanding on the theory and principle of using one over the other, ie. depth, and target id and what compromises do I induce. The reason I ask is the new V4 for XP Deus has the ability to set a minus discrimination. It kills the ability to use the "horseshoe" screen for ferrous target ID, but VID numbers are tolerable. What theoretically happens if I set a negative discrimination, but use Notch to handle ordinary ferrous trash?
As a rule do the lower vlf frequencies punch deeper than the higher ones, say 4.8 verses 14khz?
But what is the trade off? Are some frequencies better for silver coins? How does iron enter into this?
Need to understand how this all fits together!
Thanks for any and all answers.
By Steve Herschbach
When I posted the video showing the Makro Gold Racer recovery speed using two nails and a gold ring, it caused me to reflect on the various internet nail tests. Nearly all employ modern round nails, when these items rarely present issues.
The common VDI (visual discrimination scale) puts ferrous items at the low end of the scale, and items with progressively increasing conductivity higher on the scale. The problem is the size of items also matters. Small gold is low on the scale, and the larger the gold, the higher it reads on the scale. A silver quarter reads higher than a silver dime, etc.
All manner of ferrous trash including medium and smaller nails fall where they should when using discrimination and are easily tuned out. The problem is large iron and steel items, and ferrous but non-magnetic materials like stainless steel. Steel plates, large bolts, broken large square nails, axe heads, hammer heads, broken pry bar and pick tips, etc. all tend to read as high conductive targets. Usually it is just the sheer size pushing it higher up the scale.
Detectors also love things with holes, which makes for a perfect target by enabling and enhancing near perfect eddy currents, making items appear larger than they really are. Steel washers and nuts are a big problem in this regard, often reading as non-ferrous targets.
Oddball shapes cause problems, particularly in flat sheet steel. Old rusted cans often separate into irregular shaped flat pieces, and roofing tin (plated steel) and other sheet steel items are my number one nemesis around old camp sites. Bottle caps present a similar issue in modern areas. These items produce complex "sparky" eddy currents with both ferrous and non-ferrous indications. Many thin flat steel items produce remarkably good gold nugget type signals in old camp areas.
Two general tips. Concentric coils often handle ferrous trash better than DD coils. A DD coil is often the culprit when dealing with bottle caps where a concentric coil often makes them easy to identify. Another thing is to use full tones. Many ferrous items are producing both ferrous and non-ferrous tones. Blocking ferrous tones allows only the non-ferrous tone to be heard, giving a clear "dig me" signal. This was the real bane of single tone machines with a simple disc knob to eliminate ferrous objects. You still heard the non-ferrous portion of the signal. Multi tones allows you to hear the dual ferrous/non-ferrous reports from these troublesome items, helping eliminate most of them.
Certain detectors can also show multiple target responses on screen at once, like the White's models featuring the SignaGraph (XLT, DFX, etc.) and CTX with target trace. These displays show target "smearing" that stands out differently from the clean VDI responses produced by most good items. A machine with a simple VDI numeric readout can only show you one number at a time and the only indication you might get is "dancing" numbers that refuse to lock on. Usually though the predominate response overrides and fakes you out. This is where a good high end visual display capable of putting all VDI response on screen simultaneously can really help out.
I have been collecting these odd iron and steel items to practice with and to help me evaluate which machines might do best in ferrous trash. The main thing I wanted to note here is contrived internet videos with common round nails often present a misleading picture. Many machines do very well on nails yet fail miserably on flat steel.
Is Minelab the only one that uses electronic noise cancel feature??
Do they have patents associate with this feature?
Would like to see other manufacturers use some thing similar on their detectors.
Or a manufacturer should provide actual visual indication of emi levels depending on frequency used to include offsetting.
Not have the user have to use their ears to decide or even try comparing on buried targets.
Should not be trial and error.
And maybe even a system were the operator is warned,,say if emi changes and the current selected frequency is possibly not operating at optimum.
I do realize with a coil being swept over the ground, this could be difficult to do.