Log in

View Full Version : Phase Measurement and Magnetic Susceptibility

Jim Hemmingway
03-28-2011, 09:13 PM
Phase Measurement and Magnetic Susceptibiltiy


I’ve organized some notes on the subject that some members may find helpful. We’ll take a look at target ID, ground phase and magnetic susceptibility. First we’ll do a quick review about what is happening behind the scenes as we swing a VLF searchcoil over the ground, followed with a discussion about target ID and factors that influence it. Later sections look at practical applications of ground phase and magnetic susceptibility measurements. Please keep in mind that what you get here is my understanding of this topic only. You may disagree on some point or feel something has been overlooked. Please don’t hesitate to add your two cents so that everyone can benefit from a discussion.

Background Review

The VLF searchcoil contains both transmit and receive windings. An electric current is passed through the transmit winding to create an alternating electromagnetic field around the searchcoil. This electric current is reversed back and forth thousands of times per second. How often this occurs per second is referred to as a unit’s operating frequency. Gold units generally have higher frequencies compared to coin hunting units. Higher frequencies are more sensitive to smaller gold, but are more susceptible to ground mineral effects.

The transmitted electromagnetic field will induce an electric current on a metal target within its range and this electrical phenomenon is called induction. The target now generates it’s own electromagnetic field. The target’s electromagnetic field results in the formation of a secondary electromagnetic field on the searchcoil’s receive winding. This is what constitutes a target signal and this signal information is amplified and processed by the detectors circuitry / software package to ultimately provide key information to the user.

Phase shift represents a time delay with the return signal when compared to the transmit signal, a result of a given target's inductance and resistivity. Phase shift increases as target conductivity increases. Thus a silver quarter has a large phase shift while a nickel has a comparatively smaller phase shift. Phase shift is predictable for many categories of targets, and this is especially true of manmade targets such as coins.

Some ferromagnetic targets have little or no conductivity and little phase shift. These include non-conductive iron minerals found in most soils. They distort the searchcoil’s magnetic field and mask weaker signals from small or deeper targets. A result is that a target may not be detected, or if detected it may be incorrectly identified. Ferromagnetic minerals are able to return a strong signal to a searchcoil’s receive winding, often several orders of magnitude stronger than signals from typical small metal targets. A ground balance control is adjusted to minimize or cancel signals from these soil minerals.

The main thing to recognize is that a metal detector regards soil or rocks as just another “target” to be measured in terms of phase shift, and assigned a numerical value on a phase scale. For user convenience, the phase range measurements are reorganized and displayed in the form of VLF target ID for a range of target conductivities, while the soil mineral phase range measurements are displayed numerically on a separate calibrated ground balance scale on some prospecting capable units.

Target ID, Ground Phase & Magnetic Susceptibility

(a) Target ID reliability depends on ground conditions, a targets physical / chemical make-up, and effective detector operation.

An improperly operated detector impacts target ID reliability. Here are a few points to keep in mind. (a) Correct coil sweep speed enhances target ID and detection depth. (b) Avoid lingering over a target with autotrack ON, or disable it when assessing a signal. (c) Keep the gain or sensitivity control adjusted to EMI / ground conditions for reliable target ID. Operating the gain at the fringe of instability leads to erratic, unreliable target ID. (d) Ensure the detector is always properly ground balanced. This is particularly important over higher magnetic susceptible substrates.

Many factors come into play with various nuggets and ores to determine where these will locate on target ID readouts. These include the type of metal, the purity, the type of inclusions or alloys, shape, size, and structure. Below are two comparable examples. The first is a “character” native silver nugget featuring a rough, variable but very solid structure and reasonably good purity, yielding a target ID in the low pulltab range….


The second specimen contains apparent equally pure native silver and much more silver by weight. However in this instance the native silver has an intricate branching or dendritic structure that effectively restricts the target ID to the upper nickel range.


Ground conditions play a key role with how effective target ID will be at any given site. Ground factors that directly impact target ID include magnetic susceptible iron minerals, soil moisture, and disturbed / undisturbed ground conditions.

 Increased magnetic susceptible iron mineralization plays a critical role in target ID. High magnetic susceptible iron minerals reduce electromagnetic field (EMF) penetration into the ground, thereby decreasing effective target ID depth. These magnetic minerals further compound target ID by distorting target signals. As the non-conductive iron mineral magnetic susceptibility or severity increases, target ID reliability decreases. In extreme ground, target ID is unreliable.

 Some soils become more reactive as soil moisture increases. This phenomenon can reduce EMF penetration and play havoc with target ID. Some experts attribute this condition to enhanced soil mineral ionization resulting from increased soil moisture content. Soil moisture beyond “compactive” soil conditions slightly decreases detection depth in my area, a brown clay-based soil that happens to be marginally dominated by magnetite. Let’s look at a documented soil sample that is representative of soils commonly encountered throughout many southeastern states.

I have a soil sample lab report from a major manufacturer. It provides an analysis of a “reddish” soil sample submitted by a friend residing in a southeastern state. It concludes the soil in a dry state qualifies as “bad ground” but well within the ground rejection range of their current detector models. The engineer recommended that soil type should only be searched when in a dry state. Below is a direct quote about that same red soil in a wet state.

“When wet the sample becomes very had ground, difficult to reject and or penetrate. This type of iron oxide, because of its consistency, achieves a solidity or connection between iron particles making it much harder to reject when wet.”

The report does not specify the iron mineral, but the culprit is undoubtedly maghemite. We can see why the relic hunters in parts of Alabama, Georgia, Tennessee, North Carolina and Virginia have such a difficult time searching these red soils with VLF units. Target depth is reduced, and target ID is practically useless in some areas especially when the soil is wet.

 Searching mine tailings can sometimes equate with searching in “disturbed ground” for any number of reasons. Disturbed ground conditions cause many VLF units to lose some detection depth and struggle with target ID. A result is that good targets even at modest depth are more likely to be identified as iron. Science seems uncertain as to what causes this effect but many postulate that it results either from the loss of electrical alignment or continuity within the soil, or perhaps by the disruption of the magnetic fraction of a soil’s constituents. Many hobbyists relate this phenomenon to a loss of the so-called “halo effect”.

When searching any of the ground conditions described above, operating in the all-metal motion mode is normally your best operating choice. When a target is located, the best procedure with all but very shallow targets is to remove a good portion of the overlying material and recheck target signals. If these continue to indicate iron as you get near to the target, then chances are you can move on. I don’t trust discrimination under such conditions, especially when dealing with very small targets. They tend to read as iron, but it’s your choice to make.

(b) Ground Phase

Lets define ground phase measurement as a weighted-average phase measurement of all soil minerals under the searchcoil at any given sampling point. Ground phase can be viewed as a ground "target ID" measurement based on phase shift similar to any other target ID measurement. What is the significance or usefulness of the ground phase value?

The ground phase can be used for several applications we’ll discuss later, but its primary value is to adjust the ground balance control to effectively cancel out interfering ground signals. The ground balance control should be viewed as a separate discrimination control for this purpose. Proper ground balance results in improved detection depth, more accurate target ID, and more stable operation. That said, keep in mind that ground minerals continue to inhibit EMF penetration and warp the EMF despite proper ground balance. Ground balancing permits us to detect targets to the best of a given unit’s ability within the constraints imposed by ground minerals. This explains why better depth can be had in light soil minerals as contrasted to reduced depths experienced over increased magnetic susceptible soil minerals.

A full-range ground balance scale positions salt at the lower conductive end of the scale and magnetite occupies the most elevated non-conductive range of the scale. All other ground ‘targets’ lie somewhere in-between these ranges on the GB scale. Have a look at the information below. It provides a correlation between soil types and ground phase measurements. These are pretty much taken verbatim from various sections of the F75 manual.

GB 0 to 10 = wet salt and alkali
GB 5 to 25 = metallic iron. Very few soils in this range, you are likely balancing over iron metal.
GB 26 to 39 = occasional salt water beaches, but very few soils in this range
GB 40 to 75 = red, yellow, and brown iron-bearing clay minerals
GB 75 and up = magnetite and other black iron minerals

Many lighter colored clays or loams will normally range between GB50 and GB80, whereas red clays will generally range from GB35 to GB55. Gravelly or sandy soils will tend to range in the GB75 to GB95 range. As an overview the manual further states that the more highly weathered, oxidized, or finely grained the soil is, the lower the numeric GB reading will generally be. These represent generalizations based on phase measurements of many soil samples.

So, we can see that ground phase measurements have associations with ground types. A similar relationship generally applies to specific non-conductive iron mineral types as well. However, ground phase measurements on various non-conductive iron minerals do overlap. Although the type of mineral is much the weightier factor, ground phase to some extent also considers iron mineral “amounts” particularly with respect to where mixtures of maghemite and magnetite exist in the same soil.

In short, the amount of a given substance also contributes to where the phase measurement reads. To support this statement for example, magnetite and maghemite are both highly magnetic susceptible. However, they occupy distinctly different regions on the ground balance scale. Magnetite occupies the upper end of the ground balance scale whereas maghemite generally occupies the middle portion of the ground balance scale. But mix them in equal portions or amounts and that mixture will result in a ground phase measurement that falls between the ground balance range each substance separately occupies.

This ground balance readout resulting from the magnetite / maghemite mixture just described lies within a range commonly occupied by other non-conductive iron minerals. A good example is the iron mineral called “goethite”. Goethite is prevalent nearly everywhere in northern latitudes. It’s a common constituent in brown soils. The key difference is that most iron minerals such as goethite are only mildly magnetic susceptible when compared to similar proportions or amounts of either maghemite or magnetite either separately or mixed-in together in a given soil as commonly occurs.

(c) Magnetic Susceptibility

Magnetic susceptibility in a metal detecting context refers to the ability of a soil’s iron minerals to attract a magnetic field. The “follow the black sand” feature on the GMT, similar to the Fe3O4 bar graph on the F75 or other units, measures the strength of the ground’s magnetic susceptibility. This measurement is expressed as an equivalent percent volume of the non-conductive iron oxide magnetite. This is a convenient standard that places all magnetic susceptible iron minerals present in a soil into an easily understood context. It measures a soil’s magnetic susceptible strength regardless whether magnetite is present or not, although that would be a highly unlikely scenario in mining country.

What contributes to magnetic susceptibility? Once again, both the type and amount of various iron minerals determine a soils magnetic strength. Consider a soil that is dominated by goethite. Goethite has a weak magnetic susceptibility. But if we consider a different type of iron mineral, for example either magnetite or maghemite, in the same amount or quantity in a soil, the magnetic susceptibility is many times greater than is an equal amount of goethite. Therefore always keep in mind that ultimately the magnetic susceptibility measurement is entirely about the soil’s magnetic strength.

If we understand the concepts as defined above, (a) it should be clear that ground phase and Fe3O4 readings are two distinctly different measurements with entirely different implications, and (b) we are unlikely to get entangled too much in debates about “types and amounts”. Keep in mind these “types and amount” descriptions were used in related manuals as a very simplified means of describing the use of these features and necessarily are addressed in part to newcomers to the hobby. A result is that some confusion may have ensued with hobbyists.

In the Field

As described below, (a) the ground (phase) balance can be adjusted to assist in evaluating rock samples and (b) plays the key role as to whether an iron mineralized rock signals as a negative or a positive hot rock. Magnetic susceptibility is a convenient measurement to indicate how well a VLF unit will perform over a given soil, and can be used to identify shallow black sand deposits. Used in combination with ground phase measurement, the presence or absence of magnetite and maghemite dominant soils can be identified.

We’ve referred to magnetite many times and will do so again. Here is a photo of this mineral in crystalline form for newcomers to the hobby. Most often it is located in the form of black sands. Magnetite, a non-conductive iron oxide, is an abundant negative hot rock in my area that gives the well-known “boing” signal so familiar to electronic prospectors using VLF units in the motion all-metal mode. If the ground balance control is advanced above a magnetite sample’s ground balance compensation point, that sample changes from a negative hot rock “boing” signal to become a powerful, positive hotrock “zip zip” signal. Magnetite such as you see below will easily max the Fe3O4 reading on the F75.


(a) Ground Mineral Scenarios

As noted above, ground phase measurement can be used in conjunction with the Fe3O4 readout to identify some ground mineral scenarios. They can be used to identify both magnetite and maghemite where they are the dominant fraction of a substrate’s non-conductive iron mineralization either acting in unison, or acting individually. Let’s look at a few examples…

 Some operators successfully apply the magnetic susceptibility readout to locate shallow blacksand paystreaks. Frankly, I don’t use the feature for this purpose as my pursuits lie in other directions. I suggest looking for swings in both the ground phase and Fe3O4 readouts. For example, let’s say we are searching an area with GB readouts around GB70 and a fairly light Fe3O4 readout. We experience an abrupt phase swing up into the high GB80’s accompanied by a sharp increase in the Fe3O4 readout. These readouts together indicate that a black sand deposit that may or not contain precious metal values has been located.

 Another example: Let’s say we’re out searching the Ontario outback with a ground balance reading at GB75. The Fe3O4 meter reads quite mildly. We enter an area where these readings abruptly change. Now the ground balance reads at GB52 and the Fe3O4 meter has increased into the moderately high or even higher levels. These readouts indicate we are over a “burn” area where the soil is dominated by maghemite. Burn areas can be vast because of past forest fires. The heat generated from fire is sufficient to oxidize lower oxidation state (reduced Fe2+) iron minerals such as magnetite (actually formulated as Fe-Fe2O4, a ferrosic oxide) and other iron minerals into an oxidized ferric (Fe3+) form such as maghemite.

This same oxidation process occurs by means of natural processes as well. For example, small grains of magnetite exposed to the elements can “rust” into a maghemite form. Many of us have seen these “rusty” deposits in magnetite. Another example, the same natural weathering / oxidation process applies to basalt. The iron minerals contained within basalt include magnetite and as these are exposed to weathering over time they can oxidize to form maghemite.

Incidentally, maghemite may be mistaken for similar appearing hematite. The difference to metal detectors is quite distinct. Hematite has a very mild or weak magnetic susceptibility and generally ground balances between 60 and 75 on the GB scale. As a result, it has very little effect on metal detectors. Maghemite is a different matter. Its GB compensation point normally occupies the middle of the GB scale, depending partly on purity, but its magnetic susceptibility is many times greater. Maghemite affects metal detection target ID and depth similar to magnetite, although magnetite is the more magnetic susceptible of these two substances.

 A final example, let’s say we’re out detecting an area where the ground phase reads moderately say GB65 to GB70 but we notice the Fe3O4 meter reads quite high indicating a strongly magnetic susceptible soil. We can conclude we are searching an area where there is a good mix of both maghemite and magnetite. There is a separate possibility if yellow clays dominate the area, but otherwise no other conclusion fits this scenario.

(b) Hot Rocks

Hotrocks are encountered as (a) non-conductive iron mineralized rocks or (b) as electrically conductive hot rocks, usually in the form of sulfides and arsenides in my area. Both hotrock types respond to VLF metal detectors. The discussion here refers to VLF responses to hotrocks using the all-metal autotune mode. I’ll limit the discussion of conductive hotrocks to those occurring in my area here in Ontario.

Non-conductive iron mineralized hot rocks present themselves with one of two distinct responses to a metal detector. They will either respond with a positive metallic-like target “zip zip” sound, or a negative “boing” sound due to autotune threshold “overshoot” reaction to a “negative” hot rock. A positive hot rock will have a ground balance compensation point below our operating GB setting, whereas the negative hot rock will have a ground balance compensation point above our operating GB setting.

The negative hotrock “boing” signal directly results from using an all-metal autotune mode. The autotune initially goes quiet over the rock but struggles to recapture its threshold level as the coil sweeps past the rock. In doing so, it “overshoots” the threshold level briefly and yields an audio response. Thus, as the coil is swept over a “cold” / “negative” hotrock we get the familiar “boing” signal. By contrast, when using a true non-motion all-metal mode, negative hotrocks passed across the coil will simply cause the detector threshold response to go quiet. Magnetite within a rock structure is normally responsible for negative hot rock responses. These are fairly easy to recognize and can be ignored.

Positive hotrock signals can be frustrating to electronic prospectors because their signals mimic nugget signals. Many locate in the upper portion of the target ID iron range, as do many small nuggets concealed in highly mineralized ground. Fortunately, hotrocks that respond to metal detectors tend to reside very close to or on the surface. Maghemite is often the culprit responsible for positive hot rock responses.

Positive hotrocks can sometimes be distinguished from good targets because they lose their signal more quickly than precious metal targets of similar size as the coil is raised above the ground. Some amount of iron discrimination, slightly in excess of the iron discrimination level required to eliminate small metallic iron, will eliminate these pests. However, this level of discrimination will also eliminate some small nuggets that fall into a similar conductive range. Some operators prefer to ground balance to an area’s prevalent hot rock type but this technique will leave you improperly ground balanced below that required for the surrounding terrain. So, we use some judgement according to their prevalence, how far off ground balance we need to adjust the GB control to silence them, and the precious metal target size we seek. In hotrock abundant areas where the gold / silver is sufficiently large to respond, a suitable PI unit may be a better choice.

As we’ll discuss further down, we can bench test suspect rocks by adjusting the ground balance control to an appropriate setting whereby both types of non-conductive iron mineral hot rocks will give a negative threshold response regardless of their phase compensation point or magnetic susceptibility.

Conductive hot rocks respond with positive target signals to a VLF metal detector. The most frequently encountered electrically conductive rocks in Ontario’s silver country result from sulfide ore responses. Those located on the surface can usually be identified and avoided. Many ‘collectible’ conductive minerals, such as galena or iron and copper sulfides, can give variable strength responses depending on specimen size and structure, amount of a conductive mineral in a rock, and the sensitivity of the VLF unit. Some conductive minerals such as larger pieces of pyrrhotite yield wide signals, more easily distinguished from typically narrower precious metal responses.

Cobaltite (CoAsS) is a common sulfarsenide mineral in my area that responds at good depths to metal detectors. Other cobalt-nickel-arsenic minerals known as skutterudites also respond well to metal detectors. These are familiar to prospectors by the frequently observed surface oxidation of cobalt to pink, purple or even deep red ‘erythrite’ or ‘cobalt bloom’. Multi-pound specimens are frequently encountered.

Niccolite is an abundant nickel arsenide in my area that readily beeps to VLF units. A copper colored, brittle, heavy substance, specimens range in size from small bits to multi-pound pieces. These weather to a light green oxidation called ‘annabergite’ or ‘nickel bloom’. Niccolite has a wide conductive range that seems to depend mainly on size.

Pyrrhotite is the most widespread, frequently encountered conductive hotrock here in northeastern Ontario’s silver country. It occurs in variably shaped, usually multi-pound sizes. Although field specimens are weathered to a rusty or deep brown appearance, a fresh surface is a pale brass / bronze color with a metallic luster. Pyrrhotite is variably magnetic, a feature that easily distinguishes it from commonly occurring tarnished iron pyrite and chalcopyrite. It yields a very distinct sulfurous odor upon impact with a rock hammer.

There is nothing that can be practically done to mitigate its wide, blaring signal other than to recognize and ignore it when found on the surface. Rocks containing sufficient amounts of pyrrhotite cannot be ground-balanced on a VLF unit but these are normally discriminated well within the iron target ID range. Pyrrhotite is the bane of electronic prospectors because its abundance can render entire sites unsuitable for detecting.


Bench Testing

The ground (phase) balance adjustment can be used to check suspect rocks to determine whether such rocks contain precious metal values, but this technique is subject to a few conditions described below. Let’s use the F75 as our example unit. The procedure can be related to any unit that has the needed sensitivity and sufficient ground balance range to exclude maghemite or other lower GB non-conductive iron minerals from issuing a positive signal.

The F75’s calibrated numerical GB scale can be set to GB45 preferably in the more sensitive non-motion all metal mode. A smaller, more sensitive coil is more effective for testing rocks. Now, bring a suspect rock back and forth to within an inch or two of the coil’s surface. Keep in mind that no response is possible if the electromagnetic field cannot see a potential target inside a rock. Therefore be certain to run this test with the rock rotated in different profiles to the coil. All non-conductive iron mineralized rock signals will go quiet at GB45 regardless of their phase measurement or magnetic susceptibility. This is also a good method to distinguish or identify possible hotrocks in your area.

It is possible that such rocks could contain precious metal values that will never be heard because of their disseminated state, or more likely because of the overwhelming negative threshold resulting from the iron minerals in the rock. If there is sufficient metal or conductive sulfide in a given rock to overcome any non-conductive iron mineral response present, these will easily respond at the GB45 setting. If you ensure the non-motion all-metal mode is used with the highest, stable sensitivity possible, even at GB45 this is a remarkably sensitive test with this particular unit. For example, even an extreme low conductor such as iron pyrite in much smaller samples than the specimen below yield a strong distinct signal at the GB45 setting.


The important point is that non-conductive iron mineralized rocks will not issue a positive signal at the GB45 setting, and therefore can never be confused with positive sulfide or metal responses. One has only to determine the appropriate GB setting on other capable units to duplicate this test method.

Information on the subject of this essay sometimes seems inadequate or even a bit elusive. The foregoing gives you my understanding about these ground-monitoring features and how we can use them for a number of tasks. Hopefully you will find this information practically useful in the field and for evaluating samples.

Jim Hemmingway
March 2011

Steve Herschbach
03-29-2011, 09:44 AM
Wow, when I get more than a few minutes I need to digest all this. Thanks Jim!

Jim Hemmingway
03-29-2011, 02:20 PM
Hi Steve... thankyou for commenting. :) One obvious mineral responder to VLF units that was not mentioned above is graphite. I have very little experience with it over here in Ontario. That may be in part because the areas I search have a very low metamorphic ranking. In fact 'argillite' is about it. It falls between shale and slate, but occasionally contains some amount of graphite...not sufficient to be an issue. Here is a photo courtesy to me from Dr. Jim Eckert of northeastern Ontario...argillite often has a fine banding as shown in the photo below. This example has experienced extensive weathering that has bleached the argillite so that the layers stand out.


My understanding is that conductive graphite or graphitic slate rock usually gives a wider signal not resembling signals from nuggets. Similar to pyrrhotite, as I understand things, a VLF unit normally does not ground cancel rocks containing sufficient graphite, so these are likely a real nuisance.

I recollect from reading a past post that you have experienced graphitic rocks in one of your areas. How do your PI units respond or react to this mineral? Many thanks...


Reno Chris
03-29-2011, 02:32 PM
One of the strongest conductive minerals out there is cuprite - a natural copper oxide mineral. I have dug a fist sized chunk of cuprite at a depth of over 2 feet with my Minelab PI. You'd swear it was a chunk of metallic copper from the response.

Jim Hemmingway
03-29-2011, 09:38 PM
Reno Chris….thanks for bringing the cuprite information to everyone’s attention. Do you have a decent photo of cuprite that could be posted here?

Chris if you think of other minerals from your areas that respond to metal detectors would you please mention it here? With your metal detecting, prospecting, and mineralogy skills you would definitely be the one to know about this subject. Thankyou…


03-31-2011, 08:43 PM
Boy, once again a great thread! I certainly appreciate you taking the time to post such an informative thread.

04-01-2011, 05:42 PM
That's a cool website for us mineral collectors Jim. I registered just now and looked up aquamarine ? They show locations in the US but every photo of a large crystal seems to say Pakistan under it except for 1 tiny bit found in Connecticut.
I found an aquamarine crystal the size of my index finger years ago in Maine.It was at least as good as the pics of the Pakistan ones.

-Tom V.

Jim Hemmingway
04-02-2011, 10:53 AM
Well by golly Dick what a pleasant surprise! I hope that means that somehow or other I may have done a decent job of communicating a few ideas!! I didn’t realize that you used a metal detector or perhaps you use one in your mining operation…

When I started into this pursuit there was very little information available. I’ve been at it for years learning about metal detector responses to various minerals. Some leading engineers with manufacturers and a highly respected metal detector expert known to me personally have been a great help. With the advent of the internet and these subsequent forums, sharing information is dead easy and I’m sure the newcomers will especially appreciate it.

Thanks very much Dick for responding to this thread. Having the opportunity to speak with you again is more than sufficient recompense for the effort expended on this write-up. :)


Jim Hemmingway
04-02-2011, 11:00 AM
Hi Tom…I don’t know about aquamarine, but Maine is well known for tourmaline so I’m wondering if in fact that is what you may have found there. I have samples of both emerald and gemmy tourmaline called elbaite, thanks to our good buddy Strickman down in Georgia. He sent me several examples of tourmaline but my daughters only left me one sample. I guess I learned that day who the real power brokers are here. :)

After you recover from your surgery feel free to post a photo of your sample if you like. Maine is also known for detectable gold Tom. I’ll be in that area early this autumn chasing it and gemstones.

You will be prominent in my thoughts with your forthcoming surgery. Please let us know you are doing OK soon as you can comfortably do so. Can you take your computer into the hospital, might save you from boredom. See ya…


12-16-2012, 06:46 AM

Excellent thread! Once again with almost poetic prose! I really enjoy the well thought subject matter too.

My quest for the last thirty years has been to find the answer to the quesion of mineral identification (and response) to an electromagnetic field. The single question is;

What is the response?

Of course, we aren't the only ones to ever have conceived the question! There are books that have been written on the subject of electromagnetic induction as applied to geophysics. It's a wonderful study!

Drilling deeper on iron and iron minerals, I learned that iron certainly is conductive! The interesting thing about iron is that it is "diamagnetic" and causes "negative phase shift" in the secondary signal. That is what ground balancing does. Soil is composed of variable amounts of iron having variable amounts of magnetic susceptibility AND conductivity.

The electronics can measure both of those quantities. Susceptibility and conductivity are measured at different points in the receiver. As you have made "phase measurement" foremost in the title; phase measurement is the central theme of metal detecting including "mineral" detecting.

- Geowizard

12-16-2012, 09:18 AM

With reference to the image of cuprite; That looks like native copper. Cuprite is a red crystaline mineral:


I'm not sure what Chris dug up. Cuprite is 88.82 percent copper and the balance is Oxygen (Cu2O) by weight. Since oxygen is a non-conductor, cuprite wouldn't be a good candidate for metal detecting.

A PI detector uses pulse induction - doesn't use phase shift.

- Geowizard

12-16-2012, 10:24 AM

Thanks for bringing up such an interesting and important subject.

VLF metal detectors measure conductivity and/or susceptibility depending on which detector and for what purpose it was designed. More on that later. I am attaching a link to relative magnetic suscetibility for everyone's reference;


The subject cannot be covered in one page. As I mentioned earlier, volumes have been written on the subject. :)

Let's discuss or consider "minerals" vs "metals" vs "rocks" and consider the possible combinations of each. The challenge as I think you have indirectly pointed out is that there are too many permutations that when considering the overlap of conductivity, depth and orientation, one combination may "look" exactly like another combination.

- Geowizard

12-16-2012, 10:48 AM
Metal detectors are designed for different purposes. One purpose is detection of Unexploded Ordinance (UXO). UXO metal detectors detect susceptibility by measuring the secondary signal (in phase) with the reference (transmitted) signal. Reference the first four sentences of the abstract;


A metal detector has only one signal at the receiver. The phase change and amplitude (size) change are both measured to attempt to identify the target.

Note: UXO can have magnetic susceptibility or be non-metal and represent a void area. Both of these cases can be measured as a "change" in magnetic susceptibility.

- Geowizard

Jim Hemmingway
12-17-2012, 07:09 PM
Hi Geowizard… you may not have noticed my last post to this thread was April 2, 2011. I presume the forum administrator is recycling some of the older posts that may be worth another look. It’s not a bad idea… maybe other forums should think about it too. In fact I know that NASA Tom is considering restructuring his forum to improve access to older posts.

I trust you enjoyed the autumn as much as we enjoyed our stay in the North Country. Of course nothing went according to plan this season, we experienced interminable rain followed by swirling snowstorms. Invariably it seemed that the few intermittent days of good weather were spent in conversation with hobbyists visiting our camp or encountered in the field. I discovered that while these folks mostly do not comment on forums, many were familiar with my silver-related essays posted here. That was welcome information because I had entertained doubts about the utility of continuing to post to any forum.

The above essay was intended to provide hobbyists with some basic information about mineral responses to a metal detector. It’s the iron minerals found in rocks and various substrates that interest me for the reasons explored in the essay. I think that experienced electronic prospectors generally understand that a metal detector can provide only so much information and is not infallible in this application. The information is intended to be used in conjunction with some common sense and with at least a passing knowledge of local rocks and minerals in a given area.

I doubt that most hobbyists are terribly concerned with searching for minerals using advanced techniques such as identifying magnetic susceptible anomalies as referenced in one of the links you provided. It is interesting reading but I see it as beyond the scope of what was intended in the above essay. Incidentally… a recently completed article will be posted here in January that may interest you. It looks closely at phase measurement over a variety of targets utilizing the “fastgrab” feature available on many modern prospecting-capable VLF units.

Cuprite… I have a half-dollar size sample with small crystals that barely responds to a benchtest using highly sensitive VLF metal detector settings… the signal is similar to very weak responding sulfides… out to a few inches from the searchcoil but that’s it. In the field where a VLF metal detector operates within the constraints imposed by soil minerals it would not be possible to acquire any response from this sample… and certainly not from either of my PI units. I agree with you about the photo of native copper and that link has been removed.

Now should you feel inclined to add to the above essay Geowizard… by all means do so. Any additional information that will help hobbyists is most welcome but keep in mind that any information presented should be easily understood by readers with little or no technical knowledge or experience.

Meanwhile I’ll take advantage of the moment to wish you and everyone on the forum a very Merry Christmas and all the very best in the New Year. :)