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Terrestrial Use Of Impulse AQ


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Many of you have expressed a desire to know how well the Impulse AQ will function for land use. One option is to wait until the unit is released. I know, no fun! The other option is to analyze the information we do have on the unit and on PIs in general, combined with information from the scientific literature and various forum posts. I have done such an analysis which is a bit long, but I will summarize the findings followed by how I arrived at the conclusions. The places where I believe the unit will be effective include the following:

  • Black sand beaches (mainly coarse unweathered magnetite)

  • Soils containing mildly weathered granite and other felsic igneous rocks (I know this appears to conflict with Alexandre’s post, but I will elaborate below)

  • Unweathered or mildly weathered basic igneous rocks (basalt, gabbro, etc.)

Places where I think the AQ will struggle include:

  • Weathered basalt and soils derived from basalt

  • Some fine-grained volcanic rocks such as rhyolite.

The basis of my groupings above is the published magnetic susceptibilities (MS) for various minerals and rock types and on the concept of frequency dependent MS which is a very important consideration for PI detectors.

MS is a measure of the magnetization of a material in response to an applied magnetic field. Frequency dependence is when the measured MS varies when different frequencies are used for the induced field. Minerals with high MS are responsible for the “mineralization” when speaking of metal detector performance. Three minerals are responsible for most “mineralization”; magnetite (Fe3O4), titanomagnetite, and maghemite (ꝩ-Fe2O3). The MS for these minerals are orders of magnitude higher than for other iron minerals such as hematite (α-Fe2O3), goethite, biotite, pyroxenes, etc. The relative proportions of these minerals within different rock types determines the MS of the rock. Ranges for different rock types are shown in the table below.

Rock Type

Magnetic Susceptibility Range (10-6 SI)1

Andesite

170,000

Basalt

250-180,000

Diabase

1,000-160,000

Diorite

630-130,000

Gabbro

1,000-90,000

Granite

0-50,000

Peridotite

96,000-200,000

Porphyry

250-210,000

Pyroxenite

130,000

Rhyolite

250-38,000

Igneous rocks

2,700-270,000

Average felsic igneous rocks

38-82,000

Average basic igneous rocks

550-120,000

Quartzite

4,400

Gneiss

0-25,000

Limestone

2-25,000

Sandstone

0-20,900

Shale

63-18,600

1.       Compilation from Hunt et al. (1995)

 

 

Minerals with high MS are responsible for the poor performance of VLF metal detectors. Hematite within soils is typically red, but given the relatively low MS, is not particularly problematic to metal detectors. So, red soil is not always bad!

The MS of soil is a function of the parent rock from which it was formed (see table) and the degree of weathering of the iron minerals present. Soils formed from basic igneous or volcanic rocks such as basalt generally have higher MS than soils formed from felsic rocks (rhyolite, granite, etc.), but it depends on the specific rock. For example, some granites have low MS because they are dominated by ilmenite (S-type granite) as opposed to magnetite (I-type granite). Ilmenite has low MS. Geologists use MS to map different types of granite. Da Costa et al. (1999) found that the basic volcanic rocks from southern brazil produced soils containing maghemite (high MS) and hematite while the intermediate to felsic volcanic rocks produced soils containing goethite (low MS). However, there are examples of basic rocks having low MS and felsic rocks with high MS, it all depends on the mineralogy, the grain size, the degree of weathering, subsequent geochemical reactions during and after soil formation, and other factors.

Typically, the smaller the grain size, the higher the MS. Therefore, a volcanic rhyolite which has a much smaller grain size than its intrusive equivalent granite, will have a higher MS even for an identical magnetite content. Smaller magnetite particles also weather faster than coarser grains. Magnetite can weather to maghemite on exposed outcrops. Maghemite is an earthy mineral that forms very small grains. The small grains produce a superparamagnetic domain which results in frequency-dependent MS which causes problems for even PI metal detectors, especially PIs which do not have the ability to ground balance (such as the Sand Shark and Impulse AQ). Magnetite can also form very small grains, and if small enough can also be superparamagnetic. However, magnetite tends to be coarse-grained while maghemite tends to be very fine-grained.

Maghemite tends to form from magnetite and other minerals in tropical climates or where tropical climates once existed. The “bad ground” in Australia is due to the presence of maghemite, which is a brown to brick red mineral. Maghemite is less common in the US but is present. Magnetic anomalies found at the National Laboratory at Oak Ridge TN were found to be natural deposits of iron-bearing colluvium (sediment which has accumulated at the base of a mountain range) which has oxidized to maghemite (Rivers et al., 2004). Maghemite and hematite can be created from goethite (α-FeOOH) in response to the heat generated by forest fires and slash and burn agriculture (Koch et al., 2006). Therefore, poor detecting conditions can be created in such areas.

The bad ground at Culpepper VA is probably due to maghemite, but I have seen no information to confirm this. Geologic maps of Culpepper Co. do show the presence of basic bedrock, such as basalt and dolerite.

The granite that Alexandre mentioned as giving the Impulse AQ problems may be an I-type granite (magnetite rich) in which the magnetite has partially weathered to maghemite.

The reasons for why I think the Impule AQ will or will not work in various soils/rock types is summarized below.

Soil/Rock Type

AQ Works?

Reason

Black sand layers on beach

yes

Black sand is derived from physical weathering of igneous and metamorphic rocks in upland areas and consists mainly of relatively unweathered magnetite.

Soils derived from felsic igneous rocks

probably

Felsic igneous rocks with high MS, tend to be coarse grained and even when dominated by magnetite (I-type) do not typically produce maghemite unless highly weathered.

Soils derived from basic igneous rocks

Probably not

Soils derived from basic igneous rocks tend to be dominated by maghemite.

Basic igneous hot rocks

maybe

Basic igneous rocks such as gabbro can be a problem if weathered or partially weathered to maghemite.

Felsic igneous hot rocks

probably

Unless highly weathered, felsic rocks are dominated by magnetite which the AQ should be able to handle

Volcanic hot rocks or black sand beaches (i.e. Hawaii)

maybe

If fresh, the main source of MS is magnetite. If weathered or partially weathered to maghemite, the AQ may have problems. If very fine grained even unwethered volcanic rocks may present a problem.

 

References

Da Costa, A.C.S, Bigham, JM, Rhoton, FE, and SJ Traina. 1999. Quantification and Characterization of Maghemite in Soils Derived from Volcanic Rocks in Southern Brazil. Clays and Clay Minerals, v. 47, no. 4, p. 466-73.

Hunt, CP, Moskowitz, BM, and SK Banerjee. 1995. Magnetic Properties of Rocks and Minerals. In Rock Physics & Phase Relations: A Handbook of Physical Constants, Volume 3.

Koch, C.B, Borggaard, OK, and A. Gafur. 2005. Formation of iron oxides in soils developed under natural fires and slash-and-burn based agriculture in a monsoonal climate (Chittagong Hill Tracts, Bangladesh). Hyperfine Interact 166, 579–584.

Rivers, JM, Nyquist, JE, Terry, D.O., and W. E. Doll. 2004. Investigation into the Origin of Magnetic Soils on the Oak Ridge Reservation, Tennessee. Soil Science Society of America Journal, Vol. 68 No. 5 p. 1772-1779.

fisher-research-impulse-aq-metal-detector.jpg

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thank you for your great work !

A highly interesting study.

Note that, on the beach, the black sand in non volcanic regions are mostly composed by standard silice and limestone (light brown). The black matter which surrounds the grains is mostly organic (like mud). So, it’s the organic matter that causes the color. The upper part of the sand is often clear because washed and here, the only real problem is the conductivity of salt water. When the sand is hardened, it's due to the high manganese and iron oxyde content (called alios).

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People sometimes misread the term “black sand”. Black sand is a mining term and actually has more to do with weight than color. Black sands are the heavy mineral concentrate recovered when running any form of placer mining concentration system. The most basic version is the heavy mineral concentrate left in the bottom of a gold pan.

From the Glossary of Placer Terms in Placer Examination Principles And Practice:

BLACK SAND Heavy grains of various minerals which have a dark color, and are usually found accompanying gold in alluvial deposits. (Fay) The heavy minerals may consist largely of magnetite, ilmenite and hematite associated with other minerals such as garnet, rutile, zircon, chromite, amphiboles, and pyroxenes. In Western gold placers, the black sand content is commonly between 5 and 20 pounds per cubic yard of bank-run gravel.

Black sands are also concentrated by winnowing action on beaches derived from terrestrial sources like volcanoes or granitic intrusives... any source that can supply the requisite heavy minerals.

In this context simply looking at a beach or in a stream and seeing dark or black colored material is not finding black sand. Amateur gold prospectors often do this, thinking that seeing black colored material in a stream is a positive sign for gold. All they are often seeing is just material like black slate or shale gravels and sands, not actual black sand. Unless it is heavy material concentrated by gravity action and typically with a high magnetic component, it’s not black sand in the context of the discussion.

True black sands are usually very fine though I have encountered coarse grain varsities. The fine grain variety often has a glittering appearance due to the presence of many sharp edged crystals of the constituent materials, chiefly magnetite.

Sands concentrated by gravity action and containing a high enough portion of heavy garnet material which confers a reddish color are referred to as “ruby sands.”

Again, the key thing is material derived by gravity concentration and therefore very heavy, not simply color.

Here is a picture of some gold I recovered in a Garrett 10" gold pan along with the resulting heavy magnetic black sand concentrates. Click picture for larger view.

magnetic-black-sand-gold-panning-concentrate.jpg
Placer gold and black sand concentrates

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Thank you for clarifying this Steve. I would also like to point out that the black sand on Hawaii beaches may be different from the typical black sand layers found on quartz sand beaches (the material which Steve has defined above). If I am not mistaken the black sand beaches in Hawaii are composed of sand-sized basalt.

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This is an interesting subject that few people know how to deal with even at the metal detector companies.

Impulse AQ must pass over all types of magnetic soil, fe2o3 and fe3o4.

On the other hand it will not take small gold nuggets, it is not planned for that.

The impulse gold model will be provided for this research ...

Impulse AQ does not detect below 0.1 gr / 0.2 gr, however it cuts very very hard this type of ground.

Especially in volcanic mode.

http://www.zhinstruments.com/sm_30.htm

I have enough stones from all over the world to open a stone quarry.

I was able to volcanic sand 10 times more intense than the most difficult of my volcanic stones.

Image associée

HOT ROCK AND GROUND.jpg

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20190507_172000.thumb.jpg.2414227b0401f4d17c5b87c346dca50d.jpg

HERE 325 x 10^-3 SI No problem with AQ :

 

WP_20150519_036.jpg

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Excellent work ALEXANDRE ... I think ... that little pack of "Black Sand" from Fuerte Ventura has at least 70%-80% magnetite.

HOT ROCK AND GROUND.jpg

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19 minutes ago, EL NINO77 said:

Excellent work ALEXANDRE ... I think ... that little pack of "Black Sand" from Fuerte Ventura has at least 70%-80% magnetite.

 

You are probably right because on the sand  -23.9 x 10-3 SI  we have 51% of magnétite

In the table above I don't see anything comparable to 325,000 x 10 ^ -6 SI...

And yet the impulse AQ easily detects through when we speak of jewelry greater than 0.3 grs

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Merci ...ALexandre...

Black Sand with 51% volume of Magnetite will be a big challenge for any VLF detector ..., some detectors can work on such terrain, but I will have a very limited detection range ..

I use for testing 3 compact "Black sand" boxes containing 4.4%, 12%, and 33% Magnetite .. boxes are 4 "high = it means 10cm thick Black Sand ..

 12% magnetite allows still acceptable detection .. with reduced depth...33% magnetite in the Box is a strong test /"Limit" /for the VLF detector ..The signal attenuation factor through mineralization is too high...

It's a place ... where the "IMPulse" Detector ...is going to have a great advantage..and will excel...

...

 

 

januar 6 Iphone5S  2020 001.JPG

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45 minutes ago, ALEXANDRE TARTAR said:

Impulse AQ does not detect below 0.1 gr / 0.2 gr, however it cuts very very hard this type of ground.

This equates to around a 3.0 grain gold nugget, which for me is plenty small. There are many places in Nevada in particular where there are gold nuggets larger than this and where the ground is not very mineralized with magnetic minerals. There are however large areas where alkali salt really troubles some detectors, like the GPZ 7000 in particular. A detector tuned specifically to work on alkali/salt while still having sufficient sensitivity to small gold may work very well in some gold nugget locations, salt flats in particular. There may be areas where hot rocks prove problematic, but there will be areas I am sure where the Impulse AQ can find gold nuggets. I plan to be one of the first to find out. :smile:

The photo below has 24 grams of Nevada gold I found with the GPZ 7000, with none of it exceptionally large or small, but all more than the 0.2 grams we are talking about as a cut off point for the Impulse AQ.

24-grams-gold-nevada-2017-steve-herschbach.jpg

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