It feels very good that this nice forum is at high speed with all discussions about the AQ with all different subjects about the AQ.We are now in the middle of january and still nothing from the Fisher.No reports from any tester, no videos, no manual, nada...it feels kind of depressing without knowing any informations at all.I know that LE.JAG and Alexander can't say anything about these informations even though the know for sure.Is there any thought or any guesses about these questions?
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.
Magnetic Susceptibility Range (10-6 SI)1
Average felsic igneous rocks
Average basic igneous rocks
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.
Black sand layers on beach
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
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
Soils derived from basic igneous rocks tend to be dominated by maghemite.
Basic igneous hot rocks
Basic igneous rocks such as gabbro can be a problem if weathered or partially weathered to maghemite.
Felsic igneous hot rocks
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)
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.
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.
I'm new on this forum and like Alexandre Tartar, I live in north of France.
I was a young prospector in the 90's and asked my father (electronic engineer with good knowledge in magnetic field theory) to build a PI to hunt the beaches. So we have made, in a few months, an home-made PI metal detector 25 years ago, based on the technology of the old White's Surfmaster PI (mono coil). I remember the use of FETs (Field Effects Transistors to make 200 volts pulses). It worked, but unfortunately, my father was afraid by a so powerful magnetic fields and has continued his research on VLF detectors, until today !
After this short presentation, here's my question :
Is the Impulse AQ a bipolar detector ?
Le Jag has explained us on the french forum "detecteur.net" this technology developped by Alexandre :
Positive and Negative pulse are alternatively sent.
The positive one light the gold ring but magnetize the soil.
The negative one demagnetize the soil.
What about it ?
I think the Garrett ATX challenges the AQ most and maybe TDIBH come close after ATX only in raw depth.I have myself an ATX and hunt my beach with it. Max sensitivity givs me around 15 inches for a normal coin size object.My Garrett sea hunter is absolutly no match and even my Equinox has no chance.The fair comparisson is ATX (PI , almost same coil size,waterproof ) I think the AQ has only one tone for both high and low conductive targets and one for iron.The ATX has one tone for high and one tone for low conductive targets but you have no tone for iron which makes one to digg all.