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BigSkyGuy

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  1. Hi FlyFish, How deep underwater can it go (3m? 10m? 60m?). Also, can you do a video showing how it works in black sand? Thank you.
  2. Here are some MS values for minerals from the same table. Nice collection of hot rocks Alexandre! Thank you for inventing the Impulse AQ! Mineral Magnetic Susceptibility Range (10-6 SI)1 Pyrite 35-5,000 Pyrrhotites 460-1,400,000 Hematite 500-40,000 Maghemite 2,000,000-2,500,000 Ilmenite 2,200-3,800,000 Magnetite 1,000,000-5,700,000 Titanomagnetite 130,000-620,000 Graphite -80-200 Calcite -7.7--39 1. Compilation from Hunt et al. (1995)
  3. 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.
  4. 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.
  5. Very interesting find and analysis GB. The proper identification of finds is half the fun of this hobby!
  6. Hi GB, What I think you have found is a clad quarter which was minted with a poorly bonded planchet. Prior to minting the nickel layers of the sandwich are bonded to the copper layer creating the sandwich. When impurities are present between the layers, the bonding process is ineffective. Sometimes the layers separate prior to coining (stamping the impressions), resulting in a copper colored coin on one or both sides. These types of mint errors are quite rare and valuable. In your case, I believe the bonding was weak and separation of the layers occurred after coining, resulting in a blank copper core planchet. Kent
  7. Simon, I have noticed this as well, particularly for one particular site. My theory is that the copper leaches from the coin into the soil moisture. The copper solution is toxic to the bacteria and fungi responsible for the decay of the grass fibers, preserving the grass. At this same site I have also noticed that copper coins come in what I call "coin clumps", in which the coin is weakly cemented into a copper-stained clump of soil. I believe the copper solution evaporates in the dry months depositing the copper cement (possibly malachite?). This does not occur with the silver coins.
  8. Here is an article that I wrote for Lost Treasure magazine before it went under that you guys (and gals) may be interested in. Not sure how to bring in the photos, but here is the text. Do Coins Really Sink? Charles Darwin’s Contribution to Metal Detecting By Kent Whiting During my 30+ years metal detecting I have often wondered how coins become buried in the ground and what determines how deeply these coins are buried. There are two main theories; the first advocating that coins sink through the soil and the second that coins become covered by plant matter that decays and forms layers of soil on top of the coins. Over the years I have observed huge differences in coin depths from one site to the next or even within the same park. In 2011 I found an 1894 Barber quarter in good condition laying right on the surface of the ground at a site that for many years has been a dry poorly vegetated field. The following year I found, an 1894-O Barber quarter in good condition at a depth of over 10 inches in a well-watered park lawn. Both coins were probably lost in the 1930s, so why was one on the surface and the other over 10 inches deep? One possible explanation is that the coin lost in the park was buried by years of accumulation of lawn clippings. One problem with this theory is the soil above the coin was not pure organic humus, but also contained sand and silt grains. Could it be that the coin in the park was able to sink because the soil was moist? I have seen theories that attribute coin sinking to density differences between the coins and the soil. True, such processes can and do operate within streams. However, such density segregation does not occur within a soil. The frictional forces between soil grains are far too strong to permit sinking of a coin. So, if coins do not sink due to density differences and they are not covered by grass clippings how do they get so deep? Further complicating the situation is the fact that within the same park beneath a spruce tree I found a 1905 barber dime, at a depth of only about 4 inches. Fortunately, these observations can be explained by the work of one of the greatest scientists in history, Charles Darwin. Darwin is best known for his theory of natural selection to explain the diversity of life. In October 1837, about a year after he returned from his famous voyage on the Beagle, Darwin made a trip to visit his uncle, Josiah Wedgewood, at his country estate at Staffordshire, England. Wedgewood related that 12 years previously the surface of a certain pasture had been covered by a layer of lime, but the layer had since become buried by the action of earthworms. Darwin found the white layer of lime at a depth of 3 inches. According to Darwin, the burial of items by earthworms is related to their burrowing activity, which is intimately related to the feeding habits of certain species. Deep burrowing earthworms, such as the common night crawler, feed on both organic material within the soil, and on surface material, such as leaves and grass clippings. They drag their food down their burrows and deposit their waste, called “casts”, at the surface. They ingest organic-containing soil, including small grains of silt and sand, which become part of the casts. Darwin proposed that an item on the surface becomes buried due to the collapse of the subsurface burrows beneath the item, causing subsidence. The resulting surface depression, then becomes filled in by the granular casts. Darwin calculated the burial rate of the lime based on the application date and on the depth of the layer. He came up with a rate of 2.5 inches per decade. Darwin presented his findings in a paper read before the Geological Society of London in 1837. He concluded his presentation with the remark “it is probable that every particle of earth in old pasture land has passed through the intestines of worms”. Unfortunately, the paper was poorly received by Darwin’s geologist colleagues, who had expected great things from the man who, even then, was considered a celebrity following his voyage on the Beagle. Darwin soon moved on to other interests, but never completely abandoned his earthworm research. He published a few more papers on the subject, as well as beginning a few long term experiments. He spread a layer of lime over one of the pastures at his country estate in 1842 with the view of monitoring the rate of burial over time. In 1871 Darwin checked on the status of his lime layer, finding that it had become buried to a depth of about 7 inches over the 29 year period, for a burial rate of 2.4 inches per decade. He calculated the mass of soil material brought to the surface by carefully monitoring the burrows over time and found that the mass of casts brought to the surface matched reasonably well with the burial rate of surface material. The burial rate of an object due to worm activity is related to the population of deep burrowing worms for a given area. Darwin found that worm populations were considerably lower beneath beech trees compared to other areas. In 1881 he published his findings in the form of a 326 page volume which he referred to as his “little book”. The book was a commercial success, but was never embraced by the scientific community and was soon forgotten. Darwin’s work was rediscovered by scientists when the book was reprinted in 1945, and by the 1950s a few archaeologists began to recognize the importance of his work. The concepts described by Darwin are now well accepted by many archaeologists. Within the last few years new scientific terms have been coined to describe these processes. The action of earthworms on the soil is referred to as “bioturbation”, and the animals responsible for the bioturbation are the “ecological engineers”, which include not only earthworms, but any burrowing animal that deposits dirt on the surface. So how can Darwin’s work explain the depth differences between the two Barber quarters? The dry field was an inhospitable environment for earthworms. There was little or no organic material in the soil and the sparse vegetation provided little or no surface litter for them to eat. On the other hand, the park provided an ideal environment for earthworms. The soil was moist, and rich in organics, and abundant food was provided by grass clippings, and leaves. The area under the spruce tree was a relatively unfavorable environment. As mentioned previously, Darwin found that the areas beneath beech trees were unfavorable for earthworms. More recent studies have found the same to be true for conifers such as spruce and pine, and for larch trees, but not for oak or maple trees. So what does all of this mean for coinshooting? First, the idea that denser or larger items should become buried faster than smaller or less dense items can be discarded. How many times have you dug a can at over a foot deep? If cans are this deep, coins of a similar age will be just as deep. The number of remaining deeply buried half dimes and trimes in many “worked out” city parks is probably mind boggling. Another important take away is that under ideal conditions for deep burrowing earthworms, most all of the old coins could be out of range of most metal detectors. Earthworms are most active when the soil is warm and moist, so most of the burrowing and coin burial, occurs in the spring and fall. Clearly, some areas of the country are more favorable to earthworm activity than others. Warm and wet climates are particularly well suited to year round earthworm activity and potentially very deep coin burial. So, do coins sink or are they covered up? Clearly the answer is BOTH; with the help of a few “ecological engineers”. Sources Atkinson, R.J.C. 1957. Worms and weathering. Antiquity, v. 31, p. 219-233. Butt, K.R., Lowe, C.N., Beasley, T, Hanson, I, and R. Keynes. 2008. Darwin’s earthworms revisited. European Journal of Soil Biology, v. 44 p. 255-259. Darwin, C. R. 1837. On the formation of mould. Proceedings of the Geological Society of London, read November 1, 1837, v. 2 p. 574-576. Darwin, C. R. 1881. The Formation of Vegetable Mould, Through the Action of Worms, with Observations on Their Habits. John Murray, Albemarle Street. London. October, 1881. Desmond, A. and J. Moore. 1991. Darwin. Michael Joseph, Penguin Group, London. Feller, C., Brown, G.G., Blanchart, E., Deleporte, P. and S.S. Chernyanskii. 2003. Charles Darwin, earthworms and the natural sciences: various lessons from past to future. Agriculture, Ecosystems & Environment, v. 99, p. 29–49.
  9. Very much appreciated Mike. You have some great tips in there. Look forward to seeing your finds.
  10. I am considering a Gold Kruzer for jewelry hunting. Mike, I know you were on the search for the ultimate "micro machine". How do you like your Gold Kruzer now that you got all of the issues worked out? Is it hitting the stud earrings and chains? Input from other would be appreciated as well. Thank you!
  11. Did the conversion hardware for the Equinox come with the Golden Mask telescopic shaft? Thanks.
  12. I will have the EQ800 with me as well, so I will have some options. I will let you all know what I come up with. Thanks
  13. Thanks for the advice. I think I will take the 11" DD, 5x10" DD, and 11" AI. I also have an Equinox 800 to take.
  14. Gerry, Wow great find. I think I would attribute the "L" to "Company L", but I am no expert. Congratulations!! BSG
  15. Hello all, I am gearing up for a relic hunting trip in the southeast US. I have permission at a nice CW camp, that I used to hunt over 30 years ago with my White's 6db and 5000-D. I think this would be a great place to take the GPX-5000 (nice red dirt). However, a power line runs right through the middle of the camp. This was a minor issue for my White's vlfs, but may be a bigger issue for a PI. I have two AI coils, an 11" and a 14" inch which work great for the EMI which typically occurs around, or in, houses. However, I have never tried them beneath or near a power line. I want to make sure I have a number of options if the AI's do not work. I have read the manual pretty thoroughly and searched the forums and came up with a list of methods to minimize EMI (see Table below). One of the techniques was mentioned by Jonathan Porter in a Treasure Talk article on the GPZ-7000 (link below). He mentioned that "Bogene's Settings" are effective for the GPZ-7000 and have been used in the past for the GPX line for reducing EMI and ground noise. https://www.minelab.com/treasure-talk/using-the-gpz-7000-in-high-emi-conditions-and-audio-smoothing After a bit of searching, I came up with the original thread (link below), which dates back to 2008 and was originally applied using the GPX-4000 and GPX-4500. The method consists of turning down the threshold (until there is no "hum") and turning up the gain and/or stabilizer. http://www.finders.com.au/forum/viewtopic.php?t=3442&postdays=0&postorder=asc&start=0 The technique appears to have been originally developed for very hot ground in Australia, but Jonathan Porter indicates that it can be used for reducing EMI, at least for the GPZ-7000. My question is, has anyone used this method on the GPX-5000 for reducing EMI, and if so how effective was it and what were the disadvantages if any? If this topic has already been covered, I apologize in advance. Thank you! Adjustment Disadvantage of Using Use AI Coil Loss of depth compared to same size DD Use smaller DD coil Loss of depth Decrease gain Loss of depth Use Sens Extra, Normal, or Smooth timing Loss of depth (compared to Sharp timing) Decrease motion medium → slow → very slow Must swing slower Use “Quiet” Audio Type May miss faint signals Decrease stabilizer setting May miss faint signals Lower threshold and raise gain/stabilizer (i.e. Bogene’s Settings) Unknown if this will work for GPX-5000 or how effective it is for EMI. Use Cancel CoilRX setting Inability to discriminate, loss of depth
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