Yeah, sorry, I initially just had a legit question, one that hasn't been answered. Maybe I've sort of answered myself but I don't think I really have, still waiting lol. Honestly the information in this thread is brutally technical, magnetism is one from the physics texts for sure. Sorry lol
Yeah, that knife is probably magnetic enough to be detectable no less than a foot and a half away if you were using a graphical output, i wouldn't be surprised if I got a bump on the magnetogram from something on the order of 2.5 feet, maybe further. 773 mT is a huge number, you're finding out how to use it to gauge the strengths of fields there. Actually the mag can be used to help assess mineral composition especially in the case of hot rocks. I have a decent chunk of hematite I found metal detecting that also registers on the mag, granted it's on the order of a mT or two so you have to use a graphical output to actually see it.
Actually, after you calibrate the thing, re-test that knife and see what numbers you get then. 700+ mT is a pretty high number, I expect that from a neodymium magnet, not a pocket knife🤣
Oh, here's one for the visual aid here. If you're using graphical output you can detect this business from a good ways off, the magnetic field is like a 3d fabric, a pull in one place kinda causes the whole field to move. A car driving past literally kicks off an ELF range EM wave that'll travel a little ways before it peeters out almost like striking the strings of a harp. It literally produces a low frequency radio wave as it drives along. It'll cause more noise in the data set if you're close enough, it'll cause big number jumps or drops if you're really close or the vehicle is a big one. Boinnnng! You should consider downloading Phyphox if you're really intrigued, you can characterize all kinds of subtle changes all around you, lightning bolts pull a neat looking pulse train if you have graphical output for the record, caught one yesterday actually. Likewise you can study the magnetic spectrum, check out the Schumann resonances, see how local rotating magnetic fields (spinning wheels in your car, a rotating motor and etc.) causes frequency spikes (elf radio frequency!) and more.....like finding buried iron pipes which is what I'm doing with it lol
Oh, and here see, one more graph for the moment. See that up and down in the y axis? That's the movement of an oscillating fan about six feet away from me. The thing only has a weak field but I can detect it pretty strongly at range. Those are literally radio waves, the movement of the fan produces straight up radio waves and legit. They don't travel far but they are detectable for something as sensitive as a mag. Look at the numbers on the chart, see how small these changes are, using absolute numbers you'd never catch this, like ever, much too faint but there it is, proof of concept not that geophysics needs it, they already know but that's why I use graphical output.
All good and very interesting. Hey, who knows maybe it'll spark some new method in detecting. It's pretty much accepted that apart from bells n whistles, the current tech has near reached its limit.
So what you discovered is what we were doing in geophysics 30 years ago when trying to determine the best way to detect, discriminate, and classify unexploded ordnance (UXO). We were trying to decide if magnetometry or metal detectors were the best approach.
This was tried perhaps 10+ years ago by an experimenter in Australia. I'm pretty sure he used a fluxgate sensor (which adds dead metal to the coil) and found that it didn't really help because he could still detect iron deeper with the detector than the mag sensor could identify.
That's a bit more helpful. Thank you. The background baseline I've been able to navigate experimenting around, using a magnetogram plot tells you what's going on with the local field and then any dips or spikes are read against that background. The measurements I've been using are relative in nature but to do it that way it really requires a strong technical understanding of what a mag does and what the graph is telling you. For instance: if I'm gridding at the park with it and I come into a field distortion created by a fence 15 yards away I see the change in baseline and then simply watch for sudden changes within that baseline, an anomalous change that comes and goes as I pass over a given spot regardless of the overarching field numbers. I could see that being a fiddly issue for many to deal with.
Actually I was hoping this thread would catch your eye, I've read some of your past posts and the work you do is kind of incredible. So I take it that it's a logistical/cost nightmare incorporating a mag into a MD? We now have better tech available? The gadget I've been experimenting with isn't exactly scientific grade, it's a noisy cell mag lol. It does have some capacity to detect thermal remanence but I'm still exploring the limits there, it's only going to get 2-3" depth on most such features. It does great with basalt, basalt sets off the MD too but the mag responds rather more strongly. My intention is to attempt to locate the point of a lightning strike with the hope of finding fulgurites, I believe the sensor should be up to the task provided the LIRM is fresh enough. A second intention is to find out if it can trace the outline of an ancient earth work (unexcavated pueblito), if it'll respond visibly to magnetic soil build up in the foundation line surrounding the ruin. Don't know what I'll see there but it's worth trying if for nothing beyond the value of the experiment.
The issue with magnetometer data that people often don’t understand is the nature of the source and the resulting secondary magnetic field.
As to whether magnetometers have been superseded by metal detectors, the answer can be yes, no, or it depends.
I had to go research a bunch of geophysics and of course the subject of magnetometry itself to be able to use it so running it like some kooky metal detector isn't exactly turn-on-and-go. There's a lot of nuances to reading the information the magnetogram is spitting out at you so I could see how there are probably a lot of variables to consider when putting it on an MD.
I would say it's quite easy if, say, a magnetoresistive sensor has enough sensitivity. Metal detectors already have a powered PCB in the coil for a preamp, accelerometer, and security micro. It would be trivial to add a mag chip. For best results you probably want to put a reference sensor in the control box and run it as a gradiometer, and if your using a 3-axis mag chip you can do a software cal for the ambient magnetic field. However, even a slight change in the coil tilt will mess up the calibration so it could be really difficult to use. And, as Ryan points out, the swept response of an iron target can be confusing depending on the Earth field inclination. But still cheap and easy to add on to a detector.
Different tools for different jobs. In some cases using a mag makes no sense, in some cases it's a far better tool than a metal detector.
Yeah, the mag, as near as I can tell, serves two primary functions:
