The influence of magnetometer frequency on detection depth

[jeep6789]

Everybody is good! Does anyone want to talk about the frequency of the magnetometer sensor, what is the appropriate frequency to use? Is the higher the frequency, the better the sensitivity? What is the frequency of FGM3?

[Krzysztof]

Here you have an extensive report:
Microsoft Word - 04062018.doc (fgsensors.com)
Within the range of the Earth's magnetic field +/- 50000nT FGM3 gives a signal in the range of 40 - 120 kHz.
So when you set it vertically, the frequency measurement will indicate (depending on which end is up) 40 kHz or 120 kHz.
For each one measuring the frequency, you will get a better resolution by measuring the higher one - with the same small change in the magnetic field, you get 3 times better resolution with the same sensor.
Since you are using a factory sensor, you have no other influence on its frequency.

[jeep6789]

You mean if you switch the sensor up the other side, the frequency will change from 40 to 120, or from 120 to 40 KHZ? Will there be such a big change?

[Krzysztof]

Yes. See for yourself the characteristic f(H), it is not entirely linear, but sufficient for exploration purposes.
Just note that the change of 80 kHz occurred from the range of field change by 100000nT, and the average response to iron underground is in the range of a few to a dozen or so nT - so you need the highest resolution to notice anything but a buried tank.
Therefore, in exploration, you keep the sensor in one position so that the change of the field when moving the hand does not cover the impact from buried artifacts.
Therefore, differential measurement in the gradiometer system is most often used to reduce the impact of changes in position and rapid changes in the Earth's field caused by changes in the magnetosphere under the influence of the Sun.

[jeep6789]

So, what is the accuracy of the microcontroller to detect this frequency change, 1Hz or 0.1Hz? If the image is not needed, use two sensors, frequency voltage conversion, can be expressed in the form of sound

[Krzysztof]

Forget about DAC conversion if you already have everything in digital form, a possible sound will produce you processor as another additional form of result.
The processor measures the frequency of FGM3, (physics is absolute), when the measurement lasts 1s then at a frequency of 100 kHz the counter will charge 100000impulses, the resolution is 1 pulse (in electronics it is called 1LSB), when the field is then (estimated) 50000 nT then 1 pulse falls on a change of 0.5 nT. We are talking about a resolution of +/- 0.5 nT, so in practice then we get the certainty of measurement up to 1 nT (because you do not know which part of these 0.5 nT you are in during the measurement).
The frequency of the processor itself is non-critical, the nominal excitation of 16 MHz is all too sufficient.
If you wanted to measure faster (measurement time 0.1 s), the accuracy drops to 10 nT.
If you wanted to measure for 10 s, theoretically you will get greater accuracy, but then thermal influences, changes in the magnetosphere and sensor positioning will play a big role and the measurement will not be so certain (EMI).

[jeep6789]

Is the magnetometer effective for a 0.5m? hole three meters underground?

[Krzysztof]

Of course - it all depends on the amount of iron, a metal box, thick iron pipes, a metal fuel tank - no problem; but nails are not, not enough iron.

[jeep6789]

There are no holes in metal

[bernte_one]

i think jeep want to know if the mag setup is sensitive enough to locate an anomalie like underground hole , not iron

[jeep6789]

i think jeep want to know if the mag setup is sensitive enough to locate an anomalie like underground hole , not iron

yes

[Krzysztof]

A good quality magnetometer will see the damaged soil structure - a dug ditch, then buried - then, at the boundaries of the trench, there is a change in soil magnetization and this is detectable for at least a dozen or so years.
If it were just a cave or a natural cavity, the magnetometer would be powerless.
The soil subjected to the Earth's natural magnetic field for thousands of years is uniformly magnetized (uniform arrangement of magnetic domains), every digging of a place violates this continuity and this can already be detected.

[jeep6789]

Does increasing the sensor coil increase the sensitivity?

[jeep6789]

hi