Power vs detection depth

[green]

I've read it takes 64 times the power to double the detection depth. The plotted data shows it takes about 10 times the current to get the same signal for each 5 inch increase in depth. If the detection depth is 5 inches it would take 10 times the current to double the distance. If detection depth is 10 inches it would take 100 times the current to double the depth. Is my thinking wrong or are the plots wrong?

[Silver Dollar]

Well it depends... On my current Bandito build I found better air depth with less power.

Others with the Natilus have reported greater ground depth with less power.

You see it's all about seeing the target not getting a signal to a certain depth. A higher
power can magnetized the soil making it hard to see deep...

Now a PI on beach sand might be able to use your 100x power to see a bit deeper...

[Tec]

Hi,

Also, If you move the coil slower you will get more signal vs noise , the average of the random noise will tend towards zero.

So, if you moved the coil really slowly, say 1 cm every hour, and take lots of measurements, and store it in a PC.

You may have more depth... without any increase in power.

If you put the coil on a GPS position plotting cart, and allowed it to slowly autonomously scan a track over a few hours, maybe we would get more depth without any increase in power (perhaps)
Or, if you sense the peak current in the target during the pulse switch ON or OFF, rather than the decaying remnants of this signal a few 10's of usecs later, you could possibly get more depth.

[Nexus]

Detection depth is also increased by the use of larger search coils.
To double the detection depth each loop of any given IB coil design have to be 4 times larger diameter.
So if you could pick up a quarter dollar at 15" with figure of 8th search coil with 8" loop diameter, to double that depth you will need the diameter of the loops to increase to 32", four times the diameter of the small loops.
There is no need to raise the power consumption in this case, but the coil loops would be way too big.

[detectormods]

Detection depth is increased by obtaining an increase in signal to noise ratio, reduce the transmit power as not to excite the minerals in the soil and improve the the receiver performance, most metal detectors have pathetic front end receivers.

[Nupi]

Can someone explain to me why the detection depth increases with increasing coil diameter?

[dfbowers]

Can someone explain to me why the detection depth increases with increasing coil diameter?

The coils magnetic field gets larger with increasing coil size but the power density get weaker. You trade off finding smaller targets with increasing coil size.

[Nupi]

The coils magnetic field gets larger with increasing coil size but the power density get weaker. You trade off finding smaller targets with increasing coil size.

Does that mean if we would be increases the power so that the power density get bigger that we small particles could detect?
Or do I not understand it right.

[dfbowers]

Does that mean if we would be increases the power so that the power density get bigger that we small particles could detect?
Or do I not understand it right.

Yes, but more power is not always a good thing. The ground is most often the limiting factor.

[Nupi]

Yes, but more power is not always a good thing. The ground is most often the limiting factor.

I'm not sure I understand you correctly. If we, with a large coil the same power density maintaining as the small coil power density the ground can not be a problem. It is the same power.
Or am I missing something. But anyway thanks for your explanation.

[dfbowers]

I'm not sure I understand you correctly. If we, with a large coil the same power density maintaining as the small coil power density the ground can not be a problem. It is the same power.
Or am I missing something. But anyway thanks for your explanation.

The ground is usually the big challenge. If the coil is bigger it will see more ground in relation to the target.

[Nupi]

The coils magnetic field gets larger with increasing coil size but the power density get weaker. You trade off finding smaller targets with increasing coil size.

Now I can understand you better.
So it is not that the power density get weaker but that the larger coil receives more signal than the small coil and masked the small target, right!
Besides a great website you have.

[Nupi]

I sat there thinking about it, but that can not be true what I think.

-- With increasing coil size but the power density get weaker but we would be increases the power so that the power density get bigger than the small particles could be detect. I think not.

-- The ground is usually the big challenge. If the coil is bigger it will see more ground in relation to the target. I think not.

-- That the larger coil receives more signal of the ground than the small coil and masked the small targe. I think not.

-- If it were true then a large coil is able to detect a small target in a air test and that is not.

I go to sleep first few hours. Maybe then I understand it better. Only for interested here a nice pdf.

Modeling the Response of Electromagnetic
Induction Sensors to Inhomogeneous Magnetic
Soils with Arbitrary Relief

[Tinkerer]

I sat there thinking about it, but that can not be true what I think.

-- With increasing coil size but the power density get weaker but we would be increases the power so that the power density get bigger than the small particles could be detect. I think not.

-- The ground is usually the big challenge. If the coil is bigger it will see more ground in relation to the target. I think not.

-- That the larger coil receives more signal of the ground than the small coil and masked the small targe. I think not.

-- If it were true then a large coil is able to detect a small target in a air test and that is not.

I go to sleep first few hours. Maybe then I understand it better. Only for interested here a nice pdf.

Modeling the Response of Electromagnetic
Induction Sensors to Inhomogeneous Magnetic
Soils with Arbitrary Relief

I like your way of thinking.
Indeed there are many questions that beg for an answer. Here are some points to ponder:
- The magnetic field density has an impact on the target signal amplitude. For the same field strength, a large coil has less field density. There is also the magnetic moment di/dt to be taken into consideration.
- There is a relationship between the surface area of a target as presented to the magnetic field lines and the target signal amplitude.
- Homogeneous soil has a resistive and a reactive TC (PI) or frequency susceptibility (CW). A target of the same TC as the soil is difficult to detect.

Any measurement we make, is subject to the influence of the tool we use to measure.

[Davor]

There are also some geometry-related constraints often neglected in theory of magnetism. The field is in 99% of cases in literature shown for the very centre of a coil, while the largest field is in the very vicinity of the coil windings. Trouble is that the variation of the field across the coil iris is not something you can do with pen and paper math, and the exact values are either measured or simulated in FEM magnetics or similar programs.
Because of this, the soil response is much milder with Rx coil being separated from Tx coil, as it does not pick all the ground response created by Tx coil wires near ground.