salt response

[green]

Was wondering if I could chart salt response.Used some beach sand and salt water that I mixed(1/4 teaspoon salt, 1/4 cup water). I have a coil that a 35mm film canister fits into. A canister of California clay reads .3505 volts. A canister of sand and salt water doesn't change the reading. (integrator out, 5.5usec delay). Any suggestions on things to try?

[crane]

You need a very large quantity of wet salty sand if you hope to see a signal. The signal increases as the volume increases and also with increased coil diameter (assuming a very large volume of sea water or wet sand to start with). It behaves like most other conductive targets and, unlike non-conducting mineralised ground, is also orientation dependent.

[Funfinder]

You can't simulate this with mini-coil and not even with salt-rocks.

There is a huge shielding effect involved on the wet salty sand beach especially if its loamy.
The amount of salt can be much higher concentrated in the sand as its in the water
and it adds up depending how high is the sands mineralization.

The best bet is going directly to the beach and do some tests there.
Outside and insides of the water if the waves are not too strong for those test.

Because of the high mineralization (salt = alkaline, there can be also basic mineralization,
see more at https://en.wikipedia.org/wiki/Soil_pH ) keep in mind while testing that the
detection of iron and steel will be always much better as for the other stuff.

Also the coilsize-find-object-relation counts alot and how good someone or the
detector is able to distinct between good signals and wrong ones.

[Davor]

You'll find salt water tc to be well below 1us. As conductivity is quite low, the skin thickness is quite large, and therefore nothing short of a 1/2 meter thickness is not going to represent salty beach soil.

Because of the short tc, every regular frequency will be well below the corner frequency of such "target" and therefore the response is proportional with frequency. As you go higher with frequency, closer you get to the corner frequency, and your response climbs a rising slope. In multifrequency devices this is (or may be) used to eliminate salts by subtracting samples multiplied by their respective 1/f, and what remains is a response without water signal.

Same goes for conducting component of soil.

[Qiaozhi]

Was wondering if I could chart salt response.Used some beach sand and salt water that I mixed(1/4 teaspoon salt, 1/4 cup water). I have a coil that a 35mm film canister fits into. A canister of California clay reads .3505 volts. A canister of sand and salt water doesn't change the reading. (integrator out, 5.5usec delay). Any suggestions on things to try?

As Davor said, the time constant is probably around 1us, but could be higher in some areas. PIs used on the beach usually have a sample delay of between 10us and 15us. Below that they can start to produce false signals. In which case the time constant for sea water could be as high as 2us to 3us, and maybe more.

The conductivity of sea water is
So, calculating for a 35mm diameter sphere of sea water gives: 187ps (0.187ns). I know you're using a 35mm diameter cylinder (so this is just an approximation) but it does demonstrate quite clearly why the reading doesn't change.

Working backwards, it's a simple process to calculate the diameter of a sphere of sea water required to produce a of 1us. The answer is 2.56m.

Note: the average salinity of sea water is 35g/kG @ 20 Deg.C.

[crane]

You'll find salt water tc to be well below 1us. As conductivity is quite low, the skin thickness is quite large, and therefore nothing short of a 1/2 meter thickness is not going to represent salty beach soil.

Because of the short tc, every regular frequency will be well below the corner frequency of such "target" and therefore the response is proportional with frequency. As you go higher with frequency, closer you get to the corner frequency, and your response climbs a rising slope. In multifrequency devices this is (or may be) used to eliminate salts by subtracting samples multiplied by their respective 1/f, and what remains is a response without water signal.

Same goes for conducting component of soil.

The same applies for all conductive material, ie, the larger the volume, the slower the decay. A very sensitive PI such as Minelab's won't detect the magnetic ground signal if the coil is bobbed up and down at waste height but it will detect wet sea side sand or sea water.
The signal at the sea's surface increases with depth and reaches maximum when the volume of water surrounding the coil reaches maximum, it then drops as the coil approaches the sea floor.
The amplitude of the signal increases with coil diameter.

[green]

Thanks for the replies. Explains a lot.

[Davor]

Working backwards, it's a simple process to calculate the diameter of a sphere of sea water required to produce a of 1us. The answer is 2.56m.

And there is your limit. You may assume that maximum thickness of soil affected effectively by a coil is ~1.5x coil diameter. Hence, you'll hardly find a beach responding slower that 1us.

The same effect happens with aluminium foil. It is thinner than the skin thickness at any workable frequency, hence, it will present itself as a target with low tc.

[Qiaozhi]

And there is your limit. You may assume that maximum thickness of soil affected effectively by a coil is ~1.5x coil diameter. Hence, you'll hardly find a beach responding slower that 1us.

For a typical 10" (254mm) diameter coil, the works out as 22ns, assuming it's exposed to an area 1.5x the coil diameter.
However, this conflicts with the fact that falsing does occur in salt water with a PI if the sample delay is at 10us. So something's amiss.

[Tinkerer]

For a typical 10" (254mm) diameter coil, the works out as 22ns, assuming it's exposed to an area 1.5x the coil diameter.
However, this conflicts with the fact that falsing does occur in salt water with a PI if the sample delay is at 10us. So something's amiss.

As I remember from the diving days, the salt water response increased when diving down to about 50 ft. Once near the bottom, it would diminish again. I think we need to consider the responsive field of a 250mm coil to be a sphere of maybe up to 25ft radius.

That PI detector was an Eric Foster design, with a first sample at about 40us, if I remember right.

[Koala]

I tried testing a VLF with a bowl of water and and salt.

Deferentially not the same as the beach. Always meant to fetch some of the beach back for testing.

[Qiaozhi]

That PI detector was an Eric Foster design, with a first sample at about 40us, if I remember right.

If the coil is under water, then the target is effectively surrounding the coil, and a spherical area around it would have its strongest magnetic field at the centre. How strongly did that particular detector respond to the salt water?

[Tinkerer]

If the coil is under water, then the target is effectively surrounding the coil, and a spherical area around it would have its strongest magnetic field at the centre. How strongly did that particular detector respond to the salt water?

It is 30 years ago, so don't expect memory details to be too precise, but I can give you schematic details. This was a detector specifically designed and built for diver use in the ocean. I remember that when I tuned it in mid water, I had to re-tune it on the bottom. Going over rock, the signal went negative and it had to be re-tuned.

[Davor]

For a typical 10" (254mm) diameter coil, the works out as 22ns, assuming it's exposed to an area 1.5x the coil diameter.
However, this conflicts with the fact that falsing does occur in salt water with a PI if the sample delay is at 10us. So something's amiss.

Once I went through the whole calculation assuming a single frequency (VLF) and a complete skin thickness, and came up to ~1/4us or so for sea water. Perhaps limiting the affected volume in relationship with coil diameter is not the way to observe this problem. Sea water is a complete half-space below coil, or entire space when submerged. If there is a notable difference with approaching sea bed, it must be that a much larger mass of water is at play.

[Davor]

So how about an analogy with a bell... you may strike it front, back, or sideways, and it responds with the same timbre. Now, what if we should observe the full available thickness of an object up to the skin depth - if not obstructed?
I'm wrapping my head around this one and it seem plausible.