Detection in wet soils

[1843]

Hi all,
Why the depth of detection reduces in the wet soils?
Also, my MD beeps in the wet areas although there isn't any target...and first sample is about 40us:rolleyes
Why???
Thanks in advance

[Max]

Hi all,
Why the depth of detection reduces in the wet soils?
Also, my MD beeps in the wet areas although there isn't any target...and first sample is about 40us:rolleyes
Why???
Thanks in advance

Hi,
salts contained in the soil could give to wet matrix a good conduction and so also eddy-currents may flow in due to lage magnetic pulses. Normally 40us are more than enough to avoid false signals on PIs , and many of these, expecially underwater types, uses that range of first delay (>30us). What could happened with yours is that maybe there is also a bad shielding too... and so effects due to capacitive coupling sum with normal eddy-currents in wet soil. Also depth is a function of S/N ratio...and gnd signals can be considered as noise...so your S/N become smaller...and you can't find small objects or also bigger ones at medium depth...your integrators have to mix too much noise produced by eddy-current effects in soil. There is also the magnetic effect to keep in mind. If there is maghemite or magnetite or other magnetic crystals in soil you get also nois coming from magnetization of matrix...that you could notice e.g. also on bricks, pottery and similar fragments really easy in wet soil.
Solutions are related to coil shielding/shape (e.g. DD suffers less of that effects) and to integrators ability of eliminate noise without degradating too much S/N ratio...so also frequency is a big issue in wet soil.
The lower the frequency...less problems you have in these conditions. Some PIs (e.g. deepstarII) uses some KHz for beach hunting...but they have really good designed (and tuned) integrators to deal with S/N degradation better than other PIs. Also most of underwater machines now can mantain good S/N ratio even on wet soil...even in inland sites.

Best regards,
Max

[Woody.au]

Thats an excellent post Max. Now here is one to puzzle you and I have found this when beach detecting. Why is it that I get greater depth in wet sand than in dry sand?

[Max]

Thats an excellent post Max. Now here is one to puzzle you and I have found this when beach detecting. Why is it that I get greater depth in wet sand than in dry sand?

Hi Woody,
really good question.

In PI detectors the signal due to salt wet soil disappears (if no black sand and the like) before signal will be sampled cause of timing settings.

Most of the times the first delay is at 15uS timing (minimum like e.g. in Deepstar)...or more and with that and a fast monocoil (of course shielded) it's possible having a good detection of e.g. small items even in wet soil/sand (rel. hi-conductive than dry soil).

I think that if your detector works better in wet sand than in dry probably there is a part of the wet sand conductivity that superimpose with useful signal, but that only it isn't enough to give you a false signal.

If so, in effects, you could benefit from that condition of the rise of the signal curve due to wet sand effect. That way maybe a threshold detector "see" first other increase due to the useful target, but without detecting just the wet sand, so whitout giving you false signals.

But I'm sure it isn't black sand if things go that way.

Kind regards,
Max

[revmaster]

Hi,
salts contained in the soil could give to wet matrix a good conduction and so also eddy-currents may flow in due to lage magnetic pulses. Normally 40us are more than enough to avoid false signals on PIs , and many of these, expecially underwater types, uses that range of first delay (>30us). What could happened with yours is that maybe there is also a bad shielding too... and so effects due to capacitive coupling sum with normal eddy-currents in wet soil. Also depth is a function of S/N ratio...and gnd signals can be considered as noise...so your S/N become smaller...and you can't find small objects or also bigger ones at medium depth...your integrators have to mix too much noise produced by eddy-current effects in soil. There is also the magnetic effect to keep in mind. If there is maghemite or magnetite or other magnetic crystals in soil you get also nois coming from magnetization of matrix...that you could notice e.g. also on bricks, pottery and similar fragments really easy in wet soil.
Solutions are related to coil shielding/shape (e.g. DD suffers less of that effects) and to integrators ability of eliminate noise without degradating too much S/N ratio...so also frequency is a big issue in wet soil.
The lower the frequency...less problems you have in these conditions. Some PIs (e.g. deepstarII) uses some KHz for beach hunting...but they have really good designed (and tuned) integrators to deal with S/N degradation better than other PIs. Also most of underwater machines now can mantain good S/N ratio even on wet soil...even in inland sites.

Best regards,
Max

Hi Max,

I use my PI @ 500hz, if i reduce the freq will i get better response in soil / wet soil and also how do i improve SN ratio cause the moment you increase the gain the detector starts shouting without any signal....

Thanks