Low Versus High Frequency

[Chase Goldman]

 

On 11/2/2024 at 1:07 AM, CTidwell said:

I've heard that if you run low freqs you will likely miss gold, especially small gold, but does the pendulum swing the other way?

Lower freqs should hit silver better, right?  Does that mean that in high freqs you are likely to miss silver?

This is true in general because of two effects -

1) Higher frequencies resonate better with low and mid conductors like gold alloys and brass, aluminum, small lead so at a given depth, those targets will have a higher signal strength/intensity at 40 khz vs. 4 khz.

2) Higher frequencies are also able to resolve targets with a small cross section (of any metal) like micro jewelry, tiny bits of natural gold, and even high-conductive coins on edge.

Note that high frequencies are attenuated more by the ground meaning that a detector operating at higher frequencies cannot achieve the raw depth of detectors operating at lower frequencies.  This is the primary reason you are more likely to miss DEEP silver.  Silver within the depth penetration range of the low frequency transmitted magnetic field will hit the silver at similar intensities regardless of frequency used, with the higher frequency mode having a shallower edge of detection depth than the lower frequency.  Silver (not on edge) is generally the easiest target to hit with any detector.  That's why it is set up for cherry picking and is generally the first to disappear from any site.

It is primarily for this reason (deeper signal penetration) that low frequencies are used to chase deep targets, especially high conductors and targets with large cross sections. 

And since low frequencies are less sensitive to small targets and low to mid conductors, high conductors like deep silver (not on edge), in general, can be detected at depths greater than the depths you can detect similarly sized low to mid conductors or high conductors on edge.  

Also, as GB_Amateur stated:

There is a wider variation of IDs associated with gold but also with lead and aluminum because these targets' phase shift can vary with the mass (and, by association, target cross section) of the target and gold alloy composition. 

Metal detectors also hit round, symmetric targets like coins, rings, iron rings and discs, bottle caps, screw caps, freshness seals and aluminum can bottoms hard because the target eddy currents are high and they focus the magnetic fields near the center of the targets increasing their detectability. 

SMF detectors like the Legend attempt to even out this affect through the use of multiple transmitted frequencies, but different frequency profiles and sophisticated signal processing can still enable SMF to be set up to favor low to mid conductors or optimized for deep high conductor recoveries as described by JCR above.

HTH

[Geotech]

16 hours ago, Chase Goldman said:

Note that high frequencies are attenuated more by the ground meaning that a detector operating at higher frequencies cannot achieve the raw depth of detectors operating at lower frequencies.  This is the primary reason you are more likely to miss DEEP silver.

A single frequency detector uses the ground-free R channel as the targeting channel. Even though a Big Silver Coin has an overall stronger response at higher frequencies, it has a lower R response which means it looks like a weaker target. The peak response of any target occurs when its phase shift is 45° and for a silver US quarter that occurs at about 1kHz. For any frequency higher than that the quarter response diminishes. With MF detectors it depends on how the frequencies are processed but generally it seems to about the same.

[Chase Goldman]

2 hours ago, Geotech said:

A single frequency detector uses the ground-free R channel as the targeting channel. Even though a Big Silver Coin has an overall stronger response at higher frequencies, it has a lower R response which means it looks like a weaker target. The peak response of any target occurs when its phase shift is 45° and for a silver US quarter that occurs at about 1kHz. For any frequency higher than that the quarter response diminishes. With MF detectors it depends on how the frequencies are processed but generally it seems to about the same.

Thanks for popping in Carl for the tip in.  Can't wait to get my Christmas present for this year...the 3rd Edition of your book.  

[Geotech]

Here are measured plots of a US nickel & quarter response vs frequency (1-2-5-10-20-50-100kHz):

The R-axis value is what triggers the detector and for a nickel the peak R is at about 20kHz whereas it's about 1kHz for the quarter. The magnitude of the quarter response grows at higher frequencies but the R component is dropping. Ideally, a single-tau target will exactly follow the dashed semicircle response but coins are not exactly ideal.

 

[Chase Goldman]

1 hour ago, Geotech said:

Here are measured plots of a US nickel & quarter response vs frequency (1-2-5-10-20-50-100kHz):

The R-axis value is what triggers the detector and for a nickel the peak R is at about 20kHz whereas it's about 1kHz for the quarter. The magnitude of the quarter response grows at higher frequencies but the R component is dropping. Ideally, a single-tau target will exactly follow the dashed semicircle response but coins are not exactly ideal.

 

Carl, 

Can you verify whether the transmitted magnetic field strength attenuation with depth in ground at a fixed transmit power level is greater with higher frequency fields?  If true, is the effect result in a noticeable detection depth difference when operating at 5 khz vs. 50 khz?  Thanks.

[Geotech]

I have read many times that it is, but I suspect it depends on the type of ground. Mundane ferrite-type mineralization (black sand and the like) has a response that is pretty independent of frequency, at least up to 100kHz. So for TX penetration I would expect that frequency matters little. The response of viscous mineralization (the bad stuff) can vary with frequency so I suspect it can alter TX penetration. It is on my priority list to do more testing on this.

[EL NINO77]

Lately I have gotten used to using coins with different conductivity but the same size / 19-20mm / in my depth and other tests to exclude, let's say, the factor of the influence of the target size on the detection depth...I continue to use Another very small/5.5mm/ low-conductivity target for testing sensitivity and range on extra small low-conductivity targets.

 Coin targets are..

coin 1 Spanish Real 1700 -high conductor..
coin 1 Dukat 1915 gold 0./986 -medium conductor
coin 10 koruna FJ 1905 gold 0./900-low conductor
coin Roman Denarius Emperor Trajan- Silver/Lead alloy - Very low conductors..

My colleague Pasadette .. detection... left me 2 Frequency detector AKA Intronik with 3 different coils: 13" coil 2/14 kHz, 13" coil 5/40 kHz and 9x12" coil working at 7/60 kHz.. so I had time to devote myself to this topic more deeply...

Since this is a very powerful detector.. I could test on lightly mineralized terrain at really great depths.. / depth 16" -40 cm / where the effect of the optimal frequency on a given conductive target should be best manifested.-

AIR TEST...

..AKA Intronik in program2 "Standart"- Airtest   9x12" 7/60khz.. 

 

https://youtube.com/shorts/-5LTrlJ1jI4?si=oP3RbNQO4Tuqm3as

 

In this Airtest ...you can see the detection distance for coins with different conductivity...
Very low conductive coin- Roman Denarius is detected at a distance of 40.5 centimeters..

Next.. Low conductive coin and medium conductive coin are detected at a distance of 42 centimeters and 42.5 centimeters..

Also pay attention to the highly conductive coin 1 silver real 1700... detected at a distance of 37.5 cm..

The differences for different conductive coins are not extra large.. because the coil operates at a frequency of 7 kHz and a frequency of 60 kHz... but it is still visible that this coil can maintain an excellent range for a very low conductive Roman Denarius.. Why?

Because this target is demanding to use a really high frequency..so that the detector has a really good range..

----------------------------------------------------------------------

CTX3030 - Big Coil 17x13"  .. Sens 25 , Deep ON.../canal 2/..AIRTEST *

with CTX  3030 the situation is reversed.... the range of the detector on a very low conductive Denar is 28cm,,, while on a High conductive 1 Spanish silver Real the range is up to 34.5 centimeters...

The difference is so significant that where you will be able to detect a highly conductive coin at a depth of say 35 centimeters,,, the same size very low conductive coin will only be detectable at a depth of 28 centimeters..!!!

.......................................................................................................................

TERAIN TEST

.Terain Test ...-HF 7/60khz  coil in the test on a very low conductivity Roman Denarii coin at a depth of 40 cm..

As you can see, this very low conductivity target can be detected at a depth of up to 40 centimeters in light terrain as a weak but good and regular signal...

In the case  /7/60khz coil/of detecting a similarly large highly conductive coin such as a 1 Spanish Silver Real from 1700, the depth range will be somewhere around 38 centimeters.. which is not bad..but if you use a 2/14 kHz coil, this depth range will improve significantly..