Are VLF's better on small gold than PI's?

[Charles (Upstate NY)]

I don't know much about PI's but I read this quote on another forum. The guy seems to suggest that a VLF (Tesoro Tiger Shark) will go deeper on small gold than a PI. Is this accurate?

"PIs generally don't see the small pieces of aluminum. By the same token, they will not react to the small gold jewelry unless the phase angle of the mineralized conditions raises it to a point where there is a subtle response.

I recently located a White Gold Diamond Ring. Located it at about 4" in the Black Sand at Funston Beach in SF...... BARELY got a SIGNAL. A VLF Detector would have heard it probably twice as deep and three times as loud."

 

[Anonymous]

n/t

 

[Prospector Al]

Howdy, There have been a number of questions recently about the relative merits of VLF and PI detectors. I'll throw in my two cents'
worth.

A comparison between the two technologies is more complicated than it appears--one must consider the operational characteristics, before a fair comparison can be made. So let's just look at one aspect--the sensitivity in air.

(I apologize for being more technical than seems warranted, but by showing the theoretical foundation, anyone can verify the accuracy of the statements by consulting his friend, the physicist...)

The magnitude of the eddy currents generated in the target is directly proportional to the time-rate-of-change of the transmitter coil current.

So, let's look at a VLF detector with a 1 Ampere coil current, operating at 5,000 Hz. The mathematical representation of the current is:
I = I(0) x sin omega t where omega = 2 pi f ; f = the operating frequency, which we assumed to be 5000 Hz, in this case.

The rate of change is obtained by derivating the expression for the current:
dI/dt = I(0) x omega x cos omega t

From the above one can calculate that the rate of change is:

2 x 3.14 x 5000 = 31,400 Amperes/Second

Now, lets look at a PI detector with a coil pulse current of 1 A.

The target is energized (mostly) by the magnetic flux change that occurs at the trailing edge of the coil pulse, and the flux is directly proportional to the current.

So how fast must the 1 A coil current return to zero, to yield the same rate of change, i.e., the same sensitivity as a VLF detector at
5,000 Hz?

1/31,400 = 40 micro S

The above exercise can be summarized as follows: If the fall time of the PI coil pulse is shorter than 40 micro S, then the PI is more sensitive than a VLF detector, running at 5,000 Hz. (Thus, to make a valid comparison, one must plug in the appropriate numbers into the above formulas.)

I don't know how fast the coil pulse returns to zero in Eric's detectors, but I bet it's a lot faster than 40 microseconds...

I'll throw another two cents' worth in a later post--this is getting too long...

Prospector Al

 

[bakergeol]

First we are discussing ring size gold not very very tiny gold nuggets where a high frequency VLF such as the GB2 would have an advantage.

The guy probably has no experience with high end PIs such as the Goldscan 5 or the GP3500. If he had he won't have made such a outlandish statement. Similar to VLFs all PIs are not created equal as there are good and bad ones. Not all PIs use a pulse delay of 10 as the Goldscan 5 does which is necessary for small gold.


George

 

[Prospector Al]

Hi George,

Of course, the size of the target makes a big difference, as far as the preferred technology is concerned. I'm glad you pointed this out to me...

My primary experience has been with industrial detectors for detecting metal contamination in iron ore, e.g. To detect any small targets in a load of iron ore, a VLF detector is worthless...

A PI detector with a proper pulse delay can ignore the effect of the continually changing concentration of magnetic material near the coil
system. A VLF detector would have to be rebalanced every few milliseconds, which is not possible with current technology...

Happy Hunting,

Allan

 

[Prospector Al]

Another two cent's worth:

The maximally possible sensitivity in air can never be realized when the target is in the ground. The conductivity and permeability of the ground make it necessary to move the location of the gating pulse in
a VLF detector from its optimal point. The idea is to have the gating pulse located symmetrically around the point where the composite signal from the ground crosses zero. That is not the best position for detecting gold. The loss of sensitivity depends on the phase angle of the ground and in some areas as much as 50% may be lost.

The PI detector has a different problem: The target signal can not be sampled immediately after the coil pulse, where the signal amplitude is at a maximum. A delay must be inserted to allow the spurious signals generated in the coil system to dissipate.

Owing to the delay, target signals with short time constants lose amplitude: A one-gram gold nugget may lose ca. 86%, if the delay is 10 microseconds. A gold ring, however may lose much less, especially if it coplanar with the search coil.

So, there is no clear cut answer to "which detector is best for small gold nuggets". It all depends on the size of the nuggets and how much time you are willing to spend "balancing" a detector.

Personally, I would opt for a PI with a short (or adjustable) gate delay.

VLF detectors are, in a way, self-defeating: The lower you make the operating frequency to minimize the ground effect, the lower the sensitivity, since it's directly proportional to the operating frequency. (Some folks call this "the law of diminishing returns".)

When you reach DC, the detector is totally immune to the ground effect. Unfortunately, it also has zero sensitivity to metal targets...

Four cents is all I can afford at this time--you may not hear from me for a while...

Prospector Al