20/ Understanding P.I. Interference
Having encouraged several dyed-in-the-wool VLF hunters to give the Dual Field a try, I’ve had the opportunity to hear their amazement at just how much deeper it goes. However, it’s important to recognize that pulse technology is not infallible. Just as with a VLF, pulse performance is operator and conditions-dependant. By this I mean that just as with a VLF, the amount of work that the detector has to do to keep a steady threshold and the work that the operator does with the coil are what determine if a deep target will or won’t be heard. Let’s look at some of the in-ground factors that will reduce P.I. performance:
1/ Depth of Water
The deeper the water you hare hunting, the denser the electromagnetic signal that it produces will be. This is more of a concern in salt water which is more conductive to begin with. Try this test:
At a salt water site, find a spot where the water drops off steeply. Now take the coil and run it down hill. Notice how the tone rises and becomes louder. Try placing a target on this surface and note how much fainter and harder to acquire it is on the downwards sweep. Remember, it’s not so much the depth of water, but the change as you take the coil down into deeper water the causes interference. P.I’s. lose depth as they tune. (More on this below).
2/ Current or Wave Action
This is dealt with more thoroughly in “Pulsepower…” Salt water produces a signal that is just below foil and iron. The faster water is rushing past the coil, the more of this signal is present for the detector to react to. In effect, it competes with any smaller targets that you hope to find. At the water’s edge, small droplets of water on the coil also produce this effect--summating to mimic a larger signal. It used to be a popular method amongst early pulse hunters to use a silicone spray to minimize this effect, although the technology for shielding coils has improved a lot since then.
Just as when the coil is moved downhill into deeper water, wave action increases the amount of “work” that the detector must do. In fact, a VLF machine in discriminate mode is better at making the distinction between water and metal than a pulse is. For a P.I., salt water is a signal, no different than any other--save for a slower onset.
The combination of current and changing water depth (hill at the edge) can really affect pulse performance. This is important in that these steep inclines are where a lot of stuff is lost or comes to rest.
Remember that it’s only at the ideal range of threshold tone that the detector is performing at its best. Where you have no threshold or the raised amplitude that wave action causes you are loosing depth--a lot of it. I’ve had this illustrated to me in a very plain and obvious way: At one Caribbean site I decided to work along the shoreline to try and cover some prime ground quickly before my competition showed up. I though I could compensate for the steep incline and incoming waves with a slowed sweep. I found only a few surface targets--despite being accurate with my grid rows. It was only when I tried an in / out pattern that I heard several deep signals and saw that I had missed a 17 gram gold ring on my first search!
3/ Ground Conditions
A pulse also reacts to changes in the ground’s composition. These may be contours or changes in the material that is under the coil (gravel, soft sand, solid weed beds…). In fresh water, these changes can take the form of pockets (or ripples) of iron-laden “black” sand, or ground with a varied grade that exposes different kinds of material.
This is one of the great things about using a pulse—instead of riding over those features of the conditions that would cause you to loose depth with a VLF, you have the advantage of being made aware of them by changes in the tone. Using this awareness to react accordingly is the way to get maximum depth performance from the Dual Field.
4/ Sweep Speed
The deepest signals that I’ve gotten with the Dual Field have all been sweep speed contingent. These tiny murmurs or threshold breaks disappear when the coil is not moved in such a way as to keep a light, steady tone. This relates to the wave action as well. Just as with our experiment above, a passing wave can bring you up too high or a lull can knock the tone out altogether resulting in no signal. This skill is highly site-specific and some of the best results I’ve gotten with the Dual Field have come from re-working a productive patch combining different sweep speeds and Gain settings. The ideal is to maintain a sweep speed that does not overrun or lag behind the machine’s ability to process what’s under the coil. This takes a
lot of practice and is one of the most important skills in using a pulse induction detector. This is something that you can familiarize yourself with
by acquiring a deep signal then altering your sweep speed. Notice how the response has a “best” speed where it can be heard the loudest.
Another good test to try is to take a coin-sized target and run the coil over it--and back. Notice how the more quickly you make the return pass, the less of a signal there will be. This is exactly what is happening when the machine is trying to “catch up” as it compensates for the various sources of interference while detecting targets at the same time. There is a continuous “lag” to be dealt with--which is where listening to the ground and coil control come in.
From
“The White’s Surfmaster P.I. Dual Field: An Advanced Guide” (2010) by Clive James Clynick
clivesgoldpage.com
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