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Date Posted: 00:52:38 04/12/02 Fri
Author: Steve Herschbach
Subject: Pulse Induction vs Induction Balance and Multi-Frequency

Hi Guys,

Here is something I wrote in response to a question on the Alaska Gold Forum. It is of course just my take of what I've read in lots of places, including this forum. I'd like to use it on my website, but I'm asking for your expert advice on anything I may have oversimplified or am simply wrong on. The main goal was to try and keep it simple, however.

Here it is:

What follows is a layman's description of the technologies involved. I'm trying to do this in as few words as possible while hopefully imparting some of the general concepts accurately.

There are two types of detector technology currently available for hobby use. Today's metal detectors are basically radio transmitter and receiver devices.

The first, Induction Balance (IB) technology, relies on a transmitter and receiver coil operating simultaneously in a state of electrical balance. Any conductive or magnetic object entering the field disturbs the balance and generates a signal. Plus, eddy currents are induced into conductive targets creating a detectable signal.

Most people refer to these as VLF detectors, but all VLF means is Very Low Frequency and refers to a particular subset of IB detectors. Some detectors referred to as VLF today are actually operating in the LF (Low Frequency) range. Induction Balance is more accurate as it refers to the technology, not just a frequency range. Older style T/R (transmitter/receiver) detectors are also IB units.

The second, Pulse Induction technology, has been around a long time and every one of the major manufacturers makes a PI machine. There is the Fisher Impulse, Garrett Sea Hunter XL500 Pulse, Tesoro Sand Shark, and White's Surf PI. These are all units designed for use in saltwater/black sand beach environments.

Simple Pulse Induction units use a single coil to alternately transmit and receive. The transmitter segment creates an electromagnetic field that induces eddy currents in conductive targets. These eddy currents actually take some time to ''decay'' and so when the coil switches to the receive mode it picks up the weak eddy currents just induced in the transmit mode.

Induction balance units constantly bathe the search area in an electromagnetic field. Smaller items can be detected because of this, but the IB units also detect more ground mineral. If there is too much ground mineral it makes it hard for the machine to detect the target. IB detectors have superior target id capabilities as they use a combination of the eddy currents and the electrical imbalance in the search field as information to identify targets.

Pulse Induction units use eddy currents only. When the transmitter shuts off the electromagnetic field collapses and so there is only the weak eddy currents to work with. The good news is that eddy currents flow much longer in metals and for a much shorter period of time in ground minerals and salt water. A delay between the transmit and receive modes allows the eddy currents in low conductive targets to decay enough they will not be detected. The stronger eddy currents in more conductive items is detected.

This makes most saltwater and certain mineral environments almost invisible to PI detectors. It also means very small targets are harder to detect as they cannot hold an eddy current for long. Since PI units have less information to work with than IB devices they have little or no discrimination capability by comparison.

Although common PI units do better with certain mineral environments that IB units they still can be affected by mineralization. By using multiple pulse periods and using the information gained from some of the pulses to modify the others, Minelab developed ground balancing pulse induction technology. The called this MPS for ''multi-period sensing''. This allows the Minelab units to work in mineralized soils other PI units would have problems with. This in turn also allows for higher powered units with better sensitivity to small gold. And so the Minelab SD/GP series was born.

The Explorer, Fisher CZ series, and White's DFX are all Induction Balance units but are working with information received from multiple frequencies. A very rough generality is that low frequencies detect larger objects better at deeper depths. Higher frequencies tend to hit smaller items better at shallower depths. Multi-frequency units attempt to use information received from multiple frequencies to achieve superior detection characteristics including better target id. Most of what multi-frequency is about is how many frequencies are received and compared, not how many are transmitted.

But the key thing is that there is always a signal being transmitted and received simultaneously. The DFX is my favorite example. It can be run in 3kHz only or it can be run in 15kHz only. Or it can be run at both 3kHz and 15kHz simultaneously. Multi-frequency technology is just another type of IB technology, and they are not PI units.

Steve Herschbach

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