There is a tendency for some objects to read upscale on a target ID detector in mineralized ground. There are a number of reasons why this can happen. I would like to explain one in particular that I am familiar with.

There are any number of methods for a designer to compute a Target ID reading. All are based upon determining the relationship between a target’s X and R received signal components. A targets phase is related to the X and R relationship we desire to determine. Therefore, if we can compute this relationship we will determine the targets phase information. This resultant phase data, or ID reading, is used to operate the discrimination circuitry.

A straightforward method would be to use a trigonometric function to determine this relationship. However, I discovered a easy method that involves a division computation. A circuit was devised based upon division that was easier to implement than a trigonometric circuit. This division computation is given by X / (X + R). In other words we add the Targets X and R signal components. Then divide that result into the targets X signal. In a practical detector design the X and R signals are applied to filters to remove ground mineral information. The filter outputs are then applied to a circuit that computes the X / (X + R) relationship. The Mark 1 and Big Bud series use similar circuitry based upon this relationship. Here are several examples of how this computation operates. The assumption here is that there is no mineral ground to influence the ID readings.

The Target ID circuit computed X / (X + R) to approximate a target's phase. An R component of zero corresponds to a target phase of 90 degrees. The computation X / (X + R) then was X / (X + 0) = 1. The "1" result will cause the meter to read full scale. In this case a full scale ID meter reading represents a target with a phase of 90 degrees.

As another example....let the target phase equal 45 degrees. In this case the target’s X and R components are equal, X=R. If in the computation X / (X + R), X=R the result is X / (X + X) = 1/2. The meter would then read half scale. A half scale ID meter reading represents a target with a phase of 45 degrees.

The last example is a target of phase zero. Salt water is this type of target. In this case the target’s X component is zero. In the computation X / (X + R), X=0 the result is 0 / (0 + R) or 0 / R = 0. The meter would then read zero. A zero scale ID meter reading represents a target with a phase of 0 degrees.

As you can see the computation X / (X + R) will compute the correct target phase perfectly for three values....0, 45 and 90 degrees. At all other phase values are computed with a small error.

As you know targets read differently in mineralized ground. Here is a realistic example of how this circuit works in mineralized ground. Here we will see the upscale reading phenomena that I spoke of.

For a real mineral ground the X signal within the detector is (Xm + Xt). In other words the sum of the Ground and Target’s X signals. Like wise, the actual R signal is (Rm + Rt) or the sum of the Ground and Target’s R signals. We can now substitute these real signals into our X / (X + R) computation. It looks something like this.......

Meter Reading = (Xm + Xt)/(Xm +Xt + Rm + Rt)

Here Xm = Mineral ground X reactive voltage Xt = Target X reactive voltage Rm = Mineral ground R resistive voltage Rt = Target R resistive voltage

If our detector is correctly ground balanced then the ground resistive voltage Rm will be zero or Rm=0. This component then drops out of the above equation. Or.........

Meter Reading = (Xm + Xt)/(Xm + Xt + Rt)

Now here is where I am going with this. If the target is deep the target's Xt and Rt components are both near zero, or at lease very small compared to the magnitude of Xm. Therefore, our computation can be simplified to this........

Meter Reading = Xm / Xm Meter Reading = 1

As you can see the meter will read full scale. We of course have forced our condition of zero Xt and Rt signal components. If Rt were truly zero we wouldn’t have picked up the target in the first place. Therefore, in practice the Xt and Rt signals can be very small. The tendency is then to have an up scale ID reading on weak (deep) targets.

We can make several conclusions regarding detectors who use circuits like that in the Mark 1 and Big Bud. Targets that have a small R signal component (example low conductivity) will tend to read upscale in mineral ground. In addition, if the target is deep it will more that likely tend to read upscale in mineralize ground.

The circuits that compute X /(X + R) all use a blocking circuit that eliminates the -1 meter reading. Only upscale readings are allowed.

The Tek 9000 and 8500 designs were the first target ID detectors. As such these circuit were less refined. They computed (X / R) in both -X and +X directions. This unbounded signal arrangement proved difficult. Therefore, the log(X / R) was used for display purposes. However, they still retained the -1/+1 reaction. Depending upon where you set the ground balance the ID reading could slew upscale or downscale on weak targets.

The X / (X + R) computation is a factual but simple explanation of how that circuit performs in the actual detector. The Tek circuits were biased for a zero meter reading. In other words the circuit itself would tend to produce a zero meter reading for weak target signals with no mineral. The later X / (X + R) design was biased for a meter reading of half scale. Here weak targets, in an air test, would tend to center scale. Therefore, it was on average more accurate. As mentioned the latter design was prohibited from reading down scale. Since it was also biased upscale it definitely has a tendency to read upscale.

As mentioned the Tek TID designs could read up or down scale with equal probability. However, on the Mark and Big Bud circuits the R signal input for the X / (X + R) computation is from a ground balanced R channel that is preset slightly positive. This action biases the X / (X + R) computation where R is always positive. By design it can never go negative. In an air test you cannot see this form of biasing since the ground is not present. What you do see in an air test is the previously mentioned circuit bias to center scale. Therefore, when mineral is present the preset positive R will eliminate TI spreading in the negative direction