The GPZ 19 coil – does it really detect 30% deeper?

With the imminent release of the GPZ 19 accessory coil, I thought it was worth discussing its depth capability and how our 30% performance statement was derived. Typically these types of claims are commonly greeted with much concern and doubt, before many are even able to get their hands on the product and test it for themselves. Even then, there are a wide range of settings to choose from and soils to test in, so results can vary greatly depending upon the test conditions. Also, larger objects will typically be detected deeper for the same size coil. Anyway, some background information first…

We all basically know that large coils detect deeper than small coils. Why is that? The relationship between the size of a coil and the detection depth depends on a number of factors related to both the construction of the coil and the particular target itself. Starting with the coil itself, to compare the relative sensitivity of a large coil to that of a smaller coil, we need to determine the strength of their respective transmitted magnetic fields and the receiver sensitivities. These can be calculated by applying the Biot-Savart law, which describes the magnetic field produced by an electric current.

Based on this, we can understand that a small coil with more turns creates stronger localised fields, while a large coil creates a field that is weaker in the immediate vicinity of the coil, but decays more slowly with the distance from the coil. Thus, further away from the coils, a larger coil has a stronger field.

We now need to consider the target, for example a particular nugget: if we use one coil as a reference, at what depth can we just detect the nugget with it? Once we know this, we can utilise the previously calculated relative sensitivity to determine at what depth we can detect the same nugget with the other coil. Doing this for a range of nuggets, we find that small coils are better suited for very small nuggets near the surface, while the larger coils are better for intermediate and deep nuggets.

Is this all there is to this comparison? Of course not, there are other second order effects that come into play and, while they do not alter the conclusions dramatically, they nuance the previous calculations. As an example, the ground noise that each coil picks up depends on the detailed geometry and construction of the coil, but also on the composition of the soil (e.g. how inhomogeneous it is, on what spatial scale, etc.). For example, larger coils tend to have less ground noise, due to their reduced sensitivity close to the coil and this means even greater depth than the sensitivity calculations alone would imply.

Now, to the practical test info everyone is after – comparing the large GPZ 19 coil to the standard GPZ 14 coil. We’re derived a composite graph from our test results. This is shown below; and also on the back of the product carton and the product webpage.

*When compared to the average performance of the GPZ 7000 detector with the GPZ 14 coil in typical environments. Actual performance depends upon prevailing conditions. 

The information displayed in this diagram is a coil-for-coil comparison, is indicative only, and is based on the results of laboratory measurements and field testing undertaken by, and for, Minelab, using appropriate and identical detector settings. Please be aware that the depicted results give a relative and realistic comparison of the two GPZ coils for typical goldfields conditions for detecting the weight ranges of gold shown, but do not represent performance under all conditions, and should not be regarded as conclusive. Minelab does not warrant or represent that the performance levels depicted will always be achieved, as performance of the two coils will vary depending upon prevailing conditions. Relevant factors in detector plus coil performance include, but are not limited to, detector settings, ground type, mineralisation levels and type, electromagnetic interference, gold nugget size, shape and composition, and operator skill level.

Let’s take a more detailed look at what this shows. These results were obtained from 8 specific gold nuggets ranging in weight from 1.9g up to 60.5g. Each nugget was tested in both low and high mineralised ground, with the GPZ 7000 detector set in High Yield Gold Mode and General Gold Mode. Therefore the results in the diagram above are an averaged combination of 4 data sets:

• High Yield setting, lower level of ground mineralisation
• High Yield setting, higher level of ground mineralisation
• General setting, lower level of ground mineralisation
• General setting, higher level of ground mineralisation

Also, these particular results show that there was no ‘crossover point’, with the GPZ 14 performing better at depth for the smaller nuggets, and there was not a uniform depth increase across the nuggets – it did vary up and down, with some results less than 30% and some greater. This is likely due to the individual characteristics of the nuggets under test. Here are some more detailed measurements.

As you can see, there’s a lot of comparative information here, hence the simplified version shown in our marketing materials. We carry out significant testing and all of the results build up to give an overall picture of a new coil’s performance. This includes verifying computer modelling, taking measurements from our four test lanes and a large variety of results from field testers around Australia and overseas. For those of you that buy the GPZ 19 coil, you should expect it to be 30% deeper*, on average, compared to the GPZ 14 coil. It will also cover more ground and pick up less ground noise, but it is heavier and can also pick up more EMI. As with choosing any coil for any detector, it is still a trade-off decision, but you should find that the GPZ 19 really will unleash the full power of the GPZ 7000 detector! 

Minelab Note (23/11/2016): Further to the information from Phil above, the 53% additional area/coverage of the GPZ 19 coil over the GPZ 14 has been calculated by SolidWorks CAD from the combined area of the respective receive coil windings. This is shown in the diagram below:

Green is the area covered by a 14 Inch coil for a given sweep length.

Brown + green is the area covered by the GPZ 19 for the same sweep length.

The brown area nominally represents the extra 53% over the green area, irrespective of sweep length, that a user will gain in ground coverage and therefore is a better depiction of the extra benefit gained than by comparing the full physical size of the coils.

For comparing actual physical coil sizes, by overlaying the 19" onto the 14"  (using an ellipse of best fit at the outer edge of the Tx and Rx windings), the plan view area of the 14” is 80,371 sq mm, and for the 19” is 157,542 sq mm, so 96% larger.