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In the ground is where detecting really counts!

June 22, 2016 12:00pm

Minelab Electronics
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Early in December 2015, Minelab relocated to a new shared facility at Mawson Lakes with our parent company Codan. As part of this move we designed and constructed a series of test lanes specifically to measure the performance of new metal detectors under various ground conditions to supplement existing laboratory and field testing.

We have four test lanes that measure 10m × 3m × 1.6m (33ft × 10ft × 5ft). The reason for these being so deep is to make sure the metal detector is only energising one soil type - the soil in the test lane, and not the native soil beneath.

The test lanes are used for a number of purposes during product development:

  • Testing of algorithms for new technology that will primarily improve ground balance and discrimination, and also other auxiliary algorithms such as pinpointing, built-in calibration, etc.
  • Comparison of various detectors to help determine the realistically obtainable performance of a new detector.
  • Performance evaluations of new prototypes; preliminary concept demonstrators as they emerge from R&D, mature prototypes as they're refined through Engineering, and finished products from Manufacturing.

It’s very important to have a significant amount of variation between the lanes to get the widest range of results before further external testing. Whilst having a variety of conveniently accessible test lanes allows for rapid evaluation of new products, it doesn't eliminate the need for wider field testing in real detecting conditions.  Artificial test lanes tend to be quite homogenous in their composition, whereas the soil conditions in goldfields can be extremely variable. The four test lanes are:

 

The SEVERE Lane

This is a very mineralised lane used to simulate some of the harsher grounds that a gold prospector may encounter. It is a good representation of the grounds from some of the local goldfields, such as Jupiter Creek and Mount Crawford. The terms 'mineralised' or 'hot' refer to the very strong magnetic response of the ground, which for a metal detector means the sensor head couples well to the soil. This means that the received signal from the sensor head will contain a reasonable contribution from the ground, which can mask the signal from small or deep targets and hence reduce the detection.

Lane: Severe
Equivalent to: Australian goldfields
Detecting for: Gold nuggets
Ground Strength: 1,000,000
Phase angle: <1 degree @25kHz

The MODERATE Lane

This ground is much less mineralised than the Severe lane, and it represents a more common general purpose ground. However, in the Moderate ground there is small amount of energy loss, which is also referred to as a 'lossy' soil. The losses can be generally either magnetic or electric in nature. This means that the received signals from the sensor head become slightly distorted and will start to have some target-like characteristics, or become more difficult to ground balance.

Lane: Moderate
Equivalent to: USA goldfields
Detecting for: Gold nuggets
Ground Strength: 50,000
Phase angle: 4-6 degrees @25kHz

The NATURAL Lane

The natural lane is another common general purpose type of ground, which has about the same mineralisation strength as Moderate, but it has much more energy losses. This means discrimination is more difficult because as the amount of loss in the ground increases, the more difficult the ground balance, the more target-like characteristics the ground will exhibit, making separating ground and targets tricky.

Lane: Natural
Equivalent to: Parks & Fields
Detecting for: Coins & Treasure
Ground Strength: 50,000
Phase angle: 8-12 degrees @25kHz

The BENIGN Lane

As the name suggests, Benign soils have very little mineralisation and only a small amount of loss. However, because the mineralisation is so low, even a small amount of loss can dominate, for example, if there is salt in the ground and it is damp or wet. One of the purposes of the Benign lane is for a development-proving environment where new detectors/algorithms can be tested outside the lab, but without the influences of ground. Additionally, the performance of a detector in the Benign ground should be very close to that in air, which is the best-case reference condition.

Lane: Benign
Equivalent to: Reference level (similar to air)
Detecting for: Development purposes
Ground Strength: 4,000
Phase angle: ~0 degrees @25kHz

What is Phase Angle?

The Phase Angle (ø) of the soil refers to the ratio of loss components (generally magnetic and electric) to the magnetic components. If we consider a purely magnetic lossless soil, such as if a soil was solely made from a ferrite material, then we would get a purely magnetic response from the soil. In metal detector terms, this is the instantaneous magnetic response. As this is (almost) instantaneous, there is no delay between the transmitted waveform and the received waveform, thus they are 'in-phase' with no time lag and therefore the phase angle would be zero degrees. So from this description, we can say the phase angle for a ferrite should be zero degrees. And since the phase angle from the Severe ground is close to zero, we can deduce that it is quite magnetic with very little loss.

Now consider the case where we introduce some losses into the soil, either by making it slightly conductive with the addition of moisture and salt, or by adding magnetic lossy material to the soil, or both. This creates a delay and/or distortion on the received waveform relative to the transmitted waveform. In other words, the received waveform is out of phase with the transmitted waveform. The amount that the transmitted and received waveforms are out of phase is the phase angle. The larger the losses in the soil, the larger the phase angle will be. For example, if we had a purely conductive soil with no magnetic material, such as the wet salty sand on a beach, the phase angle would be almost 90 degrees.

Apart from these two extremes (i.e. a ferrite for a lossless magnetic soil, and wet salty sand for a lossy non-magnetic soil), typical prospecting ground will consist of a mixture of magnetic and lossy components, the ratio of which will determine the phase angle. For example, if there was an equal contribution of magnetic and lossy components, then the phase angle would be 45 degrees. However, in practice, the magnetic component for soils is usually much stronger or more easily has the potential to be much stronger than the lossy components, so a typical phase angle will be between 0 and 10 degrees.

This brief explanation hopefully gives you some insight into the fact that there’s a lot more “ground work” involved in developing high performance detectors than just sensing the metal targets that you’re all looking for – and, as Mark Williams says in the accompanying video, “We’ve barely scratched the surface…!”

You can also read more about the technicalities of metal detectors and ground, in the Basics and Theory Knowledge Base Article, by Bruce Candy  and more about detecting practicalities and ground, in the Treasure Talk blog, by Steve Herschbach.

For more videos and information, SUBSCRIBE to Minelab YouTube channel: https://www.youtube.com/MinelabDetecting      

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