Ground Probe
The ground probe is just another tool. Many people do great and never use it. It is used to help tell the type of soil you are hunting in. This can help make a choice of gains, frequency, filter, or coil.
The right side of the screen reports the phase and strength of the residual signal, and does so for each individual frequency. This is useful for analyzing the characteristics of the ground, but only if the residual signal consists purely of ground signal. This will be true if (1) we can get rid of other residual signals like an imperfect loop null, and (2) if we put the loop over target-free ground. To get rid of any residual loop null signal, hold the loop in the air and away from metal, TAB to select the Zero button, and press ENTER. Ground Probe is now zeroed and you should see the Strength numbers go to 0% and the phase numbers jump around a lot. Now you can lower the loop to the ground, careful to avoid targets, and the Phase and Strength numbers will be that of the ground.
"PHASE ANGLE" is the measured difference between the transmitted signal and the signal received from the target. Different materials have different characteristics (resistivity, reflectivity, conductivity, density, etc) which affect the signal that is returned from that specific target. It is this difference between the transmitted and returned signal (phase angle) that is measured. The VDI reading at the top of the screen is converted from the phase into VDI units like you'd see for normal targets such as (VDI = 19 = a nickel.
The detector only reports VDIs converted from positive phase angles (0 to 180 degrees). The negative angles aren't valid. In the ground probe screen, you may see negative angles if you are holding the coil in the air so there is no real signal coming in, just noise, or you haven't zeroed it yet. When you zero the detector, it removes the system and coil offsets and enables the user to achieve a more accurate reading of the ground. Once you zero the machine and lower it to the ground your phase angles should come in between 0 and 180 degrees.
A ferrite bead will come in very close to 180 degrees (-95 VDI). Most ground that we see is 178 degrees (-94 VDI) to 165 degrees. Salt water, wet beach or fields with lots of fertilizer come in close to 90 degrees (0 VDI).
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The VDI reading is converted from the ground phase normalized to standard VDI units. It's either going to be from the strongest signal if the detector is running best data, or the average of the two strongest signals if correlate is selected.
The phase tells you composition (between purely ferrous and purely salt), while the signal strength tells you the concentration (more mineralized). If you have ground coming in at -94(VDI) with a signal strength of 2% and another ground coming in at -91 with a signal strength of 7%, it would seem that the ground coming in at -91 is more mineralized even though it is a less negative number. A signal strength of the ground as follows 0 to 10% weak, 10% to 20% Moderate and 20% and up to be strong. The lower the mineralization, the slower one should sweep - which may mean a lower filter should be selected.
