OK, I think I better understand what you are driving at. Be careful what you ask for...
Alright, let's get ready to explode some heads...
A common misconception regarding how metal detectors work is that they rely solely measuring the specific conductivity of the target to determine the nature of the target. In other words, many folks think that the inherent high specific conductivity of metals like silver versus other less conductive metals like aluminum is the primary differentiator that a metal detector uses to determine target ID.
In fact, metal detectors look at two electromagnetic properties - inductance and conductance (or more accurately, the inverse of conductance - resistance). The inductance of a target is influenced primarily by its inherent magnetic properties while the resistance/conductance is influenced primarily by the mass/shape of the target (that is why a large aluminum can can often give a visual and tone target ID similar to a silver quarter, even though aluminum is a lot less conductive than silver. Both of these properties result in a phase change or shift in the detected magnetic field waveform that is emitted by the target and detected by the receive coil versus the transmitted field waveform that is sent into the ground by the transmit coil. The characteristics of the phase change are what detector signal processing systems use to infer the target ID of the target but more sophisticated detectors are also looking for uniformity/symmetry in the magnetic field signal as the coil passes over the target and tend to factor-in the symmetric field given off by round targets along with their phase shift because they are likely coins or round jewelry. The magnitude of the inductive component of the phase shift varies with transmission frequency for non-ferromagnetic materials such as silver, gold, aluminum, lead, brass, copper and so on. In general, the magnitude of the inductive component signal of mid-conductive targets (e.g., brass, lead, aluminum, gold alloys) tend to peak at higher transmission frequencies and the the magnitude of the inductive component signal of high-conductors (e.g., copper, silver) tend to peak at lower frequencies.
On the other hand, ferromagnetic targets such as iron nails and horseshoes tend to have a large, constant inductance signal component that is both largely independent of frequency but also results in a large phase shift that is opposite that non-ferromagnetic materials. This component term dominates the phase shift signal even though iron has much higher resistance than most non-ferromagnetic metals. Discrimination circuits/algorithms use this unique response of ferromagnetic metals to identify and filter out ferrous targets. The upshot is that frequency has a much greater effect on the signal response of non-ferrous targets than ferrous targets although it is possible to induce "false" non-ferrous responses from ferrous targets at high frequencies, especially if they have a high mass and/or are round in shape.
The total picture of target response with frequency is actually even more complex than I have described above, and there are exceptions to the simplifications I used above that even I do not fully understand.
Bottom line, set your frequency to optimize the target of interest rather than worrying about the small effect that frequency has on nearby ferrous target signal strength. Hope that helps.
