Multiple Working Frequencies for VLF/IB

[Oseania]

Many expensive VLF metal detectors (e.g. Whites V3i, Minelab E-Trac) use multiple working frequencies to provide better response for differently conducting targets. I guess that lower frequencies work for higher conductive targets and higher frequencies work for lower conductive targets.

I was wondering how these multiple working frequencies affect the coil and electronics design? At least the electronics cannot anymore use resonance with the coil so resonance frequency of the coil has to be much higher than the working frequencies. Doesn't this make the number of windings in the coil smaller than usually if VLF single frequency coils?

Has anyone ever done a VLF with multiple working frequencies as a hobby project or are there any commercial schemas around?

[Geo]

Many expensive VLF metal detectors (e.g. Whites V3i, Minelab E-Trac) use multiple working frequencies to provide better response for differently conducting targets. I guess that lower frequencies work for higher conductive targets and higher frequencies work for lower conductive targets.

I was wondering how these multiple working frequencies affect the coil and electronics design? At least the electronics cannot anymore use resonance with the coil so resonance frequency of the coil has to be much higher than the working frequencies. Doesn't this make the number of windings in the coil smaller than usually if VLF single frequency coils?

Has anyone ever done a VLF with multiple working frequencies as a hobby project or are there any commercial schemas around?

If you will search on forum you will find a schematic of the Tx stage and preamplifier with coil of the Sovereign who is a multi-frequency detector. I did not see anything else