Carl MFD article ,5KHZ frequency ?

[aft_72005]

Hi to carl and all
In MFD article ,carl used 5khz frequency for gold prospecting .
Some of other commercials almost also using this frequency.
My questions now, can anybody tell me where are come 5 khz ?
How it calculated?
why don’t using 10 khz or 15khz or other range of frequency?
What is relation between this frequency with materials resonance frequency ?
Anybody know This relation formula ?
Regards.