BVI: The salinity of water can rapidly change in only a short distance or period of time.
Depending on the change of tide and currents from rivers both above and underground.
In addition to the water being saltier or less salty, you may notice a small change (a few degrees) in temperature.
The electromagnetic field of both VLF and PI balanced inductions detectors is generated by rapidly switching the current on which flows through a loop of wire.
The resulting magnetic fields radiate through the top of the search coil as well as the bottom. Plus there is spill over of approximately one to two inches from the edge of the search coil.
The key to understanding the difference in depth capability of a Very Low Frequency (VLF) verses a Pulse Induction (PI) detector in salt water is that the VLF generates a continuous electromagnetic field while the field from a PI is not continuous.
Since salt is a conductor, it is easily detected a VLF detector. That is why you may get a false signal when you sweep over a piece of seaweed on dry sand. It would take a very smoggy day to get a return from the top of your detector on dry sand – if there are that many particles in the air you would be better off in home sucking on oxygen from a respirator.
When you go into the water, the coil on a VLF detector will detect the salinity in the water above it and detect it as a target. The detection depth of such a detector is reduced as the return from the salt above the detector (noise) must be subtracted from the return of the target.
As this is impractical with most single frequency metal detectors , multifrequency metal detectors are used for shallow water hunting where there is salt. You may be able to get by with a single frequency VLF in clean fresh water (stirred, not shaken).
Since the return from the salt water above a detector rapidly decays, you will not detect that return with a PI as your receiver will be turned off, unless you are using a very very short pulse delay. Since you do not have to subtract returns from above the search coil, you are able to detect deeper in salt water.
If you are diving or shallow water hunting with an all terrain metal detector you have to be very careful about depth. If you drop it and it sinks deeper than the depth it was designed (usually ten feet) or hit by a wave/surging water you will end up with a very expensive paper weight as it most likely will be destroyed by water seeping in past the seals of the control box.
I maybe wrong, but was told once its to do with the water pressure at depth? is this the case? (Isn't the ocean as salty at 10ft as it is at 100ft?)
So, is there a general guide to machine / depth?... ie - use SH2 from surf to 30ft, then AQ 1b from 30ft onwards etc etc.
