A "VLF" or very low frequency type metal detector (like yours),works extremely well on dry sand and in fresh water. It does not work as well in saltwater, or on wet ocean-water beaches. In highly conductive saltwater conditions pulse induction or "PI," and multi-frequency VLF "BBS" metal detectors excel.
The VLF metal detector sends 4,000 - 80,000 radio waves per-second into the ground. When the radio waves hit something conductive - like an iron nail, gold ring, coin or aluminum pull-tab, a magnetic field sets up around the object and a particular signal frequency is transmitted back to the detector's receiving coil. VLF metal detectors have the ability to "discriminate," or tell what type of metal they are seeing by "reading" the return signal frequency.
Radio or "sine" waves bounce off everything that is conductive in the sand or water. This is why VLF detectors must be "ground balanced" to work effectively in highly mineralized soil, or on highly conductive saltwater beaches. You must tune or adjust the machine to see through the "fog," or white-noise created by the salt and iron in the sand or water you are detecting. Unfortunately, this usually leads to a loss of depth and stability with most VLF detectors.
The Minelab Excalibur uses Broad Band Spectrum, or “BBS” technology (multi-frequency), and retails for about $1,500.00. According to Minelab, their BBS operating system, “simultaneously transmits, receives and analyses a broad band of multiple frequencies to deliver substantial detection depth, high sensitivity and accurate discrimination for a wide range of target types.” The key takeaway here is “multiple frequencies.” Unfortunately, radio waves regardless of their frequency still have to be filtered and balanced in heavily conductive wet-ocean sand and highly mineralized saltwater. That limits the systems depth capabilities.
The magnetic iron sands (“Black Sands”), salt, and high concentrations of other minerals in the water and sand conspire to bounce the radio waves away from the target. Conductivity and mineralization act like a shield around the target and create white noise that must be filtered electronically. Think of it as turning on your bright headlights in a heavy fog at night. All that powerful light is diffused and causes a complete white out – you can’t see anything three-feet past the hood of your car! However when you turn on your yellow fog lights, you can see a little further – not as far as you could in clear daylight, but further. That is why all radio wave machines must be “ground balanced” or tuned, to maximize their depth potential, and why BBS filters and multi-frequencies are so effective – yet still limited.
Unlike BBS and VLF metal detectors which constantly send and receive thousands of low frequency radio waves per second, a Pulse Induction (PI) metal detector fires high-voltage pulses into the sand several hundred times per second. If no metal is present the electric pulse decays at a uniform rate with no anomalies. When metal is present a small “eddy” current flows through it causing the voltage decay time to increase, which creates a measurable anomaly. Unlike VLF radio waves, electronic pulses are impervious to the effects of conductivity and mineralization, and are unaffected by salt or black sands.
Using the same heavy fog at night metaphor that I referred to earlier, pulse induction is like headlights that cut completely through the fog as if it were not there at all. The trade-off for that added depth and clarity is the inability to discriminate, or block out iron targets that you generally don’t want to waste time and energy digging. While a pulse induction machine detects all metals without discrimination, the minute differences in the signal tone and quality can give a skilled and experienced operator a clue as to what the target may, or may not be.
So basically, your machine will false and chatter in the wet sand and salt water.
