Estimating Target Size And Depth - Fisher Labs F 75 User Manual

F75
Comprehensive Operating Manual & Guide to Metal Detecting
Search Techniques (continued)
Estimating Target SIZE, DEPTH, and SHAPE
When the trigger is pulled to activate Pinpoint, the LCD displays estimated depth. The
estimate is based on the presumption that it is a coin-sized target.
But what if it is not a coin-sized target? The most common example is that of an aluminum can.
Flattened aluminum cans are usually identified as quarters. Their large size will produce a
strong signal, tricking the microcomputer into thinking that it is a shallow coin.
The following explains techniques for differentiating buried aluminum cans from coins. Sweep
back and forth to get a feel for the target, keeping the searchcoil close to the ground. Now,
continue to sweep back and forth as you slowly raise the searchcoil higher and higher. If the
response diminishes quickly and never gets very broad, the target is probably a coin. If the
response diminishes slowly as you lift the searchcoil, and you get a broad response, the target
is probably an aluminum can. If you practice this by laying a coin and a flattened aluminum
can on the ground, you will quickly understand how to differentiate the two and you will
probably never have to dig another aluminum can. And, you will know whether it was deep or
shallow. This technique works well in the All Metal modes, and to a lesser extent in
Discrimination mode.
Objects which are ring-shaped, or flat and round like coins, tend to give a narrower, crisper
response than objects of similar size with irregular shapes. The easiest way to demonstrate
this is with an aluminum screwcap from a soda bottle. In its normal shape, it occupies a
volume, and gives a somewhat broader response than that of a coin. But if you flatten it, the
response will be crisper and more like that of a coin. Again, these differences are most readily
noticed in the All Metal modes.
Long skinny iron or steel objects such as nails usually produce a double response when
scanned lengthwise, and a weaker single response when scanned crossways. This is most
noticeable in the Motion All Metal mode. However, a coin lying on its edge can produce a
similar response, so rely on both the target ID as well as target feel to distinguish between
different kinds of objects. Objects within 2 to 3 inches of the searchcoil will often produce
multiple responses as you sweep across them, because the response field close to the
searchcoil is irregular.
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