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Mining exploration is difficult because most of the earth's land surface is covered by soil. The purpose of soil sampling is to define what is in the underlying bedrock. Soil sampling is usually in the middle of the exploration cycle. In modern gold mining, the gold is in such tiny particles that they cannot even be seen, much less panned out. But soil samples, with proper analysis, can reveal clues to the location of large gold deposits.
Gold exploration in a region follows a progression of stream-sediment sampling to soil sampling to bedrock sampling. It starts with sampling stream beds because stream sediments represent a large area. You sample upstream, always looking for a higher gold concentration. If that shows promise, you start soil sampling around "hot" stream sample locations. All sample locations must be carefully marked and then plotted on your map. If the soil samples are encouraging, then you start digging to bedrock and sample that.
The depth of the soil sample is important! Soil is in layers called "horizons." Often there are three horizons, usually designated "A," "B," and "C," from the surface down. The A horizon has a lot of humus (topsoil). Below the humus is often a zone of leaching where rainwater, which has turned acid from the humus, leaches out the metals and other solubles. Then comes the B horizon where the metals accumulate. Often this is where you want to take your sample. Below this is the C horizon, which often has partly decomposed chunks of bedrock in it. But first dig a hole or trench large enough so that you can see what the soil profile looks like. It is obviously critical that all your samples come from the same horizon.
Soil sampling and geochemistry is of course a full-blown science, but it is not so technical that you cannot use it for hobby or weekend prospecting. In fact, it is doubtful if you will find a mine without it. For ten to twenty dollars you can have pretty good assays done for as many as thirty-five to forty separate metals (including gold and silver) from a single sample.
In summary, soil sampling is a method which can allow you to find the invisible.
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