Soil sampling is the process of extracting a small volume of soil for subsequent analysis at a lab. The results obtained measure the concentration of certain metals in the soil, and are useful for geologists in areas where rock does not naturally outcrop at the surface, as it can be used to infer the geology beneath the soil. Because of this, it is an important early step in the exploration for ore bodies in Ireland.
Soil sampling is a low-tech, inexpensive and relatively quick method of sampling at regular intervals along predetermined lines or grids. The spacing of the sample intervals will depend on the scale of the survey and on small programmes may be one sample per 50-100m, whereas large surveys e.g. the nation-wide Tellus programme the spacing may be 1-2km per sample.
The sample is recovered by making a small penetration into the ground by manually twisting a hand-held auger through the topsoil, and so does not involve the use of electrical equipment of machinery. Surveys are usually conducted by a team of two geologists who will then pack the sample there and then, to be taken to the laboratory. After recovering the sample, the geologists will then place the displaced topsoil back into the auger-hole to fill the cavity to ensure the stability and aesthetic of the ground is not disturbed. The auger itself is composed of steel, and there are no uses of chemicals, therefore making this sampling method – like mapping – what is termed non-intrusive.
Geologists taking a soil sample in a pasture field. On a detailed grid scale, a couple of samples would be taken per field; on a regional scale, one sample would be taken every few fields.
In mineral exploration, the desired depth will be beneath the top soil to intersect the mineral-rich layer where metal ions from the underlying rocks naturally accumulate. This depth is subjective and decided by the field geologist, however over the majority of pastures and woodland, it will likely be between 0.1-1m depending on the thickness of the topsoil. Over peat bogs, the geologist will attempt to penetrate beneath the peat up to a depth of 3-4m, although if it exceeds this thickness then it is usually too difficult to recover a high quality sample. Due to lab analysis only needing a small volume of sample to generate an accurate measurement for the soil chemistry, each field sample will amount to approximately 1kg.
Soil sampling is a cheap and effective way of kick-starting a mineral exploration programme over an area previously highlighted by historical mineral occurrences, mapping/prospecting or regional airborne geophysical surveys (see respective page). In the vast majority of cases, the results from analysis of the soils will prove insufficiently enriched in certain metals to warrant further investigation. However, for areas which may have the potential for hosting a large ore body, soil sampling may be followed up by a closer-spaced soil sampling survey, and – if this also proves interesting – eventual drilling.