Soil and overburden gases

Even where these general problems do not appear to arise, the shallow depth of sampling attained with a probe may fail to provide an appropriate sample, due to the high diffusibility of He, which promotes equilibration of He in near-surface soil gas with the atmosphere. Thus, Butt and Gole (1985) found that He concentrations increased with depth and that samples from less than 3 m had lost most, if not all, of any excess He. Similarly Hinkle (1994) found no significant variations in samples collected at various depths in the 

Deep sampling was also recommended by Jones and Drozd (1983), who collected soil gases from holes drilled to 4 m. There are, however, obviously many circumstances in which deep sampling is not possible, such as areas of shallow water table. Whilst it may be argued that shallow sampling must then suffice (Gregory and Durrance, 1985), it may be that in such circumstances gas sampling is not an appropriate procedure. 

Samples may be retained in syringes if analysed within 24 hours, apparently without loss of He. For storage over longer periods, glass or metal containers should be used. Metal cylinders, fitted with septa for injection and sealed by brass nuts with lead washers, can retain He for several weeks. Prior to use, the cylinders are evacuated and then filled with sample to about two atmospheres pressure. Possible errors due to preferential gain of He when evacuated, or loss when filled, can be detected by duplicate sampling. 

Helium analysis is most conveniently carried out by mass spectrometry and even very simple instruments are capable of sensitivities better than 0.01 ppm at concentrations of 5-6 ppm He, i.e., those commonly found in soil and overburden gases and waters. 

Mass spectrometers with electromagnets can scan a wide mass range and hence, with suitable inlet systems, can analyse for a number of gases. They are thus more versatile than modified leak detectors, but are more costly and unsuited for field use. 

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The concentrations of He and other gaseous components of soil and overburden gases and waters varies over periods of hours, days or months. This variation has importance in interpreting survey data and, by recognising significance in the variations themselves, in earthquake prediction. 

Soil and overburden gases are the most accessible of possible sample media for He surveys and their use has been widely tested. Because of the high diffusibility of He, however, sampling of the atmosphere itself has rarely been attempted. 

Experimental investigations have determined the dififusivity of different gases in various porous soil and overburden materials. The periods calculated for gases to travel a particular distance vary considerably (Table l-II). Mercury vapour diffuses in 15 days through 10 m of sand whereas 5.7 years are required for Kr to pass through 10 m of fine-grained playa sediments. 

Even if such false anomalies are present, the technique would be of value if it were certain that U mineralisation did yield He anomalies in overburden gases. Since this is not so, however carefully samples are collected and analysed, the technique must be considered to have very little application. This conclusion is supported by the mathematical modelling by Novikov and Kapkov (1965) and Jeter (1980), described above, which suggest that excess He contents in soil and overburden gas due to underlying, concealed U mineralisation would be far too low to be detected reliably. 

Subsurface formations can be divided into the overburden (unconsolidated) and bedrock according to its solidarity. The upper subsurface can be further divided into the unsaturated zone and the saturated zone depending on pore structure and moisture saturation. The saturated zone is the zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is at the top of a saturated zone in an unconfined aquifer. The unsaturated zone is the zone between the land surface and the water table, and is also called the zone of aeration or the vadose zone. The pore spaces contain water at less than atmospheric pressure, air, and other gases. This zone is unsaturated except during periods of heavy infiltration. 

A further complication is that the ideal dispersion hemisphere of a gas is prone to distortion. The source may not liberate gas uniformly over time, producing fluctuations in m. The rock and overburden column above the source may comprise lithologies of variable porosity, which may be cut by faults and fractures, and these various voids may be (partially) occupied by liquids, thus producing several different values of p in the column. The voids themselves may be occupied at different times by liquid (usually water) or by gas (usually soil air) of variable barometric pressure, with the result that the capacity of the voids to disperse gases from depth changes with time. 

The immediate source of acid-released H2S in soil is sulphide anions, for example,, HS, Me(HS)n , and/or mobile sulphur compounds, such as HjS, COS, CSj. At depth, circulating groundwater provides a medium through which sulphide anions are readily transported in aqueous solution. The results of in-vitro experiments indicate that, in the open pore spaces of overburden and soil, it is unlikely that H2S exists in the gas phase. This and the other sulphur gases are discussed further in Chapter 8. 

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