Page 249 - Geochemical Remote Sensing of The Sub-Surface

222 T. Ruan and Q. Fei

of Palaeozoic age, transported by a modern river from an area of outcrop tens of
kilometres to the west. Similarly, Price (1986) cites an example from the oil fields of
western Alberta, where high concentrations of gases released by acid treatment from
soils were attributed to carbonate fragments glacially transported from the Canadian
Rocky Mountains to the west, and not to the underlying oil fields.
Finally, gases are incorporated within the lattices of clay minerals, from which they
can be extracted only by destroying the mineral structure. This mode of gas occurrence
had attracted little interest because the lattice positions are likely to be syngenetic rather
than indicative of underlying sources.

PRACTICAL METHODS

For hydrocarbon gases, the concentration measurement tool is exclusively gas
chromatography with a packed column and an FID detector, capable of precisely
determining 10 .`7 levels of C~-C5 in less than 5 minutes. Heavier hydrocarbons are
sometimes determined using a quadrupole mass spectrometer. As these instruments and
the techniques for loading gas samples onto them are described elsewhere (e.g., Chapter
5), only field methods and, where applicable, sample pre-treatment methods for releasing
gases from soils are discussed here.

Soil air

Interstitial gases in soil air are first extracted by driving a probe into the soil and
sucking the soil air into a measurement device or a pre-concentration medium. A variety
of procedures have been reported (Devine and Sears, 1977; Jones and Drozd, 1983;
Richers and Jones, 1986). The main operational parameters are the probe depth, isolation
of the soil air from the atmospheric air, the amount of negative pressure applied, the
protection of the probe from blockage and the reduction of the dead volume of the
tubing. These have to be optimised in terms of both cost and effectiveness. Of course it
is not possible to extract interstitial soil air from waterlogged soils.
If the soil air is not pumped directly into an instrument for in situ determination of
gas concentrations, it is passed through an artificial adsorbent in order to pre-concentrate
the gases to be determined later in the laboratory. Several kinds of artificial adsorbents
have been tested and activated charcoal has been found to be the most suitable, although
it is not entirely satisfactory for C~ and C2. The activated charcoal is commonly used in
the form of a thin film coated on a ferromagnetic wire (Klusman et al., 1986) or in the
form of commercially-available fine-grained activated charcoal. Any adsorbent should
be pre-treated in an inert environment (vacuum or N2) at 400~ to release all possible
adsorbed gases before use. After sample absorption in the field, the adsorbed gases are