Page 37 - Geochemical Remote Sensing of The Sub-Surface

14 M. Hale

Samples obtained through probes reflect the soil air composition at a particular time and
the composition of soil air is prone to fluctuation. Other soil air sampling methods take
advantage of a time-integrated measurement of the soil air flux by leaving a simple
collection device at the sample site for a period of days or weeks. Inverted cups placed just
under the surface have proved the most popular design. An adsorber (e.g., activated
charcoal) or detector (e.g., film that is scarred by particles emitted through radiodecay) fixed
in the uptumed base of the cup effectively collects or records the amount of one or more
gases that find their way into the cup from the underlying soil air. After the cup is recovered
from the sample site, quantitative measurement is carried out in a laboratory.
Active surfaces on soil particles are able to adsorb some of the gases with which they
come into contact. These surfaces are normally in equilibrium with the contents of the pores
that surround them and their adsorbed gas concentrations are therefore representative of the
gas concentration in the pores. Soil samples are a particularly convenient medium for
collection and transport, but they must be treated with care to avoid losses or additions of
gases during transport and storage.
After transport to a laboratory, gases are introduced into an analytical instrument for
quantitative determination of the constituents of interest. Soil air in a container is introduced
directly to the instrument, whilst adsorbed gas is released by thermal of chemical desorption.
The instrumental methods most widely used for gas analyses include gas chromatography,
mass spectrometry and atomic absorption spectrophotometry. For quantifying the radiation
scars on film, image analysis methods are employed.
Gas concentration measurements are most usually reported as a volume ratio, that is, the
volume of the measured gas as a fraction (typically ppm) of the volume of the gas mixture
on which the measurement was made. Since, by virtue of the ideal gas law, equal volumes
of any gas at constant temperature and pressure contain equal numbers of molecules, the
volume ratio is also the molecular ratio. If necessary, the weight of gas can be obtained from
the relation that one mole occupies 22.4 litres at 0~ and 1 atm. When gas concentration
measurements are made by soil desorption, they are more conveniently reported as a weight
ratio. Radioactive gases are usually quantified in terms of "counts" of radio-decay events,
and more rarely in terms of the curie, which is the amount of the radioactive element that
produces 3.7 x 10 ~~ disintegrations per second.

CONCLUSIONS

Gases commonly occupy the pore voids m rocks, overburden and soil. Elements existing
as (components of) gaseous molecules possess, in principle, a high degree of geochemical
mobility compared to elements in solids and liquids. However, the ways in which gases
experience dispersion in the subsurface natural environment are more diverse and less well
characterised than mechanisms of dispersion in the solid and liquid phases. Nevertheless,
the application of gases in the search for deeply-buried resources is attractive.