Page 31 - Geochemical Remote Sensing of The Sub-Surface

8 M. Hale

the atomic state by sulphide oxidation, this Hg has a very high vapour pressure. In addition,
Hg readily lends itself to analytical detection at extremely low concentrations, and so it has
been widely used as a gaseous pathf'mder in mineral exploration (Chapter 12). Unique to Hg
is the possibility of discriminating between anomalies derived from mineralisation and those
of anthropogenic origin (Chapter 13).
Finally, the very process of sulphide oxidation at depth can provide geochemical signals
at surface. Sulphide oxidation consumes O2 that is ultimately drawn from the aerated rock
and soil voids in the immediate vicinity. The chemical reactions of oxidation create a low
pH environment in which any carbonate minerals break down with the liberation of CO2,
some of which finds its way into the neighbouring rock and soil voids. Thus anomalous
concentrations of O2 and CO2 in the near-surface soil air provide an indication of oxidising
mineralisation at depth (Chapter 14).

MECHANISMS OF GAS MIGRATION

The way in which gases migrate after their generation has a bearing on the detection of
indicator and pathfinder gases and the rate at which they experience dilution in background
gas mixtures such as the atmosphere. For migration in the gas phase, diffusion and mass
flow are both well-established mechanisms and each has a role to play in gas transport in the
crust. Their relative contributions to gas migration at different positions in the crust is less
certain. A more controversial mechanism, applicable to gas transport through groundwater,
is gas streaming. Finally, most gases are to some extent soluble in groundwater and may
experience dispersion in solution.

Diffusion

Diffusion is the most fundamental mechanism of gas migration in that it requires only a
partial pressure (concentration) gradient. The rate of diffusion of a gas is then determined by
the medium in which diffusion takes place, its temperature and absolute pressure, and the
diffusion coefficient of the gas. The diffusion coeffiecient is a function of molecular
weight, the shape of molecules, and their intermolecular attraction. Every gas thus has a
different diffusion coefficient. The influence of the medium in which gas diffusion occurs is
related to the density of the medium: gases diffuse less quickly through a solid than through
another gas. The rate at which a gas diffuses in a specified medium is sometimes termed its
diffusivity.
In rocks and soils, the only appreciable diffusion of gases occurs in the voids or pores,
which may be occupied by air, water or a mixture of both. Migration over any appreciable
distance is possible only if the soil pores are continuous with each other. Collisions of the
gaseous molecules with liquids or solids impede their progress, so that diffusion in a porous
medium is slower than in a free space. The important factors are the shape, size, tortuosity