Page 27 - Geochemical Remote Sensing of The Sub-Surface

4 M. Hale

exploration guide because it is potentially broad in extent and occurs at absolute elevations
above its source.
The gaseous constituents of the Earth are of course not conf'med to gases exuded by
exploration targets. Gases have accumulated near the outer rim of the Earth and tend to
occur in mixtures, the constituents reacting with one another, with solids and liquids with
which they come into contact, and responding to changes in pressure and temperature. The
contribution of mineral deposits and petroleum accumulations to these mixtures is extremely
small. Any attempt to recognise them with the confidence required for exploration
investment prompts careful consideration of the occurrence and behaviour of not only those
gases that might prove suitable for exploration but also the gas mixtures in which they have
to be detected.
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 GEOCHEMICAL BACKGROUND

The atmosphere

The most widespread gas mixture in the Earth is the atmosphere. The atmosphere is
estimated to weigh 5.1 x 10 ~5 tonnes. It comprises 0.016% of the mass of the combined
crust, hydrosphere and atmosphere, and less than 0.0001% of the mass of the whole Earth
(Henderson, 1986). Despite being poorly represented as a proportion of the composition of
the Earth, or even its outer shell, the atmosphere is omnipresent at the surface of the crust,
and partially permeates it, occupying faults, pores and other voids in rocks and overburden.
This is the very zone in which almost all exploration takes place. Consequently the
atmosphere is a major part of the geochemical background against which gases employed in
exploration must be recognised.
The atmosphere is a physically and chemically dynamic system. A mixture of primeval
gases was expelled to the outermost shell of the Earth during its exothermic accretion. Over
geologic time the chemical composition of this mixture has changed, mainly as a result of
photochemical dissociation, interaction with water and its dissolved constituents (e.g., the
oxidation of Fe 2§ to Fe 3+ during the formation of the Proterozoic banded iron formations)
and biogenic processes (especially photosynthesis). The composition that the atmosphere
has now reached is shown in Table 1-I. Although at least 17 gases are regarded as