Page 28 - Geochemical Remote Sensing of The Sub-Surface

Genesis, behaviour and detection of gases in the crust

constituents of the present atmosphere, 99.96% of its overall composition is made up of only
three- N2, O2 and Ar.
The atmosphere is still experiencing continual change in its bulk composition. For the
most part these bulk changes are on such a small scale, or are so slow, that they are of no
consequence in exploration. Localised changes to the flux of gases in the near-surface crust
are more pronounced and of much more significance to the application of gas geochemistry
to exploration. Continuing out-gassing of the mantle brings gases to the surface through
conduits such as deep faults and volcanic vents. These gases include HE, He, At, N2, O2,
CH4, CO, CO2, SO2, H2S, S:, COS, HF, HC1, CHaC1, CHaBr and CHaI.

TABLE 1-I

Composition of the atmosphere

Gas Formula ppm by volume ppm by weight
Nitrogen N2 780840 755100
Oxygen 02 209460 231500
Argon Ar 9340 12800
Carbon dioxide CO2 330 460
Neon Ne 18.18 12.5
Helium He 5.22 0.72
Methane CH4 2.0 0.74
Krypton Kr 1.14 2.9
Hydrogen H2 0.5 0.035
Nitrous oxide N20 0.5 0.8
Sulphur dioxide SO2 0-1 -
Xenon Xe 0.087 0.36
Ozone 03 0.05 -
Nitrogen dioxide NO2 0-0.02 -
Ammonia NH3 0-trace -
Carbon monoxide CO 0-trace -
Iodine I2 0-trace -

Although the bulk composition of the atmosphere may be experiencing barely
perceptible changes, some of its physical characteristics, such as temperature, pressure
and turbulence, fluctuate locally with periods varying from annual to diurnal or even
shorter. At ground level these properties greatly influence the degree and rate of change
of atmospheric aeration of the pore voids in the uppermost layers of the lithosphere.