Page 34 - Geochemical Remote Sensing of The Sub-Surface

Genesis, behaviour and detection of gases in the crust 11

to development of gas dispersion patterns. At the other extreme, close to the ground
surface and along conduits in the upper crust, mass flow plays a prominent role.

f
Mass low
Whereas diffusion of gas at depth is widespread, mass flow is often localised (near
the ground surface, in faults) or intermittent (volcanic eruptions). Mass flow is an
important consideration in the application of gas geochemistry to mineral because of its
significant role in the interchange of atmospheric air and soil air, and therefore its
influence on gas composition in the shallow subsurface from where most samples and
measurements are taken. Lovell (1979) reviewed soil aeration in this context.
Mass flow through a porous medium is influenced by the porosity of the medium in
much the same way as diffusion is influenced by porosity. Thus, mass flow proceeds
faster in a high-porosity sand than in a low-porosity clay. In addition, many of the
physical properties of atmospheric air influence the aeration of soil and porous
overburden by mass flow. Baver et al. (1972) estimate their contributions (Table 1-III).
In regions that experience regular precipitation, rain draining downward through the
soil induces most of the mass flow of gases in soils. In extreme cases water may dislodge
soil air from the soil pores themselves, but typically water displaces soil air from the
inter-crumb matrix into neighbouring macropores (Currie, 1960). Seasonal rises in the
water table might displace soil air upward. Variations in saturation of the soil also affect
the extent to which gases are dissolved in soil moisture.

TABLE 1-III

Mechanisms of soil aeration

Source of soil aeration Percentage of total air exchange
Rainfall flushing of soil air 6.25-8.3
Barometric pressure variations 1
Wind action 0.1
Temperature gradient, soil-atmosphere 0.2-0.4
Diurnal temperature variation in soil 0.13
Total mass flow 7.68-9.93
Balance (mainly diffusion) 90.07-92.32
Total 100

Aeration of the soil due to absolute pressure changes also leads to mass flow of gases
in soil. Continuous meteorological pressure variations in the atmospheric air above the
soil are the principal driving force. This barometric pumping causes atmospheric air to