Page 35 - Geochemical Remote Sensing of The Sub-Surface

12 M. Hale

flow into the soil in response to a pressure increase and to leave the soil in response to a
fall in pressure. These pressure changes are relatively slow and the effects in the soil
tend to show little detectable time lag. An increase in barometric pressure compresses
downward the soil air originally occupying the pores whilst a decrease in barometric
pressure induces egress of soil air into the atmosphere. Turbulent wind blowing as gusts
across the surface of soil produces slight but numerous changes in pressure and adds to
the pumping effects of longer-amplitude meteorological pressure changes. Wind speed
has been shown to influence the rate of loss of water vapour from soil (Acharya and
Prihar, 1969) and the same is likely to apply to the rate of loss of other gases.
Temperature affects the volume that air occupies and hence its pressure. Diurnal
temperature variations are rapid but confined to the near-surface zone; seasonal
variations are more pervasive.
In the near-surface, where gas geochemistry samples and measurements are acquired,
mass flow is a source of background variations that tend to obscure any signal arriving
from depth. The interplay of the many different causes of variation has proved a serious
impediment to the provision of interpretable gas data in exploration and this has
prompted a number of field investigations (Hinkle, (1990). In comparatively elaborate
studies, Klusman and Webster (1981) and Klusman and Jaacks (1987) monitored many
of the sources of variation along with emissions of Hg, Rn and He. By stepwise multiple
regression they found that air temperature, soil temperature, barometric pressure, relative
humidity and soil moisture exerted most influence on gas concentrations. However,
even if such monitoring could be used for gas data noise reduction, it is not practical to
monitor so many sources of variation as part of an exploration programme. Rather, in
practice, the problems tend to be alleviated by sampling as far as possible below the
ground surface and/or integrating the signal over a considerable period of time.

Gas streaming

The relatively slow gas diffusion rates in rocks of low porosity at depth have brought the
contribution of diffusion to long-distance gas migration into question. The half-life of Rn is
so short that its persistence and detection after transport by diffusion over tens or hundreds
of metres is extremely unlikely.
Kristiansson and Malmqvist (1982) and Malmqvist and Kristiansson (1984, 1985)
hypothesise that, in the zone of saturation, pressure gradients and pressure shocks cause
over-saturation, leading to the formation of gas bubbles. These stream upward at a
comparatively rapid rate until they reach the water table and mix with the soil air. The
resulting mixture is then driven slowly further upward by the pressure gradient caused by
the bubble stream.
Any gas that dissolves in groundwater could, given the appropriate conditions, migrate
by streaming. Groundwater is most likely to be saturated in gases dissolved in meteoric
water, i.e., N2, 02, Ar, CO2. These then are the gases from which bubble streams may form.