Page 33 - Geochemical Remote Sensing of The Sub-Surface

10 M. Hale

A B

[3

F

Fig. !-I. Gas plumes established by diffusion in porous media above a gas source: (A) small
source, homogeneous medium; (B) large source, homogeneous medium, impermeable vertical
boundaries; (C) small source, medium of low porosity overlying medium of high porosity; (D)
large source, medium of low porosity overlying medium of high porosity, impermeable vertical
boudaries; (E) small source, medium of high porosity overlying medium of low porosity; (F) large
source, medium of high porosity overlying medium of low porosity, impermeable vertical
boundaries (from Ruan et al., 1985a).

the source. Where a high-porosity medium (e.g., sand) lies above a less porous medium
(e.g., clay) the gas flux reaching the boundary is weak and, once dispersed in the more
porous upper medium, may barely be detectable (Fig. 1-1E). Even a large source
emitting gas into porous media bounded by impermeable media yields only a broad
weak dispersion halo near the surface (Fig. I-IF). These numerical modelling results are
supported by Hg data from in vitro experiments by Ruan et al. (1985a) and by a range of
field observations. For example, Ball et al. (1983b) note that anomalous concentrations
of 02, CO2 and Rn in soil air over a fault-hosted sulphide mineral deposit in England
tend to occur over the steep vertical walls to the mineralisation and in juxtaposition to
the boundary faults.
Diffusion represents an important mechanism for gas migration in the porous
uppermost crust and seemingly produces interpretable near-surface dispersion pattems.
At depth in rocks of much lower porosity, however, diffusion rates are likely to be
exceedingly slow, bringing into question the significance of the contribution of diffusion