Page 32 - Geochemical Remote Sensing of The Sub-Surface

Genesis, behaviour and detection of gases in the crust 9

and unevenness of the pores, the shape, orientation and size distribution of the solid phase
and the degree of water-saturation of the system.
Experimental investigations have determined the diffusivity of different gases in various
porous soil and overburden materials. The periods calculated for gases to travel a particular
distance vary considerably (Table 1-II). Mercury vapour diffuses in 15 days through 10 m of
sand whereas 5.7 years are required for Kr to pass through 10 m of fine-grained playa
sediments.

TABLE 1-II

Diffusion rates for gases through porous overburden

Gas Overburden Apparent diffusion coefficient (cm 2 S -1) Transit time, 10m
Kr Playa sediment 0.00387 (Robertson, 1969) 5.7 years
Rn Desert soil 0.036 (Tanner, 1964a) 225 days
Hg Clay 0.05 (Ruan et ai., 1985a) 162 days
Hg Sand 0.56 (Ruan et al., 1985a) 15 days

In an ideal case a source at depth liberating a gas into a homogeneous porous
medium that, at some distance from the source, is open to the atmosphere establishes by
diffusion a hemispherical halo in the porous medium. The time taken to establish the
halo varies up to many years, depending upon the thickness of the medium and the
diffusivity of the gas in it. Once established, however, the halo is persistent provided the
supply of gas from the source is maintained. Gas concentration in the hemispherical halo
falls with increasing distance from the source such that, near to its intersection with the
ground surface, the halo presents as a broad symmetrical zone with peak concentrations
directly above the source. To compare this ideal case with more complex settings, Ruan
et al. (1985b) employed numerical modelling techniques based on the alternating
direction method for the solution of finite difference equations. Their results allow
comparison of the shapes of halos of the same gas diffusing through media of different
homogeneity from sources of different sizes (Fig. 1-1). The ideal hemisphere (Fig. 1-
1A) is perturbed whenever the diffusing gas comes into contact with a medium of
different porosity. The vertical boundaries of the model imply contact with rocks of zero
permeability bounding a porous medium in which gas is diffusing. When the gas flux is
sufficiently strong for gas to reach these contacts, gas is then channelled upward (Fig. 1-
1B). Horizontal boundaries in the model separate media of different porosity. Where a
low-porosity medium (e.g., clay) lies above a more porous medium (e.g., sand) gas
diffuses from a source in bedrock with relative ease until it reaches the inter-layer
boundary, along which it can more easily migrate laterally than vertically (Fig. 1-1C).
Preferential lateral gas migration might proceed as far as an impermeable medium, at the
boundary of which the gas largely retained in the more porous medium at depth is
diverted upward (Fig. 1-1D). In this case, gas does not reach the surface directly above