Page 247 - Geochemical Remote Sensing of The Sub-Surface

220 T. Ruan and Q. Fei

o o .''. Soil particle

Adsorbed molecule
I Free molecule

Interstitial soil
Occluded gas
Microbubbles
[~ () "" 0 0 , o o'~

Fig. 6-3. Principal modes of occurrence of hydrocarbons in surface microseeps (from Ruan and
Cheng, 1991 ).

Free molecules

Free gas molecules in the interstitial soil air (and atmospheric air) represent the most
mobile and most recently-arrived portion of the gas. Usually, this portion accounts for
less than 0.1% of the total amount of gas in the near-surface environment. Gases in the
interstitial soil air can or should be determined either: (1) on-site, with a portable
instrument or buried sampling device; or (2) in the laboratory after on-site
pre-concentration onto an artificial absorbent. The advantage of this mode of gas
occurrence is that the composition of the gases has remained virtually unchanged since
their upward migration began. The disadvantage is that the anomalies are unstable
therefore frustrating to the investigator (Devine and Sears, 1977). The instability may be
reflected in diurnal, weather or seasonal variations. However, although absolute gas
concentrations may prove irreproducible, the concentration ratios between individual
constituents are repeatable over several years (Jones and Drozd, 1983). The eventual
dispersion of free hydrocarbon gases from the soil air into the atmosphere provides the
basis for airborne gas geochemical surveys, which are capable of covering large areas of
difficult access (Barringer, 1981 ; Sandy, 1988).

Adsorbed molecules

Non-polarised gases such as methane and helium can be adsorbed onto soil particle
surfaces via molecular attraction. This attraction is weak and the gases can be separated
from the soil by gentle heating, pressure reduction, or the combination of these two
procedures. This mode of gas occurrence is less influenced by meteorological conditions
than the free-molecule mode, and so the results are more repeatable. The samples are
easier to handle but the desorption procedures need to be strictly controlled: complete
desorption is not necessary, but artifacts in the data due to variations in desorption