Page 29 - Geochemical Remote Sensing of The Sub-Surface

6 M. Hale

The soil air

Some rocks have a natural porosity and, at the interface of the atmosphere and the
lithosphere and given the presence of moisture, all rocks tend to weather to a relatively
porous soil. In the simplest case, the pore spaces of soil are occupied by atmospheric air,
but the very moisture that enhances soil formation is also highly supportive of flora and
fauna, which interact with the air in the pores and modify its composition.
Perhaps the most obvious way in which the composition of soil air differs from that
of atmospheric air is through plant respiration, which reduces the 02 content of the soil
air and raises the CO2 content. Gases almost absent from the atmosphere are added to the
soil air by biogenic activity. According to Enhalt (1974), 80% of CH4 in soil air is of
recent biologic origin. Most H2S is biogenic (Schlegel, 1974), resulting from the
bacterial reduction of sulphate under anaerobic conditions. Both CH4 and H2S are only
meta-stable in the soil air, but biogenic activity generates them on a more-or-less
continuous basis, so at any time they may be present in significant concentrations.
Gases migrating from depth are also constituents of the soil air, their supply to the soil
air varying according to proximity to sources and conduits. Sources of these gases include,
but are not exclusively, mineral deposits and petroleum accumulations.
The resulting soil air, unlike atmospheric air, has no fixed or stable composition. It
is, however, generally regarded as carrying the most distinct expression of gases escaping
from mineral deposits and petroleum accumulations; once such gases escape to the free
atmosphere they experience extremely rapid dilution. Thus the soil air is an important
sampling medium for gases used in exploration, but the diversity of its source gases and its
variable physical properties induced by changes in atmospheric aeration make it a difficult
medium in which to obtain reproducible measurements. Nonetheless, it is in this variable
background that most samples and measurements for gas geochemical remote sensing of
the subsurface are acquired.

INDICATOR AND PATHFINDER GASES FOR EXPLORATION

There are no gases uniquely associated with mineral deposits or petroleum
accumulations. Even those that are perhaps the most obvious indicator gases (sulphur gases
and volatile hydrocarbons) can sometimes be generated by biogenic reactions in the soil. A
few gases, notably Hg and Rn, have the advantage of being naturally concentrated in or
associated with ore minerals whilst playing only a minor role in biogenic activity and
occurring in only trace amounts in the atmosphere. At the other extreme, widespread gases
such as CH4, CO2, He and 02 have exploration value, but their anomalous concentrations
have to be recognised against their relatively high background partial pressures in the
atmosphere.