Page 36 - Geochemical Remote Sensing of The Sub-Surface

Genesis, behaviour and detection of gases in the crust 13
By their very provenance, these are not gases indicative of mineral deposits or petroleum
accumulations. Rather a bubbles acts as a carrier for atoms of other elements which attach
to the surface of the bubble. The atoms that attach to bubble surfaces include not only
indicator and pathfinder gases such as Rn but also non-gaseous species such as metals.
Streams of gas bubbles therefore have the capacity to deliver to the near surface minute
geochemical samples from considerable depth.

INDICATOR AND PATHFINDER GAS DATA ACQUISITION

By virtue of their physical state, dispersion halos of indicator and pathf'mder gases are
difficult to measure compared with dispersion pattems in solids and liquids. The halos are
formed by gases migrating upwards from depth, and these usually need be intercepted
before they experience catastrophic dilution in the open atmosphere. The near-surface soil
suggests itself as being the most accessible medium in which to detect the dispersion halo,
although its atmospheric aeration and biogenic activity create undesirable levels of
background noise.
Procedures that have been devised for making measurements of gas dispersion halos
may be initially divided according to the measurement substrate, for example, atmospheric
air, free soil air or adsorbed gas. The period over which the sample is accumulated is an
additional important consideration, because it has a bearing on the representativity of the
measurement.
The atmospheric air immediately above the ground surface is clearly a convenient
medium in which to obtain measurements of gases emanating from depth, but the likelihood
of catastrophic dilution of the signal is very large. Limited success was achieved with a
vehicle-mounted Hg detector which collected large atmospheric air samples whilst on the
move. More success has been achieved by taking advantage of the exceptional olfactory
sense of dogs. Their use in prospecting, however, has been confined to detecting concealed
sulphide-bearing boulders in glacial dispersion trains (Kahma, 1965; Nilsson, 1971; Brock,
1972), and their capabilities do not seem to be readily translated into an instrumental
technique.
Measurements made on free soil air are obtained on samples extracted through probes.
A probe can be driven manually to a depth of 1-2 m below the surface and soil air extracted
through it with a hand pump. If entrainment of atmospheric air is suspected, holes can be
drilled mechanically to greater depths and sealed well below the surface; soil air is drawn
out after the hole has equilibrated with the surrounding soil air. The resulting soil air sample
may be passed directly to a portable analytical instrument or may be trapped for analysis
later. On-site measurement systems range from back-pack insmmaents (e.g., for Rn, O2,
CO2) to a vehicle mounted mass spectrometer (McCarthy and Bigelow, 1990). The
fieldwork requirement can be reduced if measurement of gas concentration is performed at a
field or central laboratory. This can be achieved by transporting samples of soil air in
gas-tight containers, or by selectively depositing the gas of interest onto a convenient
substrate (e.g., Hg vapour onto Au film).