Page 185 - Geochemical Remote Sensing of The Sub-Surface

162 V.T. Jones, M.D. Matthews and D.M. Richers

proximity to the atmosphere and the soil/air interface, where the boundary conditions
change.
Numerous sample collection methods have been devised for extracting near-surface
soil-gas samples. Any suitable mechanical device having a small internal volume can be
used to collect the sample. Because the probe sampling port must be forced into the soil,
some soil grains are shattered by the necessary mechanical force; many laboratory
studies have shown that gas is almost always liberated by this process (Collins, 1983). If
the probe volume is very small relative to the dimensions of the sample hole, then the
magnitude of the collected sample will be dominated by the gas liberated by crushing. In
such cases the volume of available gas will rapidly deplete as the soil gas is aspirated
from the hole. This effect can be reduced by collecting a larger volume of soil gas,
thereby incorporating a large portion of the natural free soil gas into the sample
measured, as compared to that gas liberated by forcing the probe into the ground.
One method of collecting gases with a shallow-probe system that has proven to be
simple and relatively reliable was developed by Burtell (1988). This probe system
consists of separate devices for sampling and for creating the probe hole. The device
used to make the hole is a pounder bar 1.2 metre (4 feet) long and 1.3 centimetre (1/2
inch) in diameter, with a sliding hammer that is used to pound the bar into and out of the
ground. The soil gas probe consists of a short hollow tube, tightly enclosed by a
concentric sealing tube of the same diameter as the pounder bar, which is inserted into
the ground through the hole made by the pounder. A hand pump or syringe is used to
evacuate the residual atmospheric gases from the hollow probe before the soil-gas
sample is collected. The soil-gas sample is collected in a 125 ml glass serum bottle with
an aluminium crimp top securing a butyl-rubber stopper. The sample bottle is evacuated
just before the sample is collected in order to reduce the possibility of contamination and
to eliminate atmospheric dilution effects. A sample of the soil gas is drawn into the
evacuated bottle. Additional soil gas is then pumped under pressure into the sample
container.
Probe sampling using this or any similar portable design can be used in a variety of
geologic terrains within the limits of surface geologic features. Since an effective soil
gas survey measures gas concentrations which have migrated into the soils, it is
important that sample locations be placed in areas with at least one metre of residual soil.
Alluvial and glacial deposits can also be sampled in most areas, provided there is not
active, high-volume sediment deposition (which would require a deeper sampling
method). Water-saturated soils and mud should be avoided because the wet sediments
clog the sampler and if the open pore spaces normally present in the soil are reduced by
water, then the amounts of free soil gas are much lower than in non-saturated soils.
Shallow probe techniques are prone to near-surface lithologic, meteorological and
barometric effects. This means that one must be careful in interpreting background
values since the absence of an anomaly in a prospective or producing area may be
related to lithology, rainfall, meltwater or barometric pumping. Areas containing