Page 47 - Geochemical Remote Sensing of The Sub-Surface

24 O.F. Putikov and B. Wen

Kb, equivalent to conditions at depths of 60-90 km, remain stable and prevail as gases
(Wang, 1994).
In regions in which there are considerable concentrations of gases at depth, those that
are poorly soluble escape preferentially from the water in the form of free bubbles and
migrate upward in the water-saturated porous system. On the whole, rocks of low
porosity are able to exude free gaseous bubbles of a corresponding diameter. As stated
by Fridman (1970), in the case of rocks of low effective porosity and insignificant
sorption capacity (thus, especially igneous and metamorphic rocks but not organic-rich
sediments), natural gases are mainly in the free state in fractures and faults, and are even
dissolved in underground water.
A wide distribution of a free gaseous phase in rocks is supported by numerical
estimates. For example, the background concentration of H2 in underground water is 0.1-
0.4ml/1 and anomalous concentrations are 3-50 ml/1, whilst concentrations of up to 50-
1500 ml/1 and higher are found with hydrogen flow of 105 m3/day (Scherbakov and
Kozlova, 1986). In an underground mine tunnel at a depth of 252 m a flow of gas
bubbles containing 76% CO 2 or 90% CO2 + Hz by volume was observed from 1961 to
1975. It was estimated that average flux of methane was 60-80 cm3/m 2 in one year and
that the source of the methane was at a depth of 15-20 km (Hitarov et al., 1979).
Regional sources of hydrogen may be situated at great depth, even in the mantle (Larin,
1980; Scherbakov and Kozlova, 1986), and some metallic elements, such as Mn, Fe, Ni,
Co, Cr and rare earths, are thought to be carried with the gas and form sulphide minerals
near the surface, for example in the vicinity of mid-oceanic ridges (Goriainov et al.,
1989).
In the laboratory of geoelectrochemical methods of St. Petersburg State Mining
Institute, a series of experimental studies has been carried out on the physico-chemical
mechanism of penetration of gaseous bubbles through the porous system (Putikov and
Wen, 1997; Wen, 1997a). In the experiments the porous system consists of a long wide
robe containing small particles of silicates or gravel and water with different
concentrations of metals and organic substances. The particles of silicates have different
fractions with diameters 1-2, 2-3 and 3-5 mm, and the particles of gravel have diameters
of 5-7 and 7-10 mm. Groups of gaseous bubbles of diameters from 5 • 10 -5 m to 2 • 10 .4
m were introduced into the bottom of the tube, and the average speeds of the leading and
rear fronts of every group of bubbles penetrating the porous system were determined.
Three forces act on a gaseous bubble in free liquid (without a solid phase):
gravitational force (G - mg - VOog); Archimedes force (F = Vog) and the resistant force
of the medium defined by Stoke's law (R = 6rtqr0v0), where, g = acceleration due to
gravity, r0 = radius of bubble, V = volume of bubble, 190 = density of gases in bubble, 9 =
density of liquid, rl = dynamic viscosity of liquid, v0 - speed of bubble at equilibrium of
the three forces.
The speed of the bubble can be calculated by the following equation: