Page 181 - Geochemical Remote Sensing of The Sub-Surface

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

CH4xl 0 -4 %

3 f .................... .........
0 ', '!
8L 6V 8V 10V 12V I'~V 16V 18V 20V 22V 2,~V 26V 28V Date
m t
,, ,,h, ,,I
' l" ' ,l'l

-5o 1- t "
Fig. 5-16. Variations in methane concentration in air with crustal displacement in a seismically-
active region (from Antropov et al., 1981).

Remote monitoring of the gas composition of the atmosphere with laser sources has
been actively pursued for over a decade, with systems actually built and used for
nitrogen dioxide, sulphur dioxide, ozone, carbon dioxide, ethylene, ammonia, hydrazine,
hydrogen fluoride and methane. A small mobile laser system capable of measuring
methane and ethane in the atmosphere has been developed (at Stanford Research
Institute for the Gas Research Institute) for detection of natural gas pipeline leaks (Van
de Laan et al., 1985). Another laser technique, based on established physical principles,
is LIDAR, which stands for light detection and ranging. The technique uses light from a
tuneable infrared monitored. The development of an airborne or truck-mounted system
CO2 laser to selectively detect methane and heavier gases by adsorption. The technology
was reviewed by Grant and Menzies (1983). Briefly, laser light is pulsed into the
atmosphere and aerosols, liquid droplets and gaseous molecules scatter or adsorb the
light in different ways. Some portion of the scattered light returns to its point of origin,
where a telescope-like receiver channels it to a photodetector, which produces an
electrical signal proportional to the optical radiation received by the telescope. The
length of time between transmission and reception indicates from what distance the light
was scattered and the intensity of the electrical signal indicates the concentration of the
particles or molecules being capable of range resolving the location and concentrations
of an atmospheric gas cloud will provide an extremely efficient and cost-effective
exploration tool for detecting both macroseeps and microseeps in frontier regions.
The third atmospheric technique analyses the residual liquid and/or condensate
hydrocarbon traces on aerosols carried into the atmosphere by thermals (Barringer,
1981). The aerosols are created by gas bubbles which exsolve into the atmosphere from
the sea in areas where microseeps create gas bubbles which reach the sea surface. The
aerosols are concentrated from large volumes of air and collected by an airborne cyclone
sampler carried aboard an aircraft which is flown at 30 m (100 feet) above the sea
surface. Hydrocarbons adsorbed on the aerosols are measured by a flame ionisation
detector which yields a total hydrocarbon signal. This system is claimed to produce