Page 176 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 15 3

l
NmW 2,3" RAIN. 5 ~ RAIN. W 4.8" RAIN
/
RAIN
1,4" RAIN.
2 ~ RAIN
N
MI
N
|
N
,//<.
J
N
N

,. _, ..-..-N" " ',e
\
~ m m
g I~o 1 1'112 I 13 1 IAr 1 Is I I~ 1 1T ~8 Ig12012rl 2zi 2312412s12~12r 128
Fig. 5-10. Changes in flux of propane concentration (dashed lines) with barometric pressure (solid
line) and rainfall (shaded bars) at two surface sample sites over an underground propane-storage
reservoir; horizontal scale shows days of month.

escape occurs despite the extensive microbiological activity that has developed over this
cavem.
As shown in Fig. 5-11, a propane profile collected over the top of the cavem requires
a log scale to illustrate the enormous range in gas leakage flux. An interesting secondary
observation taken from this example is the obvious colour changes noted on the soil
cores. These chemical changes are related to hydrocarbon seepage and might be used as
an additional exploration tool to provide evidence of where the gas leakage has occurred
around any type of storage cavern. The soil changes from red-brown to green-black
directly over the top of the cavern, where the largest seepage anomalies occur.
Thus the main difficulty with atmospheric sampling is created by meteorological
changes which can greatly displace and dilute the seepage emissions. In addition, it is
clear that the stress fields in the Earth can also influence this gas flux significantly.

Earthquakes

The fact that earthquakes may sometimes be preceded by geochemical anomalies was
discovered at about the same time in Japan (Okabe, 1956) and the then USSR (Fursov,
1968). Earthquake prediction studies in Russia, Japan and China include extensive
geochemical measurements. Chinese geochemical data are reported to have contributed,
at least partly, to the successful prediction of several strong earthquakes (Allen et al.,
1975). In contrast, the Earthquake Hazards Reduction Program in the United States
emphasises mainly geophysical data.