Page 267 - Geochemical Remote Sensing of The Sub-Surface

240 1-1. Yang, F.D. Van der Meer and J. Zhang

carbonate in soil. Four channels, at the wavelength ranges of 1.550-1.650 ~tm, 1.985-
2.085 ~tm, 2.037-2.137 ~tm and 2.039-2.193 ~tm, were used for anomaly extraction. The
anomaly overlies a subsurface heavy-oil reservoir (Zhu and Wang, 1991).

Vegetation stress

Hydrocarbon microseepage creates a reducing environment in the soil and
overburden at depths shallower than would be expected in the absence of microseepage.
The presence of hydrocarbons stimulates the activity of hydrocarbon-oxidising bacteria,
which decreases oxygen content of the soil whilst increasing its contents of carbon
dioxide and organic acids. These changes affect pH and Eh of soil, which in turn affect
the solubilities of elements that are plant nutrients and consequently their availability to
vegetation (Schumacher, 1996). This may affect the root structure of vegetation and
ultimately influence its vigour and hence its spectral reflectance (Feder, 1985).
Remote sensing of anomalous (or stressed) vegetation takes two forms. One is the
mapping of the distribution of different species of vegetation and the differences in
vigour and morphology within each species (Brooks, 1972; Siegel, 1974). Vegetation
that is typically prolific is often stunted or absent in areas of unusual soil environments.
On the other hand, some species thrive in environments that are toxic to most other
species and are recognised as geobotanical indicators. The second approach is to
determine differences in spectral characteristics between healthy and stressed vegetation.
The spectral signatures of vegetation associated with hydrocarbon microseepage have
been extensively studied. The main targets of attention are the green peak (at 0.56 ~tm),
the red trough (at 0.67 ~tm), the shift of position of red edge and the height of the
infrared shoulder.
In both cases, normal or background variability in the distribution and vigour of
various species presents complications that need to be taken into account. Factors such
as bedrock geology, soil type, slope, soil moisture and climate can have a more
pronounced effect than that due to the presence of hydrocarbons (Rock, 1984; Klusman
et al., 1992). Nevertheless, numerous accounts have been published of the detection of
hydrocarbon-induced vegetation anomalies by remote sensing.
McCoy and Wullstein (1988) analysed leaves of sagebrush and greasewood from the
Blackburn oil field in Nevada and reported a halo anomaly of high Mn:Fe ratios
surrounding the productive part of the field. McCoy et al. (1989) revisited the Blackburn
field and determined that the spectral reflectance of sagebrush from the anomalous area
was lower than that of sagebrush from background areas. Bammel and Birnie (1993)
evaluated reflectance in the visible and near infrared regions (0.45 to 1.1 ~tm) of
sagebrush in five areas in the Bighorn Basin of Wyoming to determine its usefulness in
hydrocarbon exploration. The most effective indicator of hydrocarbon-induced stress in
sagebrush proved to be a consistent blue shift (to shorter wavelengths) of the green peak
and red trough. This shift is only detected in areas where sagebrush is prolific and