Page 268 - Geochemical Remote Sensing of The Sub-Surface

Aerospace detection of hydrocarbon-induced alteration 241

perhaps in areas of profuse vertical microseepage. Spectral reflectance intensity was
found to have no significant correlation with the presence or absence of surface or
subsurface hydrocarbons.
In the Stoney Point oil field, Michigan, Fluorescence Line Imager (FLI) data,
covering the spectral range of 0.430 to 0.805 ~tm in 288 channels, were collected in both
spatial mode and spectral mode. With the advantage of the high spatial and spectral
resolutions of the two modes, three features of vegetation were detected: (1) a decrease
in the height of the infrared shoulder (Rs) due to structural damage (Rock, 1988); (2) an
increase in the reflectance at the chlorophyll absorption maximum (Ro) due to decreased
leaf chlorophyll (Singhroy et al., 1986); and (3) a shift in the position of the red edge
(~,p) towards shorter wavelengths (Fig. 7-3). A stress image with low digital numbers
representing anomalous areas was created by multiplying Rs by kp. Reid (1988) reported
encouraging relationships between C2-C4 hydrocarbons in soil and anomalies in the
stress image.
At the Mist gas field in Oregon, high-resolution reflectance spectra data of Douglas
Fir trees (obtained with a spectroradiometer built by Geophysical Environmental
Research Inc.) revealed spectral differences seemingly related to chlorosis. Douglas Firs
off-field have higher reflectivity in the 0.750 to 0.125 ~tm range and a stronger
absorption feature at 1200 ~tm than those on-field. Some Douglas Firs on-field exhibit
higher reflectivity in the 0.550 to 0.650 pm region and a shift towards shorter
wavelengths of the sharp rise in reflectivity at 0.700 ~tm. Locally, however, chlorosis is
poorly correlated with commercial gas production. Microscopic examination of needles
from Douglas Firs on-field revealed the presence of stress effects at the cellular level.
Trees characterised by both chlorosis and cell damage may be indicative of more severe
stress conditions. Crawford (1986) suggested that the poor health of Douglas Firs on-
field may be caused by geochemical changes in soil and groundwater of their root zone
induced by methane above the gas reservoirs.
Natural gas seeps occur in the central portion of the Sao Francisco Basin of central
Brazil and, in areas of soil gas anomalies, eucalyptus trees show clear signs of nutritional
deficiency. Oliveria and Crosta (1996) simulated Landsat TM data for bands 1-4 by
flying a non-imaging airborne data acquisition system (SADA) at a height of 150 metres
over healthy and nutritionally-deficient eucalyptus stands. The resulting data were
analysed alongside the corresponding Landsat TM data. Eucalyptus in areas of
hydrocarbon microseepage exhibits higher reflectance in TM band 3 (chlorophyll
absorption) compared to eucalyptus outside the anomalous areas. This is attributed to a
reduction in chlorophyll pigmentation due to hydrocarbon-related stress. The presence of
hydrocarbons seems also to produce lower reflectance in TM band 4, probably due to
individual trees imposing differential conditions on the internal structure of the canopy.
There may also be an effect on the spectral contribution from underlying soil energy; in
areas of microseepage, this effect is more pronounced due to the lower vegetation
density.