The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
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1991; Dawson and Curran, 1998). Derivative analysis of the 
near contiguous bands of field spectra enable the identification 
of narrow waveband ratios which are tightly coupled to changes 
in photosynthetic function resulting from vegetation stress 
(Zarco-Tejada et al., 2003). First derivatives were computed for 
all spectra, using the finite divided difference approximation 
method (Chapra and Canale, 1988; Tsai and Philpot, 1998). The 
derivative peaks were used to select two vegetation band ratios: 
the vegetation stress ratio of Smith et al. (2004) (exploiting a 
double peak in the red-edge) and a modified version of this 
ratio. 
The vegetation stress ratio of Smith et al. (2004) exploits the 
magnitude of the first derivative at 725 and 702 nm within the 
red-edge, which form the mid-point of the red-edge peak 
maximum and its shoulder. The ratio has enabled identification 
of stress due to short-term sub-surface gas leaks in grass, and 
long-term sub-surface gas leaks in winter wheat and bean crops 
at the canopy scale under controlled field test site conditions. 
The vegetation stress ratio is obtained by Smith et al. (2004): 
125nm 
702«w 
Where 725 nm is the value of the first derivative of the 
reflectance spectrum at 725 nm and 702 nm is the value of the 
first derivative of the reflectance spectrum at 702 nm. The 
modified vegetation stress ratio takes the form: 
723nm 
700nm 
Where 723 nm is the value of the first derivative of the 
reflectance spectrum at 723 nm and 700 nm is the value of the 
first derivative of the reflectance spectrum at 700 nm. 
2.4 Soils acquisition and analysis 
Soil acquisition was conducted in May 2006 to coincide with 
field and airborne hyperspectral acquisitions, and to minimise 
crop disturbance. Sub-soil sampling depth was kept as close as 
possible to 0.1 - 0.2 m. Soil was extracted at random locations 
within each of the disturbance zones producing a composite 
sample for each zone of approximately 500 g (Figure 2). The 
samples were analysed for organic carbon, Potassium, 
Phosphorus and pH, to establish if differences in soil fertility 
were evident between disturbed and undisturbed soil. Indirect 
measurements of soil strength, structure and compaction were 
obtained by penetrometer measurements and bulk density core 
sample extraction in order to validate the penetrometer readings 
(Bradford, 1986). 
3. RESULTS AND DISCUSSION 
Field spectroradiometry data were acquired during four field 
visits in June 2005 and May 2006. A pseudo RGB CASI-2 
image overlaid with a red-edge position (REP) blue shift 
classification (Guyot and Baret, 1988) and pipeline route was 
used to identify locations where potential vegetation stress 
coincided with the 508 mm pipeline for the 2005 campaign 
(Figure 1). Two of the sites of spring barley fields selected as 
test sites in 2005 are labelled 05B and 05C in Figure 1. 
Different cropping regimes prevented the use of the same test 
sites in 2006. New test sites were identified for 2006 based on 
expert field knowledge from local farmers (Pers. Comm. Donal 
Cullen, Bruce Mackie and Gerald Banks). One of the sites 
selected as a test site in 2006, labelled 06A, a winter barley 
field, (Figure 1) provided the most conclusive results and is the 
focus of discussion in this paper. 
Notable differences in reflectance for crop stress transects are 
exhibited for winter barley at test site 06A. An increase in 
reflectance is evident with proximity to the pipeline of up to 
-2% at the green peak between 10 and 60 m West (Figure 3). A 
blue shift in the red-edge of ~5 nm is evident with proximity to 
the pipeline at 25% reflectance. Similar blue shifts in the red- 
edge for stressed vegetation have also been reported by Jago et 
al. (1999) and Lelong et al. (1998). Pronounced steps in 
reflectance occur in the NIR with proximity to the pipeline 
range between -19% at 0 m at peak C and -48% at 70 m West 
at peak B. The steps in the NIR are most likely to be a direct 
result of the proportion of soil background to barley leaf 
coverage, lower NIR reflectance values corresponding with 
increased soil background proportions, particularly at 0 m and 
60 m (Figure 3). The same reflectance responses in the VIS and 
NIR were also observed by Smith et al. (2004) for soil gassed 
grass, bean and winter wheat. 
Exceptions to this trend occur at 10 m and 20 m West, which 
display high reflectance throughout the wavelengths sampled, 
with maximum reflectances of -54% and -48% at peak B 
(Figure 3). The higher reflectance values could be attributed to 
poor atmospheric conditions during acquisition, when cumulus 
cloud intermittently obscured the Sun. Another exception is at 
60 m West, which has comparatively low reflectance of-18% 
at peak C (Figure 3). This is likely due to a combination of 
increased soil background within the FOV of the ASD 
spectroradiometer and crop stress (Figure 3).