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The influence of weathering and lichen cover on the reflectance spectra of granitic rocks
REMOTE SENS. ENVIRON. 50:194-199 (1994)
Short Communication
The Influence of Weathering and Lichen Cover on the Reflectance Spectra of Granitic Rocks E. M. RoUin,* E. J. Milton,* and P. Roche t R e s u l t s are presented from a series of laboratory experiments that examined the effect of surface weathering and lichen cover on the spectral reflectance of granitic rocks over visible and infrared wavelengths. The effect of weathering on the rock reflectance varied with rock type. However, all the lichen-affected spectra, which included four different species, exhibited the same five diagnostic absorption features in the high resolution spectra. More general changes in curve characteristics were also evident in the lichen-affected spectra. The diagnostic absorption features were evident in spectra from samples with less than 50% lichen cover. Certain mineralogic features remained evident in the spectra despite the surface lichen cover.
INTRODUCTION Lithological mapping using remote sensing depends, in part, on the identification of rock types by their spectral characteristics. The potential for rock type and mineral identification benefits from some understanding of the way in which surface processes modify those spectral characteristics. Two important factors which merit consideration are surficial weathering effects and presence of organic growth, such as lichens on the rock surface. From a different perspective, detection of lichens by remote sensing offers a potentially useful tool for many *NERC-EPFS, Department of Geography, University of Southampton, United Kingdom *Department of Physics, University of Southampton, United Kingdom Address correspondence to E. M. Rollin, Natural Environment Research Council Equipment Pool for Field Spectroscopy, Department of Geography, Univ. of Southampton, Southampton, SO17 1BJ U.K. Received 16 November 1993; revised 25 June 1994.
1.94
ecological purposes and for such activities as identifying age dependent changes in the surface, for example, in order to date glacial retreat stages (e.g., Knight et al., 1987), or episodes of lava eruption (Rothery and Lefebvre, 1985). Lichen growth commonly occurs on rock outcrops at high latitudes and altitudes. This article reports the findings of a study aimed at identifying changes in the spectral reflectance over visible to shortwave infrared wavelengths (0.4-2.4/tm) of three granitic rocks as a result of weathering and the presence of lichen on the rock surface. Spectral reflectance was measured with a high spectral resolution instrument which enabled spectral differences in the form of relatively narrow spectral features to be examined, as well as more generalized changes in the shape of the reflectance spectrum. The study also aimed to identify any similarities between the spectra of the different lichen species present on the rocks. Weathering processes affect granitic rocks in various ways depending upon the precise mineralogy of the rock, the chemical composition of groundwater, and/ or surface water and the local climatic conditions. From the standpoint of remote sensing, surface manifestations of weathering processes are the most important, and these include discoloration of the surface due to iron compounds, the formation of alteration minerals such as sericite or kaolinite, and changes in the texture and microrelief of the weathered surface, possibly as a result of increased moisture content and attack by physical and chemical agents of erosion. To date, work on the spectral reflectance of lichens has largely been confined to visible and NIR (nearinfrared) wavelengths (0.46-1.1/lm). For example, Petzold and Goward (1988) measured the spectral reflectance in the visible and NIR of subarctic lichens in situ, focusing on differences between lichens and vascular plants. They found that lichens generally showed a 0034-4257 / 94 / $7.00 QElsevier Science Inc., 1994 (i.~.~ A,~an~a of tha Am.eritzn.¢ N~I~ York NY 10010
Reflectance of Granitic Rocks 195
greater reflectance in the visible and more gradual increase at the red edge than most green vegetation samples, and had a less marked absorption at 0.95 gm due to their lower intercellular water content compared with vascular plants. In a study of the energy budgets of lichens, Gauslaa (1984) measured the visible and NIR reflectance of several lichen species in a wet and dry state and was able to group the species according to their spectral reflectance characteristics. Satterwhite et al. (1985) used a laboratory spectroradiometer covering the visible and NIR wavelength region in their study of the effect of lichen cover on the reflectance of granitic rocks in Bands 1-4 of the Landsat Thematic Mapper. In terms of geological applications, the short-wave infrared (SWIR, 1.1-2.4/~m) wavelengths are the most important, since many diagnostic minerals exhibit characteristic spectral features in this region (Hunt, 1979). Some knowledge of the effect of lichen cover on rock reflectance in this spectral region is important for several aspects of spectral data interpretation. First, for geological purposes, the extent to which characteristic rock features may be preserved despite a lichen covering is the main concern, and some estimate of the critical level of lichen coverage, above which the spectral features associated with the rock mineralogy become obscured, would be of value. Second, in order to identify the presence of lichen cover in remotely sensed imagery, some knowledge of the spectral characteristics of lichen is necessary. For this, an estimate of the minimum level of cover required to produce diagnostic spectral features in the reflectance spectrum would also be of value. The results of this work are relevant to both aspects.
METHODS A GER single field-of-view IRIS (SIRIS or SFOV IRIS) spectroradiometer was used to measure the biconical spectral reflectance of various surfaces of three rock samples of granitic rock over the 0.35-2.5 gm range, relative to a barium sulphate (BaSO4) reference panel which was calibrated by the UK National Physical Laboratory in terms of absolute spectral reflectance. The measurements were performed in a blacked-out laboratory with the spectroradiometer head mounted on an optical bench orthogonal to and at a distance of 30 cm from the target and reference. The rock samples were positioned on the optical bench vertically beneath the optical head which was mounted for vertical viewing. Illumination was provided at 45 ° to the samples, by a 1000 W quartz lamp. Of the three granitic rock samples examined, two were from Norway and the third from Scotland. The mineralogy of each rock sample was derived from inspection of thin sections and is summarized in Table 1. All the samples were weathered to some degree and showed areas of lichen growth. For each sample, at least
Table 1. Mineralogy of Samples° 1 Quartz Plagioclase Feldspar Potassium Feldspar Biotite Others
Sample 2 25-30% 20% 20% 30% Microcline
0-15 25-30% 35-40% 10-15% Sericite Chlorite Opaque minerals
3 10-15% 35-40% 20-30% 15% Zircon Sericite
Margin of error approximately10% of listed value. three surfaces were examined: a cut surface exposed by sectioning the sample, a weathered surface and one or more areas affected by lichen cover. Table 2 summarizes the number of target surfaces examined for each sample and the number of replicate spectra obtained for each target area. Both the rock sample and reflectance panel were repositioned before each measurement. In each case the reflected flux from the panel was measured first, and then the panel was replaced by the rock sample. The size of area sampled by the spectroradiometer was approximately 7 cm x 2.5 cm. Each reference-target measurement pair was obtained in slightly longer than 2 min. Data were stored directly to diskette on the portable computer which comprises part of the SIRIS instrument. Relative reflectance was calculated from the pair of target-reference measurements. The reflectance spectra were then normalized to absolute reflectance by application of the absolute reflectance calibration for the BaSO4 reference panel. The coefficient of variation of the 25 replicate spectra of the weathered face of Sample 1 was used to assess the overall repeatability of the measurements. As both the target and reference were repositioned between each replicate, this provided a combined estimate of the sample variance and the variation due to the sensor and experimental procedure. The result indicated that the coefficient of variation of the 25 reflectance spectra was less than 1% for the visible wavelengths and slightly more than 1% at wavelengths greater than 1.1/~m. Mean reflectance for each surface was calculated as the average of all the replicates for each target surface. The averaged spectra were then resampled to a regular interval, and smoothed by a filtering procedure which removed features with a frequency of less than five data points. The resampling and smoothing were performed over the visible-NIR and SWIR sections of the spectrum separately, in order to take into account differences in the sampling resolution of the instrument.
RESULTS The Effect of Weathering In the present study the granites had all been subjected to weathering under subarctic or cool-temperature climatic regimes. The effect of this upon their spectral
196 Rollin et al.
Table 2. Summary of Samples and Surfaces Examined ~ Number of Spectra
Surfaces Examined S a m p l e 1: T e c t o n i c a l l y s h e a r e d granite, s h o w i n g a r a n g e of i n t e n s i t y of w e a t h e r i n g
Sample 2: Tectonically sheared granite or granite-gneiss- a strongly banded dark grey granite, with strong light and dark banding, composed of silicic material Sample 3: Quartz monzonite--a pink-colored, medium to coarse grained rock which showed signs of igneous foliation and flow deformation with indications that it had been subject to hydrothermal activity
1. C u t face 2. W e a t h e r e d face
25 25
3. An area of 100% cover of an orange/pink crustose lichen, Haematomma ventosum 4. An area of 100% cover of another crustose lichen, Rhizocarpon geographicum; this appeared predominantly light green in color (60% of the area) with black patches (40% of the area) 1. Cut surface 2. Weathered face 3. An area of 100% cover of Rhizocarpon geographicum, appearing predominantly black (60% of the area affected) with green patches (40% of the area) 1. Cut surface 2. Weathered face 3. An area with patches of a brown crustose lichen, Fuscidea cyathoides, with a total coverage of approximately 40% 4. An area of 100% cover of an olive-coloredfoliose lichen, Parmelia saxatalis
25 25 10 10 10 10 10 10 10
° The proportion of cover refers to the area within the sensor field-of-viewand was determined visually,as was the proportion of different-colored components in the lichen areas exhibiting such color variation.
response may be seen from Figure 1-3, which compare the reflectance of freshly cut facets of rock with that of naturally weathered facets free of lichen. It is clear from these figures that weathering has a dramatic effect upon both the overall level of reflectance and the shape of the spectral response curve, However, the effect of weathering upon spectral response is different for each of the three rock samples, with the weathered surface of Samples 1 and 2 exhibiting a generally lower reflectance than that of the cut surface, whereas for Sample 3 the reverse is true. For Sample 3 there is also some indication of enhanced absorption features associated with liquid water at 1.4 /lm and 1.9 /~m. The contrasting changes in reflectance with weathering for Sample 3 c o m p a r e d with those for the other two samples emphasizes the complexity of the physicochemical situation.
The Effect of Lichen Cover
Figure 1. Reflectance of the cut section and weathered surface of Sample 1.
Figure 2. Reflectance of the cut section and weathered surface of Sample 2.
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Figures 4 - 6 show spectra from lichen-affected areas of the three rock samples compared with spectra from weathered facets without lichen. In every case the presence of lichen significantly alters both the overall level of reflectance and the shape of the reflectance spectrum. The precise form of the altered rock spectrum depends upon the color of the lichen and the spectrally varying contrast ratio between it and the substrate. In general, reflectance was increased, although the presence of Rhizocarpon geographicum on Sample 1 reduced reflectance in visible wavelengths (Fig. 4). Wester and Lunden (1985) also reported an increase in the reflectance of lichen covered basalts compared with the weathered rock. Although this study was based on only three rock samples and four species of lichen, it is interesting to
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Figure 3. Reflectance of the cut section and weathered surface of Sample 3. note that five absorption features are present in all the lichen-affected spectra (Fig. 7). These are generally broader than mineral absorption features and are centered on the following wavelengths:
Figure 5. Reflectance of the weathered and lichen covered surfaces of Sample 2.
With the exception of ii, all these absorptions probably relate to the presence of intracellular carbohydrates; lichen walls are chiefly composed of lichenin, a carbohydrate (Hale, 1967), and cellulose may also be present in the algae. Absorption maxima of the cellulose and lignin occur at 1.45 jim, 1.93 jim, 2.09 jim, and 2.3 jim (Peterson et al., 1988; Bassett et al., 1965; Ager and Milton, 1987), with the features of 1.45 jim and 1.93 jim being associated with adsorbed water. The shallow, rounded base feature at 1.72 jim may result from the presence of a hemicellulose polysaccharide, e.g., xylan, or may not be of lignocellulose origin. A similar feature commonly occurs in dry plant material, probably as a result of a C - H stretching overtone (Elvidge, 1990). In healthy vascular plants, all four lignocellulose features, clearly identifiable in the lichen spectra, are masked by
liquid water absorption features centered on 1.45 Jim and 1.94 jim (Curcio and Petty, 1951). Apart from the absorption features at long wavelengths, the lichen spectra also all show a steep rise throughout visible wavelengths and a slight increase in slope around 0.75 jim, analogous to the red edge in vascular plants. Both spectra of Rhizocarpon geographicum exhibit strong absorption in the 400-480 nm wavelength range which may result from the presence of usnic acid, or similar substance, which absorbs strongly at ultraviolet (UV) wavelengths. It has been suggested that such substances protect the algae from high levels of UV irradiance (Gauslaa, 1984). The difference in magnitude between these two spectra probably relates to the different proportions of green and black areas noted in the lichen on the two rock samples as noted in Table 1. Features i and iii overlap with atmospheric water vapour absorption bands around 1.47 jim and 1.93 jim and are therefore obscured in data from sensors on airborne or orbital platforms. Features ii, iv, and v are located within atmospheric windows and so could be used to identify rock surfaces affected by lichen in such remotely sensed data. Some implications of the spectral
Figure 4. Reflectance of the weathered surface and lichen covered areas of Sample 1.
Figure 6. Reflectance of the weathered and lichen affected surfaces of Sample 3.
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Figure 7. Comparison of five lichen spectra with lichen as-
sociated features indicated. changes associated with lichen cover for remote visibleinfrared detection systems were considered. The effect on broadband measurements was examined by simulating the spectral bands of the Landsat Thematic Mapper (TM) sensor (Bands 1-5 and 7). Inband reflectance was calculated for each band from the full spectral data (Table 3). This was performed by integrating the product of the spectrum and the average relative spectral response function of the TM sensors in each spectral band (Markham and Barker, 1985). The calculated in-band values confirm that the presence of lichen influences reflectance in several TM bands, generally causing an increase in reflectance compared with the value for the weathered rock surface and especially so for TM Band 5. This supports previous findings of other workers (Birnie et al., 1989). These changes in in-band reflectance also impact on a range of band ratios which are frequently used in image analysis. In
particular, the N I R / I R ratio was higher for all lichenaffected surfaces, regardless of lichen color, compared with the weathered surface of the same rock. Both partial and complete lichen cover was associated with a distinctive change in the shape of the reflectance spectrum in the SWIR wavelength region which resulted in a reduction of the ratio of TM7 to TM5, partly due to absorption features ii and iv noted previously. For Samples 1 and 2 the ratio changed from above unity for the weathered surface to below unity for the lichen affected spectra. For Sample 3, the reduction was less distinct. Modification of reflectance in this spectral region may present a particular difficulty for many geological applications which make use of TM Band 7 because of its position in relation to the absorption features of the clay minerals between 2.2 p m and 2.4 pm. In addition to the broad-band effects noted above, the effect of complete and partial lichen cover on the identification of minerals based on characteristic spectral features merits examination. Figure 6 shows how a lichen cover as low as 40% is sufficient to imprint its response upon that of the host rock, causing it to deviate markedly from that measured from the weathered facet. However, it is significant that the absorption features which are related to mineral composition rather than lichen cover are preserved in the composite spectrum. Some mineral related features, notably the iron absorption at 1.1 pm, were present in both the weathered and the lichen-affected spectra of Samples 2 and 3. Other mineral features exhibit an apparent shift in the wavelength of their peak absorption in the lichen-rock spectrum, probably due to the superimposition of characteristics of the lichen spectrum. This is noticeable for Sample 3 (Fig. 6), where the absorption features associated with liquid water occurring at 1.4 p m and 1.9 p m
Table 3. Calculated In-Band Reflectance for Landsat TM Bands
TM1
TM2
TM3
31.28 13.40 17.26 10.94
34.01 16.26 20.83 13.92
33.79 18.0 24.13 15.08
19.43 6.03 18.33
19.23 6.93 22.62
18.86 7.38 24.94
TM4
TM5
TM7
TM4 TM3
TM7 TM5
31.32 18.95 27.93 16.81
28.57 20.72 47.28 26.08
29.84 22.26 39.23 31.13
0.93 1.05 1.16 1.12
1.05 1.07 0.83 1.19
18.03 7.3 27.26
12.14 7.31 45.11
12.69 8.78 41.22
0.96 0.99 1.09
1.05 1.20 0.92
13.22 25.21 29.88 33.94
10.85 24.35 38.16 46.08
11.99 22.77 28.91 41.46
0.99 1.6 1.44 1.40
1.11 0.94 0.76 0.90
Sample 1
Cut Weathered Haematomma ventosum Rhizocarpon geographicum
Sample 2
Cut Weathered Rhizocarpon geographicum
Sample 3
Cut Weathered Fuscidea cyathoides Parmelia saxitalis
9.21 10.72 15.89 15.57
11.68 12.44 18.6 19.96
13.32 15.78 20.69 24.18
Reflectance of Granitic Rocks
in the weathered rock spectra, become masked by the lignocellulose features in the lichen-affected spectra. Figure 6 also indicates that the amount of lichen cover affects the extent to which mineral features are evident; the mineral absorption feature at 2.20 pm is present in the spectrum of the rock with 40% cover of Fuscidea cyathoides but absent in the spectrum of the sample covered by the olive coloured Parmelia saxitalis, where it is apparently swamped by the broader, more intense absorption band at 2.09/lm resulting from the complete lichen cover. With higher spectral resolution data from remote systems, discrimination of rock and lichen-related spectra may be easier, and spectral processing techniques should enable identification of the lichen associated features at 1.72 gm, 2.09 gm, and 2.3/Jm, which are relatively broad in comparison with most mineral absorption features in the same wavelength region. Rivard and Arvidson (1992) concluded that the potential for mapping in arctic terrains will benefit from such finescale spectral information offered by imaging spectrometer systems, but that interpretation of such imagery will also require extensive field spectrometry to investigate subpixel mixtures of rock and lichen elements, and the results of this study confirm that view. SUMMARY AND CONCLUSION 1. Marked differences in the spectral reflectance of weathered surfaces relative to the cut surfaces of three granite rocks were noted, although the direction and magnitude of these differences varied between the three rocks. In some cases spectral features associated with liquid water absorption or mineral absorptions were enhanced in the spectra of the weathered surface. 2. The presence of lichen growth was found to affect both the overall shape of the reflectance spectrum and certain specific absorption features. 3. Absorption features associated with lichen cover tended to be stronger and broader than those associated with commonly occurring weathering minerals. 4. Five absorption features in the shortwave-infrared were found to occur only in the spectra of lichencovered surfaces, four probably relating to the intracellular carbohydrates of the lichen. Three of the five features occur in the atmospheric windows and therefore are potentially useful for lichen identification by spectral measurements from satellite or airborne sensors. Further investigations are needed to confirm the potential diagnostic capability of these features for discrimination of lichen growth against the spectral response of a range of host rock and soil surfaces.
199
The experiments were conducted at the NERC-EPFS laboratory in the Department of Geography at the University of Southampton. We are grateful to Dr. R. I. Lewis-Smith of the British Antarctic Survey for identification of the lichens, and to the Geology Department at the University of Southampton for assistance with sample preparation and analysis. REFERENCES Ager, C. M., and Milton, N. M. (1987), Spectral reflectance of lichens and their effects on the reflectance of rock substrates, Geophysics 52:898-906. Bassett, K. H., Liang, C. Y., and Marchessault, R. H. (1963), The infrared spectrum of crystalline polysaccharides. IX the near infrared spectrum of cellulose, J. Polym. Sci., Vol. 1, Part A:1687-1692. Birnie, R. W., Parr, J. T., Naslund, H. R., Nichols, J. D., and Turner, P. A. (1989), Applications of Landsat thematic Mapper and ground-based spectrometer data to a study of the Skaergaard and other mafic intrusions of East Greenland, Remote Sens. Environ. 28:297-304. Cnrcio, J. A., and Petty, C. C. (1951), The near infrared absorption spectrum of liquid water, J. Opt. Soc. Am. 41: 302-304. Elvidge, C. D. (1990), Visible and near infrared reflectance characteristics of dry plant material, Int. J. Remote Sens. 11:1775-1796. Gauslaa, Y. (1984), Heat resistance and energy budget in different Scandinavian plants, Holarctic Ecol. 7:1-78. Hale, M. E. (1967), The Biology of Lichens, Arnold, London. Hunt, G. R. (1979). Near infrared (1.3-2.4 microns) spectra of alteration minerals-potential for use in remote sensing, Geophysics 44:1976-1986. Knight, P., Weaver, R., and Sugden, D. (1987), Using LANDSAT MSS data for measuring ice sheet retreat, Int. J. Remote Sens. 8:1069-1074. Longton, R. E. (1988), Biology of Polar Bryophytes and Lichens, Cambridge University Press, Cambridge. Markham, B. L., and Barker, J. L. (1985), Spectral characterisation of the Landsat Thematic Mapper sensors, Int. J. Remote Sens. 6:697-716. Peterson, D. L., Aber, J. D., Matson, P. A., et al. (1988), Remote sensing of forest canopy and leaf biochemical contents, Remote Sens. Environ. 24:85-108. Petzold, D. E., and Goward, S. M. (1988), Reflectance spectra of subarctic lichens, Remote Sens. Environ. 24:481-492. Rivard, B., and Avidson, R. E. (1992), Utility of imaging spectrometry for lithological mapping in Greenland, Photogramm. Eng. Remote Sens. 58:945-949. Rothery, D. A., and Lefebvre, R. H. (1985), The causes of age dependent changes in the spectral response of lavas, Craters of the Moon, Idaho, USA, Int. J. Remote Sens. 6:14831489. Satterwhite, M. B., Henley, J. P., and Carney, J. M. (1985), The effect of lichens on the reflectance spectra of granitic rock surfaces, Remote Sens. Environ. 18:105-112. Wester, K., and Lunden, B. (1985), Laboratory measurements of spectral reflectance (0.4-2.3/~m) of basalts, in Proceedings of the 3rd International Colloquium on Spectral Signatures of Objects in Remote Sensing, SP-247, ESA, Noordwijk, Netherlands, pp. 523-526.
Short Communication
The Influence of Weathering and Lichen Cover on the Reflectance Spectra of Granitic Rocks E. M. RoUin,* E. J. Milton,* and P. Roche t R e s u l t s are presented from a series of laboratory experiments that examined the effect of surface weathering and lichen cover on the spectral reflectance of granitic rocks over visible and infrared wavelengths. The effect of weathering on the rock reflectance varied with rock type. However, all the lichen-affected spectra, which included four different species, exhibited the same five diagnostic absorption features in the high resolution spectra. More general changes in curve characteristics were also evident in the lichen-affected spectra. The diagnostic absorption features were evident in spectra from samples with less than 50% lichen cover. Certain mineralogic features remained evident in the spectra despite the surface lichen cover.
INTRODUCTION Lithological mapping using remote sensing depends, in part, on the identification of rock types by their spectral characteristics. The potential for rock type and mineral identification benefits from some understanding of the way in which surface processes modify those spectral characteristics. Two important factors which merit consideration are surficial weathering effects and presence of organic growth, such as lichens on the rock surface. From a different perspective, detection of lichens by remote sensing offers a potentially useful tool for many *NERC-EPFS, Department of Geography, University of Southampton, United Kingdom *Department of Physics, University of Southampton, United Kingdom Address correspondence to E. M. Rollin, Natural Environment Research Council Equipment Pool for Field Spectroscopy, Department of Geography, Univ. of Southampton, Southampton, SO17 1BJ U.K. Received 16 November 1993; revised 25 June 1994.
1.94
ecological purposes and for such activities as identifying age dependent changes in the surface, for example, in order to date glacial retreat stages (e.g., Knight et al., 1987), or episodes of lava eruption (Rothery and Lefebvre, 1985). Lichen growth commonly occurs on rock outcrops at high latitudes and altitudes. This article reports the findings of a study aimed at identifying changes in the spectral reflectance over visible to shortwave infrared wavelengths (0.4-2.4/tm) of three granitic rocks as a result of weathering and the presence of lichen on the rock surface. Spectral reflectance was measured with a high spectral resolution instrument which enabled spectral differences in the form of relatively narrow spectral features to be examined, as well as more generalized changes in the shape of the reflectance spectrum. The study also aimed to identify any similarities between the spectra of the different lichen species present on the rocks. Weathering processes affect granitic rocks in various ways depending upon the precise mineralogy of the rock, the chemical composition of groundwater, and/ or surface water and the local climatic conditions. From the standpoint of remote sensing, surface manifestations of weathering processes are the most important, and these include discoloration of the surface due to iron compounds, the formation of alteration minerals such as sericite or kaolinite, and changes in the texture and microrelief of the weathered surface, possibly as a result of increased moisture content and attack by physical and chemical agents of erosion. To date, work on the spectral reflectance of lichens has largely been confined to visible and NIR (nearinfrared) wavelengths (0.46-1.1/lm). For example, Petzold and Goward (1988) measured the spectral reflectance in the visible and NIR of subarctic lichens in situ, focusing on differences between lichens and vascular plants. They found that lichens generally showed a 0034-4257 / 94 / $7.00 QElsevier Science Inc., 1994 (i.~.~ A,~an~a of tha Am.eritzn.¢ N~I~ York NY 10010
Reflectance of Granitic Rocks 195
greater reflectance in the visible and more gradual increase at the red edge than most green vegetation samples, and had a less marked absorption at 0.95 gm due to their lower intercellular water content compared with vascular plants. In a study of the energy budgets of lichens, Gauslaa (1984) measured the visible and NIR reflectance of several lichen species in a wet and dry state and was able to group the species according to their spectral reflectance characteristics. Satterwhite et al. (1985) used a laboratory spectroradiometer covering the visible and NIR wavelength region in their study of the effect of lichen cover on the reflectance of granitic rocks in Bands 1-4 of the Landsat Thematic Mapper. In terms of geological applications, the short-wave infrared (SWIR, 1.1-2.4/~m) wavelengths are the most important, since many diagnostic minerals exhibit characteristic spectral features in this region (Hunt, 1979). Some knowledge of the effect of lichen cover on rock reflectance in this spectral region is important for several aspects of spectral data interpretation. First, for geological purposes, the extent to which characteristic rock features may be preserved despite a lichen covering is the main concern, and some estimate of the critical level of lichen coverage, above which the spectral features associated with the rock mineralogy become obscured, would be of value. Second, in order to identify the presence of lichen cover in remotely sensed imagery, some knowledge of the spectral characteristics of lichen is necessary. For this, an estimate of the minimum level of cover required to produce diagnostic spectral features in the reflectance spectrum would also be of value. The results of this work are relevant to both aspects.
METHODS A GER single field-of-view IRIS (SIRIS or SFOV IRIS) spectroradiometer was used to measure the biconical spectral reflectance of various surfaces of three rock samples of granitic rock over the 0.35-2.5 gm range, relative to a barium sulphate (BaSO4) reference panel which was calibrated by the UK National Physical Laboratory in terms of absolute spectral reflectance. The measurements were performed in a blacked-out laboratory with the spectroradiometer head mounted on an optical bench orthogonal to and at a distance of 30 cm from the target and reference. The rock samples were positioned on the optical bench vertically beneath the optical head which was mounted for vertical viewing. Illumination was provided at 45 ° to the samples, by a 1000 W quartz lamp. Of the three granitic rock samples examined, two were from Norway and the third from Scotland. The mineralogy of each rock sample was derived from inspection of thin sections and is summarized in Table 1. All the samples were weathered to some degree and showed areas of lichen growth. For each sample, at least
Table 1. Mineralogy of Samples° 1 Quartz Plagioclase Feldspar Potassium Feldspar Biotite Others
Sample 2 25-30% 20% 20% 30% Microcline
0-15 25-30% 35-40% 10-15% Sericite Chlorite Opaque minerals
3 10-15% 35-40% 20-30% 15% Zircon Sericite
Margin of error approximately10% of listed value. three surfaces were examined: a cut surface exposed by sectioning the sample, a weathered surface and one or more areas affected by lichen cover. Table 2 summarizes the number of target surfaces examined for each sample and the number of replicate spectra obtained for each target area. Both the rock sample and reflectance panel were repositioned before each measurement. In each case the reflected flux from the panel was measured first, and then the panel was replaced by the rock sample. The size of area sampled by the spectroradiometer was approximately 7 cm x 2.5 cm. Each reference-target measurement pair was obtained in slightly longer than 2 min. Data were stored directly to diskette on the portable computer which comprises part of the SIRIS instrument. Relative reflectance was calculated from the pair of target-reference measurements. The reflectance spectra were then normalized to absolute reflectance by application of the absolute reflectance calibration for the BaSO4 reference panel. The coefficient of variation of the 25 replicate spectra of the weathered face of Sample 1 was used to assess the overall repeatability of the measurements. As both the target and reference were repositioned between each replicate, this provided a combined estimate of the sample variance and the variation due to the sensor and experimental procedure. The result indicated that the coefficient of variation of the 25 reflectance spectra was less than 1% for the visible wavelengths and slightly more than 1% at wavelengths greater than 1.1/~m. Mean reflectance for each surface was calculated as the average of all the replicates for each target surface. The averaged spectra were then resampled to a regular interval, and smoothed by a filtering procedure which removed features with a frequency of less than five data points. The resampling and smoothing were performed over the visible-NIR and SWIR sections of the spectrum separately, in order to take into account differences in the sampling resolution of the instrument.
RESULTS The Effect of Weathering In the present study the granites had all been subjected to weathering under subarctic or cool-temperature climatic regimes. The effect of this upon their spectral
196 Rollin et al.
Table 2. Summary of Samples and Surfaces Examined ~ Number of Spectra
Surfaces Examined S a m p l e 1: T e c t o n i c a l l y s h e a r e d granite, s h o w i n g a r a n g e of i n t e n s i t y of w e a t h e r i n g
Sample 2: Tectonically sheared granite or granite-gneiss- a strongly banded dark grey granite, with strong light and dark banding, composed of silicic material Sample 3: Quartz monzonite--a pink-colored, medium to coarse grained rock which showed signs of igneous foliation and flow deformation with indications that it had been subject to hydrothermal activity
1. C u t face 2. W e a t h e r e d face
25 25
3. An area of 100% cover of an orange/pink crustose lichen, Haematomma ventosum 4. An area of 100% cover of another crustose lichen, Rhizocarpon geographicum; this appeared predominantly light green in color (60% of the area) with black patches (40% of the area) 1. Cut surface 2. Weathered face 3. An area of 100% cover of Rhizocarpon geographicum, appearing predominantly black (60% of the area affected) with green patches (40% of the area) 1. Cut surface 2. Weathered face 3. An area with patches of a brown crustose lichen, Fuscidea cyathoides, with a total coverage of approximately 40% 4. An area of 100% cover of an olive-coloredfoliose lichen, Parmelia saxatalis
25 25 10 10 10 10 10 10 10
° The proportion of cover refers to the area within the sensor field-of-viewand was determined visually,as was the proportion of different-colored components in the lichen areas exhibiting such color variation.
response may be seen from Figure 1-3, which compare the reflectance of freshly cut facets of rock with that of naturally weathered facets free of lichen. It is clear from these figures that weathering has a dramatic effect upon both the overall level of reflectance and the shape of the spectral response curve, However, the effect of weathering upon spectral response is different for each of the three rock samples, with the weathered surface of Samples 1 and 2 exhibiting a generally lower reflectance than that of the cut surface, whereas for Sample 3 the reverse is true. For Sample 3 there is also some indication of enhanced absorption features associated with liquid water at 1.4 /lm and 1.9 /~m. The contrasting changes in reflectance with weathering for Sample 3 c o m p a r e d with those for the other two samples emphasizes the complexity of the physicochemical situation.
The Effect of Lichen Cover
Figure 1. Reflectance of the cut section and weathered surface of Sample 1.
Figure 2. Reflectance of the cut section and weathered surface of Sample 2.
5(3
Figures 4 - 6 show spectra from lichen-affected areas of the three rock samples compared with spectra from weathered facets without lichen. In every case the presence of lichen significantly alters both the overall level of reflectance and the shape of the reflectance spectrum. The precise form of the altered rock spectrum depends upon the color of the lichen and the spectrally varying contrast ratio between it and the substrate. In general, reflectance was increased, although the presence of Rhizocarpon geographicum on Sample 1 reduced reflectance in visible wavelengths (Fig. 4). Wester and Lunden (1985) also reported an increase in the reflectance of lichen covered basalts compared with the weathered rock. Although this study was based on only three rock samples and four species of lichen, it is interesting to
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Figure 3. Reflectance of the cut section and weathered surface of Sample 3. note that five absorption features are present in all the lichen-affected spectra (Fig. 7). These are generally broader than mineral absorption features and are centered on the following wavelengths:
Figure 5. Reflectance of the weathered and lichen covered surfaces of Sample 2.
With the exception of ii, all these absorptions probably relate to the presence of intracellular carbohydrates; lichen walls are chiefly composed of lichenin, a carbohydrate (Hale, 1967), and cellulose may also be present in the algae. Absorption maxima of the cellulose and lignin occur at 1.45 jim, 1.93 jim, 2.09 jim, and 2.3 jim (Peterson et al., 1988; Bassett et al., 1965; Ager and Milton, 1987), with the features of 1.45 jim and 1.93 jim being associated with adsorbed water. The shallow, rounded base feature at 1.72 jim may result from the presence of a hemicellulose polysaccharide, e.g., xylan, or may not be of lignocellulose origin. A similar feature commonly occurs in dry plant material, probably as a result of a C - H stretching overtone (Elvidge, 1990). In healthy vascular plants, all four lignocellulose features, clearly identifiable in the lichen spectra, are masked by
liquid water absorption features centered on 1.45 Jim and 1.94 jim (Curcio and Petty, 1951). Apart from the absorption features at long wavelengths, the lichen spectra also all show a steep rise throughout visible wavelengths and a slight increase in slope around 0.75 jim, analogous to the red edge in vascular plants. Both spectra of Rhizocarpon geographicum exhibit strong absorption in the 400-480 nm wavelength range which may result from the presence of usnic acid, or similar substance, which absorbs strongly at ultraviolet (UV) wavelengths. It has been suggested that such substances protect the algae from high levels of UV irradiance (Gauslaa, 1984). The difference in magnitude between these two spectra probably relates to the different proportions of green and black areas noted in the lichen on the two rock samples as noted in Table 1. Features i and iii overlap with atmospheric water vapour absorption bands around 1.47 jim and 1.93 jim and are therefore obscured in data from sensors on airborne or orbital platforms. Features ii, iv, and v are located within atmospheric windows and so could be used to identify rock surfaces affected by lichen in such remotely sensed data. Some implications of the spectral
Figure 4. Reflectance of the weathered surface and lichen covered areas of Sample 1.
Figure 6. Reflectance of the weathered and lichen affected surfaces of Sample 3.
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sociated features indicated. changes associated with lichen cover for remote visibleinfrared detection systems were considered. The effect on broadband measurements was examined by simulating the spectral bands of the Landsat Thematic Mapper (TM) sensor (Bands 1-5 and 7). Inband reflectance was calculated for each band from the full spectral data (Table 3). This was performed by integrating the product of the spectrum and the average relative spectral response function of the TM sensors in each spectral band (Markham and Barker, 1985). The calculated in-band values confirm that the presence of lichen influences reflectance in several TM bands, generally causing an increase in reflectance compared with the value for the weathered rock surface and especially so for TM Band 5. This supports previous findings of other workers (Birnie et al., 1989). These changes in in-band reflectance also impact on a range of band ratios which are frequently used in image analysis. In
particular, the N I R / I R ratio was higher for all lichenaffected surfaces, regardless of lichen color, compared with the weathered surface of the same rock. Both partial and complete lichen cover was associated with a distinctive change in the shape of the reflectance spectrum in the SWIR wavelength region which resulted in a reduction of the ratio of TM7 to TM5, partly due to absorption features ii and iv noted previously. For Samples 1 and 2 the ratio changed from above unity for the weathered surface to below unity for the lichen affected spectra. For Sample 3, the reduction was less distinct. Modification of reflectance in this spectral region may present a particular difficulty for many geological applications which make use of TM Band 7 because of its position in relation to the absorption features of the clay minerals between 2.2 p m and 2.4 pm. In addition to the broad-band effects noted above, the effect of complete and partial lichen cover on the identification of minerals based on characteristic spectral features merits examination. Figure 6 shows how a lichen cover as low as 40% is sufficient to imprint its response upon that of the host rock, causing it to deviate markedly from that measured from the weathered facet. However, it is significant that the absorption features which are related to mineral composition rather than lichen cover are preserved in the composite spectrum. Some mineral related features, notably the iron absorption at 1.1 pm, were present in both the weathered and the lichen-affected spectra of Samples 2 and 3. Other mineral features exhibit an apparent shift in the wavelength of their peak absorption in the lichen-rock spectrum, probably due to the superimposition of characteristics of the lichen spectrum. This is noticeable for Sample 3 (Fig. 6), where the absorption features associated with liquid water occurring at 1.4 p m and 1.9 p m
Table 3. Calculated In-Band Reflectance for Landsat TM Bands
TM1
TM2
TM3
31.28 13.40 17.26 10.94
34.01 16.26 20.83 13.92
33.79 18.0 24.13 15.08
19.43 6.03 18.33
19.23 6.93 22.62
18.86 7.38 24.94
TM4
TM5
TM7
TM4 TM3
TM7 TM5
31.32 18.95 27.93 16.81
28.57 20.72 47.28 26.08
29.84 22.26 39.23 31.13
0.93 1.05 1.16 1.12
1.05 1.07 0.83 1.19
18.03 7.3 27.26
12.14 7.31 45.11
12.69 8.78 41.22
0.96 0.99 1.09
1.05 1.20 0.92
13.22 25.21 29.88 33.94
10.85 24.35 38.16 46.08
11.99 22.77 28.91 41.46
0.99 1.6 1.44 1.40
1.11 0.94 0.76 0.90
Sample 1
Cut Weathered Haematomma ventosum Rhizocarpon geographicum
Sample 2
Cut Weathered Rhizocarpon geographicum
Sample 3
Cut Weathered Fuscidea cyathoides Parmelia saxitalis
9.21 10.72 15.89 15.57
11.68 12.44 18.6 19.96
13.32 15.78 20.69 24.18
Reflectance of Granitic Rocks
in the weathered rock spectra, become masked by the lignocellulose features in the lichen-affected spectra. Figure 6 also indicates that the amount of lichen cover affects the extent to which mineral features are evident; the mineral absorption feature at 2.20 pm is present in the spectrum of the rock with 40% cover of Fuscidea cyathoides but absent in the spectrum of the sample covered by the olive coloured Parmelia saxitalis, where it is apparently swamped by the broader, more intense absorption band at 2.09/lm resulting from the complete lichen cover. With higher spectral resolution data from remote systems, discrimination of rock and lichen-related spectra may be easier, and spectral processing techniques should enable identification of the lichen associated features at 1.72 gm, 2.09 gm, and 2.3/Jm, which are relatively broad in comparison with most mineral absorption features in the same wavelength region. Rivard and Arvidson (1992) concluded that the potential for mapping in arctic terrains will benefit from such finescale spectral information offered by imaging spectrometer systems, but that interpretation of such imagery will also require extensive field spectrometry to investigate subpixel mixtures of rock and lichen elements, and the results of this study confirm that view. SUMMARY AND CONCLUSION 1. Marked differences in the spectral reflectance of weathered surfaces relative to the cut surfaces of three granite rocks were noted, although the direction and magnitude of these differences varied between the three rocks. In some cases spectral features associated with liquid water absorption or mineral absorptions were enhanced in the spectra of the weathered surface. 2. The presence of lichen growth was found to affect both the overall shape of the reflectance spectrum and certain specific absorption features. 3. Absorption features associated with lichen cover tended to be stronger and broader than those associated with commonly occurring weathering minerals. 4. Five absorption features in the shortwave-infrared were found to occur only in the spectra of lichencovered surfaces, four probably relating to the intracellular carbohydrates of the lichen. Three of the five features occur in the atmospheric windows and therefore are potentially useful for lichen identification by spectral measurements from satellite or airborne sensors. Further investigations are needed to confirm the potential diagnostic capability of these features for discrimination of lichen growth against the spectral response of a range of host rock and soil surfaces.
199
The experiments were conducted at the NERC-EPFS laboratory in the Department of Geography at the University of Southampton. We are grateful to Dr. R. I. Lewis-Smith of the British Antarctic Survey for identification of the lichens, and to the Geology Department at the University of Southampton for assistance with sample preparation and analysis. REFERENCES Ager, C. M., and Milton, N. M. (1987), Spectral reflectance of lichens and their effects on the reflectance of rock substrates, Geophysics 52:898-906. Bassett, K. H., Liang, C. Y., and Marchessault, R. H. (1963), The infrared spectrum of crystalline polysaccharides. IX the near infrared spectrum of cellulose, J. Polym. Sci., Vol. 1, Part A:1687-1692. Birnie, R. W., Parr, J. T., Naslund, H. R., Nichols, J. D., and Turner, P. A. (1989), Applications of Landsat thematic Mapper and ground-based spectrometer data to a study of the Skaergaard and other mafic intrusions of East Greenland, Remote Sens. Environ. 28:297-304. Cnrcio, J. A., and Petty, C. C. (1951), The near infrared absorption spectrum of liquid water, J. Opt. Soc. Am. 41: 302-304. Elvidge, C. D. (1990), Visible and near infrared reflectance characteristics of dry plant material, Int. J. Remote Sens. 11:1775-1796. Gauslaa, Y. (1984), Heat resistance and energy budget in different Scandinavian plants, Holarctic Ecol. 7:1-78. Hale, M. E. (1967), The Biology of Lichens, Arnold, London. Hunt, G. R. (1979). Near infrared (1.3-2.4 microns) spectra of alteration minerals-potential for use in remote sensing, Geophysics 44:1976-1986. Knight, P., Weaver, R., and Sugden, D. (1987), Using LANDSAT MSS data for measuring ice sheet retreat, Int. J. Remote Sens. 8:1069-1074. Longton, R. E. (1988), Biology of Polar Bryophytes and Lichens, Cambridge University Press, Cambridge. Markham, B. L., and Barker, J. L. (1985), Spectral characterisation of the Landsat Thematic Mapper sensors, Int. J. Remote Sens. 6:697-716. Peterson, D. L., Aber, J. D., Matson, P. A., et al. (1988), Remote sensing of forest canopy and leaf biochemical contents, Remote Sens. Environ. 24:85-108. Petzold, D. E., and Goward, S. M. (1988), Reflectance spectra of subarctic lichens, Remote Sens. Environ. 24:481-492. Rivard, B., and Avidson, R. E. (1992), Utility of imaging spectrometry for lithological mapping in Greenland, Photogramm. Eng. Remote Sens. 58:945-949. Rothery, D. A., and Lefebvre, R. H. (1985), The causes of age dependent changes in the spectral response of lavas, Craters of the Moon, Idaho, USA, Int. J. Remote Sens. 6:14831489. Satterwhite, M. B., Henley, J. P., and Carney, J. M. (1985), The effect of lichens on the reflectance spectra of granitic rock surfaces, Remote Sens. Environ. 18:105-112. Wester, K., and Lunden, B. (1985), Laboratory measurements of spectral reflectance (0.4-2.3/~m) of basalts, in Proceedings of the 3rd International Colloquium on Spectral Signatures of Objects in Remote Sensing, SP-247, ESA, Noordwijk, Netherlands, pp. 523-526.