- Email: [email protected]
RADARSAT data applications: radar backscatter of granitic facies, the Zaer pluton, Morocco
ELSEVIER
Journal of Geochemical Exploration 66 (1999) 413–420 www.elsevier.com/locate/jgeoexp
RADARSAT data applications: radar backscatter of granitic facies, the Zaer pluton, Morocco Ahmed Mahmood Ł , Surendra Parashar, Satish Srivastava RADARSAT Program, Space Operations, Canadian Space Agency, Saint-Hubert, Quebec J3Y 8Y9, Canada Accepted 24 February 1999
Abstract Radar backscatter in active microwave remote sensing is influenced by such physical properties of the ground as the surface roughness, shape and water contents, and varies with the wavelength and the angle of incident energy. Canada’s RADARSAT is the first spaceborne radar which provides flexibility of incidence angle and ground resolution. Its imaging device is a C-band Synthetic Aperture Radar (SAR), which is particularly well suited for interpreting micro-roughness and soil conditions by the appropriate choice of imaging parameters. This article describes a test case in which quantitative RADARSAT backscatter data were related to spatial variations in a granitic pluton. The Zaer pluton, one of the several Hercynian granitoids in northwestern Morocco, consists of medium- to coarse-grained biotite granodiorite and two-mica monzogranite. These are highly weathered and the rock surfaces are largely covered with granitic soils of comparable mineralogical composition. The soils on the biotite granodiorite are richer in micaceous and clayey minerals than those overlying two-mica monzogranite. The variation in radar backscatter response, measured with calibrated RADARSAT data, is attributed to the different grain size and water content of the granitic facies. It is concluded that with RADARSAT data, the radar backscatter response differences can be highlighted for the purpose of facies mapping and geochemical interpretation. 1999 Elsevier Science B.V. All rights reserved. Keywords: spaceborne radar; RADARSAT; facies mapping; geochemical interpretation
1. Introduction Radar remote sensing, which relies on the physical and electrical properties of ground targets, such as surface roughness, terrain geometry and degree of water saturation, has been used to map rock and soil units (Sutinen et al., 1991). Active radar remote sensing is independent of solar illumination and angle. The information received from radar imaging is thus different from remote sensing in the visible or infrared regions of the electromagnetic spectrum. The Ł Corresponding
author. E-mail: [email protected]
basic operating principles involved in active radar remote sensing consist of transmitting microwave energy (pulses) from the imaging device (an antenna) to a ground target and measuring properties of the signal reflected back to the device. The reflected signal is then processed into an image output. Regardless of the properties of the target, the backscattering of radar waves impinging on a surface is modulated by their wavelength ½, polarization, and angle ./ of the incident beam. For example, radar waves with wavelengths longer than the grain size of high-density target material will be subjected to specular reflection rather than ran-
0375-6742/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 6 7 4 2 ( 9 9 ) 0 0 0 0 9 - 6
414
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
dom scattering. Changes in the beam incidence angle will affect radar backscatter, which will decrease as the angle increases, particularly so if the surfaces are relatively smooth. The choice of incidence angle will also determine whether the backscatter response is dominated by dielectric constant, topography or roughness of the target. In areas of appreciable relief, topographic effects at steep incidence angles will be as apparent in radar imagery as radar beam sensitivity to surface roughness. The influence of the wavelength on radar imagery is equally important (Van Zyl et al., 1992; Schmugge et al., 1992). Images taken at shorter wavelengths (C-band) are more sensitive to vegetation and soil moisture changes than images at longer wavelengths (L- or P-band). Furthermore, the radar response will be different for the same vegetation type depending on the polarization of radar waves. Thin layers of vegetation with branches preferentially oriented vertically, for example, would attenuate vertically polarized waves more than horizontally polarized waves. In the past, it was not possible to adjust the imaging parameters of spaceborne radar systems to terrain conditions, but this changed with the launch of Canada’s RADARSAT satellite in late 1995. As a result of electronic beam steering, RADARSAT has provided for the first time the opportunity to select incidence angles and ground resolutions appropriate to the purpose for which the remote sensing data are collected (Mahmood et al., 1996). The purpose may require enhancing either surface roughness or mesoscale topography at the expense of the other. RADARSAT’s operational characteristics consist of a Synthetic Aperture Radar (SAR), which images at a frequency of 5.3 GHz (C-band) with horizontally transmit and receive (HH) polarization. Details on RADARSAT satellite can be found in Parashar et al. (1993). This terrain reflectivity of the radar beam may be expressed quantitatively with a degree of certainty by working with calibrated data and the quantitative measure may be used as a discriminant function for distinguishing ground units. RADARSAT data are calibrated and were used in this study to derive terrain reflectivity values to separate different granitic rock facies previously mapped on the basis of their grain size, texture, colour, mineralogical constituents and geochemical composition. The re-
flectivity due to slope was suppressed by the proper choice of incidence angle and by eliminating slope effects from the reflectivity values. Moreover, for the purpose of discriminating the granitic facies based on their physical properties, the slope effects in radar reflectivity were assumed to be constant because of the uniform slope variations across the various rock units in the study area.
2. Description of the study area The studied area is located in the Zaer region of northwestern Morocco and is formed of weathered peraluminous granites. They are part of an elongated pluton, approximately 50 km long and 15 km across, which intruded Ordovician to Silurian metasediments during the Hercynian orogeny. The pluton was forcefully emplaced, wedging apart the surrounding sediments, which is remarkably displayed on the RADARSAT image (Fig. 1). As elsewhere in the region, the granitic rock is highly weathered, covered with granitic ‘sands’, gravels, pebbles and boulders, compositionally similar to the underlying bed rock. Rock outcrops occur locally where the surface is strewn with coarser regolith. The soils are generally bare except during the spring to late summer growing season. The granitic terrain is by and large a subdued topography in relation to its surrounding country rocks. The topography is marked by gently undulating surfaces and a few ravines. Slope variations are identical over the entire area. Internally, the pluton can be divided into three main units (Fig. 2). Facies I is a dark-coloured, medium- to coarse-grained, sub-porphyritic, biotite granodiorite occurring all along the western margin of the pluton as well as in its northern and southern oval-shaped extremities. Facies II is represented by a light-coloured, coarse-grained, equigranular granodiorite outcropping in the central portion of the northern half of the pluton. Facies III comprises a two-mica coarse-grained monzogranite which occupies the central two thirds of the pluton. The weathered surface rock material coarsens from the outer biotite facies to the inner two-mica facies as a direct consequence of the texture of the underlying bedrock.
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
Fig. 1. RADARSAT Standard 3 beam image of the Zaer pluton, northwestern Morocco.
415
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A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
Fig. 2. Generalized facies map of the Zaer pluton (after Mahmood, 1985).
Figs. 3 and 4 summarize the major compositional trend within the pluton, which is marked by a decreasing calc-alkaline and mafic index from the mar-
gins inward (Facies I to Facies III). Mineralogically, this translates into a significantly higher content of minerals such as biotite and its clayey alteration
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
417
Fig. 3. Normative anorthite (plagioclase) variations in the Zaer pluton (contours at 10%, 20% and 30%).
products, and plagioclase, in the outer facies relative to the inner facies, which are more alkaline and richer in free silica.
3. Experiment and results The experiment is based on an October acquisition of RADARSAT during an afternoon pass over the region. This was selected to avoid the morning dew which prevails in the region during this time of
the year. The acquired data were processed with calibrated payload parameters (Srivastava, 1997) to a georeferenced image product of 100ð100 km, sampled at 8676 pixels on 8047 lines. The image was taken with a RADARSAT Standard 3 beam, with image resolution of a square pixel of roughly 25 m and a mean incidence angle of ¾32º. This angle was considered optimum given ground conditions for suppressing topographic effects while enhancing surface roughness. In the southern half of the pluton, the granitic rock outcrops sporadically, particularly on Facies III
418
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
Fig. 4. Mafic oxides (Fe2 O3 C FeO C MgO) variations in the Zaer pluton (contours at 1%, 2.4%, 4%, 5.5% and 7%).
backslopes of the Oued Chbeika fault scarp and the downthrown Facies I block in the south of the pluton (Mahmood, 1996). The scarp represents the faulted contact between the two-mica facies to the north and the biotite facies to the south. It can be clearly seen on the RADARSAT scene as an irregular bright fringe because of its corner reflector effects, and its backslopes are in the radar shadow. These areas have a totally different radar response and were excluded from the study which focuses on the soil overlying the altered granitic bedrock.
The radar backscatter was sampled on the part of the scene covering the pluton following a grid interval of 40 lines by 40 pixels. Each value was plotted at the centre of the grid squares situated within the pluton boundary drawn approximately on the RADARSAT scene. The radar energy reflected back from the ground target is given by its backscatter coefficient, expressed conventionally as ¦ 0 in units of decibels (dB), and is a measure of the reflective properties of the surface in relation to the reflected and incident energy. The coefficient ¦ 0 is normalized
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
419
Table 1 Beta nought statistics for the Zaer pluton facies Facies
Sample size
I II III
91 92 103
Mean (dB) 7.68 6.94 6.65
Summary description of facies Dark-coloured, medium- to coarse-grained, sub-porphyritic, single-mica, biotite-rich, granodiorite Light-coloured, coarse-grained, equigranular, single-mica, biotite granodiorite Two-mica, equigranular monzogranite
in terms of local incidence angle, which is a function of both the angle of the incident beam as well as the slope. The local slope angle effects are generally unknown and, for the purpose of this study, needed to be removed from the reflectivity estimates measured in relation only to the physical properties of the terrain. Therefore, a different reflectivity value known as beta nought .þ 0 /, or ‘radar brightness’, which is not dependent on the local incidence angle was used. This value is related to ¦ 0 by a simple equation (see Raney et al., 1994 for details). The mean beta nought values for the three facies are given in the Table 1, together with the sample sizes and summary description of the facies. The mean radar brightness value increases distinctly, about half a dB in each case, from the outer medium-grained biotite facies through the inner coarse-grained biotite facies to the two-mica facies. The radar brightness difference is more pronounced between the outer and the inner biotite facies than it is between the inner biotite and the two-mica facies. This is because of the higher surface roughness effect on radar backscattering of the facies. The facies with coarser weathered rock surfaces should result in stronger backscattering and consequently higher brightness, and this accounts for the higher beta nought values obtained for Facies III compared to Facies I. The outer biotite facies has more compact textures caused by euhedral phenocrystal grain shapes than the textures of inner twomica facies, which is dominated by rounded quartz and subangular feldspar grains. As a consequence of its compact textures, and thus reduced pore space, the outer biotite facies has lower water permeability and higher surface water saturation. The low brightness values for this facies are therefore also attributed to their higher surface water content. Results of radar backscatter experiments conducted on silty and clayey soils of variable surface roughness have
shown a marked attenuation of radar signal related to increase in surface water saturation of soils (Merot and Chanzy, 1991). The first water added to a soil is tightly bound as adsorbed water and has dielectric properties somewhere between ice and liquid water (Schmugge et al., 1992), but as more water layers are added, it acquires the dielectric properties of a liquid. The most important compositional difference presently observed is the more clayey nature of the biotite facies soils and predominantly sandy (quartz-rich) nature of the two-mica facies soils. The dielectric constants of clayey soils are generally higher, due to their higher total water content (Sutinen et al., 1991). The variation in brightness values reported here is therefore a combined effect of surface roughness and water contents of the various facies. The outer biotite facies have lower radar brightness because of their finer grain size and higher volumetric water. Most secondary alterations in granitic and surrounding terranes result in the enrichment of hydrous and argillaceous minerals. In the study area, mineralization is associated with zones of hydrothermal and less frequently metasomatic alteration. The alteration zones therefore represent discrete targets that can be recognized on the basis of their radar response.
4. Conclusions The experiment carried out on the Zaer pluton is a preliminary study to demonstrate that calibrated RADARSAT data can be considered as an effective parameter to discriminate rock and soil types. These data can be obtained as new acquisitions or from RADARSAT data archives. In the present case, the rock facies had already been mapped for their compositional and textural characteristics. The study has revealed that they have also characteristic radar
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A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
backscatter responses. Mean radar brightness values derived for the main facies of the Zaer pluton are significantly different. In areas of unknown geology where slope effects can be properly modelled, radar brightness plots on geometrically rectified images should furnish the initial information on the spatial distribution of rock and soil variations and thus enable to reduce the costs of subsequent field surveys and laboratory analysis.
Acknowledgements The authors are grateful to Drs. Ed Langham of the Canadian Space Agency and Ken Jezek of Ohio State University for their review of the manuscript.
References Mahmood, A., 1985. Emplacement of the zoned Zaer pluton, Morocco. Geol. Soc. Am. Bull. 96, 931–939. Mahmood, A., 1996. Lineaments as groundwater exploration
guides in hard-rock terranes of arid regions. Can. J. Remote Sensing 22, 108–116. Mahmood, A., Carboni, S., Muller, J., Parashar, S., 1996. Potential use of RADARSAT in geological remote sensing. Proc. 11th Thematic Conf., Las Vegas, Vol. I, pp. 475–484. Merot, Ph., Chanzy, A., 1991. Mesure de l’humidite´ d’un sol nu par radar dans des conditions d’exces d’eau. Proc. 5th Int. Colloq. Physical Measurements and Signatures in Remote Sensing, Courcheval, pp. 293–296. Parashar, S., Langham, E., McNally, J., Ahmed, S., 1993. RADARSAT mission requirement and concept. Can. J. Remote Sensing 19, 280–288. Raney, R.K., Freeman, T., Hawkins, R.W., Bamler, R., 1994. A plea for radar brightness. IGARSS ’94, Pasadena, pp. 1090– 1092. Schmugge, T., Jackson, T.J., Kustas, W.P., Wang, J.R., 1992. Passive microwave remote sensing of soil moisture: results from HAPEX, FIFE and MONSOON 90. ISPRS J. Photogramm. Remote Sensing 47, 127–143. Srivastava, S., 1997. RADARSAT calibration strategy, program and results. Proc. Workshop RADARSAT Data Quality, CEOS Working Group on Calibration and Validation, Saint-Hubert, Que. Sutinen, R., Ha¨nninen, P., Mickelson, D.M., 1991. Dielectric properties of tills. IGARSS ’91, Espoo, pp. 1171–1174. Van Zyl, J.J., Dubois, P., Guerra, A., 1992. MONSOON ’90: preliminary SAR results. IGARSS ’92, Houston, pp. 71–73.
Journal of Geochemical Exploration 66 (1999) 413–420 www.elsevier.com/locate/jgeoexp
RADARSAT data applications: radar backscatter of granitic facies, the Zaer pluton, Morocco Ahmed Mahmood Ł , Surendra Parashar, Satish Srivastava RADARSAT Program, Space Operations, Canadian Space Agency, Saint-Hubert, Quebec J3Y 8Y9, Canada Accepted 24 February 1999
Abstract Radar backscatter in active microwave remote sensing is influenced by such physical properties of the ground as the surface roughness, shape and water contents, and varies with the wavelength and the angle of incident energy. Canada’s RADARSAT is the first spaceborne radar which provides flexibility of incidence angle and ground resolution. Its imaging device is a C-band Synthetic Aperture Radar (SAR), which is particularly well suited for interpreting micro-roughness and soil conditions by the appropriate choice of imaging parameters. This article describes a test case in which quantitative RADARSAT backscatter data were related to spatial variations in a granitic pluton. The Zaer pluton, one of the several Hercynian granitoids in northwestern Morocco, consists of medium- to coarse-grained biotite granodiorite and two-mica monzogranite. These are highly weathered and the rock surfaces are largely covered with granitic soils of comparable mineralogical composition. The soils on the biotite granodiorite are richer in micaceous and clayey minerals than those overlying two-mica monzogranite. The variation in radar backscatter response, measured with calibrated RADARSAT data, is attributed to the different grain size and water content of the granitic facies. It is concluded that with RADARSAT data, the radar backscatter response differences can be highlighted for the purpose of facies mapping and geochemical interpretation. 1999 Elsevier Science B.V. All rights reserved. Keywords: spaceborne radar; RADARSAT; facies mapping; geochemical interpretation
1. Introduction Radar remote sensing, which relies on the physical and electrical properties of ground targets, such as surface roughness, terrain geometry and degree of water saturation, has been used to map rock and soil units (Sutinen et al., 1991). Active radar remote sensing is independent of solar illumination and angle. The information received from radar imaging is thus different from remote sensing in the visible or infrared regions of the electromagnetic spectrum. The Ł Corresponding
author. E-mail: [email protected]
basic operating principles involved in active radar remote sensing consist of transmitting microwave energy (pulses) from the imaging device (an antenna) to a ground target and measuring properties of the signal reflected back to the device. The reflected signal is then processed into an image output. Regardless of the properties of the target, the backscattering of radar waves impinging on a surface is modulated by their wavelength ½, polarization, and angle ./ of the incident beam. For example, radar waves with wavelengths longer than the grain size of high-density target material will be subjected to specular reflection rather than ran-
0375-6742/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 6 7 4 2 ( 9 9 ) 0 0 0 0 9 - 6
414
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
dom scattering. Changes in the beam incidence angle will affect radar backscatter, which will decrease as the angle increases, particularly so if the surfaces are relatively smooth. The choice of incidence angle will also determine whether the backscatter response is dominated by dielectric constant, topography or roughness of the target. In areas of appreciable relief, topographic effects at steep incidence angles will be as apparent in radar imagery as radar beam sensitivity to surface roughness. The influence of the wavelength on radar imagery is equally important (Van Zyl et al., 1992; Schmugge et al., 1992). Images taken at shorter wavelengths (C-band) are more sensitive to vegetation and soil moisture changes than images at longer wavelengths (L- or P-band). Furthermore, the radar response will be different for the same vegetation type depending on the polarization of radar waves. Thin layers of vegetation with branches preferentially oriented vertically, for example, would attenuate vertically polarized waves more than horizontally polarized waves. In the past, it was not possible to adjust the imaging parameters of spaceborne radar systems to terrain conditions, but this changed with the launch of Canada’s RADARSAT satellite in late 1995. As a result of electronic beam steering, RADARSAT has provided for the first time the opportunity to select incidence angles and ground resolutions appropriate to the purpose for which the remote sensing data are collected (Mahmood et al., 1996). The purpose may require enhancing either surface roughness or mesoscale topography at the expense of the other. RADARSAT’s operational characteristics consist of a Synthetic Aperture Radar (SAR), which images at a frequency of 5.3 GHz (C-band) with horizontally transmit and receive (HH) polarization. Details on RADARSAT satellite can be found in Parashar et al. (1993). This terrain reflectivity of the radar beam may be expressed quantitatively with a degree of certainty by working with calibrated data and the quantitative measure may be used as a discriminant function for distinguishing ground units. RADARSAT data are calibrated and were used in this study to derive terrain reflectivity values to separate different granitic rock facies previously mapped on the basis of their grain size, texture, colour, mineralogical constituents and geochemical composition. The re-
flectivity due to slope was suppressed by the proper choice of incidence angle and by eliminating slope effects from the reflectivity values. Moreover, for the purpose of discriminating the granitic facies based on their physical properties, the slope effects in radar reflectivity were assumed to be constant because of the uniform slope variations across the various rock units in the study area.
2. Description of the study area The studied area is located in the Zaer region of northwestern Morocco and is formed of weathered peraluminous granites. They are part of an elongated pluton, approximately 50 km long and 15 km across, which intruded Ordovician to Silurian metasediments during the Hercynian orogeny. The pluton was forcefully emplaced, wedging apart the surrounding sediments, which is remarkably displayed on the RADARSAT image (Fig. 1). As elsewhere in the region, the granitic rock is highly weathered, covered with granitic ‘sands’, gravels, pebbles and boulders, compositionally similar to the underlying bed rock. Rock outcrops occur locally where the surface is strewn with coarser regolith. The soils are generally bare except during the spring to late summer growing season. The granitic terrain is by and large a subdued topography in relation to its surrounding country rocks. The topography is marked by gently undulating surfaces and a few ravines. Slope variations are identical over the entire area. Internally, the pluton can be divided into three main units (Fig. 2). Facies I is a dark-coloured, medium- to coarse-grained, sub-porphyritic, biotite granodiorite occurring all along the western margin of the pluton as well as in its northern and southern oval-shaped extremities. Facies II is represented by a light-coloured, coarse-grained, equigranular granodiorite outcropping in the central portion of the northern half of the pluton. Facies III comprises a two-mica coarse-grained monzogranite which occupies the central two thirds of the pluton. The weathered surface rock material coarsens from the outer biotite facies to the inner two-mica facies as a direct consequence of the texture of the underlying bedrock.
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
Fig. 1. RADARSAT Standard 3 beam image of the Zaer pluton, northwestern Morocco.
415
416
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
Fig. 2. Generalized facies map of the Zaer pluton (after Mahmood, 1985).
Figs. 3 and 4 summarize the major compositional trend within the pluton, which is marked by a decreasing calc-alkaline and mafic index from the mar-
gins inward (Facies I to Facies III). Mineralogically, this translates into a significantly higher content of minerals such as biotite and its clayey alteration
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
417
Fig. 3. Normative anorthite (plagioclase) variations in the Zaer pluton (contours at 10%, 20% and 30%).
products, and plagioclase, in the outer facies relative to the inner facies, which are more alkaline and richer in free silica.
3. Experiment and results The experiment is based on an October acquisition of RADARSAT during an afternoon pass over the region. This was selected to avoid the morning dew which prevails in the region during this time of
the year. The acquired data were processed with calibrated payload parameters (Srivastava, 1997) to a georeferenced image product of 100ð100 km, sampled at 8676 pixels on 8047 lines. The image was taken with a RADARSAT Standard 3 beam, with image resolution of a square pixel of roughly 25 m and a mean incidence angle of ¾32º. This angle was considered optimum given ground conditions for suppressing topographic effects while enhancing surface roughness. In the southern half of the pluton, the granitic rock outcrops sporadically, particularly on Facies III
418
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
Fig. 4. Mafic oxides (Fe2 O3 C FeO C MgO) variations in the Zaer pluton (contours at 1%, 2.4%, 4%, 5.5% and 7%).
backslopes of the Oued Chbeika fault scarp and the downthrown Facies I block in the south of the pluton (Mahmood, 1996). The scarp represents the faulted contact between the two-mica facies to the north and the biotite facies to the south. It can be clearly seen on the RADARSAT scene as an irregular bright fringe because of its corner reflector effects, and its backslopes are in the radar shadow. These areas have a totally different radar response and were excluded from the study which focuses on the soil overlying the altered granitic bedrock.
The radar backscatter was sampled on the part of the scene covering the pluton following a grid interval of 40 lines by 40 pixels. Each value was plotted at the centre of the grid squares situated within the pluton boundary drawn approximately on the RADARSAT scene. The radar energy reflected back from the ground target is given by its backscatter coefficient, expressed conventionally as ¦ 0 in units of decibels (dB), and is a measure of the reflective properties of the surface in relation to the reflected and incident energy. The coefficient ¦ 0 is normalized
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
419
Table 1 Beta nought statistics for the Zaer pluton facies Facies
Sample size
I II III
91 92 103
Mean (dB) 7.68 6.94 6.65
Summary description of facies Dark-coloured, medium- to coarse-grained, sub-porphyritic, single-mica, biotite-rich, granodiorite Light-coloured, coarse-grained, equigranular, single-mica, biotite granodiorite Two-mica, equigranular monzogranite
in terms of local incidence angle, which is a function of both the angle of the incident beam as well as the slope. The local slope angle effects are generally unknown and, for the purpose of this study, needed to be removed from the reflectivity estimates measured in relation only to the physical properties of the terrain. Therefore, a different reflectivity value known as beta nought .þ 0 /, or ‘radar brightness’, which is not dependent on the local incidence angle was used. This value is related to ¦ 0 by a simple equation (see Raney et al., 1994 for details). The mean beta nought values for the three facies are given in the Table 1, together with the sample sizes and summary description of the facies. The mean radar brightness value increases distinctly, about half a dB in each case, from the outer medium-grained biotite facies through the inner coarse-grained biotite facies to the two-mica facies. The radar brightness difference is more pronounced between the outer and the inner biotite facies than it is between the inner biotite and the two-mica facies. This is because of the higher surface roughness effect on radar backscattering of the facies. The facies with coarser weathered rock surfaces should result in stronger backscattering and consequently higher brightness, and this accounts for the higher beta nought values obtained for Facies III compared to Facies I. The outer biotite facies has more compact textures caused by euhedral phenocrystal grain shapes than the textures of inner twomica facies, which is dominated by rounded quartz and subangular feldspar grains. As a consequence of its compact textures, and thus reduced pore space, the outer biotite facies has lower water permeability and higher surface water saturation. The low brightness values for this facies are therefore also attributed to their higher surface water content. Results of radar backscatter experiments conducted on silty and clayey soils of variable surface roughness have
shown a marked attenuation of radar signal related to increase in surface water saturation of soils (Merot and Chanzy, 1991). The first water added to a soil is tightly bound as adsorbed water and has dielectric properties somewhere between ice and liquid water (Schmugge et al., 1992), but as more water layers are added, it acquires the dielectric properties of a liquid. The most important compositional difference presently observed is the more clayey nature of the biotite facies soils and predominantly sandy (quartz-rich) nature of the two-mica facies soils. The dielectric constants of clayey soils are generally higher, due to their higher total water content (Sutinen et al., 1991). The variation in brightness values reported here is therefore a combined effect of surface roughness and water contents of the various facies. The outer biotite facies have lower radar brightness because of their finer grain size and higher volumetric water. Most secondary alterations in granitic and surrounding terranes result in the enrichment of hydrous and argillaceous minerals. In the study area, mineralization is associated with zones of hydrothermal and less frequently metasomatic alteration. The alteration zones therefore represent discrete targets that can be recognized on the basis of their radar response.
4. Conclusions The experiment carried out on the Zaer pluton is a preliminary study to demonstrate that calibrated RADARSAT data can be considered as an effective parameter to discriminate rock and soil types. These data can be obtained as new acquisitions or from RADARSAT data archives. In the present case, the rock facies had already been mapped for their compositional and textural characteristics. The study has revealed that they have also characteristic radar
420
A. Mahmood et al. / Journal of Geochemical Exploration 66 (1999) 413–420
backscatter responses. Mean radar brightness values derived for the main facies of the Zaer pluton are significantly different. In areas of unknown geology where slope effects can be properly modelled, radar brightness plots on geometrically rectified images should furnish the initial information on the spatial distribution of rock and soil variations and thus enable to reduce the costs of subsequent field surveys and laboratory analysis.
Acknowledgements The authors are grateful to Drs. Ed Langham of the Canadian Space Agency and Ken Jezek of Ohio State University for their review of the manuscript.
References Mahmood, A., 1985. Emplacement of the zoned Zaer pluton, Morocco. Geol. Soc. Am. Bull. 96, 931–939. Mahmood, A., 1996. Lineaments as groundwater exploration
guides in hard-rock terranes of arid regions. Can. J. Remote Sensing 22, 108–116. Mahmood, A., Carboni, S., Muller, J., Parashar, S., 1996. Potential use of RADARSAT in geological remote sensing. Proc. 11th Thematic Conf., Las Vegas, Vol. I, pp. 475–484. Merot, Ph., Chanzy, A., 1991. Mesure de l’humidite´ d’un sol nu par radar dans des conditions d’exces d’eau. Proc. 5th Int. Colloq. Physical Measurements and Signatures in Remote Sensing, Courcheval, pp. 293–296. Parashar, S., Langham, E., McNally, J., Ahmed, S., 1993. RADARSAT mission requirement and concept. Can. J. Remote Sensing 19, 280–288. Raney, R.K., Freeman, T., Hawkins, R.W., Bamler, R., 1994. A plea for radar brightness. IGARSS ’94, Pasadena, pp. 1090– 1092. Schmugge, T., Jackson, T.J., Kustas, W.P., Wang, J.R., 1992. Passive microwave remote sensing of soil moisture: results from HAPEX, FIFE and MONSOON 90. ISPRS J. Photogramm. Remote Sensing 47, 127–143. Srivastava, S., 1997. RADARSAT calibration strategy, program and results. Proc. Workshop RADARSAT Data Quality, CEOS Working Group on Calibration and Validation, Saint-Hubert, Que. Sutinen, R., Ha¨nninen, P., Mickelson, D.M., 1991. Dielectric properties of tills. IGARSS ’91, Espoo, pp. 1171–1174. Van Zyl, J.J., Dubois, P., Guerra, A., 1992. MONSOON ’90: preliminary SAR results. IGARSS ’92, Houston, pp. 71–73.