Preliminary analysis of shuttle multispectral radiometer data for Southern Egypt

ildv. Space Sec. Vol.3, No.2, pp.125—132, 1983 Printed in Great Britain. All rights reserved.

0273—1177/83/020125—08$04.0O/O Copyright ©COSPAR

PRELIMINARY ANALYSIS OF SHUTTLE MULTISPECTRAL RADIOMETER DATA FOR SOUTHERN EGYPT Lawrence C. Rowan*, Alexander F. H. Goetz** and Marguerite J. Kingston* *U. S. Geological Survey, Reston, Virginia, U.S.A. **Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.

ABSTRACT The Shuttle Multispectral Infrared Radiometer (SMIRR) is a spectroradiometer covering the region from 0.5 to 2.5 pm in 10 channels that acquired data from spots 100 m in diameter along the subspacecraft ground track. It was flown aboard the second flight of the space shuttle Columbia, November 12—14, 1981. Data collected during orbit 16 over southern Egypt show that carbonate rocks, kaolinite, and possibly montmorillonite can be identified by their SMIRR spectral signatures and limited knowledge of the lithologic units present. Detailed analysis of SMIRR data for this area indicates that calcite, kaolinite, and montmDrillonite rocks give rise to absorption features that result in characteristic 10 channel spectra. INTRODUCTION The Shuttle Multispectral Infrared Radiometer (SMIRR) was designed to (1) obtain spectroradiometric measurements from orbit of land surfaces in 10 wavelength channels known to be useful for mineral and rock identification, (2) determine the spectral response of known rock types under different climatic conditions, and (3) establish the utility of orbital narrow—band radiometry in the region from 2.0 to 2.5 pm for direct identification of rocks and minerals and establish the requirements for future narrow—bend imaging systems [1]. During the second flight of the space shuttle Columbia on November 12—14, 1981 [2], the SMIRR acquired approximately 400,000 spectra along 17 orbits under cloud—free conditions over the Eastejn United States, Mexico, southern Europe, North Africa, the Middle East, and China. ~Each 10—channel spectrum (fig. 1, table 1) represents a 100—m diameter area on the ground. Planned coverage of Australia, South America, and South Africa was precluded by unfavorable lighting conditions resulting from the 2—hour launch delay on November 12, 1981. Initial analysis of SMIRR data acquired over western Egypt during orbit 16 indicated that carbonate rocks, as well as kaolinite— and, perhaps, n~ntmorillonite—bearing sediments and rocks, can be identified in this very sparsely vegetated region 131. Further analysis and compilation of geologic maps and lithologic information along the ground track of this orbit substantiate these initial results and suggest that many of these sedimentary units are characterized by different features in the SMIRR spectra that express mineralogical variations. This paper briefly describes the SMIRR instrument, the calibration and data— reduction procedures, and the results obtained thus far along orbit 16. INSTRUMENT The main elements of the SMIRR consist of two bore—sighted 16—mm cameras, coaligned radiometer optics, a spinning filter wheel, two thermoelectrically cooled detectors, and associated timing and signal conditioning electronics. Data were recorded on the shuttle payload tape recorder. The ground location of the 100—m diameter, instantaneous field of view (IFOV) of the radiometer was determined by the use of two 16—mm bore—sighted cameras containing glass platens bearing fiducial marks. The SMIRR telescope and the cameras were aligned with a laboratory collimator 2 years before this mission. Timing information to the nearest 0.01 second is printed on one edge of the film to allow postflight location of the ground track

125

126

L.C. Rowan, A.F.H. Goetz and M.J. Kingston

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TABLE 1

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SMIRR ground track showing overlapping 100—rn diameter samples (IFOV’s, instantaneous fields of view).

Shuttle Multispeceral Infrared Radiometer Data

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Southern Egypt

127

by (1) determining in the radiometer output the locations of high—contrast boundary crossings, both perpendicular and oblique to the flight path, and (2) correlating them with the timed 16—mm photography. The postflight measurements show that the laboratory collimation remained intact to within ±1IFOV. During flight, both cameras were triggered at 1.28—second intervals. The data were oversainpled by 30% along the ground track (fig. 2) so that resampling can be used to realign the individual filter measurements to common ground locations.

CALIBRATION Two types of calibration

procedures were applied to the SMIRR. The absolute calibration of the instrument was carried Out in the laboratory by means of a calibrated light source referenced to a U.S. National Bureau of Standards standard. Calibration during the flight was obtained by the use of internal lamps. A variation in gain of as much as 10% was measured in the 10°Crange in electronics and filter—wheel temperatures encountered during flight. The source of the gain change is not yet understood. Prior to integration of the SMIRR on the payload pallet, aircraft [4] and ground tests were conducted to determine the response of the instrument to known targets. Figure 3 shows the results of ground tests on various minerals known to have absorption bands in the 2—2.5 pm region. Continuous laboratory spectra were obtained on a Beckman 52401 spectrophotometer modified for digital recording. In order to obtain spectral reflectance within the SMIRR filter bandpasses, the continuous spectra were weighted with the filter transmission spectra produced on the same instrument according to: k

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the continuous spectrum of the sample used to cover the complete filter standard for reference, whereas the correction for the Flberfrax/BaSO 4 ratio was also incorporated. The difference in absolute reflectance between the laboratory and SMIRR measurements (fig. 3) probably can be attributed to the fact that the laboratory measurements are made with an integrating sphere and the SMIRR measurement is bidirectional with the sun as a source. However, the absolute values of reflectance are not as important to this analysis as the shape of the SMIRR spectra. DATA REDUCTION In order to acquire an overview of the data and for ease of analysis, the continuous radiometer data (fig. 4) have been broken into line segments of 1.28—seconds. Each 1.28— seconds segment is referred to as a line (L), and each point on the line represents a spectrum (S) of a 100—rn diameter spot on the ground. Each line segment contains 128 spectra, and the beginning of each line coincides with the center of the 16—mm frame of the camera containing black—and—white film. In this paper, the analysis is based on plots of radiometric profiles for individual channels or ratios of channels as a function of time, as well as on individual 10—channel spectra. The production of surface—reflectance spectra from the SMIRR requires knowledge of the intervening scattering and absorbing atmosphere. The LOWTRAN—5 atmospheric model [5] can be used to describe the atmospheric absorption from the source to the SMIRR, but this approach was not used because the model requires variables such as visibility and water content at various altitudes that are not available for orbit 16. Therefore, the technique used for the preliminary analysis was to acquire surface samples and calibrate the SMIRR signal on the basis of laboratory spectra of the samples. This method is imperfect, because the natural surface cannot be preserved during sampling and because position location using prelaunch orbital predictions was imprecise. However, an experimental calibration was performed using samples taken near the ground track of orbit Any use of trade names in this report is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.

128

L.C. Rowan, A.F.H. Goetz and M.J. Kingston

POSITIONS OF BANDS. MICROMETERS

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Shuttle

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Data

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Southern

Egypt

129

16 in the vicinity of Kharga, Egypt, and the results obtained are consistent with the mapped surface units. Dune sand samples consisting of iron—oxide—stained quartz as well as abundant calcite and minor kaolinite and gypsum and a kaolinite—rich playa sample were examined. The procedure used for normalization is outlined by Goetz and others [31. In the future, in—situ spectral—reflectance measurements, using a radiometer equipped with the SMIRR filter set, should provide the most accurate calibration attainable.

DESCRIPTION OF GEOLOGIC UNITS The study area between Kharga and Aswan, Egypt, consists of nearly horizontal Cretaceous and lower Tertiary sedimentary rocks which are partially overlain by Quaternary units including extensive north—oriented dunes, playas, and gravel mounds, sheets, and spreads (fig. 5). Nubia strata (Kn, fig. 5) underlie most of the Kharga depression, but exposures are sparse owing to the presence of extensive sand dunes and plays deposits [61. In the southeastern part of the study area, tabular crossbedded sandstone typical of the Nubia Formation [8] is exposed where the ground track of orbit 16 crosses the lower slopes of a small upland area. Cyclic sequences are common in outcrop there, each cycle consisting of a thin basal layer of medium— to coarse—grained sandstone, a thick middle unit of tabular crosabedded granular sandstone, and a thin upper unit of laminated fine—grained sandstone and ailtstone containing thin beds of clay [8]. Kaolinitic massive sandstone containing root traces commonly constitutes the uppermost bed in these sequences. Kaolinite—rich sandstone has been described in other facies of the Nubia Formation [91,but these facies are not well exposed along this part of the ground track of orbit 16. Unit Ku (fig. 5) consists mainly of shale, carbonate rocks, and phosphatic beds which are commonly limonitic. These rocks are exposed on the lower slopes of the escarpments both east and west of Kharga and in the southeastern part of the study area (fig. 5). Paleocene and Eocene limestones (Tp and Te, fig. 5) are widespread in the study area. The best exposures are on the Kharga escarpment, where a siliceous, highly polished, hackly— weathering limestone caps the escarpment [6, 7]. Although unit Te underlies most of the central part of the area traversed by orbit 16, dunes and gravel deposits cover much of this region. The gravel deposits, shown as unit Q near the center of figure 5, are especially interesting because, as discussed below, the SMIRR spectra indicate that their mineral content is different from that of the other units. The gravel mounds consist of angular pebbles and gravels of silicified limestone, chert, and flint and are cemented by red— brown clay [7]. Gravel sheets are also common in the central part of the study area, but they either are not indurated or are cemented with tufa [7]. ANALYSIS OF SMIRR SPECTRA The initial analysis of SMIRR spectra collected during orbit 16 resulted in identification of kaolinite in a plays in the Kharga depression (L262, S41—50, fig. 5) and limestone in unit Ku west of the depression (L261, S56, fig. 5). These identifications were verified through discussions with Dr. Farouk El Baz (National Air and Space Museum, Smithsonian Institution, Washington, D.C., U.S.A.) and Dr. Rushdi Said (Southern Methodist University, Dallas, Texas, U.S.A.), both of whom are conducting research in this area. In addition, montmorillonite appears to have been identified in a gravel mound east of the Kharga escarpment (pers. communication, Dr. Said). Although this mound is not shown on the geologic map in figure 5, the location of the mean SMIRR spectrum for S80—128, line segment L272 indicates its approximate location. The procedure used in this preliminary analysis of SMIRR data was first to examine the continuous plots of calculated ratios (fig. 4) for locating oscillations above or below the background values. Next, normalized spectra were calculated for the part of the line segment where these oscillations are present. Then the cause of the features observed in these spectra was determined by comparing the normalized SMIRR spectra with SMIRR spectra obtained in the laboratory for approximately 60 commonly occurring rocks and minerals. Care was taken to avoid shadows, because shadowed areas yield unreliable data. An important consideration in the analysis was the general lithology of the study area. Although specific mineralogic information was not available for any particular 100—rn SMIRR measurement, the range of mineralogic causes for the SMIRR spectral features was limited because only sedimentary rocks are present. For example, the occurrence of a 2.35—pm absorption band could be attributed to carbonate rocks, rather than to serpentine or some other mineral commonly associated with igneous or metamorphic rocks. Examination of the continuous plots of the 2.20 pm/2.35 pm and 2.22 pm/2.35 pm ratios in

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Orbit 16 ground track on geological map of the Kharga—Aswan, Egypt region [10]. In the geologic map, heavy solid lines represent faults. The spectral reflectance relative to sample KSII is plotted with respect to wavelength. In spectral plots, solid lines represent mean spectral reflectance, and dashed lines represent standard deviations, except in the upper left plot which is a single spectrum.

Shuttle Multispectral Infrared Radiometer Data

—

Southern Egypt

131

the west—central part of the study area (fig. 4) indicated the presence of carbonate rocks where these ratios increase abruptly. This increase arises because the mean SMIRR spectrum shows a low reflectance in the 2.35 pm SMIRR channel (L267, S58—74), similar to the SMIRR spectrum obtained in the laboratory for calcite (fig. 3). These ratios are also relatively high in the southeastern part of the study area (not shown in fig. 4). The highest ratio values in the southeastern part of the area are represented by the mean SMIRR spectrum calculated for spectra S91—121 in line segment L288 (fig. 5). Again, the 2.35 pm reflectance is very low, strongly indicating the presence of carbonate rocks. The large standard deviations in this and the L267, S58—74 mean SMIRR spectrum are undoubtedly due to brightness variations induced by topographic slope. The L288, S91—121 mean spectrum and the L261, S56 spectrum discussed by Goetz and others [3] appear to represent carbonate rocks in unit Ku (fig. 5). The carbonate feature of the L267, S58—74 mean spectrum is due to the Paleocene and the Eocene limestone sequences exposed on the escarpment east of Kharga (units Tp and Te, fig. 5). Southeast of mean spectrum L288, S91—121, the 2.20 pm/2.35 pm and 2.22 pm/2.35 pm ratio values decrease concurrent with a slight decrease in the 2.20 pm/2.22 pm ratio (not shown in fig. 4). The mean SMIRR spectrum calculated for L292, 51—50 has a slightly depressed 2.20 pm reflectance (fig. 5). Because of the similarity between the shape of this mean SMIRR spectrum (fig. 5) and the SMIRR laboratory spectrum for kaolinite (fig. 3) and the presence of kaolinite in the Nubia strata in this region, we attribute the 2.20 jim absorption feature to molecular vibrations in kaolinite. Also, note the similarity of these SMIRR spectra to the mean SMIRR spectrum obtained for the kaolinite—bearing plays deposit (L262, S41—50, fig. 5). The 2.20 pm feature is only faintly evident in SMIRR spectra representing the area of Quaternary deposits southeast of L292, S1—50 (fig. 5), but it is consistently present in spectra obtained for the area of Nubia strata situated between the eastern boundary of the Quaternary deposits and the Nile River. The Quaternary deposits shown within the broad area of unit Te near the center of figure 5 appear dark in the SMIRR 16—mm photographs and show low reflectance in the continuous plots of the 1.60 pm and 2.20 pm channels. In this respect, these areas are similar to the gravel mound represented by the mean SMIRR spectrum for L272, S80—128 (fig. 5). Therefore, mean SMIRR spectra were calculated for these two areas. In both mean spectra (L278, S62—112 and L283, S123 to L284, S44, fig. 5), as in the L272, S80—128 spectrum, the difference between the 2.17 pm and 2.20 pm band reflectance is larger than this difference in the other spectra obtained. This feature is attributed to the presence of montmorillonite because it is similar to the SMIRR laboratory spectrum for montmorillonite (fig. 3). The red—brown clay matrix in sandstone of the Nubia Formation, which underlies this area, may be montmorillonite, which commonly is coated with ferruginous oxides. This and the other mineralogical assignments proposed above must be verified by detailed field and laboratory studies. CONCLUSIONS The results of this preliminary analysis of SMIRR data obtained along orbit 16 over Egypt substantiate the initial conclusions concerning the identification of carbonate rocks, kaolinite, and montmorillonite [3]. In addition, many of the exposed lithologic units can be mapped along the shuttle ground track for considerable distances because some of the minerals contain~d give rise to characteristic absorption features that can be detected through analysis of the SMIRR data. However, detailed field sampling and mineralogical analysis in the laboratory are needed for verification of these conclusions. ACKNOWLEDGEMENTS The authors wish to thank Jet Propulsion Laboratory personnel Mary Brownell and Cohn Mahoney and the other members of the SMIRR engineering team for their support in building, calibrating, and integrating the SMIRR; Helen Paley for her many years of work with the preparation, flight planning, and flight operation of the SMIRR; and John Reimer and Elsa Abbott for data—reduction support. Our thanks to Dr. Farouk El Baz and Dr. Rushdi Said for contributions made to the interpretation of the data. The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the U.S. National Aeronautics and Space Administration, and by the U.S. Geological Survey.

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L.C. Rowan, A.F.H. Goetz and M.J. Kingston

REFERENCES CITED

1.

A. F. H. Goetz and L. C. Rowan,

2.

J. V. Taranik and M. Settle, Science 214, 619 (198l)~

3.

A. F. H. Goetz, L. C. Rowan, and M. J. Kingston, in: International Geoscience and Remote Sensing Symposium (IGARSS ‘82), Munich, Germany, (in press).

4.

A. F. H. Goetz, Society of Photo—Optical Instrumentation Engineers Proceedings 268, 17 (1981).

5.

F. X. Kneizys, E. P. Shettle, W. 0. Gallery, J. H. Chetwynd, Jr., L. W. Abreu, J. E. A. Selby, R. W. Fenn, and R. A. McClaskey, U.S. Air Force Geophysical Research Laboratory Technical Report AFGRL—TR—80—OO67(1980).

6.

R. Said, The Geology of Egypt:

7.

R. Said,

in:

Science 211, 781 (1981).

New York, Elsevier, 1962.

The Prehistory of the Egyptian Sahara:

Academic Press, New York,

1981,

p. 281. 8.

W. C. Ward and K. C. McDonald,

American Association of Petroleum Geologists Bulletin

63, 975 (1979).

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10.

E. Klitzsch, J. C. Harms, A. Lejal—Nicol, and F. K. List, American Assocociation of Petroleum Geologists Bulletin 63, 967 (1979), Egyptian Geological Survey, Geologic Map of Egypt:

19, scale 1:2,000,000 (1981).