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The application of remote sensing techniques to regional geological survey in China
AJv. Spae~!?ea. Printed
Vol.3, No.2, pp.91—94, 1983
in Great Britain.
All
rights
02731177/83/020091—04$03.0O/O
reserved.
Copyright
© COSPAR
THE APPLICATION OF REMOTE SENSING TECHNIQUES TO REGIONAL GEOLOGICAL SURVEY IN CHINA Huang Sihuang Geological Bureau of Sichuan Province, Ministry of Geology, The People’s Republic of China
ABSTRACT Information is provided on the applications of remote sensing to regional geological survey and mapping on medium and small scale over recent years in China. The work began from establishing interpretation key for strata, magmatic rocks and regional slructures. preliminary results have been obtained. It is explained through case histories ix, Sichuan Province and Xinjiang Uygur Autonomous Region. By means of remote sensing the time of compilation has been shortened and the cost was reduced, the accuracy and quality of geological maps was improved, and abundant basic geological data were provided for exploration. In addition, prediction of various ore targets were defined along with new ideas for geological scientific research. INTRODUCTION In recent years, some of the remote sensing techniques (mainly the geological interpretation of aerial photo and Landsat images) in China were extensively employed both in regional geological surveys at different scales and in reconstructing maps of geology and mineral resources, on a small to medium scale, thus enabling geologists to double and redouble the efficiency of map compilation, with reduced cost. New geological knowledge was developed and both accuracy and quality of maps improved. Furthermore, a good number of new keys fundamental geological information for prospecting for various ore minerals and geological research work were developed. 1. STRATIGRAPHIC INTERPRETATION From 1977 to 1980, a program of regional geological survey at a scale of 1:200,000 was carried out in an area of about 150,000 square kilometers within the Sichuan Basin. Careful analysis of the imagery and the establishment of interpretation indicators helped define strata of different geologic ages into mapping units. It was possible to delineate the boundaries of 39 mapping units ranging in age from the Cambrian to the Quaternary, using a combination of aerial photos and Landsat images to trace and interpret along stratigraphic trends. Good results were achieved with respect particularly to the widely distributed Mesozoic red bed deposits, which were characterised by complicated variations in lithology and lithofacies (plate 1). More than 80% of the defined boundaries were proven by actual field examination, with part of them completely corresponding to the actual field situation. Thus, one could get rid of the disorder arising from complicated stratigraphic correlation, with some generalization attained throughout the region. At the same time, according to the circular zoning displayed on the image in combination with field investigation, the Cretaceous ‘Jiading Group with thickness of more than 1,000m. in southern Sichuan Province was divided into four stratigraphic units. Detailed subdivision was also made with respect to the widely developed Quaterriary deposits over the Chengdu Plain, which had been the difficult ones to be subdivided and mapped. Valley terraces of individual rivers and ancient channels in the basin were further delineated. It took less than four years to compile 20 maps of regional geology at a scale of 1:200,000, using the results of interpretation mentioned above in conjunction with the field investigation, as compared to at least ten years which previously would have been required for compiling the same number of maps employing conventional mapping approaches. In the Xinjiar.g Uygur Autonomous Region the same method was also successfully used in regional geological survey on the scale of 1:50,000 and 1:200,000 as well as in recompilation of maps on the scale of 1:500,000 to 1:1,000,000. It is well known that this part of the country is vast in territory of difficult access and sparse in population. It was impossible to run a sufficient number of traverses in previous regional geological survey, consequently some omissions oF’ individual layers and mistakes in ~tratigraphic correlation could not ho 91
Huang Sihuang
avoided. Geological interpretation of images in combination with field investigation allowed us to correct some mistakes in stratigraphic correlation. For example, in some localities, Tertiary had been mistaken for Quaternary, Triassic for Carboniferous, Permian for Sinian, Sinian for Cambrian or vice versa. Some omitted layers of different ages were added. For examples, in Kuluketage area, 8—9 stratigraphic formations or mapping units, ranging from the Sinian to the mid—Ordovician were identified within a large area of the Sinian. The presinian strata or rocks defined before, with their extensive distribution and structural morphology were accurately delineated (Fig.I); the large area east of Shanshan originally taken for the lower Carboniferous was actually of Triassic age; Paleogene rocks were identified within a large area of the Quaternary on the eastern bank of Lop Nur; in the eastern part of Aqi Mts. the unconformable boundary between the lower and middle Carboniferous had been assigned to the mid—Carboniferous, while the lower part of the mid—Carboniferous was assigned to the lower Carboniferous. The remote sensing images showed that the unconformable boundary ought to have been moved downward. As a result, not only the unconformable boundary between the middle and lower Carboniferous was revised, but so also was the stratigraphical age. In the course of interpretation, facies changes were also studied according to the imagery variation of the same stratigraphic unit. For example, the Triassic deposits tended to change from the east to the west in a more complicated manner. The Feixianguan Formation of the lower Triassic series was gradually changed from limestone into sandstone and mud stone; the Leikoupo Formation of the mid—Triassic series was gradually changed from sandstone and mud stone into limestone; the Xujiahe Formation of the upper Triassic series was gradually changed from a single thick sandstone into multi—layered sandstone intercalated with shales and coal beds. All these features mentioned above were well indicated on mages. The same result came about in the Triassic southwest of Guizhou Province where the abrupt change from sandstone into limestone was also well indicated on images. 2. GEOLOGICAL INTERPRETATION OF MAGMATIC ROCKS All kinds of magmatic intrusive bodies and dykes were distinguished from surrounding sedimentary deposits or metamorphic rocks on remote sensing images, especially in areas where bedrocks were exposed. For instance, in the plateau country of western Sichuan Province where accessibility was rather difficult, granite bodies intruding into the Triassic black slates were accurately located by means of images. Due to the limitation of observations along ground traverses, some mapping at a small to medium scale was dune in areas where less developed magmatic rocks in the Xinjiang Uygur Autonomous Region frequently resulted in omission of part of the rock bodies, especially small ones. The delineated bodies were distorted to some extent both in size and morphology and presented uncertainties in their relationship. Based on the analysis and geological interpretation of features indicated on images, some distinguishing characteristics were built up with respect to the middle and late Variscian intrusives (mainly granitic bodies). The middle intrusives (rd) were charac— tensed by different size, occurring as elongated bodies with more complicated configuration whose extension generally followed the main tectonic trends. Vein rocks were very developed within the bodies with intensive alteration along their outer contacts; they appeared in the scene to be of darker tone, while the late intrusives (ri) occurred typically as circular or elliptical and generally distributed along fracture zones. There were few vein rocks cutting through them; alteration along contacts appeared quite weak; tone was light to white. On this premise all these bodies were redelineated with some previous mistakes corrected. At the same time, some lithofacies zones and contacts within rock bodies themselves were located more accurately. Furthermore, a number of new small intrusive bodies and dykes were also found in the course of interpretation, with their relationship defined, thus providing more reliable information for the study of magmatism and its effect on ore formation throughout the region. In addition, some rock bodies half covered by desert (Gobi) whose boundaries could hardly be delineated accurately in a survey on the ground, appeared to be quite clear on images (Plate 2). Volcanic eruptive rocks of Cenozoic age were easily mapped due to their well—preserved original eruptive morphology. Delineation of volcanic cones of different shapes and the extent of distribution of lava flows were defined. Precenozoic volcanic rocks were mapped on the basis of their geomorphology, circular structure, radial fracture and drainage pattern, the paleovolcanic craters could be identified and the configuration or the extent of the distribution of paleovolcanic lava flows could be delineated. The interpreted paleovolcanic apparatus of the early—and mid—Carboniferous in the Xinjiang Uygur Autonomous Region was confirmed by field examinations. 3. INTERPRETATION OF REGIONAL STRUCTURE The definition of regional structure based on the analyses of images probably was the most effective aspect of the geological interpretation as a whole. In mountainous country one could identify folds of different morphology and faults extending in different orientations. Joints were mapped which might be readily omitted in a field survey. Unconformable boundaries between sedimentary layers were delineated aecurately an’ extended under o~r~rhiirden
Remote Sensing
—
Regional Geological Survey in China
93
consisting of the Quaternary bess, desert sand and gravel horizon and other less consolidated sandstone. A great number of hidden folds of different morphology and faults were mapped in different direction. It was obvious that the discovery of these features not only allowed to correct some mistakes of all kinds of maps of structure and geology compiled previously, but also to provide abundant materials for structural analysis and study of structural control of mineralization throughout the region. For instance, in a regional geological survey carried out in the Sichuan Basin, a variety of folds and their relationships (including bifurcation, oblique intersection and superimposition) were defined precisely thanks to the geologic interpretation of images with their extension and size revealed more pronounced. In the meantime, a number of northwest and east—west structures and some buried domes and hidden faults covered by a thick sandstone horizon were also disclosed (Plate 2). In the course of map compilation of the Xinjiang region, not only a large number of north—west and north—south faults of different nature were revealed, but some major faults of 500——1,SOO kilometres long, which were of geological significance, were also reconstructed. Furthermore, dozens of buried faults of a considerable size were found to extend under the desert cover over the north—eastern margin of the Talimu Basin from the Bositeng Lake to the Lop Nur (Plate 3). In addition, a great number of ring—like fractures, circular structures and circular colour halos of varied sizes almost never reflected on the geologic ma a published before, were discovered. These features were most likely to be related to some of the endogenetic mineral deposits or oil and gas accumulations. CONCLUSION Remote sensing techniques undoubtedly have a great advantage when used in regional geological survey. Not only do they save time and money, but the result of geological interpretation can enrich the contents of a map of geology and mineral resources, making it more reasonable and more consistent with reality. Improved knowledge of regional geology can provide more reliable basis for mineral genetic analysis and prediction, relevant to some of’ the mineral resources and to guide further prospecting and exploration. II. should be noted, however, the geological application of remote sensing techniques has certain limitations, at least for the time being. The imagery of the same geological unit may be quite different, depending on the variety of physiographical environments, the density of vegetation and the intensity of weathering. For example, a super thick vegetation cover can obscure indicators for geological interpretation, bringing about difficulties in interpretation, and resulting even in misleading information. Therefore, it should be particularly emphasised that the geological interpretation of images must be combined with field investigation. If possible, some kind of integrated interpretation involving geophysical and geochemical data as completely as possible is desired. Only in this way, a better result and a satisfactory map of geology and mineral resources can be expected.
__
Plate 1. Silurian, Perrniau—Jurassjc Strata image S—shale; q ~limestone; P~—flint limestone; ~ —siltstone and mudstone; T 1~—limestone; T3,, —sandstone intercalated shale and coal beds; J—mudstone, finesandstone and siltstone inter— bedding. (Sichuan basin, aerophoto)
~
Plate 2. A granite intruded into the anticlinal axis. 1—granite porphyry; 2—red granite; 3—hornstone; 4—dikes of basic rock. (Northwest of Gansu Province, aerophoto)
94
Iluang Sihuang
~
~
~1YIUi!2~1~~IIII c
~
~
TQ
/
Ia
3~4 ~ ~
J
‘~‘
Figure I. The comparing maps of before and after re—compilation. It forms only a small part of a 1:1,000,000 scale in Wulumuqi area. Left—mapping in 1964; right—map recently recompiled.
Vol.3, No.2, pp.91—94, 1983
in Great Britain.
All
rights
02731177/83/020091—04$03.0O/O
reserved.
Copyright
© COSPAR
THE APPLICATION OF REMOTE SENSING TECHNIQUES TO REGIONAL GEOLOGICAL SURVEY IN CHINA Huang Sihuang Geological Bureau of Sichuan Province, Ministry of Geology, The People’s Republic of China
ABSTRACT Information is provided on the applications of remote sensing to regional geological survey and mapping on medium and small scale over recent years in China. The work began from establishing interpretation key for strata, magmatic rocks and regional slructures. preliminary results have been obtained. It is explained through case histories ix, Sichuan Province and Xinjiang Uygur Autonomous Region. By means of remote sensing the time of compilation has been shortened and the cost was reduced, the accuracy and quality of geological maps was improved, and abundant basic geological data were provided for exploration. In addition, prediction of various ore targets were defined along with new ideas for geological scientific research. INTRODUCTION In recent years, some of the remote sensing techniques (mainly the geological interpretation of aerial photo and Landsat images) in China were extensively employed both in regional geological surveys at different scales and in reconstructing maps of geology and mineral resources, on a small to medium scale, thus enabling geologists to double and redouble the efficiency of map compilation, with reduced cost. New geological knowledge was developed and both accuracy and quality of maps improved. Furthermore, a good number of new keys fundamental geological information for prospecting for various ore minerals and geological research work were developed. 1. STRATIGRAPHIC INTERPRETATION From 1977 to 1980, a program of regional geological survey at a scale of 1:200,000 was carried out in an area of about 150,000 square kilometers within the Sichuan Basin. Careful analysis of the imagery and the establishment of interpretation indicators helped define strata of different geologic ages into mapping units. It was possible to delineate the boundaries of 39 mapping units ranging in age from the Cambrian to the Quaternary, using a combination of aerial photos and Landsat images to trace and interpret along stratigraphic trends. Good results were achieved with respect particularly to the widely distributed Mesozoic red bed deposits, which were characterised by complicated variations in lithology and lithofacies (plate 1). More than 80% of the defined boundaries were proven by actual field examination, with part of them completely corresponding to the actual field situation. Thus, one could get rid of the disorder arising from complicated stratigraphic correlation, with some generalization attained throughout the region. At the same time, according to the circular zoning displayed on the image in combination with field investigation, the Cretaceous ‘Jiading Group with thickness of more than 1,000m. in southern Sichuan Province was divided into four stratigraphic units. Detailed subdivision was also made with respect to the widely developed Quaterriary deposits over the Chengdu Plain, which had been the difficult ones to be subdivided and mapped. Valley terraces of individual rivers and ancient channels in the basin were further delineated. It took less than four years to compile 20 maps of regional geology at a scale of 1:200,000, using the results of interpretation mentioned above in conjunction with the field investigation, as compared to at least ten years which previously would have been required for compiling the same number of maps employing conventional mapping approaches. In the Xinjiar.g Uygur Autonomous Region the same method was also successfully used in regional geological survey on the scale of 1:50,000 and 1:200,000 as well as in recompilation of maps on the scale of 1:500,000 to 1:1,000,000. It is well known that this part of the country is vast in territory of difficult access and sparse in population. It was impossible to run a sufficient number of traverses in previous regional geological survey, consequently some omissions oF’ individual layers and mistakes in ~tratigraphic correlation could not ho 91
Huang Sihuang
avoided. Geological interpretation of images in combination with field investigation allowed us to correct some mistakes in stratigraphic correlation. For example, in some localities, Tertiary had been mistaken for Quaternary, Triassic for Carboniferous, Permian for Sinian, Sinian for Cambrian or vice versa. Some omitted layers of different ages were added. For examples, in Kuluketage area, 8—9 stratigraphic formations or mapping units, ranging from the Sinian to the mid—Ordovician were identified within a large area of the Sinian. The presinian strata or rocks defined before, with their extensive distribution and structural morphology were accurately delineated (Fig.I); the large area east of Shanshan originally taken for the lower Carboniferous was actually of Triassic age; Paleogene rocks were identified within a large area of the Quaternary on the eastern bank of Lop Nur; in the eastern part of Aqi Mts. the unconformable boundary between the lower and middle Carboniferous had been assigned to the mid—Carboniferous, while the lower part of the mid—Carboniferous was assigned to the lower Carboniferous. The remote sensing images showed that the unconformable boundary ought to have been moved downward. As a result, not only the unconformable boundary between the middle and lower Carboniferous was revised, but so also was the stratigraphical age. In the course of interpretation, facies changes were also studied according to the imagery variation of the same stratigraphic unit. For example, the Triassic deposits tended to change from the east to the west in a more complicated manner. The Feixianguan Formation of the lower Triassic series was gradually changed from limestone into sandstone and mud stone; the Leikoupo Formation of the mid—Triassic series was gradually changed from sandstone and mud stone into limestone; the Xujiahe Formation of the upper Triassic series was gradually changed from a single thick sandstone into multi—layered sandstone intercalated with shales and coal beds. All these features mentioned above were well indicated on mages. The same result came about in the Triassic southwest of Guizhou Province where the abrupt change from sandstone into limestone was also well indicated on images. 2. GEOLOGICAL INTERPRETATION OF MAGMATIC ROCKS All kinds of magmatic intrusive bodies and dykes were distinguished from surrounding sedimentary deposits or metamorphic rocks on remote sensing images, especially in areas where bedrocks were exposed. For instance, in the plateau country of western Sichuan Province where accessibility was rather difficult, granite bodies intruding into the Triassic black slates were accurately located by means of images. Due to the limitation of observations along ground traverses, some mapping at a small to medium scale was dune in areas where less developed magmatic rocks in the Xinjiang Uygur Autonomous Region frequently resulted in omission of part of the rock bodies, especially small ones. The delineated bodies were distorted to some extent both in size and morphology and presented uncertainties in their relationship. Based on the analysis and geological interpretation of features indicated on images, some distinguishing characteristics were built up with respect to the middle and late Variscian intrusives (mainly granitic bodies). The middle intrusives (rd) were charac— tensed by different size, occurring as elongated bodies with more complicated configuration whose extension generally followed the main tectonic trends. Vein rocks were very developed within the bodies with intensive alteration along their outer contacts; they appeared in the scene to be of darker tone, while the late intrusives (ri) occurred typically as circular or elliptical and generally distributed along fracture zones. There were few vein rocks cutting through them; alteration along contacts appeared quite weak; tone was light to white. On this premise all these bodies were redelineated with some previous mistakes corrected. At the same time, some lithofacies zones and contacts within rock bodies themselves were located more accurately. Furthermore, a number of new small intrusive bodies and dykes were also found in the course of interpretation, with their relationship defined, thus providing more reliable information for the study of magmatism and its effect on ore formation throughout the region. In addition, some rock bodies half covered by desert (Gobi) whose boundaries could hardly be delineated accurately in a survey on the ground, appeared to be quite clear on images (Plate 2). Volcanic eruptive rocks of Cenozoic age were easily mapped due to their well—preserved original eruptive morphology. Delineation of volcanic cones of different shapes and the extent of distribution of lava flows were defined. Precenozoic volcanic rocks were mapped on the basis of their geomorphology, circular structure, radial fracture and drainage pattern, the paleovolcanic craters could be identified and the configuration or the extent of the distribution of paleovolcanic lava flows could be delineated. The interpreted paleovolcanic apparatus of the early—and mid—Carboniferous in the Xinjiang Uygur Autonomous Region was confirmed by field examinations. 3. INTERPRETATION OF REGIONAL STRUCTURE The definition of regional structure based on the analyses of images probably was the most effective aspect of the geological interpretation as a whole. In mountainous country one could identify folds of different morphology and faults extending in different orientations. Joints were mapped which might be readily omitted in a field survey. Unconformable boundaries between sedimentary layers were delineated aecurately an’ extended under o~r~rhiirden
Remote Sensing
—
Regional Geological Survey in China
93
consisting of the Quaternary bess, desert sand and gravel horizon and other less consolidated sandstone. A great number of hidden folds of different morphology and faults were mapped in different direction. It was obvious that the discovery of these features not only allowed to correct some mistakes of all kinds of maps of structure and geology compiled previously, but also to provide abundant materials for structural analysis and study of structural control of mineralization throughout the region. For instance, in a regional geological survey carried out in the Sichuan Basin, a variety of folds and their relationships (including bifurcation, oblique intersection and superimposition) were defined precisely thanks to the geologic interpretation of images with their extension and size revealed more pronounced. In the meantime, a number of northwest and east—west structures and some buried domes and hidden faults covered by a thick sandstone horizon were also disclosed (Plate 2). In the course of map compilation of the Xinjiang region, not only a large number of north—west and north—south faults of different nature were revealed, but some major faults of 500——1,SOO kilometres long, which were of geological significance, were also reconstructed. Furthermore, dozens of buried faults of a considerable size were found to extend under the desert cover over the north—eastern margin of the Talimu Basin from the Bositeng Lake to the Lop Nur (Plate 3). In addition, a great number of ring—like fractures, circular structures and circular colour halos of varied sizes almost never reflected on the geologic ma a published before, were discovered. These features were most likely to be related to some of the endogenetic mineral deposits or oil and gas accumulations. CONCLUSION Remote sensing techniques undoubtedly have a great advantage when used in regional geological survey. Not only do they save time and money, but the result of geological interpretation can enrich the contents of a map of geology and mineral resources, making it more reasonable and more consistent with reality. Improved knowledge of regional geology can provide more reliable basis for mineral genetic analysis and prediction, relevant to some of’ the mineral resources and to guide further prospecting and exploration. II. should be noted, however, the geological application of remote sensing techniques has certain limitations, at least for the time being. The imagery of the same geological unit may be quite different, depending on the variety of physiographical environments, the density of vegetation and the intensity of weathering. For example, a super thick vegetation cover can obscure indicators for geological interpretation, bringing about difficulties in interpretation, and resulting even in misleading information. Therefore, it should be particularly emphasised that the geological interpretation of images must be combined with field investigation. If possible, some kind of integrated interpretation involving geophysical and geochemical data as completely as possible is desired. Only in this way, a better result and a satisfactory map of geology and mineral resources can be expected.
__
Plate 1. Silurian, Perrniau—Jurassjc Strata image S—shale; q ~limestone; P~—flint limestone; ~ —siltstone and mudstone; T 1~—limestone; T3,, —sandstone intercalated shale and coal beds; J—mudstone, finesandstone and siltstone inter— bedding. (Sichuan basin, aerophoto)
~
Plate 2. A granite intruded into the anticlinal axis. 1—granite porphyry; 2—red granite; 3—hornstone; 4—dikes of basic rock. (Northwest of Gansu Province, aerophoto)
94
Iluang Sihuang
~
~
~1YIUi!2~1~~IIII c
~
~
TQ
/
Ia
3~4 ~ ~
J
‘~‘
Figure I. The comparing maps of before and after re—compilation. It forms only a small part of a 1:1,000,000 scale in Wulumuqi area. Left—mapping in 1964; right—map recently recompiled.