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Home , Geologic hazards, Remote sensing, Remote sensing (geology)

Proc. Indian Acad. Sci. (Engg. Sei.), Vol. 6, Pt. 4, December 1983, pp. 297-31 !. ~) Printed in India.

Applications of techniques to

S K BHAN and K KRISHNANUNNI* Geosciences Division, National Remote Sensing Agency, Balanagar, Hyderabad 500 037, India * PGRS Division, Geological Survey of India, 29 Jawaharlal Nehru Road, Calcutta 700 016, India

Abstract. Remote sensing is a new emerging of technological development and has made a very significant impact on the geological surveys and studies. The work done so far in geological remote sensing has indicated the scope, utility and limitations of these modern techniques in different geological problems. The utility of airborne surveys and aerial has now been well established whereas remote sensing at present has two main constraints--resolution and lack of . With the developments in sensor technology to provide sensors with improved resolution, more spectral bands and stereoscopy, substantial new results are anticipated in the geological remote sensing from space. Brief overview of applications of remote sensing techniques to geology is discussed in this paper.

Keywords. Remote sensing; aerial photo interpretation; multispectral data: multispectral ; resolution; geological mapping; lithologic mapping; structural mapping; geomorpho- logical mapping; exploration; multispectral scanner

1. Introduction

Search for mineral deposits has been a constant pursuit right from ancient civilisation. The growth of civilisation through ages has witnessed increasing use of mineral resources and the economic prosperity of any region is governed by the mineral resources which can be exploited, because these resources constitute important raw materials for industrialisation. Mineral resources are finite in extent and non-renewable in nature and so are limited in supply. The fact that known resources are getting depleted at a much faster rate, due to rapid rate of growth, calls for newer methods of search and exploration of additional resources at an equally fast rate so that atleast the present rate of growth can be sustained. Rapid industrialisation is also causing increasing stress on natural environment which leads to geologic such as flooding, landsliding, etc. The most vital factor in the exploitation of mineral resources is geologic information. Mineral resources are products of geological processes and the exploitation strategy is governed by geological factors and environment in which these deposits occur. But the most important geologic information is the geological map without which no exploration programme can ever be thought of. Thus geological mapping is a primary activity not only for search of but also in geotechnical investigations and mapping etc. Geological mapping methods have been undergoing continuous change along with technological and scientific advances in other relevant fields. Remote sensing tech- niques are now being increasingly used to prepare geological maps and obtain the basic geological information on which further detailed work is based.

297 (Enog. Sci.)--3 298 S K Bhan and K Krishnanunni

2. Evolution of remote sensing in geology

The application of remote sensing techniques to geology and mineral exploration started with . The main thrust of development in aerial photography took place between the two world wars and since then has made tremendous contribution towards obtaining geological information. Further impetus was added by the development of colour and colour photography. The main advantages of aerial photography viz high spatial resolution, stereo coverage, choice of data acquisition with exact specification and extremely large quantities of information that can be recorded, were sufficient to promote its utilisation in geosciences. Moreover geological studies were so far carried out by conventional ground surveys which had certain disadvantages such as the long time spent on surveys, high manpower, inaccessibility and unreliability in areas with limited exposures and/or covered with etc. Many of these constraints were overcome by aerial photographs and other remote sensing methods (Varadan 1977; Krishnanunni 1982). The technological development of space programmes saw the introduction of high- altitude space photography during the sixties. This was a great step towards understanding the regional geological studies and correlations of geologic and geomorphic features over long distances compatible with modern theories of plate etc. The synoptic view allowed the scientists to see the terrain model in its entirety and helped them to think in the regional context for better understanding of geological and geomorphological problems. Geologic evaluation of small-scale Mercury, Gemini and Apollo photographs confirmed its utility in regional geological mapping (Lowman 1965; Raina 1972, 1978) The development of multispectral scanner during the seventies which followed the aerial and space was perhaps one single factor which made significant contribution to remote sensing, as it enabled for the first time sensing beyond the photographic range of the . The launching of LANDSATseries of in the seventies with multispectral scanner gave the geologic community enormous data of great utility and which was made available at reasonable cost and free from political and security restrictions (Rowan 1975). The -synchronous polar orbital geometry of LANDS^r satellites along with characteristics of multispeetral scanner (Mss) such as spatially registered multispectral imaging over large areas (synoptic view), rel~titive coverage with narrow look angle under uniform conditions and scale made the data unique. This has been useful in the observation and study of regional geological and geomorphological features, extra- polation of local observation to the regional scale, delineation of critical areas for further detailed work, comparison of areas and monitoring of geologic hazards (Bhan 1982b, 1983)

3. Application potential

Aerial photography has proved its worth in geological applications. However the satellite remote sensing at present has two limiting factors--resolution (spatial and spectral) and lack of stereoscopy. Poor spatial resolution is a handicap in many geological investigations, particularly in areas where there are frequent variations within short distances and where the size of mappable units is much smaller than the Remote sensing---geological applications 299 spatial resolution. Moreo~/er the spectral resolution has not been optimum for geological work so far. Recent advances made in sensor technology with the development of sensors having finer spatial resolution, more spectral bands (particularly in the region of middle infrared and thermal infrared with sharper spectral separation), improved geometric fidelity and greater radiometric accuracy along with stereoscopy will provide geologic community improved capability to extract more useful information from the satellite data. In this context utilization of data in middle infrared (1.55 to 2-35) and thermal infrared (1(~4 to 12.5) bands will be watched with greater interest as these bands will have a significant impact in delineation of hydrothermally altered zones, lithologic discrimination, soil moisture, flood mapping and geothermal mapping etc. The development taking place in technology hold promise, particularly in structural mapping. The most significant development will however be the stereoscopic space images which could possibly lead to a better depiction of geological structures and terrain features thus leading to improved geologic and geomorphic mapping. The high resolution which could be attained is also expected to displace aerial remote sensing techniques to a secondary position.

4. Case studies

In all earth resources programmes the most vital factor is geological information. In recent years more and more remote sensing techniques are being applied to obtain geological information depending upon the needs, sphere of activities and availability of resources (Dhoundial 1983). The impact of these techniques has been significant because these methods give continuity of geological interfaces. This is otherwise difficult in ground surveys due to the fact that even in areas of good rock exposures nearly 30 to 40 % of ground is covered by soil or vegetation and to a large extent geological maps based on ground surveys are made on the basis of inferences drawn from random observations or observations along traverses (Ramaswamy 1983). One of the main objectives of geologic remote sensing is to discriminate among different materials and ultimately to identify them, thus aiding the mapping process which is fundamental to many investigations. Images displaying progressively larger areas show that different scales and types of information are revealed by changing the perspective. For example, major crustal breaks or faults are rarely exposed continu- ously for great distances; in fact major faults are usually zones of many smaller faults and related deformational features rather than a continuous single fracture. While the details of such features are better seen on larger scale airborne images and photography, the full significance of such features might escape the notice of the interpreter because the airborne images may only show a small segment of a major zone (Rowan 1975). Remote sensing techniques no doubt hold very good promise and if utilised properly can supplement the ground surveys to maximise the results. Conjunctive use of remote sensing techniques with limited conventional ground observations may work out to be a better strategy for natural resources survey. A few specific case studies from remote sensing are given to substantiate its utility in geosciences. 300 S K Bhan and K Krishnanunni

4.1 Geological mapping

Good geological maps are essential not only for exploration and exploitation of natural resources but also for a variety of other purposes such as location of power plants, dams, alignment of highways and railway lines, tunnelling etc. Even in areas where geological mapping has been done in great detail, remote sensing techniques have contributed information hitherto unknown such as identification of new faults and fractures. Amongst all the remote sensing methods, the interpretation of aerial photos have been widely accepted as a tool of geological mapping. With the advent of space images particularly the LAr~DSAr images much larger perspective became available for geological studies and regional geological mapping--resolution and scale of satellite data limiting the geological mapping to scales smaller than 1:250,000. Capability of space images for regional geological mapping has been demonstrated through interpretation of Gemini and Apollo photos and LANDSATimages (Raina 1978; Rakshit 1976, Bhan 1982b). However, LANDSATdata has been of limited use in the preparation of geological map of virgin or unmapped areas. But in areas of difficult access such as Himalayas or north eastern region of the country regional geological maps of considerable usefulness can be prepared on the scales of 1:250,000. Figure 1 shows the LANDSArimage of the highly rugged and mountainous terrain covered with forests in northeastern India. Producing geological maps with stratigraphic and lithologic units in such regions by conventional methods and even by aerial photo interpretation is not only a difficult task but cannot be accomplished within a reasonable time. It is in such areas that regional geological maps of considerable utility can be prepared by interpreting LANDSATdata (figure 2). Due to poor spatial resolution only broad lithologic groups could be delineated. Most of the stratigraphic units could not be recognised and delineated as they are usually of much smaller thickness although complete structural and tectonic pattern showing doubly plunging folds and longitudinal faults (figure 3) could be delineated. Since the region is favourable for oil accumulation, delineation of anticlines and domes assumes significance in oil exploration because these are often the loci ofoil accumulation in such regions. The LANOSAT data have been found to be useful in revising and updating the geological maps even in areas which have been mapped in great detail and in areas where rock exposures are well defined as in arid and semi-arid areas. The geological map based on interpretation of LANOSATdata of Andhra, Orissa and Singhbhum (NRS^ 1978, 1981 and Rakshit 1982) clearly indicate the utility of LANDSATdata in the revision of geological maps.

4.2 Lithologic mapping

In geological mapping from space the limiting factors have been stratigraphic and lithologic discrimination. The latter has only limited success because of the generally less pronounced spectral differences amongst most rock types in the bands provided in these images, particularly in LANDSAT. Moreover LANDSAT MSS measures only the colour and brightness of rock which are not diagnostic in rock type discrimination since one rock type can have many colour variants and secondly different rock types can have Remote sensing--geological applications 301

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{~2JP/N77 C" N2=~3B'EEI92-23 N N2a'38/E092 28 ~ISS D SUN EL30 RZI3~ 189-99eE,'• ;'N-P-2L NRSR ERTS E'2711"O32aS -~ 01

Figure 1. LANDSATimagery--false colour composite of parts of Tripura and Mizoram States same colour and brightness. The lithologic discrimination is further limited by heterogeneous surface features and cover factors. Due to these factors only broad lithological groups such as basalts, granites and horizontally disposed sedimentary rocks covering extensive areas can be delineated with ease from space data. Further discrimination is based mainly on additional properties such as relative erodibility expressed as , pattern, vegetation association etc. (Bhan 1983). Now with digital enhancement of Mss data using computer techniques it is possible to discriminate further between different rock types on the basis of subtle tonal variations. Further research on spectral responses of rocks at different levels (ground, air and space) has been carried out in detail to optimise suitable spectral channels and their combinations for lithologic discrimination (Krishnanunni et al 1982a, b).

4.3 Structural mapping

Remote sensing techniques particularly from space have greatly contributed to structural mapping. A synoptic overview of space images from great altitude enabled 302 S K Bhan and K Krishnanunni

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L E G E N D KhOwoi Formation Upper Bhuban Formation ~'~ Dupitllo Formation Lower Bhuban Formation ~ Tipom Formation BaSement

Bekgbil Formation Lineament

Figure 2. Geologicmapping--Based on interpretation of LANDSATimagery of figure 1 the scientists to view the entire regional structure and at the same time allowed critical study of the inter-relationships with other geological features. Figure 4 shows the LANDSAT imagery of the northern part of Orissa where large regional structure of Simlipal basin is seen in the image. Many other important structural features such as Pal Lahara fault, folded iron group rocks, extension of Singhbhum shear zone, fracture pattern in Singhbhum granite and the number of dykes (some extending for over 100 km in length) etc are seen in the same image (figure 5) thus facilitating correlation of structural features and extrapolation of information (Bhan 1982b). This is obviously due to the synoptic overview of features which cannot otherwise be discerned due to their great size and magnitude. In fact space data have provided a wealth of information on lineaments ever since the Remote sensing--oeological applications 303

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Figure 3. Doubly-plungingfolds on LANDSATimagery of figure 1

lineaments were observed for the first time on the synoptic . The analysis of lineament pattern has assumed great significance as lineaments often pass through mineralised areas and mineralisation is often localised at the lineament intersections. Location of deep-seated features such as epicentres and thermal springs aligned along lineaments give credence to the fact that some of the lineaments, if not all, are of deep-seated tectonic nature. It is these which can assist in refining the metallogenic models by delineating boundaries of crustal blocks, and redefining the tectonic set-up of the region, as done in the case of Orissa (NRSA 1981). Tectonic patterns in NE India have been analysed from lineament study on LANDSAX imagery (Nandy 1980) and in parts of Manipur there is definite correlation between magnetic trends and lineaments (Hegde & Bhan 1981). The lineament map of India based on the interpretation of LANDSATdata on the scale of 1:2 million will soon be published by the Geological Survey of India. 304 S K Bhan and K Krishnanunni

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Figure 4. LANDSATimagery--black and white band 7 (infrared) of northern parts of Orissa

4.4 Geomorphological mapping

Geomorphological mapping in India is a comparatively recent phenomena and not much work has been done. The importance of geomorphological mapping and terrain analysis was realised while interpreting the remotely-sensed data as the landforms were the most conspicuous features on the images and photographs, which could be observed directly. Their interpretation helped in solving many of the geological and environ- mental problems such as lithologic discrimination, quaternary geologic mapping, mineral and exploration and geologic hazards like , sedimentation in lakes, dams, mass movements and wasting, flood mapping, volcanic activity etc. Geomorphic mapping also calls for analysis of the terrain for better understanding of geomorphic domains, features and processes. For this reason small scale satellite data particularly from LANDSAT have proved its utility in regional and semidetailed geomorphic mapping (Bedi 1978; Bedi & Bhan 1978; Rao 1979; Bhate 1980, Rakshit 1980) in diverse terrain types. Regional geomorphic maps have proved useful in evaluating the groundwater potential of each geomorphic unit so delineated as done in Andhra, parts of Tamil Nadu, parts of Orissa and Bundelkhand region. LANDSATdata Remote sensing---geological applications 305

L E G E N D [~ Uncloellflod@nellll ~_~ Younger Gr4nlte ,.. o,, ,rouw ~ oyk,, 81flqlhllhva Orenlte ~ norlpod4 Bed ~] Kelre $1mllpel (~eule []~] keterite Oeal|pur Ilqkl)kmn Oreup ~ AlllvlQIIUUI Motovolcanice,M-,~kro4,Anefthoeiloe ~ Trend Lle,O

Figure 5. Geological map~interpreted from LANDSATimagery of figure 4.

have also been found useful in monitoring the dynamic geomorphic features such as coastal environmental features, migration and evolution of rivers (figure 6), drainage development and river geometry and desert features etc. McKee's study of sand is one of the classic examples of mapping of large areas of aeolian features around the world (McKee et al 1973) on the basis of LANDSAT data, providing for better understanding of aeolian deposits. Aerial photography has been used quite successfully to prepare detailed geomorphic maps covering small areas on scales varying between 1:15,000 and 1:50,000 as delineation of geomorphic features and evidences of recent tectonic activities are more 306 S K Bhan and K Krishnanunni

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precise on aerial photos and such maps have been useful in mapping natural zones, groundwater exploration, geotechnical investigations and geoenvironmental studies etc.

4.5 Mineral exploration

One basic objective of geologic, geomorphic and tectonic maps is ultimately to identify features or guides associated with mineralisation and to delineate target areas for exploration. Though remote sensing methods cannot replace proven methods, they can definitely provide useful inputs in recognition and delineation of mineral provinces and target areas by identifying surficial indicators or guides. Aerial photography is widely used for delineation of some of the mineral guides. The delineation of bauxite-capped plateaus in the of Andhra and Orissa was greatly facilitated by aerial photographs in the rugged mountainous terrain of difficult access. Recently air photo interpretation techniques have been used to locate buried deposits of calcareous nodules, locally called 'Kankar' in the alluvial tracts of Uttar Pradesh and desert terrain of Rajasthan by Gsl (Basu & Duara 1982). Studies carried out by the Central Road Research Institute, New Delhi have proved that these kankars can be used as substitutes for road metal. This is significant in areas such as Indo Gangetic plain and Rajasthan desert where hard rocks are not locally available. On the basis of photo tone, texture, vegetation, drainage and unique geomorphic associations, 28 potential deposits of nodular and bedded Kankar, occurring beneath an overburden of 1-5 m and aggregating an area of 3-4 km 2 out of 2500 sq. km could be delineated thereby reducing the target area by 735 times (figure 7). Subsequent ground checking of 23 deposits, covering an area of 2.7 km 2 confirmed them as actually kankar bearing, giving a success rating of 80 %. Similar studies carried out over an area of 3500 km 2 in Remote sensing--geological applications 307

Bikaner and Nagaur districts of Rajasthan have brought to light 103 workable deposits of kankar and dhandla with a success rating varying from 77-90 %. Now airborne multispcctral scanner data are also being used, particularly thermal infrared data, in locating zones of geothermal energy by identifying surface manifes- tations like hot springs and hot spots etc, oil and gas leaks, structural mapping in covered areas and lithologic discrimination. Thermal infrared images particularly have been successfully used in lithologic discrimination (Watson 1975) and mapping of lineaments (Sabins 1969). Spaceborne data particularly the LA~DSAThave been used in mineral exploration in two ways, viz identifying features directly related to mineralisation such as alteration zones, gossans, specific host rock or characteristic mineral association and secondly as delineation of features favourablr for localisation of mineralisation such as folds,'faults and fractures. The study of lineaments has assumed great significance because of their correlation with known mineral deposits all over the world helping in establishing the control of mineralisation in many areas. Potential ore bodies occur along or in close proximity of major lineaments representing fracture zones and often at the intersection of two or more lineaments in the mineralised province. In Khetri and Singhbhum areas it has been postulated that intersection of different sets of lineaments are the most favourablr zones of localisation of mineral deposits. Limits and possible extension of mineral belt could also be assessed through lineament studies and identification of lithological and mineralogical guides as done in Singhbhum Shear zone in Orissa on LANDSATdata (naSA 1981) and on digitally enhanced LA~DSAT data (Rakshit 1982). Besides structural features the LA~DSATdata have also proved its utility in locating anomalous tonal areas directly related to mineral deposits, subject to limitation of

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Figure 7. Kankardeposits in alluvial plains around Sitapur, UP 308 S K Bhan and K Krishnanunni spatial resolution, such as identification of gossans, blooms, alteration zones etc. In the Eastern Ghat terrain of Orissa, bauxitic-lateritic plateaus could be picked up by visual interpretation of LANDSA'rdata as such plateaus show a distinct tone and occur as fiat surface on top of hills and ridges (Bhan 1982a, Bhan & Hegde 1983). Delineation of anomalous tonal areas is particularly significant in sulphide ore deposits and mineralisation as these deposits form distinctive alteration zones at surface. Techniques such as band ratioing have been developed to enhance or distinguish these altered zones on images. These ratio colour composites bring out spectral differences which aid in lithologic discrimination and delineation of such altered zones (Rowan et al 1974; Schmidt 1975) for mineral exploration. LANDSATimagery and its enhanced products, particularly the ratio images and colour composite, exhibit some interesting geological features in Badampahar area which are significant from the stratigraphic point of view in establishing the relationship between the Singhbhum granite and the iron-ore of rocks and correlation "of the iron-ore group formations east and west of the Singhbhum granite craton (Rakshit 1982; Rakshit & Swaminathan 1983). The Singhbhum granite in the area has probably two components, one forming the base for iron-ore formations and the other being a later granite which may be equivalent to the Mayurbhanj granite-granophyre igneous complex. No physical continuity of the iron-ore formations of Badampahar group in the east to the Noamundi group iron-formations has so far been established though Dunn & Dey (1942) has indicated correlation between the two groups. Iyengar & Murthy (1982) and others have however postulated these groups as separate ones in their stratigraphic position, suggesting Badampahar group as much older than the Noamundi formations of Koira group, deposited in different basins. From LANDSATimagery (figure 8) the extension of the Badampahar iron-ore formation towards west through a series of detached and discrete outcrops with a folded configuration ultimately passing into the Noamundi formation, could be suggested. This continuity is well depicted along a rugged, curvilinear, densely-forested zone on the enhanced LANDSATimagery. Field verification showed this zone to be mainly composed of granite gneiss, granite, granophyre and pegmatite with numerous basic to ultrabasic dykes and enclaves of hornblende gneiss. Bands of basic lava, hornblende gneiss, banded ferruginous shale and quartzites and banded hematite quartzite occur sporadically. Laterite mounds found here and there contain pieces of banded hematite quartzite and banded ferruginous shale and quartzite. Banded hematite rocks found are similar to that of Badampahar area and the basic lava and emphibolites are also similar to those found in the iron-ore formation of Badampahar and Noamundi area.

5. Conclusions

Even with certain constraints remote sensing techniques in geological applications and mineral exploration have proved their value. In fact satellite remote sensing has become an important tool in the preparation of regional geological and geomorphological maps of areas not previously covered, in updating and revising already existing maps, in correlating large scale geological features and structures over widely spaced separate areas and in delineating target areas for detailed work by airborne and ground methods in mineralised belts. Remote sensing--geological applications 309

Two main constraints in satellite data have been resolution (spatial and spectral) and lack of stereoscopy. Developments now taking place in sensor technology to provide sensors with better resolution, more spectral bands with sharper spectral separation (LANDSATthematic mapper), stereoscopy (SrOT) and in thermal infrared and microwave region of the electromagnetic spectrum are expected to have a significant impact in improving the capability to extract more useful geologic and geomorphic information from satellite data. It is evident that the geologic community will utilize more and more of remote sensing data in future for all programmes.

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Figure 8. u. LANDSATdata enhanced; Band ratio black and white image of MSS Band 6/7. 2a--Singhbhum granite. 2e--Mayurbhanj granite. 4--Dhanjori group. 8g--Iron ore SHQ/quartzite/shale. 8f--lron ore metavolcanic/metabasic/newer dolerite dyke. 310 S K Bhan and K Krishnanunni

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Figure 8. b. Geological interpretation based on LANDSATdata enhanced.

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