Possibilities of Using Remote Sensing in Archaeological Research in Bengal Delta in Bangladesh - A Human-Influenced Alluvial Terrain

In archaeology, research design involves selecting a region or site for survey, excavation and recovering data. Excavation is costly and is increasingly seen as last resort [1]. Remote sensing gives a synoptic view of the surveying sites, frequently, and at low cost, of often inaccessible areas, saving time and money. In favourable circumstances and with the efficiency of the interpreter, it can record details of the buried sites revealed by discolouration in the overlying soil or vegetation [2]. Threedimensional effect from the terrain can be achieved with the help of stereo pair aerial photographs. However, optimum utilization of remote sensing technology depends on clear understanding of the interaction among electromagnetic spectrum, atmosphere and terrain characteristics.


Introduction
In archaeology, research design involves selecting a region or site for survey, excavation and recovering data. Excavation is costly and is increasingly seen as last resort [1]. Remote sensing gives a synoptic view of the surveying sites, frequently, and at low cost, of often inaccessible areas, saving time and money. In favourable circumstances and with the efficiency of the interpreter, it can record details of the buried sites revealed by discolouration in the overlying soil or vegetation [2]. Threedimensional effect from the terrain can be achieved with the help of stereo pair aerial photographs. However, optimum utilization of remote sensing technology depends on clear understanding of the interaction among electromagnetic spectrum, atmosphere and terrain characteristics.
Bangladesh is covered with Tertiary folded sedimentary rocks (12%), Pleistocene soil (8%) and Holocene alluvial sediments (80%). Moreover, it is in a tectonically active region of the world. These sediments, consisting mainly of unconsolidated sand, silt and clay in varying amounts, come from different geological environments and deposit in different geomorphological conditions. Besides these, the country is tackling anthropogenic activities where population become double within 40 years.
Climate is another important factor, which has relevance in shaping of the earth's surface because the processes that act upon the surface material are different in different climate zones [3]. Bangladesh enjoys a tropical monsoon climate [4]. Mean annual rainfall is 1250 mm in the centre-west, more than 2500mm in the north-west and near the coast and exceeds 5000 mm in the north-east [5]. Mean temperature is about 25 °C, and that ranges from 18 °C to 30 °C in winter and summer respectively. Wind is generally light, but it goes 50-100 km/hr or more during pre-monsoon or cyclones. Evaporation is about 50-75, 100-175 and 100-125 mm/month in winter, pre-monsoon and monsoon respectively.
Application of remote sensing in Bangladesh geology started during the middle of 1950s [6]. For practical reasons aerial photographs were used for geological mapping till the availability of satellite remote sensing data, which is continuing. Scientists of the country have been working with aerial photography, Landsat (MSS, TM, ETM+), SPOT, IRS, Radarsat, JERS-1, Lidar, CORONA satellite photograph, RapidEye, Pleiades data in different fields of geology [6]. Other fields of application include forestry, soil, environment, agriculture, water resources,

Prehistoric Settlements (Neolithic Settlements)
Archaeologists identified several potential paleolithic, mesolithic and neolothic sites in Bangladesh. All the reported locations of pre-historic archaeological records are confined to the Pleistocene lateritic terraces of Lalmai Hills of Cumilla and uplands of Narshingdi, Sylhet, Habiganj, Rangamati and Chattagram districts [9]. Discovered materials are pre-historic artifacts made mainly from silicified or fossil wood or fossil. These areas lie mostly in the north-eastern and eastern parts of the country that are geologically occupied by hills of Tertiary sedimentary rocks, except those of Cumilla and Narshingdi which are uplifted terraces of Pleistocene time (Figure 1). techniques were adopted after 700 BC onwards. Exactly from this period, the human settlements in Bengal started undergoing qualitative changes along with the other parts of the subcontinent. The use of iron played a significant role in the economic, social, cultural and political life. It is consistent to mention here that the Megalithic culture is a specialized form of the Iron Age [7].

Megalithic Culture
The remains of Megalithic culture are found in various countries of Europe, Africa and Asia. In the Indian subcontinent megalithic culture started in Neolithic period, however, it flourished during the Iron Age. In Bangladesh, evidences of megalithic culture are found in Jaintapur (Figure 1) of Sylhet district, which includes upright stone (menhir) and slab stone (dolmen) horizontally resting on several stone posts [9]. This area lies in the north-eastern hilly region of the country and occupied by younger Tertiary sedimentary rocks. Brief descriptions of few important archaeological sites/places of Bangladesh are given below.

Important Historical-Period Settlements
Wari-Bateshwar, Narshingdi: The Wari-Bateshwar ( Figure  1) region in Narshingdi, Bangladesh is the site of an ancient fort city dating back to 450 BCE. The 2500-year-old ruins lie near the Old Brahmaputra River. The sites are located on the flat-topped surface of the Madhupur Tract [11] underlain by Madhupur clay residuum [12]. Wari-Bateshwar is the rich, well planned, ancient emporium (a commercial city) "Sounagora", mentioned by Greek geographer, astronomer, mathematician Ptolemy in his book Geographia [13].
Wari-Bateshwar, in 600m X 600m enclosure with four mud ramparts, is called a fort-city or urban centre. So far 50 archaeological sites have been discovered in and around Wari-Bateshwar. From pattern of archaeological sites, discovered artifacts and sandwich glass bead, it is evident that the settlement was developed in flood free zone and it was affluent trade centre and its inhabitants depended on agriculture [14]. The discovery of Janapada coins places Wari-Bateshwar back to the Sodosha Maha Janapada (ca. 600-400 BC) kingdom of Indian subcontinent. In recent archaeological excavation evidence of human settlement has been discovered which bears the character of Chalcolithic culture, the most important discovery of this culture is black and red ware and evidence of pitdwelling. Several neolithic tools have been discovered from this region, though all the tools are chance finds and no prehistoric settlement has yet been identified. The discovery of prehistoric tools indicates the prehistoric settlement in the region which is waiting to be explored [14].

Mahasthangarh, Bogura:
Mahasthangarh is one of the oldest archaeological sites of Bangladesh, lies on the right bank of the Karatoya River, about 12 km north of Bogura town ( Figure  1). The spectacular site is an imposing landmark in the area, having a fortified, oblong enclosure measuring 1524m x 1370m with an average height of 5m from the surrounding areas. Like the Wari-Bateshwar area, similar settlement patterns are evident at Mahasthangarh (Pundranagar) [14]. The elevation of the Mahasthangarh ranges from 15m to 25m and is at a higher in elevation than the surrounding flat plains and relatively flood free. The site lies on the eastern part of the Barind tract covered by Pleistocene Barind clay residuum [12] and sub-Recent sediments borne by Bangali and Karatoya rivers. Karatoya-Bangali meander floodplain has a complex landscape containing sediments of Holocene age. Other important rivers that drain the tract are the Atrai and Karatoya rivers discharging into the Jamuna River to the southeast, and a few small and monsoon-fed seasonal rivers e.g. Ichamati, Bangali and Nagar. Many swampy areas, locally known as beels are also found around the area [14].
Somapura Mahavihara, Paharpur, Naogaon: Somapura Mahavihara was one of the most famous Buddhist monastic institutions of ancient Bengal. The excavated monastic complex at Paharpur (Figures 1 & 2) has been identified with the Somapura Mahavihara built by the second Pala king Dharmapala (c 781-821 AD). The monastery flourished until the 11 th century AD. The monastery was repaired and renovated during the reign of Mahipala (c 995-1043 AD), and it is recorded in the Tibetan work, Pag Sam Jon Zang that the same king used to visit Somapura Vihara to offer his homage to it. Somapura Mahavihara gradually declined and was finally abandoned during the 13 th century, when the area came under Muslim occupation [14]. This site has been included to the UNESCO declared World Heritage. the western scarp, differential soil development, rapid filling of closed depressions at the toe etc. Nateshwar, Munshiganj: Several archaeological evidences of a Buddhist city older than a thousand years have been discovered at Nateshwar (Figure 1) of Tongibari Upazila in Munshiganj district. These include an entrance and walkway, prayer hall, mortar floor, octagonal stupas, pot shreds, baked clay materials and burnt bricks. The aesthetic of these stupas is unique in architectural style. More than 5,000 square meters were unearthed at Nateshwar and a series of significant results were achieved by this time [17]. C14 dating shows that there were two phases of human habitat in the area -first from 780 to 950 AD and the second from 950 to 1223 AD [13].

Coastal Area (Sundarbans):
Recently an archaeological site ( Figure 1) has been discovered in the Sundarbans mangrove forest in the south-western coastal region of Bangladesh. So far five spots have been identified there. An estimated age of this archaeological site goes back to about one thousand five hundred years. The sites are scattered on the coast to the Bay of Bengal and on the river banks of the area. From the first impression it is thought the site was developed for salt industry and trade centres, but there were permanent human settlements. These sites are in Shyamnagar of Satkhira and at Katka part of Khulna districts [18].
Another archaeological site ( Figure 1) was discovered few years ago in the coastal Sundarbans, a huge UNESCO-protected mangrove forest. Discovery includes submerged salt-producing kilns; built just above the winterly spring high-tide level of time but their bases are currently located ~155 cm below the corresponding modern level. Age of the site was calculated from OSL dating and supported by C14 dating to ~300 yr ago [19].
Regional Tectonic Setting, Geomorphology and Geology of Bangladesh

Regional Tectonic Setting
Bangladesh constitutes the eastern continuation of the central broad Indo-Gangetic plains of India between the Peninsular (shield) area to the south and the extra-Peninsular (Himalayan mountains) region to the north and northeast. At the eastern part of the plain the Bengal Basin is located, having varied tectonic history. The Basin consists of major part of Bangladesh and small portion of West Bengal of India. Bengal Basin lies north of the Bengal Deep-Sea Fan and is bounded on the north by the Shillong Plateau, in the west by shield and in the east by Indo-Burma Folded belt. The Dauki Fault Zone forms an active major east-west tectonic element [20] that separates the Shillong Plateau from the subsiding Surma basin of the Bengal Basin [21]. Tectonically, Bangladesh lies on the Indian plate that is gradually closing to Eurasian plate to the north at a rate of approximately 36-45 mm/year [22] and its eastern margin is bounded by Indo-Burma Folded Belt where an atypical continent-continent subduction is going on [23], the Burmese plate ( Figure 3) is moving westward at the rate of 20 mm/year.

Geomorphology and Geology
Geomorphologically, major part of the country is occupied by one of the largest deltas of the world formed by the Ganges-Brahmaputra-Meghna river system originated from the uplift of the Himalayas. The delta prograded south accompanied by rapid subsidence in the basin resulting deposition of huge thickness of deltaic to fluvio-deltaic sediments. The delta building process is continuing into the present Bay of Bengal and broad fluvial front of the major system gradually follows it from behind. Major landforms of the country are being produced by fluvial, tidal and estuarine processes resulting floodplains of Ganges-Brahmaputra-Meghna rivers, deltaic and coastal plains, piedmont i.e. alluvial fans, and estuarine plains. Apart from these, there are three tracts viz. Barind Tract, Madhupur Tract and Lalmai Hills [4]. The north-eastern and eastern parts of the country are occupied by hills ( Figure 4).

Figure 4:
Simplified and generalized geomorphological map (left) and generalized geological map (right) of Bangladesh (Information taken from [12,34] for geomorphological map; taken and simplified from [12] for geological map).
A large alluvial fan, known as Tista alluvial fan, has been developed in the north-western part of the country, rich in archaeological heritage. The alluvial fan is divided into western as old and eastern as young fans. Besides these, a series of small alluvial fans have been developed at the south of the Shillong Plateau and affected by neotectonic activities [25], and at the base of the eastern hills a narrow piedmont plain has been developed. The floodplains and delta are studded all over with clusters of swamps (depressions) locally known as haor or bil/ beel [26]. The most important cluster is Sylhet trough or Surma basin in the north-eastern part of the country. It is a tectonically active and subsiding basin [27]. Another large swamp lies in the north of the Ganges River, known as Chalan beel. Other swamps exist in the central delta.
The coastal region of the country is divided into old Ganges delta plain in the west, Meghna estuarine plain in the middle and Chittagong coastal plain in the east. Each of these units has their own morphological and sedimentary characteristics. Among other factors, active tectonic nature of Bengal basin, occupying major part of the country, results in a highly complex geomorphology [28]. Several studies show that many of the changes in river courses, deposition of sediments and landform development are affected by tectonics operating in the region. Result of the on-going tectonic movements at the Himalayan front is development of alluvial fans developed at the south [29]. Part of one of these vast fans lies in Bangladesh, the Tista fan. Half of the country is lower than elevation of 12.5 m above sea level. The elevation of hilly areas, occupy the eastern and north-eastern parts, lies between 70 and 1000m. The alluvial plains have the elevation from about 90m in the north-western part of the country to 0 m along the coastal part. Among the uplifted tracts, the maximum elevation, about 40m, was observed in the Barind Tract [30], however, Reimann [4] found about 45m above mean sea level.
Geologically, 80% of the surface and near surface of the country is formed of Holocene deposits [12]. The Holocene deposits, consisting of unconsolidated sand, silt and clay of varying amounts, are the products of piedmont, alluvial, fluvial, deltaic or coastal processes. Eight percent area is covered with Pleistocene clay residuum in the three uplifted terraces ( Figure  4). These three tracts are underlain by red to brown coloured oxidized soil. Alam [31] found foundry, chimney with slag and laterite deposits etc. on the Madhupur Tract, and he thought that those are products of iron manufacture in the early days of the history of Bangladesh. The Tertiary sedimentary rocks; consisting mainly of sandstone, siltstone, shale and clay; cover 12%. The oldest exposed rock is the Tura Sandstone of Paleocene age but older rocks like Mesozoic, Paleozoic and Precambrian Basement have been encountered in the drill holes in the northwestern part. Through a long geological time (Permian to Recent) the basement of Bengal Basin, below a thick sedimentary cover, has been severely faulted and fractured which are covered under Holocene surficial deposits.

Factors Controlling Changes in Landforms in the Context of Bangladesh
Fluvial environment is dynamic and constantly changing. Constant deposition of sediments and erosion; slow lateral migration and avulsion of channels due to usual geomorphological processes and tectonic process contribute to the changes in the physical appearance of floodplains. But the common factors that control the supply and deposition of sediments are climate change over time, active tectonics, sea level changes, and change in land use due to anthropogenic activities.
The coastal environment, the most dynamic of environment on the earth, was one of the earliest choices for human habitation. Changes are brought about by interplay between marine and terrestrial processes, tectonic activities resulting uplift and subsidence, climate effect and meteorological events like storms. Bangladesh coastal zone frequently experiences cyclones, in many cases with surges. Syvitski [32] give an estimated stormsurge area on the delta of Bangladesh to be 10,500 sq. km.

High Sedimentation Rate
The Ganges, Jamuna-Brahmaputra and Tista rivers carry huge amount of sediment whereas the Meghna River carries much less amount. In an estimate, Kuehl [33] showed the total amount to be of one billion ton of which 30% are deposited on floodplains. Most of these sediments are carried during the rainy season. Sometimes episodic floods bring enormous amount of sediments that suddenly bury the depositional surface thickly ( Figure 5).

Geomorphic and Neotectonics Processes
Usual fluvio-deltaic processes are continually shaping this largest delta of the world. However, geomorphology of the delta becomes complex because of ongoing tectonics. Many workers [25,30,[34][35][36][37][38][39][40][41] show the uplift of the Madhupur Tract, a Pleistocene tract composed of oxidized soil and stands above the active floodplain. with distinct scarp along the western side, hanging and beheaded valleys, incised drainage on the tract and flow direction indicating tilting in the east, west and south-west. Islam [11] detected evidences from the Madhupur tract in central Bangladesh that indicate Holocene tectonics. Bakr [15]  Islam [21,25] identified several evidences in the areas south of Shillong Plateau indicating recent tectonics.
Shifting of the Tista River from southerly direction to southeast direction occurred in 1787 and that of the Old Brahmaputra River from east of Madhupur tract to west of the tract taking present Jamuna channel occurred sometimes between 1765 and 1830. These changes probably occurred after the major flood of 1787 [34]. As the two large rivers came to the Jamuna valley from opposites sides, it is logical to believe that ongoing tectonics is responsible for these changes. The Jamuna valley follows the Jamuna fault [42] and another significant point is occurrence of two ~7.0 magnitude earthquakes within the valley in 50 years. Besides these, many examples of shifting of rivers courses can be cited although not all of them are due to tectonic effect.

Natural Hazards
Flood occurs every year submerging floodplains of Bangladesh to which people are adapted, but in some years, flood turns abnormal submergence. The later causes damage to property and crops, communications, disrupts economic activity and endangers lives [34]. Floods in Bangladesh are normally associated with annual monsoon rainfall into the Ganges-Brahmaputra-Meghna basin. Monsoon rain and high river flows across the country's boundaries are the dominant cause, but other factors such as flat and lowlying topography, bank erosion, siltation, earthquake, storm surge, neotectonics activity, man-made-structures also contribute to inundation and sedimentation. In the living memory, the country experienced worst flooding in 1974, 1987, 1988, 1998 and 2004. Sometimes one flood event may carry enormous amounts of sediments, capable of burying the surface deeply ( Figure 5).
The Bangladesh coast experienced 149 cyclones between 1891 and 1998 [43]. The storm surge height attained 10.6m in 1970 cyclone, but its range was 3.0-6.7 during 1960-2007 [43].  This region of the world was shaken by 7.0 or greater magnitude earthquakes during the last two hundred and fifty years e.g. 1762, 1885, 1897, 1918, 1930, 1934, 1950 [25]. These earthquakes had significant impacts on the geology and geomorphology, and on the lives, properties and infrastructures as well. During first half of this decade the country was shaken by 6.9 Sikkim (2011) and 7.8 Nepal (2015) earthquakes, although there was no damage, but Alam [44] and Alam & Ahsan [45] clearly showed the relationship between earthquake shaking and response of geologic materials i.e. local soil condition. Even low magnitude earthquakes, in some instances, were responsible for morphological change, shifting of streams, generation of earth fissures and liquefaction [25,46] causing changes in landform. The above evidences show that natural hazards were capable enough in changing landforms in the past like the present.

Subsidence
The delta is suffering from subsidence due to usual geological processes and tectonic reasons as well. Alam [47] outlined that the Ganges-Brahmaputra delta is subsiding at a rate of 2-4 mm/ year. Higgins [48] show a subsidence rate of 0 to >10 mm/yr in Dhaka. Hanebuth [19] gave a subsidence rate of the outer delta to be 5.2 ± 1.2 mm/yr. From the above results it is clear now that different parts of the delta subside at different rates. The subsidence causes changes on geomorphology.

Anthropogenic Activities
Population of the country increased from 75 million in 1971 to about 150 million in 2011 [49] with a density of population of 976/sq km in 2011. Due to the rapid increase of population land covers are being continually and rapidly changed for urbanization (Figures 7 & 8), cultivation, construction of roads and industries. This is happening throughout the country. Landfills for land development for construction and dumping municipal wastes, digging for soil and sand collection, cutting for levelling lands and hills are common practice, lead to landform changes.

Climate change
It is now widely believed that Global warming, ozone layer depletion in the atmosphere and rain forest destruction are some of the crucial problems. These changes would affect the environment in different ways. Since it is a global phenomenon Bangladesh cannot escape from it.
Thus, retreats of sea, subsidence, natural hazards, anthropogenic influence are the causes of the ruins of many of the archaeological sites/places in the delta.

Introduction
Broadly, remote sensing is defined as the acquisition of information about the surface of the land and ocean, and the atmosphere, by air-or space-borne sensors. It includes both passive and active sensors, receiving reflected and emitted electromagnetic radiation respectively. Remote sensing of earth has come a long way from nineteenth century aerial photography, but now it means satellite remote sensing, started with the launch of Landsat-1 in 1972 for civilian applications. Operation of these varies from low altitude Unmanned Aerial Vehicles (UAVs) to high altitude satellites orbiting the Earth. The electromagnetic radiation includes a very wide range of energy, from X-rays through visible light to radio waves. However, only a portion of the huge electromagnetic spectrum is actually used for remote sensing. This paper mainly deals with the satellite remote sensing. Over the time, the field of satellite remote sensing has passed through new developments e.g. higher spatial resolution optical and radar systems, hyperspectral sensors having the capability of generation of digital elevation model (DEM). processing using computers. However, both techniques have their respective advantages and disadvantages. Knowledge of the specific geographic region depicted on an image can be equally significant because every locality has unique characteristics that influence the patterns recorded on an image [50].

Visual Image Interpretation
Much interpretation and identification of targets is done manually or visually. This requires recognizing targets, which is the key to interpretation and information extraction. Observing the differences between targets and their backgrounds involves comparing different targets based on any, or all, of the visual elements of tone (or hue), shape, size, pattern, texture, shadow, and association (site and situation) by human knowledge and experience. If a two-dimensional image can be viewed stereoscopically to simulate the third dimension of height, visual interpretation will be much easier. Aerial photograph gives such opportunity. Radar response is controlled by wavelength, incidence angle, polarization, and surface roughness and dieelectric constant of the soil, depends on moisture content. For these Radar images have certain characteristics that are fundamentally different from optical and thermal images [51], include speckle, texture and geometry. During image interpretations this should be borne in mind.
Tone (or hue) refers to relative brightness or colour of objects on an image. To the interpreter it appears as relative lightness or darkness in the region under study. Shape refers to the general form, configuration, or outline of individual objects. Size of objects on images must be considered in the context of the image scale. Pattern refers to the arrangement of individual objects into distinctive recurring forms that facilitate their recognition [50]. The repetition of certain general forms or relationships is characteristic of many objects. Texture is the arrangement and frequency of tonal change on an image. It gives a feeling of apparent roughness or smoothness to the interpreter. It is produced by an aggregation of unit features that may be too small to be discerned individually on the image. Shadows are important, affords an impression of the profile view of the object and subtle variations in terrain elevations. As a rule, images are more easily interpreted when shadows fall toward the observer. Site refers to topographic or geographic location. Association refers to the occurrence of certain features in relation to others [52] but not like pattern. However, collateral information and non-image information are required to assist in image the interpretation [50].

Digital Image Processing
The digital image usually contains of millions of discrete picture elements known as pixels. A digital image is not the same as a photograph, and only becomes a picture when converted from digital to analogue form on a display screen or in a photographic print. The minimum area covered by a pixel is known as resolution of the sensor, also known as ground resolution or spatial resolution. There is another resolution in remote sensing known as spectral resolution that defines which wavelength is used for the pixels of an array. A multispectral or multi-band image is made up of a series of digital images, one for each waveband images. The third resolution is the temporal resolution which indicates how often the sensor of a satellite acquire image over the terrain. Resolution in remote sensing data is very important to the users' objectives.
Digital image processing is the application of algorithms on digital images to perform, processing, analysis, and information extraction. Before extraction of information from earth surface data, as seen by the sensors in different wavelengths, need to be radiometrically and geometrically corrected. Digital image processing may involve numerous procedures including formatting and such corrections of data, digital enhancement to facilitate better visual interpretation, or even automated classification of targets and features entirely by computer. Modern archaeology increasingly crosses academic boundaries by combining different new methodologies in order to answer research questions about ancient cultures and their remains [53]. Thus, another advantage of modern-day technology may be taken that includes integration of data from multiple sources to extract better and/or more information, and even multitemporal, multi-resolution, multi-sensor or multi-data type may be integrated, depending on the objectives of the work. The use of remote sensing has been applied to archaeological prospection and monitoring since the early days of aviation [53][54][55]. Lambers [56] found great potential for a truly semantic analysis of remote sensing data for archaeological purposes.

Remote Sensing Application in Archaeology
At present a vast array of active and passive remote sensing techniques are available. Since the launch in 1999 of IKONOS, the first civilian spaceborne Very High Resolution (VHR) multispectral sensor, archaeologists have access to relatively low cost VHR optical data over areas where airborne campaigns may be difficult or expensive to organize. However, many similar VHR multispectral spaceborne sensors are also available today, such as Quick Bird, WorldView-1 to 4, GeoEye-1, and Pleiades-1A and 1B. Some available satellites are given in Table 1.

Survey for Archaeological Sites
Visual image interpretation has proven particularly useful in locating sites whose existence lost to history [52]. Surface features include visible ruins, mounds, rock piles and various other surface markings. Subsurface archaeological features include buried ruins of buildings, ditches, canals, and roads. When such features are covered by agricultural fields or native vegetation, they may be revealed on aerial or satellite images by tonal anomalies resulting from subtle differences in soil moisture or crop growth. The traces of ancient human transformations of landscape create very subtle spatial features, namely surface anomalies that are only visible from a bird view. The characteristics of these archaeological features strongly depend on vegetation cover and phenology, pedology, soil types and topography, and named soil, shadow and crop marks [57,58]. Soil-marks can appear on bare soil as changes of tone/colour or texture. Shadow marks can be seen in presence of variations of micro-topographic relief visible by shadowing. Crop marks can be evident for vegetated areas, covered by crops or weeds. They can appear as differences in height or tone/colour in crops which are under stress due to lack of water or deficiencies in nutrients. There may have different in texture.
However, Chen [58] describe in detail the rational basis for archaeological marks. Past human occupation and activities have left traces and marks on landscape alterations and environmental changes that can be recognized even after centuries and millennia. For example, archaeological materials e.g. artifacts, bone, pottery or clusters of building materials would cause such changes. Erosional and depositional processes also contribute to changes. All these marks can be detected on remote sensing images, since the alteration they produce can be revealed by satellite sensors as they can influence spectral response and radar return. Bini [59] could identify different types of colour anomalies from SPOT 6 and SPOT 7 images as well as from Sentinel-2 image. Radar signal return from the object is dependent on radar look direction also. Modern day digital image processing technique can enhance the visibility. Micro-topographic relief variation, sometimes important in archaeology, produces shadows on optical images acquired under low angle sunlight condition. But in case of microwave images i.e. shadow producing mechanism is different, depends on angle of incidence. On Radar image subtle relief can provide shadows, significant in neotectonics and useful in terrain analysis. From high resolution satellite images construction of digital elevation model (DEM), such micro-relief can be identified [60].
Vegetation type and pattern on near-surface archaeological site would show different tone and texture from its surroundings on the image. These happen because compositions of the buried archaeological materials are different; this would have different water holding capacity. Thus, the vegetation gets different water and nutrient from underlying materials showing this difference on the image. There are two kinds of crop marks, including negative marks above wall foundations and positive marks [57] above the damp or nutritious soil of buried pits and ditches. The crop marks in SAR images are indicated by backscattering anomalies instead of the Near Infrared Red (NIR) spectral separability of optical remote sensing [58]. The occurrence of archaeological remains can change soil types and contents compared with their surroundings resulting in the well-known phenomenon of soil marks showing change in tone and texture in the absence of vegetation cover. Damp marks appear on bare ground because buried archaeological-remains can alter drainage capability and, in turn, water distribution. The detectability of damp marks is linked to soil type, climate and meteorological conditions. Bangladesh examples: Examples from two archaeological sites/places may be cited here. Efforts have been taken to evaluate the visibility on remote sensing images for two large archaeological sites in Bangladesh, situated in two different geological environments. These sites/places are at Dharmapalgarh, Nilphamari and Wari-Bateshwar, Narshingdi. The first site is on Young Tista Alluvial Fan that contains Recent unconsolidated sediments and the site is younger than the later. The second site is on the uplifted terrace containing Pleistocene oxidized soils and about 4 th BC old.
The walls of the archaeological site at Dhamapalgarh, Nilphmari ( Figure 1) are visible on CORONA satellite photograph, Landsat TM and Google images. On the CORONA satellite photograph acquired during late 1960s and Google image acquired in 2011 rampart of this fortified city of Dhormo Pal and structure of another nearby-site in Nilphamari district are easily identifiable (Figure 9). It is a rectangular structure (1mile (N-S) x ½ mile (E-W)) [61], but its eastern wall has been eroded away by one of the paleo-courses of the Tista River. However, on the Landsat TM bands 3 and 4 north and west walls are identifiable, south wall is not visible; whereas the site is not visible on bands 1 and 2, this needs more study. Obviously, Landsat TM FCC shows the north and west walls clearly as white colour in the red coloured background. It is also noticeable the changes that took place during the 1960s and 2010s period due to natural (vegetation and channel morphology) and anthropogenic (agriculture, settlements and road construction) activities. During a field visit, detection of the continuity of the walls was found not easy. This area experiences inundation from annual flooding that brings sediments, deposit on the plain. The area also experiences subsidence due to usual geologic processes. In the field visit to a nearby mound site an elderly local resident informed that the mound was at a higher elevation about 45-50 years ago than that of today. Main anthropogenic activities include intense agriculture and reuse of the bricks for local purposes. The fort-city or urban centre of Wari-Bateshwar is enclosed with four mud ramparts in a 600m x 600m area [13]. This site ( Figure 10) is clearly visible on CORONA satellite photograph and Google images, from their shape, size and association. However, the site was not identifiable on Landsat TM image. This result is only from visual interpretation; enhancement in digital images may help in identification of such features. Due to the position of the site above flood level there is no or very little sedimentation. Little amount of sediments may come from surface erosion. Anthropogenic activities include cultivation of land, cutting red soil for building local mud-house, digging pit for dumping garbage etc. by local people. Luo [62] found GoogleEarth very high-resolution imagery to be powerful for a range of different archaeological and cultural heritage applications.  Comparison of situations at two different times -before and after excavations at an archaeological site/place at Madhabgaon, Dinajpur district is shown in Figure 11. Geologically the site is on the Old Tista alluvial fan, surficial sediments are of early Holocene time. The image before excavation (left) shows only a round-shape land with smooth texture and light tone surrounded by vegetation, although no significant archaeological evidence is identifiable. But the image acquired after excavation (right) gives light tone with rough texture. Substantial changes on the site made during excavation give such changes in tone and texture.

Geoarchaeology
In geoarchaeology, primarily a subset of geosciences is considered [63] that include particularly geomorphology, sedimentology, pedology and stratigraphy. For geoarchaeological research in an alluvial terrain it is important to know the influence of internal (autogenic i.e. caused by infilling of sediments to storage site or sudden channel modification) and external (allogenic i.e. caused by alterations in catchment characteristics and tectonics) forces in channel change. But, changes in catchment characteristics and alteration of sediments in depositional site are related with climate change. Mehdi [64] concluded, based on the available data on sediments, that the buried channels, identified from Landsat ETM+ and SRTM and ASTER DEM data, at depths 6-13m were last active between 6 and 3 Ka B.P., when they supported the archaeological settlements along their banks. Ferentinos [65] opined that geological evidence for disappeared habitation sites can be identified from remote sensing.
Rivers and associated floodplain depositional environments are important in archaeology [66]; they served as notable significant loci for past occupations [67]. These dynamic landscapes exhibit a variety of local sedimentary environments [68] producing terraces, wealth of stratigraphic, paleoenvironmental and geoarchaeological information [67]. Alam & Islam [69] successfully mapped the terraces on the Brahmaputra floodplain using SPOT Pan Imagery. Such information contributes to infer past conditions from floodplain sediments comes from the analysis of contemporary rivers and their sedimentation [68] i.e. the present is the key to the past. The utility of SAR remote sensing to detect the subsurface in sand covered areas has long been known [70,71]. Stewart [72] evaluated X-band SAR data and found to have limited capability for prospection of archaeological structures buried in sand covered areas. Its success depends also on the terrain condition, among other controlling factors.
In Bangladesh, SAR data have so far used in flood assessment [73,74] using RADARSAT data and for measuring subsidence rate [48] in the delta using SAR interferometry (ALOS L-band InSAR). In geomorphological study, Alam [75,76] could identify different features on RADARSAT (C-band) and JERS-1 (L-band) SAR images e. g, dry valleys (not buried) on the uplifted terrace and active channels but lithological discrimination was not possible. In another study, even geomorphological features were not identifiable [25]. Probably high moisture content, influences die-electric property of the materials hampers the process of image interpretation. As the terrain condition of Bangladesh is different from the above-mentioned examples [70,72], potentiality of available SAR images from RADARSAT and ERS (C-band) or JERS may be evaluated and same for the ground penetrating radar (GPR) dependency on water or moisture [66] among others.
Bangladesh examples: Earliest work among the above was done by [15] where he mapped drainages and landforms in the eastern part of Bangladesh from Landsat imagery and aerial photographs and described in detail about landform evolution. Bakr [15,77] showed the relationship of archaeological sites of 8-10th century and landscape, how the uplift of the Early Recent Chandina Deltaic Plain including shifting of river courses badly affected the environment of the area. Akanda [78] studied satellite images to detect the nature of archaeological records in terms of geomorphology, geology, elementary pedology and stratigraphy of Wari-Bateshwar area in order to understand the influence of fluvial environmental variables on deposition and modification of archaeological records. They concluded that any archaeological inference on the area must be corroborated by the geoarchaeological evaluation.
Sen [79] successfully used, among others, satellite images to unfold the formative history of the floodplain and the lateral migration of rivers in the archaeological site of Somapura Mahavihara, Naogaon. Among other reasons, gradual migration of the main river in the area must have caused the settlement here to be abandoned. To understand the early Medieval settlement archaeology of the alluvial terrain of the northwestern part of Bangladesh Sen [80] carried out a detail study in a geo-spatial environment. He identified not only the various geomorphological features and potential archaeological features on the floodplain but also showed spatial patterns of archaeological places on satellite images. Paleo-channels of the Tista-Mahananda-Karatoya rivers have been identified [81,82] from visual interpretation of aerial photographs and satellite images on the Tista Alluvial Fan. Their findings help in deciphering paleo-environment, river dynamics, anthropogenic impact on environment, paleoclimate condition etc, giving clues to the causes of migration and migration pattern of human settlements.

Discussion and Conclusion
As mentioned earlier that major part of Bangladesh is covered with Quaternary alluvial deposits, the region is tectonically influenced, and the sediments have experienced impacts of climate change as well. Such factors must have influenced the human settlements in the alluvial terrain, found ideal for past settlements [54]. Factors that influences changes in the landforms of the country at present time discussed in the previous paragraphs might have also influenced in the past resulting burial of human settlements or archaeological marks on the delta. The very widely accepted principle in geology -'present is the key to the past' may be applied in search of hidden archaeological sites using remote sensing. Advantage of medium and high spatial remote sensing data may be taken to locate archaeological site, and relatively low and medium resolution image data may be used in geoarchaeological study.
Depending on the purposes, selection of band is another important point needed to detect, identify and map the active and paleo-channels, and water bodies required for evaluating rate, pattern and style of channel migration. Smaller objects may be identifiable from their shape and association, however, may need to apply image enhancement techniques on the remote sensing data during digital image processing. High spatial resolution image e.g. Pleiades (50cm), IKONOS, QuickBird (60cm) etc. will help in identification of smaller size sites and objects. Moreover, DEM can be constructed using high resolution stereo-images that would give subtle micro-relief on plain lands, important clue for archaeological research. Due to several reasons' aerial photographs, advantage of having stereoscopic visibility capability, are not easily available. In that case high resolution satellite image may be used.
Geoarchaeological techniques and approaches are widely recognized as enabling syntheses of landscape development and the archaeological record [83]. But for Bangladesh, more works [11,15,[77][78][79][80]82] need to be done to examine and understand the changing human settlements with the changes of landforms in such alluvial terrain. Use of wide ranging remote sensing data from aerial photographs, CORONA satellite photographs, medium resolution satellite images (Landsat TM & ETM+, SPOT, IRS) to high resolution (Lidar, Pleiades, RapidEye) and from optical to microwave (Radarsat, JERS and ERS) contributed much to geological, geomorphological, urban geological [84], environment, natural hazard assessment [50] etc. furnished with successful results in Bangladesh. But the relationship between SAR signal and terrain characteristics is yet to be well understood, important also for archaeological research. Because of its different geologic and geotectonic characteristics, identification of buried sites using SAR images is in doubt. Only the expert visual image interpreter can use SAR images for geoarchaeology. However, SAR look direction gives the advantage in locating linear features on the image. Anyway, research must be carried on for a better understanding of these relationships and identification capabilities.
CORONA satellite photographs provide information of the time prior to Landsat era. These photographs were acquired during 1960 and 1972, now become available for civilian use [50], with variable spatial resolution (~2-12m) and in black and white [59]. At that time, population of this country and population pressure on the land and environment, development activities were much less than those of today. The advantage of CORONA satellite photograph may be taken for archaeological research to see situation in a relatively less modified landform, favouring identification of archaeological marks. Significant development in remote sensing techniques have been made during the last few decades advantages of which should be taken to locate and identify buried features prior to excavation, and to understand the relationship between human settlements with their geological environments.
After an evaluation of archaeological works done using remote sensing data particularly in Bangladesh context application of this technology is in the initial stage and it has much potentiality. Depending on nature of problem selection of multi-spectral, multi-resolution or multi-temporal image will depend. In order to extract meaningful information, based about Bangladesh i.e. the intensively cultivated lands, huge anthropogenic influence on environment, vegetation cover, influence of geomorphic, geologic and tectonic processes, and climatic condition; appropriate image interpretation and digital image processing techniques should be performed. Successful extraction of relevant information can improve our level of understanding about our environment, which may need integration of data from different sources. Another important point is that an integrated approach of archaeologists, geologists and remote sensing experts would bring successful results to solve the archaeological problems.