Possibilities of Using Remote Sensing in Archaeological Research in Bengal Delta in Bangladesh - A Human-Influenced Alluvial Terrain- Juniper Publishers
Archaeology & Anthropology- Juniper Publishers
Abstract
Bangladesh, an alluvial country lying in an active
tectonic region of the world, has a huge population and rich
archaeological heritage. The fluvio-deltaic landforms of the country are
continually changing due to geologic processes, natural hazards and
anthropogenic activities. In such an actively changing condition it is
difficult to preserve and to detect the archaeological sites. From
archaeological point of view, it is also important to understand the
relationship between the human settlements and migration, and the
changing landforms. Remote Sensing can help both to explore the
archaeological sites and to understand the relationship. Advantages like
multi-spectral, multi-resolution and multi-temporal capabilities of
remote sensing can be taken to solve special problems. Over the last few
decades, remote sensing technology has tremendously developed, and area
of application has also widened. Possibilities of use of this technique
in the context of Bengal Delta in Bangladesh have been evaluated. Use
of remote sensing data would increase the efficiency of archaeological
discovery/research through saving time and money in a country having
intense anthropogenic activity and continuously changing alluvial
landforms.
Keywords: Remote Sensing; Archaeology; Alluvial Terrain; Anthropogenic Influence; Geoarchaeology; Bengal Delta; Bangladesh
Abbrevations:
AMSL: Above Mean Sea Level; DEM: Digital Elevation Model; UAVs:
Unmanned Aerial Vehicles; VHR: Very High Resolution; NIR: Near Infrared
Red; GPR: Ground Penetrating Radar; MIR: Mid Infrared; TIR: Thermal
Infrared; MS: Multi-Spectral
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]. Three-dimensional 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, natural hazards etc. Application in archaeology remains only
within geoarchaeological studies.
In the following paragraphs archaeological heritage [7-
9] and fundamentals of geology of the country have briefly
been described. At the later part basics of remote sensing
with information extraction procedures and requirements
for archaeological research are discussed. Current status of
remote sensing application in archaeology in the country and
elsewhere has been reviewed. Results of few such applications
and possibilities of future applications of remote sensing in the
archaeological research in Bangladesh are discussed.
Archaeological Heritage of Bangladesh
Majority of the landforms of Bangladesh are originated from
fluvial and deltaic processes forming the largest delta of the
world. Quaternary sediments and soils cover about 90% area
of the country. Among the four favoured general environments
for human habitation two -– alluvial and coastal environments
[10] exist in Bangladesh. It has huge networks of large and small
rivers with their fertile floodplain replenished with sediments
and nutrients during annual floods. The floodplain provides
food, drinking water and water for irrigation; rivers provide
communication networks, sediments are sources of important
raw material, gravels are used for constructions. The country has
a vast coastal plain that also favours for food, communication
for trade and others. All these favourable conditions attracted
humans since long. This vast alluvial land holds the settlements
having its own people and as well as people from different
countries and nations having different ethnicity, religion, race,
language and culture from pre-historic age till to date.
However, during the second half of the first millennium,
ancient Bengal including present-day Bangladesh, West Bengal
and parts of Assam and Bihar of India were roughly divided into
several political divisions which consisted of Pundra or Varendra
(North Bengal), Rahr (West Bengal and adjacent parts of Bihar),
Kamrupa or Anga (Assam), Vanga or Harikel (Southern Bengal)
and Samatata (South-eastern and eastern Bengal). However,
Dinajpur, Rajshahi and Bogra region (Figure 1) of Bangladesh
have signs of the Early Historic settlement that can be assigned to
the 3rd century BC [7], although the history of the period between
the fall of the Mauryan Empire (2nd century BC) and the rise of
the Gupta rule (4th century AD) is unclear. The Pal dynasty ruled
Bengal from ~781 to 1124 AD of which Somapura Mahavihara is
one of the important evidences. Later Sen Empire ruled during
1158 – 1205 AD. Muslim era started in 1204, Lakhnauti in
Dinajpur was conquered. The empire, centring Lakhnauti, was
supposedly stretched to the Padma/Ganges River in the south,
Rangpur and Dinajpur (Devkot) in the north, Bihar (India) in the
west, and the rivers of Tista, Brahmaputra and Karatoya in the
east. Later in 1338 AD independent sultanate was established in
Bengal which remained for 200 years. Afterwards Delhi-based
Mughal rule started that continued till the death of Aurangzeb in
1707 and afterwards became weak gradually. Bengal was ruled
by independent rulers till 1757 and British ruled from 1757 to
1947 followed by Pakistani regime of 1947-1971. Afterwards
Bangladesh emerged as an independent nation in 1971.
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).
Chalcolithic Settlements
Chalcolithic cultures are evidenced by Black-and-Red
ware and pit-dwelling were discovered in Wari-Bateshwar
area of Narshingdi district, area lies on the uplifted terrace of
Pleistocene. Hearths and wells have also been discovered there.
The Iron Age began in the middle of the Chalcolithic age (1200
BC) and lasted until the onset of the early historic period (400
BC). In Bangladesh, traces of the settlements belonging to the
later part of the Iron Age have been found in Mahasthangarh
of Bogura district (Figure 1). Though the iron tools were made
in the primitive techniques from 1200 BC to 700 BC, advanced
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 11t 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 13th century,
when the area came under Muslim occupation [14]. This site has
been included to the UNESCO declared World Heritage.
Mainamati, Cumilla Mainamati archaeological site, dotted
with more than 50 ancient Buddhist settlements of the 8th to
12th century AD [14], is located on the Lalmai Hills (Figure 1).
Lalmai Hills, lying west of the Cumilla town, is a 19 km long,
north-south trending hilly terrain and has an average elevation
of 30.50 m above mean sea level (AMSL). It is gently sloping to
the east; the western flank is cliff-like, overlooking the Chandina
deltaic plain to the west [15]. The elevation of the plain is about
6.7-9.7 m AMSL. The area is drained by three major rivers -- the
Gumti, the Dakatia, and the Little Feni and numerous rain-fed
channels. The Lalmai hill comprises the Dupi Tila Formation of
the Plio-Pleistocene age, blanketed by Madhupur clay residuum
of Pleistocene time [12]. Alam [16] gave the impression of
Recent uplift in the area based on drainage anomaly, nature of
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.
Based on the results of geophysical surveys, geological
mapping and borelog data Bangladesh part of the Bengal Basin
could be divided into two major divisions 1) the Rangpur
Platform (also known as ‘Indian platform’ or ‘Stable shelf’) and
2) the Bengal Foredeep [4,24]. The Rangpur Platform is further
subdivided into three zones (i) the elevated part of the basement
complexes the ‘Rangpur Saddle’, (ii) the northern slope of the
saddle the ‘Himalayan Foredeep’ and (iii) the southern slope
of the saddle ‘Bogra Shelf’. The Hinge Zone, about 25 km wide
with NE-SW trending, separates the Bogra Shelf and the Bengal
Foredeep, the latter is a region of great subsidence of the
crust, accumulating thick sediments. The Foredeep has been
subdivided into two major tectonic zones, (i) the deep basinal
unfolded or gently folded ‘Platform Flank’ and (ii) the ‘Folded
Flank’ comprising hilly regions of the Chittagong Hill Tracts,
Chittagong and Sylhet districts.
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).
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 northeastern
parts, lies between 70 and 1000 m. 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 storm-surge 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 [25,30,34-41] 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] described in detail 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 which lies in
the eastern part of the present-day Bangladesh. Alam & Alam &
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].
The cyclones with storm surges carry enormous sediments
from the sea and deposit on the coastal zone. Recent evidence
shows how much sediments carried by cyclone Sidr of 2007
that buried the roots of mangrove plants under about 1 m depth
in the Kuakata coast (Figure 6), SW Bangladesh. 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.
Basics of Remote Sensing
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).
Information Extraction Process
Successful use of remote sensing data depends on the ability
of extraction of meaningful information from the imagery, where
both visual image interpretation done manually and digital image
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.
Remote Sensing Application in Archaeology
The use of remote sensing has been applied to archaeological
prospection and monitoring since the early days of aviation [53- 55]. Lambers [56] found great potential for a truly semantic
analysis of remote sensing data for archaeological purposes.
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 4th 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
[11,15,77-80,82] 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.
Acknowledgement
Professor Sufi Mostafizur Rahman, Professor Syed
Mohammad Kamrul Ahsan and Professor Swadhin Sen of
Department of Archaeology, Jahangirnagar University kindly
provided support to visit the archaeological sites in the central
and fieldwork in the north-western parts of Bangladesh at
various times. Discussions with them on various aspects of
archaeology, human settlements and their relationship with
landscape etc. enriched the author’s level of understanding.
Reshad Md. Ekram Ali of Geological Survey of Bangladesh kindly
provided CORONA satellite photographs. All their supports are
gratefully acknowledged. Finally, I would like to thank Nahid
Arjuma Begum, my beloved wife, who rendered whole-hearted
and continuous support to all my scientific and research works,
especially more desirable in retired life.
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