Northwestern Part Of Bangladesh Research Articles

Inversion of Gravity Data for Imaging of a Sediment-basement Interface: A Case Study in the Northwestern Part of Bangladesh

Gravity data collected by the Geological Survey of Bangladesh are processed and interpreted for imaging of a sediment-basement interface over the northwestern part of Bangladesh. The observed gravity data are processed for discriminating gravitational fields contributed subtly from the shallow basement topographic feature with the twelve nodal piecewise cubic polynomial–based finite–element approach. In spectral analysis, the presence of a widely spread shallow basement feature has been detected and interpretation of gravity data using a two-dimensional gravity inversion technique indicates that its depth ranges from 0.041 km to 0.570 km relative to ground surface. In the northern part of the study area, the inferred basement configuration shows a general depression of the basement in the Takurgaon-Panchagar and Lalmonirhat districts and reaches a maximum depth of about 0.570 km. In the Nilphamari district and its southwestern part, the basement occurs at the most shallow depth due to its upliftment. However, the estimated sediment-basement interface depths are compared with the borehole and other geophysical interpretative information and are found to be consistent.

Clay mineralogy of archaeological soil: an approach to paleoclimatic and environmental reconstruction of the archaeological sites of the Paharpur area, Badalgacchi upazila, Naogaon district, Bangladesh

A geo-archaeological examination was carried out in and around the Paharpur area, Badalgacchi upazila of Naogaon District in the north-western part of Bangladesh. Clay mineralogy and global paleo-temperature curves were used to determine the paleoclimatic and depositional environmental conditions of the Holocene archaeological soils (Barind clay residuum) of the Paharpur archaeological spot. Clay mineralogical data show that the dominant clay mineral is illite with subordinate amounts of kaolinite and chlorite. The high illite and low kaolinite content implies a marine or estuarine deposit. The clay mineral ratio log diagram, constructed by plotting the 7–10°A integrated intensity ratios versus depth, supports this assumption. The presence of illite, together with quartz and feldspar, suggests high detrital input under generally cool to temperate and dry climates. Chlorite is another common by-product of weathering reactions with low hydrolysis, which is typical for cool to temperate and dry climates. The results suggest the existence of a marine–estuarine influence under a generally cool to temperate and dry climates around Paharpur and the surrounding region throughout the Pala dynasty around mid-century 700 a.d. to early 1100 a.d. Furthermore, this observation is consistent with the global paleo-temperature curves.

Study on the seasonal changes of land cover and their impact on surface albedo in the northwestern part of Bangladesh using remote sensing

In the present work, the dynamic behaviour of surface albedo of a coupled soil–vegetation–atmosphere (SVA) system has been studied in relation to landform characteristics, seasonally varying surface cover, and hydro‐meteorological condition using multi‐temporal Landsat TM data. Two adjacent, sharply contrasted and physiographically different land areas, (1) the semi‐arid Barind Tract area and (2) the flood plain areas of the rivers Ganges and Mahananda situated in the northwestern part of Bangladesh, have been selected for this study. Spatio‐temporal analysis of Landsat TM‐derived spectral signatures manifests quite contrasted response characteristics for different physiographic land units in the two study sites and shows a seasonal trend that is consistent with field observations. Coupling of satellite‐derived land cover information with landform and hydro‐climatic data reveals that the observed variability in surface cover and its condition, as well as seasonal transformation of surface categories (e.g. soil, vegetation, and water) over time and space, are a function of landform characteristics and the prevailing hydro‐climatic condition of the area. Particularly, depth of flooding in relation to land elevation during the wet season in the flood plain areas and soil moisture deficit during the dry season in the Barind Tract area seemed to be the two major determinative variables governing land use of the area. Climatic variables, particularly precipitation, appear to be the main driving force that brings out the dynamic changes in the geo‐environmental condition in both study sites. The changes in vegetation cover in response to the rainfall amount show a much higher sensitivity of the Barind tract areas in comparison to the flood plain areas. Land cover information as derived from multidate Landsat TM data demonstrates a relatively high change (about 68 per cent) in the aerial extent of vegetation cover in the Barind Tract and a small change (only about 8 per cent) in the flood plain areas between summer and the rainy season. The flood plain areas are found to be moderately vegetated throughout different seasons, which is in contrast to the semi‐arid Barind Tract area that remains either mostly bare (about 90 per cent) or highly vegetated (vegetation cover greater than 75 per cent), depending on the season, though both sites receive an almost similar amount of seasonal and annual precipitation. Variation in soil moisture and vegetation condition, as well as transformation of bare land into a vegetated or seasonally flooded area in response to seasonally varying precipitation, introduce sequential variability in the surface absorption level, thereby provoking seasonal rhythmic variation in the surface albedo over the area. It is found that the transformation of bare land into a crop area in the Barind Tract reduces the surface albedo values by about 60–75 per cent. Relatively higher spectral responses in summer over most of the semi‐arid area depict a deficit in moisture content in the soil layer, as compared to that in the relatively moist flood plain area, except the sandy area, whereas transformation of land into water due to seasonal flooding, especially in the flood plain areas in the wet season, associates a decrease in albedo from 13 per cent in summer to about 6 per cent in winter. Increased albedo, particularly over the river area in summer, indicates the presence of suspended sediments originating from erosion due to surface runoff and nearby riverbank shifting. As a whole, surface albedo seems to be a land type‐dependent seasonal indicator of the area.

Spatial and Temporal Variations of Precipitation in and around Bangladesh

Radar data from the Bangladesh Meteorological Department (BMD) are employed as a preliminary analysis. This is a first research work to investigate the spatial and temporal distribution of rainfall over the country for 135 consecutive days, from 16 April to 30 August 2000. Radar data are sampled in 10 km grid boxes to obtain daily rainfall over the country. Rain gauge data at 33 locations are utilized to check the radar results. The distributions of rainfall obtained by both the radar and the rain gauges are similarin pattern, but the time of the maximum rainfall determined by the radar is a few hours earlier than that determined by the rain gauges. The distribution of rainfall over the whole radar domain suggests that 21 to 09 local standard time (LST) is the most likely time for rainfall to occur in Bangladesh, while 06 LST is the most likely time for maximum rainfall to occur over the entire country. It is mentioned that no data are available between 03-06 LST. The occurrence of 21 to 09 LST rainfall is possibly linked to the local effects such as complex terrain and sea and land breeze circulations. The morning maximum rainfall at 06 LST in Bangladesh is different from that of the Indian subcontinent or of the mountain area where, generally, maximum rainfall occurs in the afternoon. The northern border of Bangladesh, close to the Shillong hill of India, is the region with the highest rainfall, while the second highest volume of rainfall occurs on the eastern border. In order to observe the characteristics of large-scale cloud activity, analyses of the Japanese GMS-5 hourly infrared data, within a larger domain of 80°-100°E and 10-30°N were conducted. The cloud activities in and around Bangladesh were obtained in 10 by 10 grid boxes. The northwestern part of Bangladesh was largely affected by pre-monsoon clouds, while the whole country was affected by the peak monsoon activities.

Studies on Surface Temperature using Remote Sensing Technique in the NW Part of Bangladesh

A simple and straightforward technique based on remote sensing was applied for the estimation of surface emissivity and surface temperature at different dates in the northwestern part of Bangladesh for the summer and autumn period (March through October). Surface temperature was estimated from Pathfinder AVHRR Land (PAL) daily data. For this estimation different split window algorithms were employed. Estimated results show a discrepancy ranging from 0.04°C to 4.0°C. Surface emissivity was calculated based on fractionation of vegetation cover and NDVI at pixel level calculated from TM data. TM data was corrected for atmospheric effect. PAL data set was provided after atmospheric and geometric correction. Both air temperature and estimated surface temperature shows a decreasing trend from south to farther north. However, no distinct trend could be recognized in the temporal variation in surface temperature and in measured air temperature. A general trend of negative relationship could be concluded between NDVI and surface temperature from this study.

Palaeoclimatic impact on the flood basin accretion and palaeosol development in northwestern Bangladesh

During the Quaternary period, a number of climatic fluctuations occurred as evidenced in the deep-sea core samples, palaeolake deposits, continuous loess profiles in China and glacial deposits in various parts of the world. The fluctuations in palaeoclimatic conditions have influenced eustatic sea-level changes as well resulting in the formation of different sediment facies and landscape evolution worldwide. In the northwestern part of Bangladesh, a thick Pleistocene alluvial sequence has left signatures of these climatic changes. Based on different field criteria such as destratification, horizon formation, colour, biogenic­ pedogenic features (i.e. rootlets or root traces, burrows, CaCO3 nodules, Fe concretions) and soil structures, 10 buried palaeosols have been recognised in the Pleistocene deposits. The odd numbered palaeosols (i.e., GS1 , GS3, GS5, GS6 and GS9) are designated as moderately developed palaeosols as inferred from their relatively finer textures, higher concentration of pedogenic soil carbonate nodules and lower abundance of iron oxides. The even numbered palaeosols (i.e., GS2, GS4, GS6, GS8 and GS 10) are strongly developed palaeosols, containing very few or no pedogenic carbonate. They also contain Fe oxide concretions, burrows and root channels. Both moderately developed and strongly developed palaeosols occur in cyclic succession. Strongly developed palaeosols occur in thick sediment sequences, which indicate that the flood basin accretion rate was quite high as a result of stronger hydrodynamic conditions. They are the direct consequence of bigger flood events. The relatively thinner sediment sequences, in which the palaeosols are moderately developed, are a consequence of consecutive smaller flood events under drier conditions. Hence, 10 buried palaeosols suggest that at least 10 alternating dry and wet climatic phases prevailed during the successive phases of both palaeosol development and sedimentation cycles. These alternating dry and wet climatic cycles most probably influenced the strengthening and/or weakening of the palaeo-summer moonson. Moreover, studies based on isotopic compositions of stable carbon and oxygen as derived from the analysis on pedogenic carbonate nodules, as well as from the measured bulk magnetic susceptibility in the palaeosol sequences, are found to be in good agreement with the above statement on the dry and wet phases.