Geological Survey

Geological Survey is concerned chiefly with the systematic geological mapping and survey of rock and mineral, fossils, oil, gas and water resources of the surface and subsurface. With the help of field geology, air photography, satellite imaging (remote sensing) and geophysical surveying it particularly puts emphasis on the economic deposits and thus usually prepares maps having increasingly larger scales for use in economic exploitation. Its purpose may also be purely scientific. The geological surveying also furnishes useful information in foundation studies for road and railway alignments, for bridges, tunnels and buildings, in town protection works and in earthquake hazards.

geological survey of bangladesh is mainly responsible for the detailed regional geological surveying (scale 1:50,000) of Bangladesh and so far about an area of 60,000 sq km of the country has been mapped. [M Abu Bakr]

History Throughout history man has recorded, analysed and communicated information in map form. The oldest map in existence is engraved on a Babylonian clay tablet dating from 3000 BC. Around fifth century BC, that Greek philosophy stimulated attempts to create a map of the world. The foundations of modern map making were introduced in the sixteenth century (1512-94). Plane surveying dates from ancient times and was highly developed in Egypt. In the case of American history plane surveying played an important role by making boundaries for settlements. Surveying was a respectable profession - both George Washington and Mark Jefferson worked for a time as surveyors. The most dramatic changes of surveying have come with the development of aerial survey techniques. Today photographs from satellite or aircraft are used together with data from ground surveys to map accurately large areas of the earth. In modern days geophysical surveying is the only tool for subsurface mapping. In Bangladesh (the then East Pakistan) geological surveying was introduced in early fifties.

Field survey (geologic) The study and interpretation of rocks, the determination and location of points where observations are made and measurement of the attitude of beds as well as the study of physical features, structures, lithology, stratigraphy, geologic history, economic geology and finally the preparation of maps usually involves the geologic field survey. With the exception of northeastern and southeastern margins, Bangladesh is covered by a vast alluvium plain. Generally, the professional geologists working with the universities, Geological Survey of Bangladesh, petrobangla, Bangladesh Water Development Board and Atomic Energy Commission carry out fieldwork regularly in the alluvium plains as well as in the hills.

For the training purposes of students field works are confined of the districts of Chittagong and Chittagong Hill Tracts, Sylhet and Comilla. Although, these districts contain several geological structures, not all of them are accessible easily. As such, only a few structures are regularly surveyed viz Lalmai Hill Range of Comilla district; Hararganj anticline, Patharia anticline, Jaflong-Tamabil-Lalakhal area of Sylhet district; Sitakunda anticline of Chittagong district; Maheshkhali anticline, Inani anticline, Waylataung anticline, Dakhin Nhila anticline of Cox's Bazar district and Sitapahar anticline, Bandarban anticline, Matamuhuri anticline etc of the Chittagong Hill Tracts. In these hilly areas, sedimentary rocks are exposed and the geological studies in these areas are done in great detail. The st martin's island of Cox's Bazar district, is also an excellent place for the study of field geology.

Extensive field works over the years have made possible the publication of the following geological maps of Bangladesh: 'Geological map of Pakistan' (inset East Pakistan), 1964 and 'Geological map of Bangladesh' (scale-1:1000,000), 1990.

Air survey the technique of precision photographing of the surface features of the earth from an aircraft. The photographs so taken are air photographs. Rapid examination of an area from the air may greatly facilitate the selection of localities to be explored on the ground, especially in countries like Bangladesh, where roads are few, accessibility is difficult and where geologic features can be seen reasonably well from the air. As such in Bangladesh air photographs of dry season, rainy season and winter season is mostly used to determine engineering, structural, petroleum, hydrogeological and Quaternary geological problems. Ministry of Defence looks after these photographs.

Satellite remote sensing Satellites such as Landsat have surveyed the earth's entire surface from orbit. In Bangladesh application of satellite data started in the early 1970's with the creation of a research organisation called Space Research and Remote Sensing Organisation (sparrso). At that time, Bangladesh used to receive Earth Resources Technology Satellite (ERTS) data, which later change its name as Landsat, on a regular basis at' free of cost. In addition EGIS, a non-government organisation is also engaged in remote' sensing works. [Sifatul Quader Chowdhury]

Geophysical survey The making and interpretation of certain physical measurements with the object of furthering the study of subsurface geological conditions of structure or material are done by geophysical surveying. There are four types of geophysical methods: gravity method, magnetic method, seismic method and electrical methods.

Gravity method determines the specific-gravity differences of rock masses by mapping the force of gravity of an area, using a gravimeter. Gravity data, usually, after necessary corrections are displayed as Bouguer anomaly maps.

During the 1950's and 1960's National Oil and Gas Development Company (OGDC) of the then Pakistan and some major foreign oil companies, Shell, Stanvac and Pakistan Petroleum Limited (PPL) carried out gravity surveys in the onshore areas of Bangladesh as reconnaissance tools. After the independence of Bangladesh, Petrobangla has continued to carry out gravity surveying in the onshore areas, mostly, in the scale of 1:50,000 and in 1981, synthesising available gravity data of the country produced a regional Bouguer gravity anomaly map in the scale of 1:1000,000. Later in 1990, Bouguer gravity anomaly map of Bangladesh was produced by Geological Survey of Bangladesh (GSB) by digitising the hand drawn regional Bouguer gravity anomaly map of 1981.

Magnetic method involves investigating subsurface geology on the basis of measured anomalies in the earth's magnetic field resulting from the magnetic properties of the underlying rocks and ore deposits.

Magnetic surveys started to be used in Bangladesh territory in 1950's and continued during 1960's by OGDC and the other major oil companies; ie Shell, Stanvac and PPL. Early aeromagnetic surveys suffered from injudicious selection flight line directions giving rise to false anomalies. Hunting rectified these shortcomings during the Petrobangla survey of 1979-80 when the entire country was covered by a grid of 3x5 km flight lines flown in north-easterly and north-westerly directions. The results were reported accompanied by a suite of maps. In 1990, an aeromagnetic anomaly map of Bangladesh was produced by GSB, digitising the total intensity aeromagnetic map of Hunting.

Seismic method In this technique artificially generated seismic waves is used in the search for economic deposits such as salt, oil and gas or in engineering studies such as determining depth to bedrock or in the presence of potentially active faults.

Seismic surveys was first introduced in the territory of Bangladesh in 1950 by Standard Vacuum Oil Company (SVOC) in the north-western part of the country. During 1950 to 1964 Geological Survey of India (GSI) as contractor of SVOC, PPL and Pakistan Shell Oil Company (PSOC) carried out seismic survey covering selected areas of north- western, middle and eastern region of the country. Since 1963 Petrobangla is carrying out seismic survey in different areas of the country. Single fold data collected during 1950 to 1978 were about 20,000-line km. From 1978 onward different surveying companies have been using multifold (6,12 and 24 folds) and digital system of recording. Dynamite sources have been used for single and multifold onshore surveys. Seismic surveys in the offshore areas were mostly carried out during mid 1970. All offshore data are multifold and digitally recorded. The results of the seismic survey have yielded important new information for exploration targets. Recently in 1997-99 Grants Geophysical Inc also conducted both reflection and refraction seismic surveys mainly in the onshore and offshore areas of Chittagong district.

Electrical method A geophysical prospecting method that depends on the electrical or electrochemical properties of rocks. First geoelectric resistivity survey in Bangladesh was carried out in the north-western part of the country by a German Group under a programme of Technical Assistance from Federal Republic of Germany in 1967-68. The survey was aimed to investigate the hydrogeological condition of the covered Rajshahi, Bogra, Dinajpur and Rangpur districts. Recently, resistivity survey has been carried out in a number of towns of the country as part of the Water Supply and sanitation Projects by different consulting firms to identify the suitable aquifer for groundwater development. [ASM Woobaidullah]