Geophysics

'Geophysics' the physics of the earth and its environment, that is, earth, air, and (by extension) space. Geophysical investigations of the interior of the earth involve measurements, taken at or near the surface of the earth, that are partially dependent on the internal distribution of physical properties. Proper analysis of these measurements can discern the variation of the physical properties of earth';s interior laterally and vertically.

The science of geophysics may be applied to a wide range of investigations from studies of the entire earth, termed as, global geophysics, to exploration of localised region of the upper crust termed as 'exploration geophysics';. In exploration geophysics, measurements taken within geographically restricted areas are used to determine the distribution of physical properties at depth and to isolate the zones with anomalous physical properties. These anomalous zones are then interpreted in terms of local subsurface geology. Based on the operative physical property following geophysical prospecting methods exist: (i) gravity, (ii) magnetic, (iii) seismic, (iv) electrical and electromagnetic, (v) radioactive, and (vi) borehole geophysics.

Early forms of modern geophysical methods were used for the first time in the territory of Bangladesh about 50 years ago. During 1950';s and 1960';s the onshore area was explored by major international oil companies (ioc), eg Shell, Stanvac, PPL, and the national OGDC (Oil and Gas Development Corporation). Gravity and aeromagnetic surveys covered almost whole of Bangladesh and in 1990, geological survey of bangladesh produced Bouguer gravity anomaly and aeromagnetic anomaly maps of Bangladesh in 1:1,000,000 scale. Single and multifold seismic survey with different density of data covered most of the onshore areas of the country. Multifold seismic survey of digital recording carried out mostly during mid 1970';s covered offshore area of Bangladesh. Electrical prospecting especially surface resistivity survey is carried out in different parts of the coastal belt and northwestern region of the country for groundwater investigation. Geophysical logging is routinely used during well drilling for the evaluation of petrophysical parameters. [ASM Woobaidullah] [Woobaidullah, ASM Professor of Geology, Dhaka University]