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Applications of Geophysics in addressing the Societal Issues for Geo-scientific point of view. Dr. DINESH GUPTA, DG, GSI V.RAMAMURTY Superintendting Geophysicst. OBJECTIVE. Geophysics in Town Planning Rangapahar area, Dimapur, Nagaland.
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Applications of Geophysics in addressing the Societal Issues for Geo-scientific point of view Dr. DINESH GUPTA, DG, GSI V.RAMAMURTY Superintendting Geophysicst
OBJECTIVE Geophysics in Town Planning • Rangapahar area, Dimapur, Nagaland. • The objective of the investigation has to delineate the overburden thickness in study area and to show the remedial measures to construct suitable structures.
Geophysics in Town Planning Two big buildings (Cinema hall building and Library building) were constructed without any geological considerations in Rangapahar area of Dimapur started to develop vertical cracks and both the parts were pulling apart. This created a panic for the authorities and they have requested the GSI to investigate the Rangapahar area, to identify the cause of development of vertical cracks and measures to construct suitable multistoried and heavy structures in the area
SITE GEOLOGY • Nagalandis apart of the Northern Extension of ArkanYoma mobile belt of Cretaceous – Tertiary age. • The rock types in the present area mainly belong to the Tipam, NamsangDihing and quaternary. • The larger and smaller hillocks that constitute the Rangapahar area composed of semi-consolidated soft-sediments.
Necessity of Geophysics Dimapur is an important town in Nagaland being the only railhead in the state and Rangapahar area has been occupied by strategic establishment figured out soil creeping, caving and developing cracks in the library hall and cinema hall area of Dimapur Town, necessitated the Geophysical surveys. In geophysical surveys electrical resistivity and shallow seismic refraction surveys were envisaged to determine the physical properties of the overburden and to map the bedrock topography.
RESISTIVITY METHOD • Resistivity soundings were carried out over the available sites and along the roads as the area is thickly populated. • Based on the resistivity data three layers of overburden have been identified. Top layer comprises of loose soil, pebbles and gravel has shown resistivity of the order of 300-1800 ohm.m and thickness varies from 2 to 10m. The resistivity range of 20-70 ohm.m has been identified as clay dominated soft sediments. High resistivity recorded in the area concealed with the compact rock. • The data has been corroborated well by boreholes drilled by Central Ground Water Board (CGWB).
Shallow Seismic Refraction Survey • Seismic refraction surveys have also brought out a three-layer sequence. Top layer velocity recorded has 400-450m/s and thickness in the range of 2 to 10m. The second layer has the velocity of the order of 800-2500 m/s and thickness of 4 to 15m. The underlying third layer with the velocity in the range could be identified by resistivity data and is 40-150m thickness and comprises clay dominated soft sediments
CONCLUSIONS • Integration of the all the available geo-scientific data sets, the conclusion of the data has been brought out: • The overburden in the area mainly consists of three layers. • Third layer i.e clay dominating soft sediments is probably becoming plastic due to percolation of water, because of poor drainage in the area may be a one of causative source of cracks developed in the constructed structures. • Thus geophysical surveys were identified the cause of development of cracks in the cinema hall building and suggested remedial measures.
Surface resistivity surveys for fresh / saline water interface and coastal aquifer studies – a case study from Neyveli area, Tamil Nadu. The area, part of Cauvery basin is occupied by Quaternary sediments over Tertiary Formation. These tertiary formations comprised of argillaceous sandstones, pebble-bearing sandstones, grits, clays, pebble bed and limestone. Geophysical surveys comprising both resistivity profiling as well as deep resisirivity soundings were conducted over700 sq.km area along the coast between Puducherry in the North and up to Coleroon River in South with 70km length and 10km average width of the area.
fresh / saline water interface • Electrical resistivity surveys were conducted to understand the nature of different aquifers and aquicludes up to a depth of 500m. • The resistivity profiling and soundings were delineated the salt/fresh water interface by sharp variations in Resistivity. Pseudo and Resistivity cross sections demarcated low resistivity zone. • These low resisirivity zones are indicative of saline zones in this area. Water samples collected and analyzed for salinity studies, analysis studies proved the saline/clay zones. This data is better corroborated with resistivity data.
RESULTS A B A – Apparenant resistivity contour map of AB/2=100m B – Apparent resisirivity contour map of AB/2 = 500m
Geophysical Surveys for evaluation of Bronzite bearing Gabbro (Black Galaxy Granite) – A case study from Chimakurti, Prakasam District, Andhra Pradesh. Introduction: Any hard rock which can be used as dimension stone is termed as black galaxy granite irrespective of its composition and geological nomenclature. The Black Galaxy Granite, which is a Bronzite bearing Noritic Gabbro also comes under Black Granite Group. Chimakurti black galaxy granite is an intrusive in the Tectonic zones between EGMB and Dharwar Craton east of Cuddapah basin,
Introduction • Geologically, the study area is occupied by Bronzite bearing Gabbro/Norite, which is light to dark gray in color, medium to coarse grained, massive with widely scattered and randomly oriented sub-total to enhedral Bronzite grains. The rock is exposed in the form of clusters of big boulders and sheets.
Efficacy of geophysics • The efficacy of the seismic and resistivity methods in estimating bed-rock depths and assessing physical characteristic of both bed-rock and overburden layers in geological and Engineering problems were well established. • Geophysical surveys employing Shallow Refraction Seismic Survey, Magnetic methods supplemented by Resistivity profiling and Soundings were conducted in Chimakurti area to delineate potential blocks of black granite bodies to assess their suitability for polishing and dimension stone industry.
Efficacy of geophysics • Magnetic survey also conducted to study the response over Gabbro bodies and to identify any control such as shears / fractures etc in the emplacement of Gabbro bodies and their disposition. The surveys were taken up for Andhra Pradesh Mineral Development Corporation (on commercial basis)
The geophysical surveys conducted in a systematic grid manner with a grid interval of 25m in this area. • Seismic Refraction survey has brought out 3-5 layer sequence, at places seismic survey depicting 3 layer case, this is a inference that, the bed-rock is shallow such locations in the study area . • Resistivity soundings brought out 4-5 layer sequence in this area. Even then there is a good correlation between both the geophysical data sets to estimate the bed-rock depths .
RESULTS • On the basis of geophysical results, a number of zones were recommended for locating compact bed-rock suitable for industrial usage. • Further these surveys have helped in delineating semi-weathered layer, which resembles bed-rock but of less commercial importance.
RESULTS • Magnetic method plays a vital role in this area to identifying clusters of Bronzite bearing Galaxies in the hard rock area. • It is interesting to note that bore holes drilled at proposed locations have encountered hard compact Bronzite bearing Galaxy granite at recommended depth, thus proving good correspondence between geophysical data and drilling results.
