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Airborne Survey and Remote Sensing: (ASRS)

      Airborne Survey and Remote Sensing (ASRS) Group is responsible for airborne (fixed wing / helicopter) geophysical survey data acquisition, processing, interpretation, modeling, integration and generation of thematic maps to delineate target areas for exploration of uranium and other atomic minerals. ASRS Group generates litho-structural maps by the interpretation of remotely sensed data products and subsurface variations in conductivity/resistivity, magnetic susceptibility and density by the interpretation of heliborne geophysical data. AMD commenced airborne surveys in 1955 with indigenously designed and developed Gamma Ray Total Count System. High sensitivity Airborne Gamma Ray Spectrometer (AGRS) with larger volume (50 litres) NaI (Tl) detector and Proton Precession Magnetometer were deployed for survey during 1972 to 1997. The AGRS interfaced with Cs-vapour magnetometer and Global Positioning System was extensively flown from 1997 to 2002.

      The calibration pads at Nagpur civil airport, constructed as per IAEA standards in 1984, are used for calibration of gamma-ray spectrometers, calculation of system sensitivities and stripping ratios before taking up airborne/heliborne surveys. These pads are the only one of their kind in Southeast Asia and are declared as “National Facility” for calibration of airborne radiometric instruments. Two test strips of natural terrain, located at Devarkonda, Nalgonda district, Telangana and Malharbodi, Bhandara district, Maharashtra are used for the determination of height attenuation coefficients for each of the radio-elements and total gamma activity. Presently, calibration of AGRS systems is also being carried out using indigenously developed transportable calibration pads, which are cost effective and can be transported to the areas of heliborne survey.

      During 2002 to 2007, AMD has accrued close spaced, high sensitivity, high resolution Frequency Domain Electromagnetic (FDEM) / Time Domain Electromagnetic (TDEM), magnetic and gamma-ray spectrometric data in various geological domains of the country. AMD has procured a Versatile Time Domain Electromagnetic (VTEM) system, advanced heliborne gamma ray spectrometer, Cs-magnetometer and GT-1A gravimeter, which are being used for acquiring high resolution data. Design and development of hardware required for updating the geophysical instruments for smooth operations is done by the Group.

      Airborne radiometric survey directly records the surface response of distribution of uranium, thorium and potassium in rocks and soils. Magnetic survey records the variations in the magnitude of Earth’s magnetic field in order to detect local changes in the magnetic properties of the underlying geological formations. Processing, 2D & 3D modeling and interpretation of magnetic data reveal subsurface structures and geological information, which are extensively used in uranium exploration. The ternary images i.e., composite images of K (%), eTh (ppm), and eU (ppm) or in combination with ratios such as eU/eTh, eU/K and eTh/K of spectrometric data are generated by digital image processing techniques. These images are of direct assistance in exploration of U, Th REE, Nb and Zr besides lithological discrimination and identification of alteration zones.

      The VTEM data are processed into an array of channel amplitudes. The channel amplitudes are interpreted after conversion into depth-related conductivities and displayed as depth sections. Images of half-space apparent resistivity or decay constant (tau) have proven to be effective in subsurface geological mapping. The results from the modelling techniques provide better approximations to the earth’s conductivity structure with depth, and can be presented in a plan view, as well as in a series of image slices at selected apparent depths along with 3D voxel models.

      AFMAG (ZTEM - Z-axis Tipper Electromagnetic) is a natural source electromagnetic geophysical exploration technique which uses the electromagnetic signal in audio frequency range. These natural fields are planar and propagate horizontally. Vertical field response is caused by the lateral conductivity/resistivity contrasts in the Earth. The Heliborne ZTEM system comprises a horizontal EM receiver coil/sensor which measures the Z-component of magnetic field in the frequency range of 1 Hz to 1 kHz. The vertical EM field is remotely referenced to the horizontal base station coils positioned within the survey area. This makes the ZTEM an effective tool for mapping deep (~ 2km) conductive/resistive zones. AMD has conducted ZTEM survey in parts of Cuddapah Basin to identify deeper targets for uranium exploration.

      GT-1A heliborne Gravimeter is used to generate Bouguer anomaly maps for delineation of litho-structural details of the area under exploration by exploration Geophysics group.

      Geographical Information System (GIS) is extensively used for integrating exploration datasets, analysing geological, geochemical, remote sensing and geophysical data for both lithological discrimination and mineral exploration applications. GIS modelling has been extensively used in identifying the target areas for uranium exploration.

      From 1978 to 2021, a total of over 10,85,543 Lakh line Km of high resolution EM magnetic and Radiometric data has been acquired, covering around 10,10,269 Sq Km area over parts of North Delhi Fold Belt, Cuddapah Basin, Bhima Basin, Singhbhum Shear Zone, North Singhbhum Shear Zone, Chhattisgarh Basin, Shillong Basin, Kaladgi Basin and Vindhyan Basin. Integrated interpretation of these data sets have facilitated in identifying several target areas for subsurface exploration. Subsequent sub-surface exploration proved uranium mineralisation in some areas. Inland heavy mineral placer deposits have also been identified using these AGRS & AM data sets. AMD has also surveyed for environmental radiation monitoring over Madras Atomic Power Plant (MAPP) at Kalpakkam and Rajasthan Atomic Power Plant (RAPP) at Kota.

      The advanced image processing software (ERDAS Imaging, ILWIS and ENVI), mapping software (ArcGIS, AutoCAD, Target, Golden Software Surfer), advanced geophysical processing, modelling softwares viz., Oasis Montaj, Intrepid, Maxwell, CSIRO modelling modules, EMaxAir, EMFLOW, Profile Analyst, UBC3D, Model Vision, Praga are being used by ASRS Group. In addition to commercial softwares, the Group also develops additional modules in GX, C and C++ programming languages for processing and modelling of geophysical data.

      Remote Sensing is one of the survey techniques that is directly used for litho-structural mapping and alteration zones studies associated with many ore deposits. Based on the principles and techniques of remote sensing interpretation, faults, fractures and geological contacts are brought out clearly using IRS-WIFS, LISS-III, LISS-IV, PAN, LANDSAT-ETM, ETM+, SPOT and IRS-Cartosat data. With the advancement of remote sensing technology and image processing tools, new sensors with better spectral and spatial resolution like ASTER and Hyperspectral- Hyperion are now used for lithological mapping in selected parts of mineral provinces. Thematic digital geological maps, utilising the various remote sensing data integrated with all published geological and other geosciences data from various sources, are generated. These thematic maps form the base for planning airborne geophysical surveys and also aid in interpreting the airborne geophysical anomalies in terms of geology.

Heliborne geopysical survey in Rajasthan (video)

Calibration  facilities

VTEM (Versatile Time Domain Electromagnetic system)

GT1A Gravitymeter

Litho-structural interpretation on IRS PAN sharpen ETM image

Interpreted radiometric ternary image K-Th-U (RGB)

Interpreted Total Magnetic Intensity image

Generated thematic integrated litho-structural map

Conductivity Depth Images (CDI) and 3D voxel models