Print Icon Aerial Survey Remote Sensing

Airborne Survey and Remote Sensing (ASRS) Group is responsible for airborne (fixed wing / helicopter) geophysical survey data acquisition, processing, interpretation, integration, modelling and generation of thematic maps to delineate target areas for further survey and exploration for uranium mineralisation. AMD commenced airborne survey way back in1955 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 was designed, developed and deployed for survey in 1972. The AGRS interfaced with Cs-vapour magnetometer and Global Positioning System was extensively flown during 1997 to 2002. A total of 5 lakh line km data was acquired with these systems using fixed wing platform.

The calibration pads at civil airport, Nagpur, which were 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 survey. Two test strips of natural terrain, located at Devarkonda, Nalgonda district, Telangana and Malharbodi, Bhandara district, Maharashtra are being used for the determination of height attenuation coefficients for each of the radio-elements and total gamma radioactivity. Presently, calibration of these systems is being carried out over indigenously developed transportable calibration pads, which are cost effective and can be transported to the areas of heliborne survey.  

Since last decade, AMD has acquired close spaced, high sensitivity, high resolution Frequency Domain Electromagnetic (FDEM) / Time Domain Electromagnetic (TDEM), magnetic and gamma-ray spectrometric data in different geological domains of the country. AMD has procured Versatile Time Domain Electromagnetic (VTEM) system, advance heliborne gamma ray spectrometer, Cs-magnetometer and GT-1A gravimeter, which have been successfully utilised in acquiring high resolution data. 

                                                                  

                                                           VTEM system                                                         Gravimeter

Radiometric method directly record the surface response of distribution of uranium and other radioactive elements in rocks and soils. Gamma-ray spectrometry provides a method of measuring concentrations of individual radioactive elements (in particular, K, U & Th). Magnetic survey records 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 modelling and interpretation of acquired magnetic data over different geological terrain can reveal subsurface structures and geological details, which are used in uranium exploration. The ternary images i.e. composite images of K (%), Th (ppm), and U (ppm) or in combination with ratios eU/eTh, eU/K and eTh/K of spectrometric data are generated by advanced digital image processing technology. These images are of direct assistance in exploration of U, Th and also for Sn, W, REE, Nb and Zr besides lithological discrimination and identification of alteration zones. 

                                             

The VTEM data are subsequently processed into an array of channel amplitudes. The channel amplitudes are most sensibly interpreted after conversion into depth-related conductivities and displayed as depth sections. Images of half-space apparent resistivity or decay constant ז (tau) have proven to be effective subsurface geological mapping tools in many areas. 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 a series of image slices at selected apparent depths along with 3D voxel models.

                                                         

                                                              Conductivity Depth Images (CDI) and 3D voxel models

AFMAG (ZTEM - Z-axis Tipper Electromagnetic) is a natural source electromagnetic geophysical exploration technique which uses the EM 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 the deeper targets for uranium exploration in association with Geotech Ltd, Canada. 

AMD uses GT-1A heliborne Gravimeter, which is compact, single sensor, vertical scalar, lightweight, low-power GPS-INS Gravimeter and has a 0.54 mGal of RMS error in the Free Air Gravity anomaly on the receptivity test line. The processed Bouguer anomaly maps are generated to delineate litho-structural details of the area under exploration.  

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

AMD has acquired 5.7 lakh line km, wide spaced high sensitivity AGRS and AM by fixed wing surveys. AMD has also acquired data for ONGC over Krishna-Godavari, Cauvery and South Rewa, Gondwana Basins. Inland heavy mineral placer deposits have also been identified using these AGRS & AM data sets. This data was also used for environmental radiation monitoring over Madras Atomic Power Plant (MAPP) at Kalpakkam and Rajasthan Atomic Power Plant (RAPP) at Kota. AMD has acquired 3.5 lakh line km of high resolution airborne/heliborne (GRS, Magnetic, FDEM/TDEM/ZTEM) data 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 facilitated in identifying several target areas for subsurface exploration. Subsequent sub-surface exploration proved uranium mineralization in several areas.

The advanced image processing software (ERDAS Imagine, ILWIS and ENVI); mapping software (ArcGIS, AutoCAD, Golden Software Surfer); Advanced geophysical processing, modelling software (Geosoft, Maxwell, CSIRO modelling modules, EMaxAir, EMFLOW, Profile Analyst); Praga (Radiometrics), UBC3D for both gravity and magnetic and Model Vision are available with the Group.   

  Remote sensing technology is one of the exploration methods that directly map the broad range of alteration zones associated with many ore deposits. Based on the principles and techniques of photo-interpretation, faults, fractures and geological contacts are brought out clearly. IRS-WIFS, LISS-III, LISS-IV, PAN, LANDSAT-ETM, ETM+, SPOT and IRS-Cartosat data have been used to map lithostructural details. With the advancement of remote sensing technology and advance image processing tools, new sensors with better spectral and spatial resolution like ASTER and Hyperspectral- Hyperion have been 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