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Unique polyphase deformational structures of Lunawada metasedimentary rocks identified from remote sensing imagery

CURRENT SCIENCE, VOL. 119, NO. 4, 25 AUGUST 2020 600 interference. MIT was conducted by keeping the magnetic sensor on ground and then the UAV was flown at different heights from the sensor while recording magnetic data. A change of ~2 nT in magnetic data was observed when the UAV was flown at ~4 m and negligible change was found at more height than 4 m. No change in magnetic data is observed when the UAV was at a height of >6 m. Based on this experiment, a separation of 5 m between the UAV and magnetic sensor was found suitable, which has been further validated during flight. An east–west trending dyke having a variable width of 50–100 m and strike length of several kilometres near Yacharam was chosen to test the UAV magnetometer along with ground magnetic survey. The dyke can be traced on either side of the Hyderabad–Nagarjuna Sagar road (on NH-65), 51 km from Hyderabad and 200 m north of Yacharam town. It is well exposed in the form of a geomorphic ridge at the test site and at places is covered by thin (<1 m) soil cover. The mafic dolerite dyke is a Proterozoic dyke intruding Neoarchean granite gneiss in a part of the Eastern Dharwar craton (Figure 1). In general, the area consists of several dykes of different orientations, size and ages. However, the Yacharam dyke is distinctly seen both in satellite images and on the ground, and therefore was chosen as the test area for the present study. An area of 1 km × 0.6 km was selected as the test location with a N–S profile length of 900 m and line spacing of 50 m (Figure 2). Data were acquired at 5 samples/sec along 12 profile lines (L01 to L12), while the UAV was flown at a speed of 5 m/sec 35 m above the ground surface. Considering average topographic variations of about a metre over the study region, the height of the UAV was maintained at 35 m above the average height of the study area. However, the UAV has an option of changing flight height during voyage and this can be planned at the starting time of flight. A ground magnetic survey was conducted along Line 02 prior to the drone magnetometer experiment for comparison of results. In addition, two lines, L02 and L04, were flown twice to test repeatability of data received from the UAV magnetometer. The standard deviation (SD) of repeat data along L02 and L04 was computed for 954 locations. The average SD for L02 was 3.09 nT and for L04 it was 2.39 nT. This would further improve with diurnal correction. Figure 3 shows the data obtained from ground and UAV magnetic surveys along line L02. A good correlation with minor mismatch can be seen. This mismatch could be attributed to the GPS position accuracy in both surveys. The measurement repeatability test was conducted along two lines (L02 and L04) and the comparison indicates good correlation (Figure 4). The data acquired by the UAV magnetometer experiment were processed through WinGLink software (Figure 5). It was observed that the intensity of contours trend in the E–W direction coinciding with the strike of the dolerite dyke. The contour pattern clearly reveals the dyke and adjacent granitic terrain. The contours are dense with high magnetic gradient over the dyke, while they are diffuse away from the dyke. The dyke is well reflected in the central part of the contour map derived from the UAV magnetic data of the study area (Figure 5). The results clearly distinguish the mafic dolerite dyke and granitic terrain in the test area. The present study has successfully demonstrated the capability of UAV-Magnetic survey, which is cost effective, faster and reliable.