Three-Dimensional Mapping of Hanseran Evaporite Bed from Bouguer Anomaly over Ganganagar Basin, Rajasthan

The Ganganagar evaporite basin, located in the northern part of Rajasthan in Thar desert, is a good source for potash minerals in Hanseran Evaporite Group(HEG). Delineation of top and bottom of the evaporite bed by scout drilling and geophysical surveys has been the main exploration methods for potash exploration during the last two decades. The evaporite bed is sandwiched between the Jodhpur Group at the bottom and the Nagaur Group on top belonging to Lower Cambrian Marwar Supergroup. The Marwars are successively overlain by Tertiary and Quaternary formations and underlain by Precambrian rocks of Malani Igneous Suite and/or the Delhi metamorphites.

The Bouguer anomaly map covering an area of about 22,500 km² (between Lat. 28°45' to 30° 15' N, Long. 73°30' to 75° E) of the basin has been prepared and analytically interpreted for determination of top and bottom of HEG as it has sufficient density contrast (+0.15 and -0.25g/cm³) across the interfaces. Three-Dimensional(3D) spectral analysis is carried out to determine the structure of the Jodhpur sandstone underlying the evaporite bed. The depth to the base of HEG varies from 600 and 840m, The upward continuation and matched filter techniques have been used to compute gravity field associated with the thickness of HEG. The resulting gravity field is then inverted to map variations in depth of HEG from Tsuboi's method considering a mean depth of 525m and a density contrast 0.15 g/cm³. The mean depth is determined on the basis of the information derived from the boreholes drilled in the basin and the density contrast determined from the core samples collected from 12 boreholes. The depth to the top of HEG varies from 340 to 680 m. The accuracy in depths determination from the present analysis has been evaluated through 37 boreholes drilled during the exploration. The results also delineated the fault zones on the central and eastern edge of the basin and helped in identifying evaporite deposits in the basin.