Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Ram Gopal ¹, R. N. Agarwal ¹

Affiliations
1 Geophysical Division, Geological Survey of India, Jaipur 302 004, IN

Abstract

Geophysical Surveys employing, magnetic, self-potential and resistivity methods in Bethunmi-Bhopalsagar belt in Rajasthan have proved to be effective in identifying and mapping the various litho-units largely concealed under soil cover. An important discovery in the area was a striking closure of a local structure to the south of Dariba by magnetic suryeys. The disappearance of the graphitic mica schist and amphibolite to the south of Dariba may be explained by the structural closure brought out by these surveys rather than by faulting as invoked from geological evidences.

Keywords

Geophysical Mapping, Bhilwara Supergroup of Rocks, Rajasthan.

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Ram Gopal ¹

Affiliations
1 Geological Survey of India, Jaipur - 302 004, IN

Abstract

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.

Keywords

Evaporite, 3-D, Spectral Analysis, Gravity Field, upward Continuation, Ganganagar, Rajasthan.

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L. N. Singh ¹, D. Gupta ², Ram Gopal ², Radhe Shyam ², S. K. Singh ¹, S. L. Singh ², A. K. Agrawal ³

Affiliations
1 Geological Survey of India, Northern Region, Lucknow - 226 024, IN
2 Geological Survey of India, Western Region, Jaipur - 302 004, IN
3 Geological Survey of India, A M S E Wing, Bangalore - 560 078, IN

Abstract

A small deposit of lead and zinc was established in southern part of Sawar metasedimentary belt during third phase of drilling. The Mise-A-La-Masse survey, a post discovery geophysical method carried out in the boreholes of this belt has yielded significant results. This method has been useful in delineating the subsurface disposition of mineralised zones and their lateral extension. This technique is also successful in the area in establishing correlation between different parts of an ore body with varying sulphide percentage, which are located distantly but are in electrical continuity. Hence the Mise-La-Masse method has been of immense help in planning the boreholes thereby optimizing drilling cost in the Sawar Belt. The composite map of equipotential line and chargeability contour has indicated that the contour pattern and disposition of anomaly axes are similar in both the cases.

Keywords

Mise-A-La-Masse Survey, Pb-Zn Mineralised Zone, Composite Map, Sawar Metasedimentary Belt, Ajmer District, Rajasthan.

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Ram Gopal ¹

Affiliations
1 Geophysics Division, Geological Survey of India, Western Region, 15-16, Jhalana Doongri, Jaipur - 302 004, IN

Abstract

This paper discusses the Self-Potential, resistivity and electromagnetic prospecting techniques used and the responses obtained over flaky graphite occurrences within laterite capping in Karimukal area, Ernakulam district Kerala. The three main factors controlling these responses are the size and concentration of the conductive body, depth from surface and the country rock.

Detailed Self-Potential (SP) surveys conducted in the area have delineated nine anomalous zones with peak negative values varying from - 145 to -680 mV having width of 30 to 50 m and extending in strike length of 100 to 300 m Graphite has been struck in five of these zones. Results of four more anomalics are yet to be tested Resistivity and Horizontal loop electromagnetic(HLEM) surveys over SP anomaly zones have yielded conductive zones, at shallow depths of 2-9 m Pitting carriedout over these zones revealed graphite occurrences at interpreted depths.

Keywords

Graphite Exploration, Self-Potential, Resistivity, Electromagnetic, Kerala.

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Ram Gopal ¹, Dinesh Gupta ¹, Radhe Shyam ¹

Affiliations
1 Geological Survey of India, WR, 15-16, Jhalana Doongri, Jaipur-302004, IN

Abstract

Downhole wide spacing IP (DHIP), Self-potential (SP), magnetic susceptibility and applied potential observations were made along the boreholes drilled for Pb-Zn-Cu sulphide in mineral belts of Rajasthan The aim was to identify mineralised zones, physical property contrast of the mineral and country rock and presence of mineralisation in the vicmity of the borehole but missed in drilling

Number of boreholes were surveyed in different parts of the mineral belts of Rajasthan and the relationship between apparent resistivity and chargeability, SP and magnetic susceptibility and lithology of the cores has been studied and analyzed Chargeability 'high', resistivity 'low', SP 'low/high' and magnetic susceptibility 'high' characterized the sulphide mineralisation The surveys were successful in detecting the presence of mineralisation in the drill hole, studying the m situ physical properties of barren & mineralised rocks and finding the correct depth of the conductor It helped in conducting raise-a-la-masse surveys and interpretation of surface geophysical data m mineral belts of Rajasthan Applied potential (Mise-a-la-masse), surveys were able to give information on the lateral extension of the mineralised zones intersected in the borehole

Five examples of detection of sulphide mineralisation in boreholes of Kayar, Pindwara-Watera and Danba-Bethumbi mineral belts of Rajasthan are presented The results indicated average appaient resistivity low value of the order of 1 to 10 ohm-m and magnetic susceptibility of order of 600-5000 x 10⁶ c g s units in the sulphide zones, while in the country rocks values recorded were m the range of 3000-24,000 ohm-m and 50-500 x 10⁶c g s units respectively The diagnostic resistivity response is simply related to the percentage, size and disposition of the sulphide mineralisation The association of high magnetic susceptibility is indicative of pyrrhotite mineralisation in these zones The SP 'high' and 'low' could be due to the electrochemical potential developed at the interface of country rock and ore body Detection IP/ resistivity measurements have also improved the prospective search radius of drill-holes and are successful in assessing qualitatively the lateral extent ol intersected mineralisation.

Keywords

Drill-Hole Induced-Polanzation (DHIP), Self-Potential, Magnetic, Geometric Disposition, Rajasthan.

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