CONCENTRIC AND RADIAL UPWARPS
AND DOWNWARPS: CRUSTAL ARCHITECHTURE OF THE INDIAN SHIELD FROM GRAVITY MAPS
M. Kesavamani, C. Ramachandran and R.M.C. Prasad
IMAGE DISPLAY AND TONE AND
TEXTURE IN AEROMAGNETIC DATA
Duncan R. Cowan, Sam Bullock, Sheila Cowan, Linda A. Tompkins and Mike Dentith
STATISTICAL ANALYSIS AND
VISUALISATION OF GOLD MINE DATA : A CASE STUDY OF KOLAR GOLD MINE
N.K. Indira and K.S. Yajnik
AN INTERACTIVE COMPUTER
PACKAGE FOR MODELLING THE CRUSTAL STRUCTURE FROM BOUGUER
ANOMALIES:METHODOLOGY AND APPLICATION
C. Visweswara Rao and E. Rajendra Prasad
CRUSTAL STRUCTURE OF
CHATTISGARH REGION, MADHYA PRADESH, INDIA
S.Srinivas, A.S.K.Murthy, G.S.Yadav and K.M.Srivastava
METEOROLOGICAL PARAMETERS ON RADON EMANATION AT PALAMPUR (H.P.)
Anand K. Sharma, Vivek Walia and H.S. Virk
Regional and residual gravity field components of the Bouguer gravity anomaly over the Indian Shield are analysed to understand its crustal structure and tectonics.
The gravity lows in the regional gravity field indicate the cratons and/or zones of subsidence, while the highs correlate with the surrounding mobile belts and/or deep gravity basement ridges. The Moho relief map shows thicker crust over the cratonic areas and a thin crust over the mobile belts possibily due to inhomogeneous upper mantle. The residual gravity high and low anomalies reveal a heterogeneous basement of mafic and felsic rocks, which respectively indicate the synformal and domal structures.
The arcuate central Indian gravity high (CIH) in the regional field, possibly defining the Narmda-Son-Brahmaputra lineament, divides the Indian shield into two major blocks. The CIH together with the regional gravity high along Godavari valley forms a 'Y ' junction near Nagpur and indicates a radial deep basement ridge. The radial gravity high zone is associated with lows flanked by highs in the residual gravity field indicating a three-armed rift system. The steep gradients, possibly due to the transitional crust with associated thrust zones and en-echelon geometry, along the peripheries of the subcontinent and the disposition of the CIH suggest strike slip (dextral?) movements.
The fundamental structural framework of the Indian shield as reflected in both the regional and residual gravity fields is concentric upwarps and downwarps, with Nagpur as the apical zone. These upwarps and downwarps form orthogonal structures to the regional N-S trends in the southern parts of the shield, while in the north they follow the near E-W regional trends. The general E-W Himalayan trend in north, the N-S trending EGMB in the east, the E-W trending Bhavani-Cauvery shear zone in south and the N-S trending Aravalli-Delhi fold belt in west together form a major concentric structural feature in the Indian subcontinent. Similar parallel inner structural features can be visualized around Nagpur.
In view of the above, the basic structural frame work of the Indian shield is a Concentric and Radial Upwarps and Downwarps (CRUD) model, centered around Nagpur.
Qualitative aeromagnetic interpretation using high-resolution filtered images is essentially a pattern recognition technique involving identification of magnetic elements from changes in magnetic relief and magnetic texture. Magnetic relief consists of anomaly amplitude and shape and is relatively objective. Magnetic texture consists of shape, size and continuity of adjacent anomalies and is more subjective. Amplitude in image processing equates to tone, the relative brightness of individual pixels. Texture refers to the spatial/statistical distribution of the tones, so it is fundamentally different from tone, which is defined at one location (pixel) while texture occurs over some finite area and hence requires some finite number of pixels to represent it. Texture can occur at several scales and larger scale aggregates are sometimes described as structure to distinguish them from smaller scale texture variations. The spatial arrangement of contrasting tones and textures within an image are referred to as patterns. An experienced interpreter can recognise type patterns characteristic of the magnetic expression of geological features.
Conventional filters such as vertical derivatives respond primarily to the amplitude content of the data and have problems with large dynamic range and have limited ability to enhance low amplitude, subtle anomalies. AGC filters while reducing amplitude dependence tend to amplify noise in the data, are relatively band limited and are mainly dominated by texture. A range of filters have been developed to try to overcome the limitations of conventional filtering and enhance tonal and textural attributes simultaneously. Separation (layer) filters, grey-level co-occurrence texture filters and the gradient tilt angle all perform better than conventional filters but have very different enhancement characteristics with varying degrees of tonal/textural information content amplification.
The various filters were tested on aeromagnetic data covering the Lake Mackay, Northern Territory, Australia 1:250,000 map sheet. It was concluded that the different filters have different amplitude and wavelength responses and the results are complementary. The GLCM texture filter probably provided the best enhancement of subtle low amplitude anomalies but had little tonal information content limiting the effectiveness of the filter in the magnetically active areas. The gradient tilt angle provides good textural content with more tonal content than the GLCM filters. Combining two filters with different bandwidths and amplification factors such as the separation filter and gradient tilt angle filter is a good compromise, providing good enhancement of textural information while preserving enough tonal or amplitude information to discriminate major magneto-tectonic units.
A case study based on the statistical analysis of the Footwall Lode of the Champion Reef Mine of the Kolar Gold Mines has been made. It has been found that the gold deposition in terms of the grade values follows a univariate lognormal distribution as does the accumulation. On the basis of the distribution of the width of the lode, the analysis is extended further to support the possible existence of two different lodes using the F-test, two sample t-test and Kolmogorov- Smirnov two sample test. The data is geometrically modelled on a graphic workstation.
---An interactive computer package for interpreting Bouguer gravity anomaly in terms of crustal structure has been developed. Power spectral analysis, regression analysis, the digital filter for separating lower and higher wavelength components and the properties of sin x/x function are used to formulate the methodology for crustal modelling. Two Bouguer anomaly profiles, viz., Koyna I (Guhagar – Chorochi) and Koyna II (Kelsi – Loni) are re-interpreted using the developed package and the results are in general agreement with those of the earlier analyses of these profiles establishing the utility of the package.
An attempt is made to discuss uniqueness of depth determination from observed potential field data on reproducing the field by simple layer density s belonging to a horizontal half-space boundary S that lies below the datum level . Assuming that the s belonging to S reproduces the field over as S gradually moves downward and crosses the causative mass m, it is theoretically shown that the s exhibits a smooth continuous behavior when S is shallower than m and an erratic behavior when S is partially or wholly below it. The depth to the shallowest S with an erratic s defines the depth to the top of the causative mass and this depth is unique.
A regional gravity anomaly profile across the Chattisgarh region is studied in terms of basin configuration and crustal structure. Two gravity models are inferred viz., a two layered crustal model and a differentiated crustal model, out of which the differential crustal model appears to be realistic. Both the models indicate a maximum thickness of 3.5 km for the sediments and an up-warp of 2 km in the moho under the Chattisgarh basin.
Effect of meteorological parameters on radon emanation in the soil-gas at Palampur station has been studied for time window March 1993 to March 1995 and the results obtained are correlated in terms of radon fluctuations. The radon value is found to be positively correlated with temperature, relative humidity and mild rainfall whereas it shows inverse relationship with evaporation, wind velocity and heavy rainfall. The overall correlation is found to be week resulting in a cumulative effect of 10-20% on radon emanation at the most. Other parameters, e.g., sunshine hours and vapour pressure cause negligible effect on radon emanation rate.