Moho boundary, discovered about a century back, is well established on the basis of seismological studies. Electromagnetic studies using very low frequencies, however, have not been able to establish this boundary due to the presence of highly conducting continental lower crust. Magnetotelluric lithospheric study over the Eastern Indian Craton (3.3 Gyr) resolves the lower crust and upper mantle boundary due to the absence of highly conducting continental lower crust underneath the craton. From the present study it can be speculated that a dynamics other than the plate tectonics possibly existed up to the mid‐Archaean.
A B S T R A C TWe present results from the resolution and sensitivity analysis of 1D DC resistivity and IP sounding data using a non-linear inversion. The inversion scheme uses a theoretically correct Metropolis-Gibbs' sampling technique and an approximate method using numerous models sampled by a global optimization algorithm called very fast simulated annealing (VFSA). VFSA has recently been found to be computationally efficient in several geophysical parameter estimation problems. Unlike conventional simulated annealing (SA), in VFSA the perturbations are generated from the model parameters according to a Cauchy-like distribution whose shape changes with each iteration. This results in an algorithm that converges much faster than a standard SA. In the course of finding the optimal solution, VFSA samples several models from the search space. All these models can be used to obtain estimates of uncertainty in the derived solution. This method makes no assumptions about the shape of an a posteriori probability density function in the model space. Here, we carry out a VFSA-based sensitivity analysis with several synthetic and field sounding data sets for resistivity and IP. The resolution capability of the VFSA algorithm as seen from the sensitivity analysis is satisfactory. The interpretation of VES and IP sounding data by VFSA, incorporating resolution, sensitivity and uncertainty of layer parameters, would generally be more useful than the conventional best-fit techniques.
[1] Remote reference (RR) magnetotelluric (MT) measurements are made to reduce the bias caused by noise in electric field E and magnetic field H at a local site. RR only works when noise at a local and remote sites are uncorrelated. A study has been undertaken to find the farthest distance of a far remote site for crustal study which maintains the effectiveness of the RR technique. From theoretical studies the conditions for valid RR estimates have been obtained. The study shows that the remote site can be kept at a considerably large distance from the local site. In a field experiment a fixed local site and several remote sites have been selected at distances of 80, 115, and 215 km away in the frequency range of 30 Hz ($0.03 s) to 0.00055 Hz ($1800 s) at midlatitudes where the wavelengths of the source magnetic fields are long compared to the site separation. The remote sites are distributed over diverse geological settings. All the data of the fixed local site have been remote reference processed with the corresponding remote sites. The study reveals that using a remote site located at as large a distance as 215 km results in unbiased observations and remains effective in improving the MT data quality for all frequency ranges. If the data were acquired at long periods and or in the high latitudes, then to extract the stable uniform field estimates of the impedance one has to carry out robust processing technique.
International audienceCharge occurrence probability (COP) and dipole occurrence probability (DOP) tomographic imaging of graphite and sulphide orebodies, which were earlier detected by a self-potential (SP) survey, were carried out. Three typical examples, two shallower graphite lenses in Bender and Berni, Orissa State, and one relatively deeper sulphide deposit in Kayar, Rajasthan State, are presented. An exploratory test hole, drilled on the basis of the SP results in Bender and Berni, struck graphite. A mise-à-la-masse survey was also carried out on these two deposits to delineate the orebody. The graphite deposits have subsequently been mined. The shapes of the graphite bodies, especially the upper parts, are now known. The Bender deposit is a single graphite lens, whereas the graphite occurrences in Berni are complex multilens-type deposits. Both the COP and DOP techniques clearly imaged the single lens at Bender, as well as the multilens deposits at Berni. The Kayar sulphide deposit, unlike the graphite deposits, has not yet been mined, and therefore it is not delineated as precisely as the graphite orebodies. However, its outline is known from the drilling data. The imaging results of COP and DOP show good agreement with the drilling results of the sulphide deposits
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