ENHANCEMENT OF ELECTROMAGNETIC ANOMALIES BY A CONDUCTING OVERBURDEN*

The effect of a conducting overburden on the electromagnetic response of sulphide ore-bodies has been studied with the help of quantitative electromagnetic model experiments. These experiments were conducted at a fixed, crystal controlled, frequency of 100 kHz using a number of transmitter-receiver configurations, though the results discussed here mostly pertain to a horizontal coplanar system. An analysis of the anomaly profiles—after accounting for the regional anomaly—indicates a general enhancement of the response. This is attributed (a) to the concentration of the current lines in the host-rock effected by the embedded target and (b) to the change in the phase and space orientation of the field vectors, brought about by the conducting overburden. The enhancement is most pronounced in the case of non-symmetrical bodies such as veins and ribbon-like structures represented by sheet models, but is also observed to a lesser degree for isometric structures. These results are expected to pave the way for a more accurate interpretation of the induction prospecting data.     

TOPOGRAPHIC EFFECTS ON ELECTROMAGNETIC DEPTH SOUNDING SYSTEMS *

Computations have been made for the effect of ground slope in combination with transmitter coil misorientation on different electromagnetic depth sounding systems and sets of frequency sounding master curves are presented for different angles. The effect of a thin air layer due to the elevation of coils above ground surface has been studied for different systems. Also discussed are the effects due to an error in transmitter-receiver coil separation.     

TIME-DOMAIN ELECTROMAGNETIC SOUNDING—COMPUTATION OF MULTI-LAYER RESPONSE AND THE PROBLEM OF EQUIVALENCE IN INTERPRETATION*

Computations of the time-domain electromagnetic response of a multi-layered earth have been carried out for different source-receiver coil systems. The primary excitation is a train of half-sinusoidal waveforms of alternating polarity. The conversion into the time-domain involves Fourier series summation of the matched complex mutual coupling ratios of the layered earth models computed by a digital linear filter method. As an example, the response of a perpendicular coil system on the ground surface for two source-receiver separations has been presented for a five-layer earth model. This has been compared with the responses of homogeneous, two-layer, three-layer, and four-layer models. Next, the investigations have been extended to study the problems of equivalence of three-layer models, the intermediate layer of which is either conductive or resistive. For an intermediate conductive layer (H-type), the studies show that in the early portion of the signal the interpretation of a true three-layer earth is possible to some extent, whereas the ambiguity due to equivalence persists in the late samples. On the other hand, for an intermediate resistive layer (K-type), the three-layer earth and its equivalent model cannot be distinguished from each other over the entire sampling period. On the basis of a computational approach, equivalence has been empirically established as √h/ρ=constant for H-type earth-sections, and as h²ρ=constant for K-type earth sections, where h and ρ are respectively the thickness and resistivity of the intermediate layer.     

Influence of shear velocity on frictional characteristics of rock surface

Understanding the fundamental issues related with the effect of shear velocity on frictional characteristics at the interface of rock surfaces is an important issue. In this paper, strain-rate dependence on friction is investigated in relation to sliding behaviour under normal load. The phenomenon of stick-slip of granite and shaly sandstone with a tribometer at constant rate of strain under normal loads was observed.     

Classification of the Didwana‐Rajod meteorite: A Mössbauer spectroscopic study

Abstract— Mössbauer spectroscopic studies of the Didwana‐Rajod chondrite, which fell on 1991 August 12 in western Rajasthan, India, are presented. The results are compared with the Mössbauer data of several enstatite and ordinary chondrites including the Dhajala chondrite for which Mössbauer data were acquired during the present study. The Didwana‐Rajod chondrite's iron phases and its oxidation states strongly suggest that it should be classified as an H‐type ordinary chondrite instead of the earlier suggestion (based on petrographic studies) that it could be an enstatite chondrite. The present study demonstrates that Mössbauer spectroscopy is a very powerful technique for aiding in the classification of meteorites.     

ENHANCEMENT OF ELECTROMAGNETIC ANOMALIES BY A CONDUCTING OVERBURDEN II *

The influence of a partially conducting overburden/host rock on the electromagnetic response of a horizontal, tabular conducting ore body, investigated with the aid of quantitative scale model experiments, was discussed in an earlier paper (Gaur, Verma and Gupta 1971), which will be referred to as I. This paper presents the results of more comprehensive experiments subsequently carried out to study the combined effect of various geological parameters, namely: the dip and depth of burial of the ore body, its electrical conductivity and that of the zone surrounding it. These results obtained for four different transmitter-receiver configurations confirm the general enhancement of response in varying degrees, brought about by a conducting overburden. However, the transformation of the shapes of the anomaly profiles with increasing overburden conductivity is observed to depend on the dip of the ore body, being more drastic for gently dipping ones. Variations in the inphase and quadrature components as well as in the phase of the anomaly have been studied for varying depths of burial of the ore body and for a number of values of the solution conductivity. Anomaly index diagrams have been constructed with a view to predicting possible values of the geological parameters from a knowledge of the anomaly components. It is felt that the notable overburden effects are caused by a drastic redistribution and concentration of currents, mostly in a narrow loop at the top of the ore body, brought about by its galvanic contact with an extended medium of relatively poorer conductivity.     

TIME DOMAIN ELECTROMAGNETIC RESPONSE OF A SHIELDED CONDUCTOR*

Velekin and Bulgakov (1967) in an interesting model experiment while studying the transient electromagnetic response of a conductive sphere placed below a thin conductive sheet found that at the earlier stages of the transience, the composite system response corresponded to the response due to the overlying sheet alone and at the later stages, it corresponded to that of the sphere alone. To examine whether such a separation of responses due to individual components can be analytically studied and applied to other source configurations, we have analyzed an idealized model consisting of two spherical shells. We find that in corroboration with the above results, the general nature of the curve consists of two humps representing the responses dominated by the outer shell and the inner shell respectively. In addition, however, we find that the two humps gradually disappear to yield a smooth decay curve for increasing values of the ratio σd₁b/σ₂d₂a (where σ₁, σ₂ are the conductivities, d₁, d₂ are the thicknesses of the outer and inner shells respectively, and b and a are their respective distances from the centre) and the effect of inner shell on the composite system response is considerably reduced.     

NOMOGRAMS TO INTERPRET CRONE PEM DATA USING A DIPPING SHEET MODEL*

The response of a rectangular plate to the Crone PEM system has been investigated for different values of depth, dip and conductance parameters. The conductance aperture diagram is presented to estimate the conductance and depth to the top of a vertical sheet. Information regarding dip and depth may be obtained using the appropriate nomogram.     

Mössbauer spectroscopic studies of the Piplia Kalan (eucrite) and Lohawat (howardite) meteorites

Abstract— Two meteorites belonging to the howardite‐eucrite‐diogenite (HED) group fell recently in Rajasthan, India. One of these, Piplia Kalan, was classified as a eucrite and the other, Lohawat, as a howardite. In this study, we present the results of Mössbauer spectroscopic investigations of these two meteorites. We also compare the results with the Mössbauer experiments reported for the Kapoeta howardite and look for systematics in the Mössbauer spectra of HED meteorites.     

ELECTROMAGNETIC RESPONSE OF FOLD MODELS IN A UNIFORM FIELD*

The electromagnetic response of anticline fold models in a uniform field has been studied using scale-modeling. With the increasing conductance the in-phase anomaly increases initially but then starts to decrease and finally attains a negative saturations, whilst the quadrature anomaly reaches a positive saturation. E.M. response of folds is similar to that of an interaction between two weakly coupled circuits in a dipolar field. The base of the fold is one of the characteristic parameters to determine the measured response.