P. V. Sanker Narayan

Interpretation of magnetic anomalies due to dikes: The complex gradient method

A method to interpret the magnetic anomaly due to a dipping dike using the resultant of the horizontal and vertical gradients of the anomaly is suggested. The resultant of both the gradients is a vector quantity and is defined as the “complex gradient.” A few characteristic points defined on the amplitude and phase plots of the complex gradient are used to solve for the parameters of the dike. For a dike uniformly magnetized in the earth’s magnetic field, the amplitude plot is independent of θF, the index parameter, which depends upon the strike and dip of the dike and the magnetic inclination of the area. The phase plot of the complex gradient is an antisymmetric curve with an offset value equal to -θF. For a dike whose half‐width is greater than its depth of burial, two maxima at equal distances on either side of a minimum value appear on the amplitude plot. For a dike whose half‐width is equal to or less than its depth of burial, the amplitude plot is a bell‐shaped symmetric curve with its maximum appe...

Relationship of magnetic anomalies due to subsurface features and the interpretation of sloping contacts

A study of the magnetic anomalies produced by sloping geologic contacts, thin dikes, and horizontal cylinders has revealed that a single relationship exists among the magnetic anomalies created by them. The magnetic anomaly due to a horizontal cylinder, the first horizontal derivative of the magnetic anomaly due to a thin dike, and the second horizontal derivative of the magnetic anomaly due to a sloping contact are found to be identical in shape. Gay (1963, 1965) presented standard curves to interpret the magnetic anomalies over long tabular bodies (1963) and long horizontal cylinders (1965). It is shown here that the same curves can also be used to interpret the total, vertical and horizontal magnetic anomalies due to sloping geologic contacts.

A study of pearl pulsations in the equatorial region of India

Abstract Pearl pulsations in the equatorial region, despite their relative paucity, are known to exhibit definite features in their diurnal, seasonal and annual occurrence patterns. In the present study, attempts are made to examine and bring out in detail some more aspects of these pulsations recorded at Choutuppal (Geomagnetic latitude: 7°28′N) over a period of nine years during 1967–1975. The mid-period (t) of pearls is found to bear a linear relation to the pearl repetition period (T) and is of the form T = 70t + 11.2. The periods of pearls are seen to be influenced by the average magnetic activity level prevailing during the preceding few days rather than that existing on the day of pearl occurrence. There is an increase in the mid-frequency of pearls that occurred succeeding a magnetic disturbance particularly when the disturbance persisted over some days. The amplitude ratio (Hx/Hy) averaged for each two-hourly interval is seen to attain a value of unity around local midnight and decrease towards the dawn and morning hours. From these results, namely the dependence of mid-frequency of pearls on the average magnetic activity (represented by (Σ Kp >) during the preceding few days and the occurrence of shorter period pearls even after a few days following a magnetic storm, it is suggested that the plasmapause takes much longer time to re-establish to the quiet time position. In the alternative, it may be visualized that possible presence of localized regions of high plasma density gradients inside the plasmasphere after the cessation of a storm might provide favourable conditions for the generation of shorter period pearls.

Structural control of emplacement of kimberlite pipes at panna — A suggestion from aeromagnetics

Abstract The aeromagnetic map of an area of about 1200 km2 around Panna has been subjected to a detailed study in order to understand the relationship of magnetic trends with the occurrence of the kimberlite pipes. The study of the original magnetic map revealed that the two known pipes at Majgama (Majhgawn) and Hinota coincide with the steep gradient zone of the original anomaly and is possibly reflecting a boundary — a structural dislocation in which the pipes might be emplaced. The residual map and the second derivative map have brought out two distinct anomalous trends namely 1. (1) ENE-WSW and 2. (2) NE-SW trends. The E-NE trend appears to have been affected by the northeasterly trend. Both the known pipes occur in the close vicinity of the meeting points of the two trends. Implications of this observation for further exploration activity for kimberlites in this area are discussed.

Interpretation of the total magnetic intensity anomaly due to a two-dimensional asymmetrical triangular prism using Fourier transforms

Abstract An expression for the total field magnetic anomaly due to a two-dimensional asymmetrical triangular prism is formulated and its Fourier transform is derived. Assuming the susceptibility contrast, the Fourier amplitude and phase spectra are analysed to evaluate the parameters of the prism. Application of this method to a synthetic anomaly yielded reasonably accurate results. With suitable modifications, this technique can also be extended to interpret the anomalies in the vertical and the horizontal components.

Detectability of an intermediate layer by magnetotelluric sounding

The recent publication by Verma and Mallick (1979) on the detectability of an intermediate layer by time domain EM sounding provides some informative answers to a problem of practical interest. That is, an attempt has been made to probe the usefulness of the sounding system with regard to its ability to detect a subsurface layer. It is worthwhile to attempt to analyze the same problem of detectability in a three‐layer sequence by magnetotelluric (MT) sounding which is finding increasing use in recent years in exploration for minerals (Strangway and Koziar, 1979) and in prospecting for geothermal resources (Hoover and Long, 1975), particularly in the audio‐frequency range. We present results of an analysis of the problem of detectability of an intermediate layer in a three‐layer horizontal sequence with several possible combinations of the layer resistivities of practical interest covering all the four types H, K, A and Q, by the AMT sounding system. The computational approach used by Verma and Mallick (19...