An electromagnetic prospecting instrument for subsurface conducting zones

Summary A portable electromagnetic prospecting unit comprising a transmitter, receiver and resolver, designed and constructed to operate with a vertical setup of coils at a single frequency of 1250 cps, measures the components of the magnetic field at the receiver station resolved in phase and at quadrature with field near the transmitter. This may be used to locate vertical or steeply dipping conductors down to a depth of about 0.6 times the maximum workable transmitter receiver separation of 120 meters, the accuracy of measurements of the resolved components being better than 2%. The variation of resolved components over a buried conductor indicates its position and depth.This paper is published by the kind permission of the Director General, Geological Survey of India.     

Elastic waves from a circular disc source

Summary Elastic waves radiated by a finite circular disc source are compared for two models by treating it as the limit of an explosion inside a cavity in the form of a circular cylinder of small height or an oblate spheroid of small minor axis.Now on leave at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, England (Priv. Address: 19 Marlowe Road, Cambridge, England).     

Using fast Hartley transform to study the free oscillations of the earth

In recent years, Hartley Transform (HT) as a substitute of much widely used Fourier Transform (FT) has been practised in science and industries. The advantage of faster computation of HT is enormous when one is dealing with very long data sets. One such application arises in computation of parameters of Free Oscillations of the Earth (FOE), where one needs to study very long period vibrations of the earth, excited after a large earthquake. We demonstrate here an application of HT to determine the parameters of these normal modes of the earth after the Minahasa Peninsula earthquake of 18 April 1990 (M_s = 7.5).     

Nutrient modeling of an urban lake using best subset method

Lakes are functionally integral and biologically complex freshwater ecosystems which provide a vast array of ecosystem goods and services to society. Nowadays aquatic ecosystems are being used, misused and abused by diverse anthropogenic activities at an unprecedented rate. The management of lake water quality is usually directed to resolution of conflicts between maintenance of desirable water quality and human-induced degradation of aquatic environment. Nutrients play a decisive role in determining lake’s environmental state through regulation of its primary production. The present study on Saheb Bundh Lake located in Purulia District, West Bengal, India, was undertaken to assess the status of nutrients (nitrogen and phosphorus) subject to certain anthropogenic activities, and to construct models using best subset method which could be adopted as a nutrient management tool. The water samples were monitored for different physicochemical parameters adopting standard methods. It was found that the set of variables including turbidity, temperature, pH, redox potential and total hardness has been championed as the best subset of water quality explaining the dynamics of total phosphorus concentration of freshwater Saheb Bundh Lake. The suite of factors comprising dissolved oxygen, pH, temperature, turbidity and total hardness has been proved as the best subset for estimating total nitrogen concentration. The models developed have been validated for total phosphorus and total nitrogen concentration. For total phosphorus, the model values were found to be very close to the measured values but the values varied widely for total nitrogen, championing the former as a very potent down-to-earth model.     

Modeling of source parameters and moment tensors of local earthquakes occurring in the eastern Indian shield

Earthquake source parameters and crustal Q are being estimated simultaneously through the inversion of S-wave displacement spectra from three-component recordings of ten local cratonic intraplate earthquakes from 3-6 broadband stations in the eastern Indian shield, wherein, an iterative Levenberg-Marquardt inversion technique is used. The estimated seismic moment (M_o) and source radii (r) vary from 7.4 x 10¹² to 7.1 x 10¹⁴ N-m and 144.2 to 211.3 m, respectively, while estimated stress drops (Δσ) and multiplicative factor (E_mo) values range from 0.11 to 4.13 MPa and 1.33 to 2.16, respectively. The corner frequencies range from 6.23 to 8.62 Hz while moment magnitudes vary from 2.44 to 3.57. The radiated seismic energy and apparent stresses range from 8.3 x 10⁶ to 2.0 x 10¹⁰ Joules and 0.06 to 0.94 MPa, respectively, wherein the estimated corner frequencies and seismic moment satisfy the relation Mo ∞ f _c ^–(3+ε) for ε = 12.7. Thus, the source scaling of these events clearly deviates from the self-similarity i.e. f^–3. Estimated Zuniga parameters reveal that all selected events satisfy the partial stress drop model, which is in good agreement with the global observations. Our estimated crustal S-wave quality factors vary from 1091 to 4926 with an average of 3006, suggesting a less heterogeneous crustal structure underlying the study region.We also perform moment tensor inversion of five selected local events using ISOLA software, which reveals that the dominant deformation mode for the eastern Indian shield is left-lateral strike slip motion with minor normal dip-slip component on an almost vertical plane. This observation suggests that neotectonic vertical movements might have played a key role in generating these earthquakes. Our modeling also depicts that the seismically mildly active Singhbhum shear zone and Eastern Ghats mobile belt are characterized by the left-lateral strike motion while two events in the Chotanagpur half graben belt suggest a normal dip-slip motion along a south dipping plane. A north-south orientation of P-axis is found to be dominant in the area, which is consistent with the prevailing north–south compression over the Indian plate.     

Seismic waves due to storm over the sea: I

Summary Elastic wave propagation problem has been studied in the composite medium — a gravitating liquid layer overlying an elastic half-space. Assuming moving normal pressure load on the free surface, an attempt has been made to study the order of disturbance on the earth's surface. In the discussion asymptotic values of the displacement, components have been considered for small values of the parameter.     

Morphological analysis and channel shifting of the Fulahar river in Malda district,West Bengal,India using remote sensing and GIS techniques

GeoJournal - The present study has been carried out to analyse and interpret the morphological changes and channel shifting along the Fulahar river in the Malda district. Fulahar river is one of...     

Geophysical surveys for identifying source and pathways of subsurface water inflow at the Bangur chromite mine,Odisha, India

Geophysical survey was carried out in an effort to solve an underground flooding problem at the Bangur chromite mine of Odisha Mining Corporation Limited, Odisha, India. To identify sources and pathways of the influx, very low-frequency electromagnetic, self-potential and resistivity surveys were performed. Geophysical studies clearly depict a major fracture zone passing through the mine and its connection to a water storage pond. The fracture zone extends further west from the pond to the Salandi River and the Salandi Reservoir. The dip of the delineated fracture zone is around 45° to the N, and it matches with the fault plane exposed in the mine. Since water enters into the mine from the west, the delineated fracture zone is thought to be the main pathway for the inflow. Geophysical studies conclude that the IMFA pond, Salandi River and Salandi Reservoir could be possible sources of water in the mine. To ascertain the source and pathway, tracer testing was conducted at two locations, based on the geophysical survey by the hydrogeological team, but tracer test results were inconclusive. However, the fact remains that the water enters to the mine through the delineated fracture and poses threat in mine operation. Dumping water in nearby pond will again enter in mine with interconnected fractures. Therefore, for safe mining operation, it is proposed to drill a slanted tube well at the delineated fracture and pump out maximum water and discharge the pumped water at canal situated about 2.5 km from the mine.     

Variation of γ-Ray and Particle Fluxes At the Sea Level During the Total Solar Eclipse of 24 October, 1995

A Total Solar Eclipse (TSE) was observed from Diamond Harbour (lat. 22.2°N, long 88.2°E) on 24 October 1995. The variation of -ray intensity was measured in the energy range of 0.3–3.0 MeV for different time spans throughout the period of the eclipse. A CR-39 detector was used to look at the change in the fluxes of neutral and charged particles. The maximum drop ( 25%) in the intensity of -ray was observed in the range 2.5–3 MeV during TSE. The CR-39 results showed the appearance of a good number of tracks and a small variation of proton and neutron flux of 10% which was not significant statistically. Low energy -ray fluxes at sea level originate from the secondary electron-photon components of cosmic rays in the atmosphere; its modulation by TSE is interpreted as follows. The cooling of the atmosphere in the path of the umbra induces a reduction of the height of the main production layer of the nuclear component, as a result of which, fewer µ^± mesons (from the decay of the^± mesons) decay to e^±. This leads to a small reduction in the flux of electron-photon component at sea level which originates from this branch; the main branch of e - component from ⁰ decay remains nearly unaffected. As the total mass of air remains the same, little or no change in the slow proton or the neutron flux at sea level is expected. These are consistent with the present observations. For a better understanding, further studies of this new phenomenon during future TSE are suggested.