DEPTH ESTIMATION FROM RESIDUAL BOUGUER ANOMALIES: A PROCEDURE BASED ON THE BACKUS-GILBERT MINIMUM DISTANCE CRITERION *

The minimum distance criterion of Backus and Gilbert is used to determine the depths of line masses approximating the subsurface two dimensional distribution of density differences responsible for the observed residual Bouguer anomaly values. The quantity minimized is the sum of the squared differences between the starting trial depths and the desired final depths of line masses. The search for the desired depths is limited to preset depth ranges.     

SIMULATIONS OF REFLECTIONS AND DIFFRACTIONS FROM TARGETS BURIED IN A MEDIUM WITH LINEAR VARIATION OF VELOCITY WITH DISTANCE*

A combination of ray theory and diffraction theory has been used to simulate reflections of seismic pulses from a full plane and reflections and diffractions from a half plane buried horizontally in earth models with linear increase of velocity with depth. Wave forms of signals reflected from a full plane in this case are modifications of the source pulse, even if the medium is assumed to be perfectly elastic. The extent of pulse modification increases with increase of the velocity gradient. The duration of the reflected pulse is always longer than that of the source pulse. The pulses diffracted from the edge of the half plane in such cases do not lie along hyperbolic curves in the time-offset plane. Diffracted signals can be observed only up to a limited distance from the edge of the half plane equalling half the horizontal range of the ray which is tangential to the half plane and has its end points at the level of the source and the receiver.     

DIVIDED LANDSCAPES, FRAGMENTED IDENTITIES: EAST BENGAL REFUGEES AND THEIR REHABILITATION IN INDIA, 1947–79

The partition of the Indian subcontinent in 1947 was followed by the forced uprooting of an estimated 18 million people. This paper focuses on the predicament of the minority communities in East Pakistan (now Bangladesh) who were uprooted and forced to seek shelter in the Indian province of West Bengal. It considers the responses of Indian federal and provincial governments to the challenge of refugee rehabilitation. A study is made of the Dandakaranya scheme which was undertaken after 1958 to resettle the refugees by colonising forest land: the project was sited in a peninsular region marked by plateaus and hill ranges which the refugees, originally from the riverine and deltaic landscape of Bengal, found hard to accept. Despite substantial official rehabilitation efforts, the refugees demanded to be resettled back in their “natural habitat” of Indian Bengal. However, this was resisted by the state. Notwithstanding this opposition, a large number of East Bengal refugees moved back into regions which formed a part of erstwhile undivided Bengal where, without any government aid and planning, they colonised lands and created their own habitats. Many preferred to become squatters in the slums that sprawled in and around Calcutta. The complex interplay of identity and landscape, of dependence and self-help, that informed the choices which the refugees made in rebuilding their lives is analysed in the paper.     

ON TRACING SEISMIC RAYS WITH SPECIFIED END POINTS IN LAYERS OF CONSTANT VELOCITY AND PLANE INTERFACES*

A polygonal ray path connects the seismic source and detector positions when the intervening medium consists solely of constant velocity layers with plane interfaces which may have arbitrary orientation. The coordinates of the ray vertices satisfy a system of coupled equations resulting from the requirement that Fermat's principle be satisfied along the ray path. Solving the system of equations is equivalent to tracing the ray numerically. A notable feature of this approach is that a ray which is critically refracted over a segment of its path requires no special handling.     

An overview of sensor calibration inter-comparison and applications

Long-term climate data records (CDR) are often constructed using observations made by multiple Earth observing sensors over a broad range of spectra and a large scale in both time and space. These sensors can be of the same or different types operated on the same or different platforms. They can be developed and built with different technologies and are likely operated over different time spans. It has been known that the uncertainty of climate models and data records depends not only on the calibration quality (accuracy and stability) of individual sensors, but also on their calibration consistency across instruments and platforms. Therefore, sensor calibration inter-comparison and validation have become increasingly demanding and will continue to play an important role for a better understanding of the science product quality. This paper provides an overview of different methodologies, which have been successfully applied for sensor calibration inter-comparison. Specific examples using different sensors, including MODIS, AVHRR, and ETM+, are presented to illustrate the implementation of these methodologies.