Volume 108, Issue 4
December 1999, pages 223-343
pp 223-231 December 1999
A method of representing surfaces and volumes by a set of geometric points and a small set of auxiliary parameters, based on generalization of Bernoulli’s notion of lemniscates is introduced. It provides for easy generation and modification of surfaces and volumes, which could be connected, disjoint or even with very irregular boundaries. This allows solving geophysical inversion problems, without constraining the anomalous volumes to some ideal or simple forms. This is illustrated by the example of joint inversion of gravity and magnetic data sets attributable to two-dimensional anomalous bodies. A nonlocal optimization algorithm calledsegmented Hamming scan is used for inversion. Comparison with nonlinear least-squares algorithm shows the advantages of the chosen approach. The concepts ofdesideratal andprocedural detours are illustrated.
pp 233-253 December 1999
The paper presents an efficient finite difference based 2D-inversion algorithm, EM2INV, for geoelectromagnetic data. The special features of the algorithm are
The algorithm is tested rigorously by setting up exercises of diverse nature and of practical significance. The stability of the algorithm is established by inverting the synthetic response corrupted with Gaussian noise. The inversion experiments are aimed at studying
It has been observed that the Magneto-telluric data deciphers better the vertical position of the target and Geomagnetic Depth Sounding data deciphers the horizontal variations in a better way. The conductive and resistive bodies are better resolved by inversion of E- and B-polarization data respectively. The results of multi-frequency inversion imply that the increase in the number of frequencies does not necessarily enhance the inversion quality especially when the spread of observation points is sufficiently large to sense the target. The study of a minimum number of observation points highlights the importance of single point inversion that furnishes useful information about the inhomogeneity.
pp 255-267 December 1999
Investigations of three plausible tectonic settings of the Kerguelen hotspot relative to the Wharton spreading center evoke the on-spreading-axis hotspot volcanism of Paleocene (60-54 Ma) age along the Ninetyeast Ridge. The hypothesis is consistent with magnetic lineations and abandoned spreading centers of the eastern Indian Ocean and seismic structure and radiometric dates of the Ninetyeast Ridge. Furthermore, it is supported by the occurrence of oceanic andesites at Deep Sea Drilling Project (DSDP) Site 214, isotopically heterogeneous basalts at Ocean Drilling Program (ODP) Site 757 of approximately the same age (59-58 Ma) at both sites. Intermix basalts generated by plume-mid-ocean ridge (MOR) interaction, exist between 11° and 17°S along the Ninetyeast Ridge. A comparison of age profile along the Ninetyeast Ridge between ODP Sites 758 (82 Ma) and 756 (43 Ma) with similarly aged oceanic crust in the Central Indian Basin and Wharton Basin reveals the existence of extra oceanic crust spanning 11° latitude beneath the Ninetyeast Ridge. The extra crust is attributed to the transfer of lithospheric blocks from the Antarctic plate to the Indian plate through a series of southward ridge jumps at about 65, 54 and 42 Ma. Emplacement of volcanic rocks on the extra crust resulted from rapid northward motion (absolute) of the Indian plate. The Ninetyeast Ridge was originated when the spreading centers of the Wharton Ridge were absolutely moving northward with respect to a relatively stationary Kerguelen hotspot with multiple southward ridge jumps. In the process, the spreading center coincided with the Kerguelen hotspot and took place on-spreading-axis volcanism along the Ninetyeast Ridge.
pp 269-275 December 1999
Nonlinear, nonlocal and adaptive optimization algorithms, now readily available, as applied to parameter estimation problems, require that the data to be inverted should not be very noisy. If they are so, the algorithm tends to fit them, rather than smoothening the noise component out. Here, use of Bernstein polynomials is proposed to prefilter noise out, before inversion with the help of a sophisticated optimization algorithm. Their properties are described. Inversion of gravity and magnetic data for basement depth estimation, singly and jointly, and without and after Bernstein-preprocessing is conducted to illustrate that the inversion of Bernstein-preprocessed gravity data alone may be slightly superior to the joint inversion of gravity and magnetic data.
pp 277-286 December 1999
Forecasting weather parameters such as temperature and pressure with a reasonable degree of accuracy three hours ahead of the scheduled departure of an aircraft helps economic and efficient planning of aircraft operations. However, these two parameters exhibit a high degree of persistency and have nonstationary mean and variance at sub-periods (i.e. at 0000, 0300, 0600,…, 2100UTC). Hence these series have been standardised (to have mean 0 and variance 1) and thereafter seasonal differenced (lag 8) to achieve almost near stationarity. An attempt has been made to fit the standardised and seasonal differenced series of Chennai (a coastal station) and Trichy (an inland station) airport into an Auto Regressive (AR) process. The model coefficients have been estimated based on adaptive filter algorithm which uses the method of convergence by the steepest descent. The models were tested with an independent data set and diagnostic checks were made on the residual error series. An independent estimation of fractal dimension has also been made in this study to conform the number parameters used in the AR processes. The models contemplated in this study are parsimonious and can be used to forecast surface temperature and pressure.
pp 287-295 December 1999
Contrary to the prevalent belief that tropical region is characterized by convective clouds rather than by layer clouds, we have suggested that deep convective clouds occur on meso-scale, but layer clouds occur on larger synoptic-scale with a relatively small region of deep convective clouds. Sustenance of deep convective clouds is inhibited by the presence of inertio-gravity waves, which have alternating layers of upward and downward motion in the vertical. We have also shown that inertio-gravity waves generate regions of relatively strong horizontal velocity, vertically separated by layers of relatively weak horizontal velocity. Layers of strong horizontal velocity are created by inertio-gravity wave system through convergence of vertical flux of horizontal momentum. We have also suggested that horizontal convergence/divergence of moisture flux is generated by inertio-gravity waves, giving rise to vertically alternating layers of high/low humidity, and visible or sub-visible clouds. Layers of high humidity become layers of strong radar reflectivity at frequency of 53 MHz at which MST Radar at Gadanki, near Tirupati, India, operates. These observations, more than 2,50,000 in number, for vertical grid points, spread over all the months of the year, have helped us, among other observations, to arrive at these conclusions. Further, the analysis suggests that the main source of strong MST radar reflectivity is not mechanical turbulence as is commonly believed.
pp 297-304 December 1999
Pn velocity has been computed across the NE India and Moho geometry constrained, using regional earthquake travel times recorded by a network of 30 seismological stations operated during February-May 1993. Using an appropriate velocity model and the arrival times at the network stations, preliminary hypocentres of 16 regional earthquakes provided by NEIC were also improved. The average Pn wave velocity in NE India has been found to be 8.5 ±0.2 km/s. It varies from 8.3 to 8.5 km/s beneath the Shillong Plateau, Mikhir hills and Assam valley, which is significantly higher than those in other parts of India. The crustal thickness in NE India is also high, varying from 45–49 km under the Shillong plateau and the adjoining region to 55–65 km in the convergence zone. The presence of a thick crust and high Pn velocity suggests that the lithosphere in NE India is colder, as also indicated by the observed deeper level (45-51 km) seismicity of the region.
pp 305-307 December 1999
In this paper an attempt has been made to search a new parameter for the prediction of the Indian summer monsoon rainfall. For this purpose the relationship of the global surface-air temperature of four standard seasons viz., Winter (December-January-February), Spring (March-April-May), Summer (June-July-August), Autumn (September-October-November) with the Indian summer monsoon rainfall has been carried out. The same analysis is also carried out with surface-air temperature anomalies within the tropical belt (30°S to 30°N) and Indian summer monsoon rainfall. For the present study data for 30 years period from 1958 to 1988 have been used. The analysis reveals that there is a strong inverse relationship between the monsoon activity and the tropical belt temperature.
pp 309-316 December 1999
A study was conducted with four traditional photosensitive and one high yielding rice varieties grown during the kharif season under rainfed conditions. The curvilinear technique was used to examine the influence of meteorological parameters on the yield of rice. Rice varieties grown in different agroclimatic regions performed differently to climatic parameters. The maximum yield was observed when rainfall ranged between 100 and 115 cm. Maximum and minimum temperature ranges of 29–32°C and 23–25°C respectively appear ideal for optimum yield. Photoinsensitive high yielding variety performed well even at low light intensity (250-350 hours of bright sunshine).
pp 317-320 December 1999
Surface processes play an important role in the simulation of desertification and climate change. The present study shows that enhancement of evaporation at a given place is due to surface temperature and wetness inhomogeneities compared to its surroundings. Between any adjacent dry and wet strips the induced advective heat transport changes the available net radiation between sensible and latent heat fluxes. It also accommodates a redistribution of surface energy between two adjacent inhomogeneous surface strips. A combined model of Tarpley (1994) and Ya Guo and Schuepp (1994) about the advective heat transport over dry and wet strips has been tested over Andhra Pradesh. Taking the Ananthpur district as a relatively drier region, the induced transport over the rest of the districts is estimated. This study has been done for two monsoon seasons of 1990 and 1991. Attempts are made to identify an indexing criteria based on this study to estimate the magnitude of the heat transport.
pp 321-326 December 1999
Surface and upper air synoptic features prevailing at the time of onset of southwest monsoon over Mumbai have been examined for a period of 50 years from 1947 to 1996. These synoptic situations have been found to fall under three broad categories. The study shows that on about 66% of the occasions, the onset of monsoon over Mumbai is caused by low pressure area forming over southeast and east central Arabian Sea as they move northwards, while on 28% of the occasions, the onset is due to the low pressure systems forming over northwest/head Bay of Bengal and their movement towards northwest/west northwest. Based on the findings of the study and the climatological aspects, a criterion for declaring onset over Mumbai has been suggested.
pp 327-332 December 1999
Rainfall variability over a river basin has greater impact on the water resource in that basin. With this in view, the variability of the monsoon rainfall over the Godavari river basin has been studied on different time scales. As expected, the monsoon rainfall in Godavari basin is more variable (17%) than the all-India monsoon rainfall (11%) during the period of study (1951–90). Similarly, inter-annual variability of the monsoon rainfall on smaller time scales is found to be still higher and increases while going on from seasonal to daily scales. An interesting observation is that the intra-seasonal variability of the monsoon rainfall has a significant negative relationship (CC= −0.53) with the total seasonal rainfall in the basin.
pp 341-343 December 1999