Determination of partial geoidal parameters over the Indian sub-continent

The paper presents values of partial geoidal parametersN, ξ and η which define the departure of the geoid from the reference spheroid, at 1° intervals over the Indian subcontinent. These values represent contributions arising from the gravity anomaly data over the entire earth's surface, except for that from a 6°×6° element around the immediate neighbourhood of the point. Complete values of these parameters at a point can be obtained simply by adding to these partial values, contributions from the 6°×6° element circumscribing that point. The objective of the study was to provide a ready basis for updating the geoidal parameters at the initial reference point as and when the density and quality of local gravity data around it improve. These computations once made, would also facilitate calculation of geoidal parameters at a fairly large number of astro-geodetic stations apart from that at the initial reference point, which should lead to considerably more precise value of the absolute datum of the Indian geodetic dasystem.     

Induction by long period geomagnetic variations in the Indian sub-continent

In the present paper storm time variations and 27-day geomagnetic periodicity have been analysed to estimate the depth of the substitute conductor, assuming an infinitely (super) conducting core model of the earth. The advantage of using data from a restricted longitude range is that the uncertainties arising from lateral contrasts in the upper mantle and contributions from Sq current systems are considerably reduced. The result of the present analysis, which has been done in the time domain, gives a value of 522 km for the depth of the substitute conductor in case of storm time variations which rises to 870 km for 27-day recurrent storms. A higher value of the depth for 27-day variations indicate that the rise in conductivity inside the earth is not like a step function rather is a gradual one. The value of 522 km for storm time variations for the Indian region is smaller than the global average. This is natural to expect because the Indian sub-continent is known to be a tectonically active region.     

Water vapour periodicities over Indian Ocean during contrasting monsoons from NOAA satellite data

The satellite-derived moisture fields during different phases of two normal and poor monsoon years have been studied. Spectral analysis was performed in different zones of the monsoon region to study the nature and modes of intraseasonal fluctuations of lower layer moisture fields. Seasonal mean fields of water vapour at low and middle layers show a dry anomaly over the Arabian subcontinent and a wet anomaly over the Bay of Bengal during good monsoon years, while the anomalies show an opposite trend during the poor monsoon years. The zonal and meridional propagation of low-frequency oscillations of moisture fields has also been examined. The southward movement of low-frequency oscillations seems to be suppressed in good monsoon years as compared to the poor monsoon years, whereas the northward movement of the same shows no particular difference. Fluctuations in the 30–50 day range are found shifted to longer time-period side in the poor monsoon years.     

Co and postseismic characteristics of Indian sub-continent in response to the 2004 Sumatra earthquake

The M_w 9.3 Sumatra earthquake of December 26, 2004 caused extensive coseismic displacements globally, measurements of which were made essentially using modern geodetic techniques. This earthquake induced considerable perturbation in stress distribution as far as ∼8000 km away from the epicenteral region, which is tending to relax to its normal rates as seen from postseismic transient deformation. The monitoring of crustal displacements from strategically located sites using GPS provides coseismic as well as postseismic deformation that facilitates the understanding of the fault geometry, elastic thickness, postseismic relaxation mechanisms, rheology and earthquake recurrence time interval.We investigated coseismic and postseismic GPS derived displacements in Indian region together with the GPS data collected from Andaman and Sumatra region. It is found that while EW displacements are significantly large in peninsular India, those in the region to the north of Central India Tectonic Zone (CITZ) are relatively small. We could delineate the postseismic transients from position time series and interpreted them in terms of viscoelastic relaxation. It is inferred that the postseismic deformation is characterized by a power-law viscoelastic flow in the mantle. In Indian peninsula region, the timescale parameter of the exponential decay (τ = 250 days) would require an extremely low viscosity for the upper mantle. Relying on the prevailing coseismic and postseismic displacement fields, the present study also reflects upon the contemporary litho-tectonics of the Indian sub-continent.     

Use of satellite data for radiative energy budget study of Indian summer monsoon

In this paper satellite-derived radiative energy budget such as shortwave radiative heating, longwave radiative heating and net radiation balance have been studied for the post-onset phase of summer monsoon 1979. Since clouds play an important role in determining diabatic heating field as well as being a reflection of status of the monsoon itself, the day to day evolution of clouds from TIROS-N satellite has been made. Satellite-derived radiative heating rates from surface to 100 hPa were computed for each 100 hPa thickness layer. These heating rates were then compared with the observed latitudinal distribution of total radiative heating rates over the domain of the study. From the results of our study it was found that the characteristic features such as net radiative heating rates of the order of 0.2°C/day at upper tropospheric layer (100–200 hPa) and cooling throughout the lower tropospheric layers with relatively less cooling between 500–700 hPa layer observed in a case of satellite-derived radiative energy budget agree well with the characteristic features of observational radiative energy budget over the domain of the study. Therefore, it is suggested that radiative energy budget derived from satellite observations can be used with great potential and confidence for the evolution of the complete life cycle of the monsoon over the Indian region for different years.     

Medium-range forecast of cyclogenesis over North Indian Ocean with multilayer perceptron model using satellite data

An attempt is made in this study to develop a model to forecast the cyclonic depressions leading to cyclonic storms over North Indian Ocean (NIO) with 3 days lead time. A multilayer perceptron (MLP) model is developed for the purpose and the forecast quality of the model is compared with other neural network and multiple linear regression models to assess the forecast skill and performances of the MLP model. The input matrix of the model is prepared with the data of cloud coverage, cloud top temperature, cloud top pressure, cloud optical depth, cloud water path collected from remotely sensed moderate resolution imaging spectro-radiometer (MODIS), and sea surface temperature. The input data are collected 3 days before the cyclogenesis over NIO. The target output is the central pressure, pressure drop, wind speed, and sea surface temperature associated with cyclogenesis over NIO. The models are trained with the data and records from 1998 to 2008. The result of the study reveals that the forecast error with MLP model varies between 0 and 7.2 % for target outputs. The errors with MLP are less than radial basis function network, generalized regression neural network, linear neural network where the errors vary between 0 and 8.4 %, 0.3 and 24.8 %, and 0.3 and 32.4 %, respectively. The forecast with conventional statistical multiple linear regression model, on the other hand, generates error values between 15.9 and 32.4 %. The performances of the models are validated for the cyclonic storms of 2009, 2010, and 2011. The forecast errors with MLP model during validation are also observed to be minimum.     

Isoseismal map of the 1755 Lisbon earthquake obtained from Spanish data

The Royal Academy of History (RAH) of Spain collected very detailed information about the effects on the Spanish territory of the Lisbon, 1755 earthquake, reporting such information within a year after the shock. The data provided by the RAH has been thoroughly analyzed, deriving an M S K intensity value for each one of the localities (more than 1000) included in the report, and an isoseismal map has been obtained from such values. Observations reported, such as tsunami, surface effects, duration of shaking and effects on the population are also discussed. Finally, a comparison is made of the data with the results of recent investigations on the tectonics of the region.     

Statistical map analysis of regional stream-sediment data from Australia

In a regional stream-sediment survey of the Seigal and Hedleys Creek areas, covering approximately 6000 km², in all 2508 stream-sediment samples were collected with an overall sample density of 1 per 2–3 km². The minus 180 μm fraction of the samples was analyzed for up to 25 elements including As, Ba, Be, Bi, Ce, Co, Cr, Cu, F, Fe, Li, Mn, Mo, Nb, Ni, Pb, Rb, S, Sn, Th, Ti, U, W, Y and Zn.Selected elements were processed by a statistical map-analysis technique to: (a) display the broad-scale regional distribution patterns of the elements using a cell-average gap-fill mapping technique followed by low-pass filtering (LPF); and (b) delineate anomalous areas using the picture frame filter (PFF) and the Kolmogorov-Smirnov filter (KSF).The regional distribution patterns of elements clearly relate to geologic belts, plutons and stratigraphic units. Anomaly filtering clearly identifies the various known mineral occurrences including U, U-Cu-Sn, and Pb-Zn. The technique also delineated several anomalies, unrelated to known mineral occurrences, having similar anomalous element combinations and occurring in similar geological settings as those related to known mineral occurrences. Some of these filter-enhanced anomalies are too subtle to be immediately apparent in the unprocessed data.     

Estimation of evaporation rates over the Arabian Sea from satellite data

Utilizing both the SAMIR brightness temperatures of Bhaskara II and GOSSTCOMP charts of NOAA satellite series, the evaporation rates over the Arabian Sea for June 1982 are estimated through the bulk aerodynamic method. The spatial distribution of evaporation rates estimated from the satellite data sets coincides well with those obtained from ship observations as well as from climatological data. The accuracy in the estimation of evaporation rates has considerably been improved after the removal of bias in sea surface temperature and is about ±0·8 mm/day.     

Mapping from space — cartographic applications of satellite image data

Digital as well as photographic satellite image data offer a high potential of topographic and thematic information. The paper discusses the application of such data for mapping purposes. For the compilation of conventionalTopographic Maps the requirements concerning the geometrical accuracy can easily be met for 1 : 50,000 or even 1 : 25,000. However, the interpretability of the features that are relevant for topography is often the critical factor. Only the Russian KWR-1000 data are suitable for mapping topographic features in 1 : 25,000. Main emphasis is laid on a new map type, theSatellite Image Map. The authors describe the satellite image mapping procedures developed at the Technical University of Berlin and applied in practice by FPK Consulting Engineers. Some related problems are discussed in details, eg mosaicking, merging of different types of data, and integration of graphical elements. An increase in cartographic applications of satellite remote sensing data is anticipated.     

Estimation of snow cover distribution in Beas basin,Indian Himalaya using satellite data and ground measurements

In the present paper, a methodology has been developed for the mapping of snow cover in Beas basin, Indian Himalaya using AWiFS (IRS-P6) satellite data. The complexities in the mapping of snow cover in the study area are snow under vegetation, contaminated snow and patchy snow. To overcome these problems, field measurements using spectroradiometer were carried out and reflectance/snow indices trend were studied. By evaluation and validation of different topographic correction models, it was observed that, the normalized difference snow index (NDSI) values remain constant with the variations in slope and aspect and thus NDSI can take care of topography effects. Different snow cover mapping methods using snow indices are compared to find the suitable mapping technique. The proposed methodology for snow cover mapping uses the NDSI (estimated using planetary reflectance), NIR band reflectance and forest/vegetation cover information. The satellite estimated snow or non-snow pixel information using proposed methodology was validated with the snow cover information collected at three observatory locations and it was found that the algorithm classify all the sample points correctly, once that pixel is cloud free. The snow cover distribution was estimated using one year (2004–05) cloud free satellite data and good correlation was observed between increase/decrease areal extent of seasonal snow cover and ground observed fresh snowfall and standing snow data.     

Long wavelength magnetic anomalies from the lithosphere: Indian shield and Himalaya

A few long-range airborne magnetic profiles flown at an altitude of 7.5 km a.s.l. across the Indian shield are analysed and interpreted in terms of magnetization in the lower crust. The wavelengths of the crustal anomalies are in the range of 51–255 km and this is used to separate them from signals originating at shallow depths. Spectral analysis of these profiles provided a maximum depth of 34–41 km for the long-wavelength anomalies and 9–10 km for the shallow sources identified as Mohorovic̆ić discontinuity and the basement respectively. The magnetic “high” recorded in satellite observations over the Indian shield is interpreted as due to a bulge of 3–4 km in the Moho under the Godovari graben, with a magnetization of 200 nT in the direction of the Earth's present-day magnetic field. Similarly the magnetic lows observed over the Himalaya are interpreted in terms of thickening of the granitic part of the crust from 18 to 23.5 km with a magnetization contrast of 200 nT in the direction of the Earth's present-day magnetic field.     

On determining reference fields from ground-based and satellite field data

The physical content of simultaneous SHA internal field models for the concentric spheres for the Earth's surface and for the satellite altitude differs because of the different distances from the internal magnetic field sources. This fact is reflected in the different convergence behaviour of both the SHA expansions that can be described mathematically by the functional behaviour of the contributions of the SHA functional terms to the N · (N+2) dimensional volume in a functional space. Thereby a regularizing criterion is derived for the downward field continuation which results in a transformation formula for the Gauss coefficients. This enables to calculate all the field components. The comparison of the ground-based SHA model with that having been downwardly continued from the satellite altitude evaluates its different physical content.     

The composite magnetic anomaly map of the East Antarctic

Airborne and marine magnetic observations in East Antarctica and adjacent seas of the Indian Ocean were compiled for a magnetic anomaly map of the Antarctic. For East Antarctica, over 260,000 line km of Russian reconnaissance magnetic data were used that had been collected since 1955 mainly at line spacings of about 5, 20 and 50 km. For the offshore areas, magnetic data from American, Australian, German, Japanese, and Russian marine expeditions were incorporated. Digitally recorded data and data digitized from published and unpublished maps and profiles were included in the compilation. Local grids of these data were developed and merged into a regional grid at an interval of 5 km. The prime product of this compilation is a shaded-relief map that shows the most complete and coherent perspective to date of the region's magnetic character. In combination with other types of data, the compilation provides new insight on the tectonic features and history of this largely inaccessible region of the world. It maps out approximately 4300 km of the Antarctic Continental Margin Magnetic Anomaly (ACMMA) related to Gondwana breakup, new cratons and mobile belts, and large submarine igneous provinces.     

Using stratiform magnetic anomalies to map near-surface architecture: insights from the Amadeus Basin

The Amadeus Basin displays subtle magnetic anomalies that trace strata for considerable distance, highlighting complex folding patterns. Magnetic modelling techniques can be utilised on these stratiform anomalies to extrapolate the near-surface structure of the basin. However, because of the mathematical trade-off between the dip and magnetisation of bodies, the dips of the bodies cannot be known unless the magnetisation is also known. Normally it would be optimal to measure the magnetisation, but this is not always possible or feasible. In this study, we investigate the relationships between dip and magnetisation using an approach that would generally be considered a little backward, i.e. constraining magnetisation direction using geological data. Three study areas were chosen to investigate a number of stratigraphic horizons, the Waterhouse Range, Glen Helen and Ross River areas. Modelling results suggest that some layers primarily retain induced magnetisation, remanence is dominant in others, but both are present in most. Remanence is mainly associated with relatively oxidised units that contain only hematite (e.g. Arumbera Sandstone), and we have demonstrated that these magnetisations predate folding of the Ross River Syncline. In some cases, the anomalies represent redox zonation within units, e.g. the Pertatataka Formation near Glen Helen, where discrete magnetic layers correspond to thin grey (reduced, magnetite-rich) horizons interbedded with more prevalent red (oxidised, hematite-rich) horizons. We also found that where magnetised units are relatively thin and occur near the surface, their magnetic response is sharp, and in aeromagnetic data such adjacent anomalies commonly overlap to form a single anomaly, thus misrepresenting the magnetic field, and mis-mapping the actual magnetic horizons. While the magnetic properties of the causative bodies are variable, we have demonstrated that a better understanding of the magnetic properties of these magnetised horizons can be used to provide insights into the structure and tectonic history of the Amadeus Basin.