January, 2005

January, 2005   ||  Volume 9 No.1

Orthogonal Dykes around the Cuddapah Basin - A Palaeomagnetic Study
G.V.S.Poornachandra Rao
National Geophysical Research Institute, Hyderabad - 500 007

The dyke swarms surrounding the Palaeo-Meso Proterozoic intracratonic Cuddapah Basin in the south Indian peninsular shield are considered to be the result of thermal events responsible for its initiation and development. The stress fields control the joint pattern with unidirectional and orthogonal fissures, which in turn provide channels for the dyke emplacements. Often the orthogonal joints are considered to be simultaneous in their development. Whenever there is dyke emplacement it acquires a magnetization parallel to the ambient geomagnetic field and therefore, a study of the magnetic expression of these intersecting dyke systems will reveal the nature of stress fields and their emplacement periods so as to constrain the tectonic evolution of the basin. Intersecting dyke sets were noticed at several locations in the N, NW, W, SW and S portions of the Cuddapah Basin and their magnetic signatures are critically examined in this study to understand the nature of stress fields. The palaeomagnetic signatures of these dykes revealed magnetic directions in unidirectional and orthogonal dyke sets indicating development of fractures due to operation of stress fields along these directions simultaneously as well as in phases. Further, it is also evident from the magnetic signatures of these dykes that there are multiple phases of dyke emplacement throughout the Proterozoic era confirming the available radiometric data on these dykes around the Cuddapah Basin.

Geoelectrical investigations for Flyover Bridge construction in marine environment of Visakhapatnam: A case study
B.Surya Prakasa Rao, V.Venkatewara Rao and N.Venkateswarlu1
Geo-Engineering, Andhra University, Visakhapatnam - 530 003
1National Geophysical Research Institute, Hyderabad – 500 007

This present study impresses the advantage of inverse slope method of interpretation of Wenner geoelectrical data for shallow investigations. In addition to geological and geomorpological data collection, geoelectrical survey has been carried out to map the depth of hard rock for pile foundations for a flyover in the Port city, Visakhapatnam. The study includes 100 geoelectrical soundings and monitoring of 252 pile borings. An excellent correlation has been obtained between the geoelectrical results and borelog data. Inverse slope method of interpretation of resistivity data is compared with curve matching technique.

Structural analysis of magnetic anomalies across Gondwana
Outlier near Tiruvuru, Krishna District, Andhra Pradesh

M.Narasimha Chary, Y.Srinivas1 and N.Sundararajan1
Atomic Minerals Directorate for Exploration and Research
Department of Atomic Energy, Begumpet, Hyderabad – 500 016
1Centre of Exploration Geophysics, Osmania University, Hyderabad–500 007, India
E- mail: sundararajan_n@yahoo.com

Geological contacts associated with an outlier in biotite gneiss and sandstones near Tiruvuru, Krishna district, Andhra Pradesh, were investigated employing magnetic method. The contacts, which are generally favourable for ground water occurrence, were precisely located based on the analysis of magnetic data by analytical signal approach and spectral analysis. Further, the data were subjected to modeling and the results are found to agree well with those obtained from analytical methods. In addition, the results were also examined by correlating the magnetic signals with known geology in order to improve the reliability of interpretation and the results are presented here.

Impact of recursive digital filtering initialization on the forecast performance of a limited area model over Indian region
A.Bandyopadhyay and S.Mahapatra
Indian Institute of Tropical Meteorology, Dr.Homi Bhabha Road, Pashan,
Pune – 411 008
E-mail: bandop@tropmet.res.in

Two versions of recursive digital filtering initialization (DFI) schemes have been applied to a limited area model to examine their impact on the model performance on short-range forecast over Indian region. For comparison, the input is also initialized by non-recursive diabatic digital filtering initialization (DDFI) technique. It is found that both the versions of recursive filter could effectively suppress the high-frequency oscillations from the forecasts. The filters are found to be more effective in controlling the noise in the first few hours of integration. The changes induced to the analysis by both the versions of recursive filters are reasonably small and are comparable with changes made by non-recursive filter (DDFI). The 24 hr forecasts from the either of the two-recursive DFI schemes are comparable with those produced from DDFI scheme and also with the forecasts obtained from un-initialized input. The main advantage of DFI is found to be its great simplicity in conception and application. Its implementation is very easy, requiring only the calculation of the filter coefficients and minor adjustments of the model code.

Location of the Dip Equator over Peninsular India
R.C.Deka, L.A.D’Cruz, V.J.Jacob, A.Iype and P.Elango
Indian Institute of Geomagnetism, New Panvel, Navi Mumbai – 410 218

The dip equator runs over only the landmass of South America, Central Africa and peninsular India apart from a small section further east of India. A ground magnetic survey was conducted to determine the absolute values of both (I) and (Z) in the southern most part of peninsular India, to know the location and migratory nature of the dip equator for the epoch 2003.7. Though no trace of dip equator was found on the said landmass even after the last possible site at Kanyakumari, based on the data of ground magnetic survey, attempts are made here to trace it statistically. A well-defined southward migration over a period of last three and a half decades is still evident.The location of the dip equator between 76°E and 78°E (Indian sector) and its migratory nature in this sector are highlighted by the ground survey data comparing with the IGRF model data along with the nature of secular trends of the vertical field from stations in the equatorial region of India.

Delineation of Fresh Water Zones in Coastal Aquifers through quality check analysis of Electrical Survey Logs
Rambhatla G. Sastry and Rahul Khanna1
Department of Earth Sciences, IIT, Roorkee – 247 667
1U.P Remote Sensing Agency, Sector-G, Janaki Puram, Kursi Road, Lucknow
E-mail: rgss1fes@iitr.ernet.in / Rambhatla_gs@yahoo.com

The separation of Short and Long Normal responses coupled with estimated Rw value delineated the fresh water bearing sands in the coastal region of Orissa, India. The log data interpretation yielded a geoelectric model of subsurface and key formation parameters. Even though Self-Potential (SP) curve is featureless, natural Gamma Ray (GR) curve has allowed estimations of shaliness (V
sh) and identification of clay beds. There seems to be a seven-layer sequence with a 37m thick fresh water zone of TDS value of 596 PPM at an approximate depth of 45m bounded by clay/ shaly sand zones. Based on derived geoelectric section, Short and Long Normal curves are synthesized by finite-difference based forward modeling. The computed logs closely match with the original data set, thereby validating the log interpretation.

Weibull and Gamma distributions for Wave Parameter Predictions
S.P.Satheesh, V.K.Praveen, V.Jagadish Kumar, G.Muraleedharan1 and P.G.Kurup2
Department of Physical Oceanography, Cochin University of Science and Technology, Cochin-682016
1Centre for Atmospheric Sciences, IIT Delhi, New Delhi-110016
2Amrita Institutions, Ashram Lane, Azad Road, Cochin-682017

Distribution of maximum wave height follows the model derived on the concept that the individual zero up-crossing wave heights follow the Weibull law. Better results are obtained when a depth factor included to accommodate shallow water attenuation effects. Certain wave height parameters such as mean maximum wave height, most frequent maximum wave height, extreme wave height, return period of an extreme wave and probability of realising an extreme wave in a time less than the designated return period are estimated and compared with computed values giving reliable results. Predicted significant wave heights are comparable with the computed value with maximum deviation of 0.21m. Theoretical and empirical supremacy of Weibull is re-established. A 100.0% empirical support is obtained in the simulation of zero up-crossing wave periods by Gamma at 0.05 level of significance. A Weibull-Gamma (with non-zero correlation) joint distribution simulates more effectively the real 3D plot.

A cost effective strategy in conducting integrated geophysical studies in trap covered country
P.Koteswara Rao and P.R.Reddy
National Geophysical Research Institute, Uppal Road, Hyderabad -500 007
Email: koteswara999@rediffmail.com, paravata@yahoo.com

Even though it is well known that seismic refraction is the most efficient technique to map the basement configuration, it is not commonly used due to the high cost involved in acquiring the seismic data. Keeping in view the importance of identifying potential hydrocarbon bearing sedimentary structures, after going through the derived success in different Trap covered terrains, it is strongly opined that a realistic data acquisition management module has to be selected to derive maximum benefit through optimal utilization of inputs. The suggested strategy, especially in the Trap covered area is as follows: To start with it is essential to look at the Bouguer/isostatic anomaly details of the area of interest to properly select the location, direction and extension of seismic refraction profiles. Once this is done, the suggested follow up action is to have Magneto-Telluric (MT) / Deep Resistivity Sounding (DRS) studies to identify broad features of potential structures, as the resistivity contrast between the Trap cover and the underlain sedimentary column is good to yield favorable results. This combined approach of having gravity and MT/DRS information prior to starting seismic studies, would then localize the area of study for seismic investigation. Similarly, the joint inversion of various data sets would narrow down the ambiguity and lead to production of better subtrappean structural details. Implementation of such a strategy has yielded very fruitful results in Saurashtra peninsula and Deccan Syneclise. Some salient sequence of events is given in this paper.

West coast rainfall and convective instability
U.S.De and Somenath Dutta1
Visiting faculty, Department of Environmental science, University of Pune.
1Meteorological office, Pune – 411 008
E-mail : udayshankar_de@hotmail.com, duttapne@yahoo.co.in

The paper examines the role of convective instability in producing spells of heavy rain at the coastal location on the windward side of the Western Ghats (WG) over the Indian west coast. For this purpose, cases have been selected, which include, days when isolated heavy rains occurred at Mumbai (Santacruz), with preceding and succeeding days reporting comparatively less rainfall. For each of these epochs, consisting of three consecutive days, we have computed Convective available potential energy (CAPE), Convective inhibition energy (CINE), Moist static energy (MSE), a non-dimensional kinetic parameter (''), vertically weighted average value of MSE (s) etc., to obtain their daily variations. A simple convective precipitation model has been developed to obtain rainfall on these days. The model has two parts. In the first part convective updraft (w) at each level, solely due to the buoyancy effect, has been computed. In the second, this ‘w’ is used to compute convective precipitation rate using the physical model (Sarker 1966) for precipitation intensity. From the study it appears that a rise in rainfall from the first day to second day is generally associated with a corresponding rise in the positive value of CAPE and the parameter '', a fall in ‘s’ and a fall in the negative value of CINE. While a fall in rainfall on the third day is associated with a corresponding fall in the positive value of CAPE. The study also shows that the proposed convective precipitation model, in general, is capable, at least qualitatively, of capturing the observed fluctuation in daily rainfall.