March, 2017 || Volume 21 No.2
Modeling of Marine Magnetotelluric Response across 85°E Ridge: A Numerical Simulation
K.S.N. Murthy1,2, K. Veeraswamy*1 and Prasanta K. Patro1
1CSIR – National Geophysical Research Institute, MT-DRS Group, Hyderabad, India
2Geological Survey of India, Training Institute, Hyderabad, India
*Corresponding Author: email@example.com
85°E Ridge in the Bay of Bengal region is one of the most interesting and enigmatic geotectonic features in the Indian off-shore region with its surprisingly low free-air gravity anomalies. As the marine field studies are highly expensive, it is proposed here, to simulate Marine Magneto Telluric (MMT) response across this ridge to estimate the resolvability of layer parameters like thickness and resistivity followed by period band to penetrate the signal to the desired depth using forward modeling. Similar to land MT measurements, the MT data acquired in the marine environment also gets distorted due to coast effect. In order to derive this response, a synthetic initial model has been considered which extends to 800 km on either side of the coast line and down to 600 km depth with a water column of 4 km. A finite element algorithm has been utilized to accommodate inclined continental shelf in the numerical model. In the period range of 40-4000 sec, Ridge response is well reflected in the apparent resistivity, phase, magnetic field components (Hx, Hy), and Tipper in TE and TM modes, whereas electric field components (Ex, Ey) are less pronounced. The present study reveals that the distortion in MMT responses due to coast effect is noticeable up to 200 km from the sea-land boundary for periods between 10 and 4000 sec. As the 85°E Ridge is located at a distance of 500 km from the coast, the MMT measurements made here are free from coast effect. Data needs to be acquired for a period of at least 8 days to get more than 10 stacks of 4096 sec and therefore for better results.
Key words: 85°E Ridge, Magnetotellurics, Finite elements, Marine Electromagnetics, Coast-effect, Northeastern Indian Ocean.
Inversion of well log data using improved shale model for determination of petrophysical parameters
Ajay Malkoti*1, Nimisha Vedanti1 and A.K. Bhattacharya2
1Academy of Scientific and Innovative Research, CSIR-NGRI, India-500007
2 Indian Institute of Technology, Kharagpur, West Bengal, India-721302
*Corresponding Author: firstname.lastname@example.org
Estimation of petrophysical parameters from well logs is an important procedure in reservoir characterization. Here we present a method, based upon improved estimates of average mineralogical composition of shale, to estimate the petrophysical parameters from the well log data of a sandstone reservoir. The shale response is dependent of its distributional characteristics which is difficult to model. Shale response has a large influence on the inversion algorithm, which affects the parameter estimation. Thus to mitigate its effect in forward modelling we used the improved estimates of an average shale-mineralogical-composition model (SMCM). A genetic algorithm (GA) based inversion was carried out for correct estimation of petrophysical parameters. This improved algorithm was tested for the applicability on the synthetic data and then applied on - well log data of Ankleshwar field, cambay Basin, India. The result for synthetics exibited good match with the assumed model and worked well even in presence of large noise. In Ankleshwar reservoir, parameters estimated using this method were compared with the industry provided values and with earlier studies. It was found that our results are in good agreement with them. The average error between the ankleshwar well data and its synthetics, generated for inverted parameters, was found to be about 6.27%. Major advantages of this approach are mitigation of cumulative error, enhanced resolution and capability to generate missing logs. This algorithm has also demonstrated its capability in delineating the finer details of the formation.
Key words: Inversion, Genetic Algorithm(GA), shale-mineralogical-composition model (SMCM), Well-log data, Ankleshwar field.
Geotechnical investigations in the southern part of Ahmedabad district, Gujarat, India
Sarda Thokchom*1, B.K. Rastogi1, N.N. Dogra2 and Vasu Pancholi1
1Institute of Seismological Research, Gandhinagar, Gujarat, India
2Department of Applied Geology, Kurukshetra University, Kurukshetra, India
*Corresponding Author: email@example.com
Geotechnical investigations of soil consisting of surface and subsurface studies are performed in the southern part of Ahmedabad district in Gujarat state of western India. Surface investigations include detailed geological and geomorphological mapping, whereas subsurface investigations involved selected drilling, soil sampling and laboratory analysis of the samples. About 700 samples were collected at every 1.5m depth interval from 64 boreholes, each of about 50 m depth for soil classification. The study area is covered by alternate layers of fine and coarse grained sediments. We used Standard Penetration Test (SPT)-N values, Soil Bearing Capacity (SBC) and Soil Bearing Pressure (SBP) for soil classification. The SPT-N values are noted during drilling while the SBC and SBP are estimated from the soil properties. The measured soil properties both physical and mechanical include: density, specific gravity, permeability, stiffness, strength, grain size analysis, liquid limit, plasticity index and shear strength parameters (Cohesion co-efficient and Friction Angle). A relationship has been obtained between SBC and N-values derived from the above studies.
Key words: Geotechnical investigation, Engineering geology, Geotechnical properties, Surface exploration, subsurface exploration.
Assessment of Liquefaction potential of soil in Ahmedabad Region, Western India
V.K. Dwivedi*1, R.K. Dubey2, S. Thockhom1, V. Pancholi1, S. Chopra1 and B.K. Rastogi1
1Institute of Seismological Research, Raisan, Gandhinagar
2Indian School of Mines, Dhanbad
*Corresponding Author: firstname.lastname@example.org
Estimation of liquefaction resistance, also called Cyclic Resistance Ratio (CRR) of soil, is an important aspect of geotechnical earthquake engineering; it is one of the most important secondary effects of earthquake which causes severe damages to engineering structures. The liquefaction potential is estimated in terms of factor of safety (FS). The present study involves evaluation of liquefaction potential of soil in the Ahmedabad city area, which dominantly consists of sandy to silty sediments and is witnessing significant constructional activities in recent times. Standard Penetration Test (SPT) data from 23 boreholes are collected for analysis. The SPT N-values were corrected for the estimation of CRR. Results of the study indicate that the soil in Ahmedabad city area is much compact, water table is much low and SPT- N values are high, which imply that liquefaction potential is much low. However, it is advisable to perform site specific detailed geotechnical investigation in case of high rise structure and or heavy engineering structures.
Key words: Geotechnical, SPT N-value, Cyclic Resistance Ratio (CRR), Cyclic Stress Ratio (CSR), liquefaction susceptibility, Factor of Safety (FS).
A note on the origin of Clinopyroxene megacrysts from the Udiripikonda lamprophyre, Eastern Dharwar Craton, southern India
Ashutosh Pandey, Rohit Pandey, Dinesh Pandit, Praveer Pankaj and N.V. Chalapathi Rao*
Electron Probe Micro Analyzer (EPMA) Laboratory, Department of Geology
Banaras Hindu University, Varanasi-221005 India
*Corresponding Author: email@example.com
Abundant sub-rounded to sub-angular and centimeter-sized clinopyroxene megacrysts constitute a conspicuous feature of the Udiripikonda lamprophyre, located in the Eastern Dharwar craton, Southern India. These clinopyroxene megacrysts, at times, are also associated with minor amounts of biotite. The megacrysts lack reaction-rim or any other disequilibrium textures generally displayed by crustal and mantle xenocrysts/xenoliths entrained in such volatile-rich magmas. Cr2O3-impoverished (< 0.1 wt%) nature of the clinopyroxene megacrysts preclude them from being chrome-diopside, derived from the disaggregation of upper mantle rocks, and commonly found entrained in kimberlites,. The clinopyroxene megacrysts (Wo47.43- 49.20En32.44- 33.64 Fs13.73- 15.03; Ac3.32- 4.69) and associated biotite (Mg#: 0.84- 0.90) are compositionally similar to the clinopyroxene (Wo43.68-47.76; En37.47-44.58; Fs8.36 – 12.31; Ac2.70- 3.38) and biotite (Mg#: 0.84- 0.88) occurring as liquidus phases within the host lamprophyre. Clinopyroxene barometry reveals an overlapping pressure estimates for megacrysts (9.8 to 12.4 kbar) and phenocrysts (8.4 to 10.1 kbar). Likewise, the Ti-in-biotite geothermometry also suggests an overlapping temperature range of 957oC to 1097oC and 904oC to 1069oC for megacrystal suite and phenocrysts respectively at pressure of ~10 kbar. The clinopyroxene ± biotite megacrysts of this study are, thus, inferred to be cognate products which crystallized under high- to medium-pressure conditions during the evolution of lamprophyre magma.
Key words: Clinopyroxene, Megacryst, Lamprophyre, Udiripikonda, Dharwar Craton, Southern India.
Bedrock Structural Controls on the Occurrence of Sinkholes : A Case Study from Chintakommadinne Area, Part of Cuddapah Basin, South India
M. Prasad1, M. Ramakrishna Reddy1 and V. Sunitha*2
1Dept of Earth Sciences, Yogi Vemana University, Kadapa, Andhra Pradesh
2Dept of Geology, Yogi Vemana University, Kadapa, Andhra Pradesh
*Corresponding Author: firstname.lastname@example.org
Heavy precipitation in the Chintakommadinne area in the Y.S.R Kadapa District, Andhra Pradesh during November 2015 has induced ground subsidence resulting in rapid increase in the frequency of 15 sinkhole occurrences. Most susceptible to future sinkhole development, determined by different factors: (1) bedrock type has the most significant impact on predicting sinkhole risk, (2) proximity to faults, lineaments (3) sudden recharge and raise in the groundwater table, and (4) depth of overlying soil to bedrock has an existent yet insignificant effect on sinkhole development. Alignment of sinkholes mainly occurs along strike of bedding. Enhanced rock solution and conduit formation correlates with carbonate units of greater limestone purity and finer grain size, suggesting some lithologic control on karst formation. Density of sinkhole occurrence tends to increase in areas where water-table fluctuations are large. Given the correspondence between geologic structures such as faults, geologic maps showing such structural data are useful tools for predicting future sinkhole occurrence.
Key words: Sinkholes, Structural control, Karst Topography, Buggavanka River, Cuddapah Basin.
Determination of Urban Dust Signatures through Chemical and Mineralogical Characterization of Atmospheric Dustfall in East Delhi (India)
Anshu Sharma, Saumya Singh and U.C. Kulshrestha*
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067
*Corresponding Author: email@example.com
The fine road dust particles contribute a dominant fraction of ambient concentration of air particulates in urban areas. The road dust carries toxic pollutants such as heavy metals and polyaromatic hydrocarbons (PAHs) etc. exerting a significant influence on air quality. This study is aimed to analyze the chemical, mineralogical and morphological characteristics of the dustfall deposition during the period from November 2013 to February 2014 in east Delhi. The research findings of this study confirm that the vehicular traffic is a significant cause for high deposition flux of the dustfall resulting in a possible health hazard for roadside population. Average dustfall flux was observed to be 310 mg/m2/day and the average pH of aqueous extract of dustfall samples was noticed as 7.7. SEM results showed the dust particles were irregular in shape representing spherical, rectangular, platy and angular shapes. Their diameter ranged from 1.5-151.5µm. Ca, Si, and Al were the major contributors in dust indicating that the dustfall is weathered material of local rock which consists of calcium aluminum silicates. Other heavy metals such as Zn, Cu contributed primarily from vehicular traffic while Fe, Al, and Mn mainly from suspended road dust. C particles seen in the dustfall samples might be due to interaction of atmospheric dust with industrial and vehicular emissions.
Key words: Road dust, vehicular traffic, SEM-EDX, heavy metals, dustfall.
Winter precipitation climatology over Western Himalaya: Altitude and Range wise study
M S Shekhar*1, N Narasimha Rao1, Surendar Paul2, S C Bhan3, G P Singh4 and Amreek Singh1
1Snow and Avalanche Study Establishment, Research and Development Centre, Sector 37 Chandigarh - 160036, India
2 India Meteorological Department Chandigarh - 160036, India
3India Meteorological Department, Mausam Bhawan, Lodhi Road, New Delhi- 110 003 India
4Department of Geophysics, Institute of Science, Banaras Hindu University, Varanasi-221005 India
*Corresponding Author: firstname.lastname@example.org
An Attempt has been made in this study to find the Climatology of winter precipitation over western Himalaya region (WH). Characteristics of Precipitation Variation Trends (PVT) with different altitude regions and Himalayan ranges have been analysed by using monthly cumulative precipitation data from 81 surface observatories of western Himalaya from 1971-2013. With respect to elevation, western Himalaya is divided into three altitude regions and four Himalayan ranges. The altitude regions considered for study are below 1500m, 1500-3500m and above 3500m respectively. Results show that more than 80 % of the stations under study indicates strong negative precipitation tendency rate over western Himalaya. Simple linear regressions have been used for trend analysis. The statistical significance of winter precipitation trends has also been studied by using Mann-Kendall method. The results confirm the decreasing trends in winter precipitation there by indicating the impact of climate change and precipitation variability in western Himalayan region.
Key words: Precipitation Variation Trend, Western Himalaya, Climatology, Mann-Kendall test, Range and altitude.
Selection of optimum wavelet in CWT analysis of geophysical downhole data
Amrita Singh*1, Saumen Maiti1 and R.K.Tiwari2
1Department of Applied Geophysics, Indian Institute of Technology (Indian School of Mines), Dhanbad-826004, India
2CSIR-National Geophysical Research Institute, Hyderbad-500007, India
*Corresponding Author: email@example.com
Continuous wavelet transform (CWT)-based scalogram analysis is appropriate for the modelling of discontinuous well log signal. The choice of appropriate mother wavelet in scalogram analysis is crucial to model the geophysical well log data precisely. Here, we examine some of the key points related to the choice of optimum wavelet for the analysis of well log signal. We used the scalogram analysis to detect the formation interface and explored the impact of each mother wavelet. Following three key indicators, e.g. (i) histogram analysis of CWT coefficient at even number scale (ii) statistical significance test of each sub- signal at even number scale and (iii) Principal component analysis (PCA) of those sub-signals are used. These key steps help to identify the precise localization of formation tops detection problem of well log data of KTB borehole, Germany. Here, five mostly used wavelet functions (Haar, Gaussian wavelet of order 1 (Gaus1), Gaussian wavelet of order 3 (Gaus3), Morlet and Daubechies wavelet of order 2 (Db2)) are employed on two sets of data: (i) spectral gamma ray (SGR) log in pilot hole and (ii) density (RHOB) log in main hole data. The comparative results suggest that Gaus1 is better among all mother wavelets rendering maximum number of occurrences of CWT coefficients in the total well log signal modelling. Statistical significance test shows that CWT coefficients and their respective scales are statistically important. The results based on PCA analysis further suggest that Gaus1 wavelet is in good agreement with the gross structure of the signal.
Key words: Continuous wavelet transform, PCA analysis, Histogram analysis, Significance test, Formation interface, KTB.