CONTENTS OF OCTOBER, 2012, Vol.XXXIII   No.4

 

Seismic Veloctties and Estimation of Gas - Hydrates Across A BSR from AVA Modeling in the Western Continental Margin of India.

Maheswar Ojha and Kalachand Sain

 

Sea Bed Logging – A Tool for Risk Mitigation in Hydrocarbon Exploration.

R. K. Khanna and P. S. Rao

 

Direct Detection of Hydrocarbon by Using Marine Controlled Source Electromagnetic Sounding.

Samir Kumar Dhar and Jayanta Basu

 

Acoustic and Electrical Logging for Evaluation of Resistivity and Shear and Compressional wave Velocities of Foundation of Kakrapur Nuclear Power Plant.

R.K. Kamble, C.K.Rani, N. Ghosh and G.A. Panvalkar

 

A Rigourous Geostatistical Analyses of the Self Potential Signals to Delineate the Contaminant Plumes.

Tanvi Arora, Shakeel Ahmed and André Revil

 

Next Tsunami in India in the Arabian Sea ?

R.K. Jaiswal and B.K. Rastogi

 

 

Seismic Veloctties and Estimation of Gas - Hydrates Across A BSR from AVA Modeling in the Western Continental Margin of India

 

Maheswar Ojha and Kalachand Sain
National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007
E-mail: maheswar_ojha@yahoo.com

 

 Abstract

   The bottom simulating reflector (BSR) commonly used marker for gas-hydrates investigation, has been identified by seismic experiment in the western continental margin of India (WCMI). Seismic velocities hold a key to understand the origin of BSR and for quantitative assessment of gas-hydrates. Here we show a new approach of Amplitude Versus Angle (AVA) modeling to estimate seismic velocities across various reflectors including the BSR in the WCMI. The result reveals the P-wave velocity of 2.25 and S-wave velocity of 0.985 km/s above the BSR. This corresponds to a Poisson ratio of 0.382 and hydrate saturation of ~30%. The comparison of estimated P-wave velocity (1.77 km/s) above the hydrated sediment to that (1.78 km/s) below the BSR implies that the origin of BSR is mainly due to gas-hydrates.

 

 

 

Sea Bed Logging – A Tool for Risk Mitigation in Hydrocarbon Exploration

 

R. K. Khanna and P. S. Rao
Oil and Natural Gas Corporation Limited, Mumbai
E-mail: psrao56@gmail.com

 


Abstract

   The resistivity of the hydrocarbon saturated formation is significantly higher than the surrounding non-hydrocarbon bearing rocks. This property of hydrocarbon saturated rocks is made use of in identifying the subsurface resisivity anomalies using the Sea Bed Logging technology as a tool and accordingly the survey is designed. Large current of the order of thousand amperes generated onboard a vessel is discharged through the electrodes kept apart by few hundred meteres (source). The refracted electrical energy from the subsurface layers and the induced magnetic energy in the earth’s magnetic field are measured by the receivers spaced at regular intervals along a profile while the source is moving from one end to the other end of the profile. The variations in the refracted electrical and magnetic energy measured and compared with that of a reference receiver kept closer to the survey area. The recorded data is plotted in amplitude Vs offset and phase Vs offset. Anomalies observed in the plots are carefully analysed and integrated with available seismic data to reduce risk, thereby improve the success ratio of the drilled locations.

 

 

 

Direct Detection of Hydrocarbon by Using Marine Controlled Source Electromagnetic Sounding

 

Samir Kumar Dhar and Jayanta Basu
Oil and Natural Gas Corporation Limited, Gujarat

 

Abstract

 

   Marine Controlled Source Electromagnetic Sounding (CSEM) is a geophysical exploration method which can identify buried resistive layers underlying deepwater conductive sediments. It is known from conventional petrophysics that hydrocarbon saturated sediments is having higher resistivity value than water saturated sedimentary rock. Therefore accurate identification of resistive sedimentary layer at the subsurface may detect directly a hydrocarbon pool. The present work addresses the problem of identifying the resistive layer by interpreting the CSEM response. Selection of suitable field specific data acquisition parameter is another point of concern in this work, to achieve noise free interpretable quality CSEM data.

 

   In this work, an attempt has been made to determine the field specific data acquisition parameter for a deep water exploration block situated at eastern coast of India. A set of synthetic model response curves are generated by simulating different possible values of acquisition parameters to select their applicable range of values for data acquisition. A 2D petrophysical forward model is constructed by considering the available geological information of the exploration block. CSEM response is then simulated over this field specific model with realistic data set. The forward model response clearly shows the presence of resistive layer buried in the conductive host rock. The response anomaly is further enhanced with increase in the resistivity contrast between the layers. The validity of the modeled response is compared with an actual CSEM response curve acquired from a field where the input geological information is kept same as that of the field. By knowing the exact cause of increase in resistivity within the sediments, one can conclude about the nature of fluid present within an entrapment.


 

 

Acoustic and Electrical Logging for Evaluation of Resistivity and Shear and Compressional wave Velocities of Foundation of Kakrapur Nuclear Power Plant

 

R.K. Kamble, C.K.Rani, N. Ghosh and G.A. Panvalkar
Central Water and Power Research Station, Pune – 411 024
E-mail: kamble_rk@cwprs.gov.in

 

Abstract

 

   The construction of unit 3 and 4 of Kakrapar Atomic Power Project (KAPP) was proposed on the Deccan traps of Cretaceous age, comprising of mostly amygdaloidal basalt overlain by about 7 – 8 m thick silty soil. Electrical resistivity and shear and compressional wave velocities of subsurface formation were required to be assessed in situ for design of foundation of crucial structures like turbo-generator and reactor building of KAPP units 3 and 4. This paper deals with acoustic and electrical logging at KAPP for assessing nature of foundation and determination of in-situ shear and compressional wave velocities of subsurface rock. The resistivity and shear and compressional wave velocities thus determined were utilized for design of crucial structures of KAPP.


 

 

A Rigourous Geostatistical Analyses of the Self Potential Signals to Delineate the Contaminant Plumes

 

Tanvi Arora, Shakeel Ahmed and André Revil
*IFCGR, National Geophysical Research Institute, Hyderabad 500007
**CNRS-CEREGE, Dept. of Hydrogeophysics and Porous Media, Aix-en-Provence, France

Abstract

 

   Accurate mapping of the redox potential and the electrical conductivity of groundwater is important in delineating the shape of a contaminant plume. Organic matter- rich contaminant plumes, e.g., associated with leakages from municipal landfills, are the source of electrical potential variations at the Earth’s surface termed self-potential anomalies. The plume can be regarded as a natural geobattery, in which the source current results from degradation reactions of the organic matter by the growth of micro-organisms. The self- potential depends on groundwater flow (electrokinetic contribution) and redox conditions (electroredox). The electrical source associated with this electroredox process adds to the contributions associated with (1) the electrokinetic conversion of ground water flow and (2) membrane or diffusion potentials associated with concentration gradients of the ionic species present in the pore water. Once removed the electrokinetic contribution, a correlation between in situ redox potentials measured in piezometers and the electrical potential measured at the ground surface using non-polarisable electrodes exists. A geobattery model can explain how organic matter-rich contaminant plumes behave as geobatteries and are the source of a natural electrical field. This field can be recorded at the ground surface and used to map the redox potential of the leachate plume using the self-potential method as an efficient non-intrusive technique. Using the variation of the piezometric head in the aquifer, the electrokinetic contribution is removed from the SP signal. A good correlation is obtained between the residual SP data and the redox potential values. This relationship is used to draw a redox potential map over the overall contaminated site.

 

 

 

Next Tsunami in India in the Arabian Sea ?

 

R.K. Jaiswal and B.K. Rastogi
Institute of Seismological Research, Sector 18, Gandhinagar-382 018, Gujarat
E-mail: brastogi@yahoo.com

Abstract

 

   Future source zones of earthquakes that can generate tsunamis in the Arabian Sea are identified based on past seismicity and gap areas along subduction zones and zones of compression. These zones are Makran coast, Indus Delta and Kutch-Saurashtra region.

 

   The Makran subduction zone of Iran and southern Pakistan, situated on the northwestern side of the Arabian Sea, generates great but infrequent earthquakes, mud volcanoes and tsunamis. Indus delta of Pakistan has given rise to large earthquakes in the past. Kutch-Saurashtra region of India has prominent coastal faults. Past great and large earthquakes in Kutch have possibly generated local tsunamis. Based on the long-term assessment of large earthquakes in these regions it is inferred that tsunamigenic earthquakes can occur in near future in these regions.

 

 

 

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