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The marine-controlled source electromagnetic method (mCSEM) is employed for studying the electrical characteristics and fluid contents of sedimentary reservoirs. However, the success rate of the method can be improved significantly by finding the sources of electromagnetic noise and addressing the challenge posed by them at larger offsets where the reservoir signal is often weak. For the need, I have first developed the theory of natural creation of electromagnetic signal by the dynamics and then later studied the data for the quantification.
The electromagnetic modes by the horizontal and vertical dynamics, is considered as a separate case. The toroidal velocity field is found to excite only the Tangential Magnetic (TM) mode. On the other hand, the poloidal velocity field excites the Tangential Electric (TE) mode. Moreover, the role of electrical charges, implication of a particle trajectory during the wave movements, and role of the damping in governing the field responses are also presented in detail. It has been found that the smaller waves of dominant horizontal motion experience only the geometrical damping. On the other hand, the waves greater than 2 km primarily attenuate by the EM damping.
Practical mCSEM data is studied and a significant electromagnetic noise is found. The strength of the noise is noted 1600 times stronger than the seafloor mCSEM signal at 0.1 Hz. The noise and the transmitted mCSEM signals are found coherent in inter-station recordings. The role of infragravity wave and microseisms is investigated. Microseism causes oscillation of the seafloor and produces electromagnetic disturbances by the dynamics of water. I have used various conditions for a proper discrimination of the noise as microseisms. This mechanism is clearly illustrated with the help of a conceptual diagram. The role of the directionality is part of the study, which is argued for having a significant role in the generation of microseisms. In this talk, a new algorithm will be presented, which is used for calculating the coherency.
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