UsingG佃eralized RRI M e t h o dKazunobuY品仏NE 1 ), S h i n j i TAKASUGI 2 ),H e e J o o n KIM 3 ) 四d K iHaLEE 4 ) l ) G e o t h e r m a l E n e r g y R e s e a r c h a n d D e v e l o p m e n t Co. , L t d . , T o k y o 103-0026, J a p a n .2)品i[C G田thermal E n g i n e e r i n g Co., Ltd., T o k y o 103-0016, J a p a n 3
MT and AMT surveys were conducted in the Minami-Kayabe area, southern Hokkaido, over a period of about two months in the fall of 1988. In this survey, 161 sites were arranged in a square lattice shape. FFT techniques were used in the initial data processing in 1998. Here we describe improved data processing using the cascade decimation technique. Data were processed efficiently by constructing a system using coherency and remote-reference. More than 10 sets of threshold values for the coherencies of (Ex, Hy) and (Ey, Hx) were then determined. Power spectra were sorted by these threshold values and then stacked. Satisfactory results were obtained for the frequency range from 3.4 Hz to 250 Hz. At frequencies below 3.4 Hz, however, the result of cascade decimation was not satisfactory because a sufficient number of stacks could not be obtained due to high local noise and limited measurement time.
Recently the marine controlled-source electromagnetics CSEM is applied commercially to the problem of detecting the presence of hydrocarbon filled layers in the sub-sea formations, and a number of companies are now providing this service. In this paper, we return to fundamentals of electromagnetic EM method. Transmission frequencies used in the marine CSEM are typically between 0.01 and 10Hz. As such low frequencies, the behavior of EM field in subsurface is governed by the diffusion equation rather than the wave equation. The EM field radiated by an electric source can be considered to consist of two different modes a TM mode component and a TE mode component. An analysis of both the TE and TM mode holds the potential to determine if the increase in the measured EM field is caused by the presence of hydrocarbon saturated reservoir or other resistive bodies in the subsurface. The physics of CSEM in shallow water is also examined by using simple 1-D models. This would show that the concept of air interaction, rather than air wave, is appropriate because of complex coupling between signals interacting with seafloor and air. We then apply the inversion analysis to frequency-domain CSEM sounding data acquired in the offshore of Australia, investigate how the marine CSEM could reconstruct resistivity image of gas bearing formation. The survey area comprises two towed lines, covering discovered gas field. The transmitter antenna is a horizontal electric dipole of length about 250m, towing along profiles at an average of 30m above the receivers. For the inversion process, we use a starting model with homogeneous background of 1.0 ohm-m below the sea bottom. No further a priori information is utilized during the inversion. The inversion reaches acceptable data misfit, produced images of the resistive target which can be interpreted gas bearing formation.
The controlled source electromagnetic (CSEM) method may be the most significant new technology for oil and gas exploration since the development of 3-D seismic 20 years ago. The promise for the technology lies in its ability to differentiate resistive, potentially oil-bearing intervals from surrounding, more conductive water-bearing units. The principal is the same as that used in well logging devices to identify hydrocarbon zones in well bores. The technique is not new but the capability to resolve relatively thin resistive intervals in the depth domain offers new promise to lower risk through direct hydrocarbon indicators in conjunction with modern seismic methodsWe evaluated the sensing capability of the marine CSEM method to detect a thin hydrocarbon reservoir in deep, intermediate and shallow marine environments by analyzing the synthetic responses and the related physics using numerical modeling techniques. We would like to investigate the physics of marine CSEM using 1-D modeling program. The scope of investigation involved, all possible source-receiver configurations, advantages and disadvantages of frequency-versus time-domain systems, advantages and disadvantages of electric versus magnetic field measurements, a range of source-receiver separations including the coincident situation, and finally the air wave effect.
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