Study On Pseudorandom Marine Controlled-Source Electromagnetic Sounding With Multi-offsets

The pseudorandom multi-frequency marine controlled-source electromagnetic (PRMF-mCSEM) method was studied in this thesis. Two types of pseudorandom multi-frequency waveform were adopted as source signal. Based on correlation identifyication the detection program for seafloor hydrocarbon reservoir both in frequency domain and in time domain was proposed.The promising marine CSEM exploration for seafloor hydrocarbon reservoir was mainly surveyed in frequency-domain with one or a few of discreted frequency now. The seafloor electrical conductivity was then maped by numerical modeling. And only the seafloor electrical conductivity was used to identify the seafloor hydrocarbon reservoir. So the pseudorandom binary signal with broadband frequency was adoped for mrine CSEM exploration in this thesis. The electric field response was simultaneous observed with multi electric dipole receivers configuring inline with source dipole. The frequency characteristic and impulse response of seafloor geoelectric system was correlation identified. So the seafloor thin resistor can be detected both in time domain and frequency domain. The transient impulse peak time can be an indicator for seafloor conductivity in time domain. The percent frequency effect, differentiation of percent frequency effect with offset and differentiation of relative phase with offset computed with multi frequency electric field response can be used to detect the seafloor thin resistor in frequency domain. So the seafloor hydrocarbon reservoir can be identified by multi parameters. The result was derived from 1D numerical modeling in this dissertation.The main works were listed as following:1. Based on correlation system identification principle, aimed at the shortage of variable-frequency method of mCSEM, two type of pseudorandom binary signal were selected as source signal for their multi-frquency and very good correlation function property similar with white noise. The step response and impulse response of the marine geoelectricity can be correlation identified with high resolution using the invert-repeated m binary sequency. Some kinds of random disturbance can be surpressed very good by properly select the signal generation parameter and map the seafloor electrical conductivity with multi parameters.2. The step response and impulse response of homogeneous earth half space , marine double half space and multi-layer earth models with multi-offsets to horizontal electric dipole source were computed using Gaver-Stehfest inverse Laplace transforms and numerical integration Hankel transforms. A concept of transient impulse time was put forward. It is confirmed that the transient impulse time of the conductive medium to horizontal electric dipole can directly indicate the conductivity of the earth. Inline dipole-dipole transient EM sounds with geometry, it is suitable survey with multi offsets. Multi receiver dipoles and transmitter dipoles can be laid in line with different space. So it is sensitive to both thin resistor and good conductor. The differentiation of transient impulse peak time with offset is more sensitive to thin resistor. The effect of sea water depth and transverse resistivity equivalent on transient impulse time was also analyzed.3. Based on Cole-Cole IP model, the IP response of seafloor hydrocarbon reservoir is analyzed. Surveying with wideband source and multi offsets electrical dipole-dipole configuration the percent frequency effect (PFE) was computed using two complex resistivities of a high and a low frequency, and differentiation of low frequency CR with offset was computed. The numerical result implies that when exciting source is a wideband signal such as invert-repeated m binary sequence, and simultaneous observation with multi offsets inline dipole-dipole configuration, the differentiation of percent frequency effect with offset(DPFE) and differentiation of low frequency CR with offset as property parameters can indicate whether or not a high or a low resistor induced polarization thin abnormality is present. The low frequency grounded dipole can generate galvanic current which is effective for detecting the deeply buried thin resistor. Simultaneous observation with multi offsets inline dipole-dipole is a geometry sounding method. If a high resistor induced polarization (IP) body is present, the differentiation of low frequency CR with offset or the differentiation of Percent Frequency Effect with offset(DPEF) will have a peak value at appropriate offset, this offset can be used to estimate the buried depth of the resistor IP body, it is empirically about 2.2 to 3.5 times of the buried depth. This method can be used for field data processing.4. The function definitions for PRMF-mCSEM survey system instrument based on pseudo-random correlation identification is designed in this paper. A novel signal generation cell, GPS synchronization program and recervers underwater positioning and orientation also were introduced. The 2~n series pseudo-random multi-frequency signal and invert-repeated m-sequence were generated by hardware using the complex programmable logic device LC4256V-100T from Lattice semiconductor and the tool of hardware description language (VHDL). It can produce different waveforms, and has twenty-five frequency options which divided from four dense frequency ratio initial frequencies. The generated 2~n series signal contains up to thirteen frequencies. And the m-signal and invert-repeated m-signal can be generated by up to 32 shift registers. There are up to 104 clock frequencyes. This design will meet the demand of application for mCSEM. The advising and next work is also mentioned in conclusion.