The Characteristic Analysis Of Forward Response And Inversion Research For Marine Controlled-source Electromagnetic Method

Based on the classical ocean 1D layered geoelectric models of MCSEM, this essay discussed the electric dipole forward response characteristics of each component, studied the forward modeling data in different transmitter orientation, field component and frequency were how to influence the resolution of the inversion for a theoretical model. In the end of this essay respectively stimulates the effects of fine-scale variational seabed and stratified seawater conductivity on the inversion models.Using TE and TM modes decomposition method and citing Schelkunoff potential to calculate respectively the electromagnetic fields generated by the electric dipole in different orientations, we can draw the following conclusions: the responses anomaly of Ey, Ez and Bx field components are more obviously than Ex, By and Bz in any transmitter orientation, air wave has little interference to Ez field component, Bz field component has no significant difference to the abnormal bodies. Analyzing the variation laws of critical offsets with the sea water depth and under sea floor conductivity knows, the depth of the sea water has played a significant role in suppression to air wave, the variation laws of critical offsets is much larger with the increasing thickness of cladding with 1Hz electromagnetic waves, the anomaly generated by high resistance in the seabed begins to disappear when the thickness of cladding more than 3000 m. MVO curves almost coincide when the product of conductivity and thickness remains the same. It is pointed out that the deeper anomalies information will be appeared when increase the offsets in the end of this section.Gauss-Newton method and Occam method are compared in the inversion research. The analysis of inversion effects using different transmitter orientation, field components and combination of some frequency shows that an inline horizontal electric dipole(HED-Y) is found to provide better resolution than either broadside or vertical electric dipoles. The horizontal electric and magnetic fields for any transmitter orientation have better resolution than vertical fields. Separate inversions of electric and magnetic fields perform equally well at recovering the abnormality, and there is no resolution improvement from jointly inverting both fields. Inverting frequencies of 0.1 and 1Hz can reach expectation well, but further increasing the number of frequency does not improve the effects much. The last simulation of Marine actual strata inversion shows that water layer had a great influence on the inversion. It is necessary to measuring sea water conductivity. The result of inversion with fine-scale variability of resistivity at sea floor can reflect the overall resistivity trend along the strata.