Remarkable progress has been made in electromagnetic (EM) techniques as applied to the petroleum industry in instruments, data acquisition, and processing and interpretation in China. Included here is equipment, such as high-power Controlled Source EM (CSEM) acquisition systems, acquisition methods, such as the three dimensional small-bin Continuous Electromagnetic Array acquisition method, Time and Frequency Domain Controlled Source Electromagnetic, Borehole-to-surface Electromagnetic technique and marine magnetotelluric method. Data processing methods, such as fast three dimensional inversion using nonlinear conjugate gradients, and data interpretation methods, like Induced Polarization and Resistivity anomalies for hydrocarbon detection, are also included. These new techniques have been applied in petroleum survey and many cases are in complicated areas. They have successfully served the investigation of deep igneous rock reservoirs, and prediction of potential hydrocarbon targets. The cases indicate that electromagnetic techniques can help seismic survey to effectively detect hydrocarbon reservoir and remarkably improve drilling successes.

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We present a new technique for inverting magnetotelluric sounding (MT) data by introducing the quadratic function approaching scheme, which is originally used in nonlinear optimization, to MT data inversion. The quadratic function approaching method (QFAM) takes the advantage that the quadratic function has a single extreme value. It avoids leading to an inversion solution for local minimum and ensures the solution for global minimization of the objective function. The method does not need calculation of sensitivity matrix and not require a strict initial earth model. In this study, the coefficients of quadratic function used for the inversion are calculated dexterously by using affine transformation and by adopting an effective method for selecting control points. Examples for synthetic data and field measurement data indicate that the proposed inversion method is effective.

[Abstract] The Directly Iterative Finite Element algorithm (DIFE) is developed to calculate the electromagnetic responses at earth surface excited by a line current source. The directly iterative formulation is deduced first, then iterative method is adopted to solve these FE equations. The compound griding method including re-interpolation scheme at the adjacent points has been realized to denser local grids near the source point, thus the precision of field calculation near the source point is assured. An iterative approach has put forward to obtain the whole range apparent resistivity from the electric field response excited by line current source. This approach has been verified through model calculations of uniform half space, layered earth and two dimensional earth model. For all these models, apparent resistivity curves very similar to magnetotelluric responses are obtained. It shows in theory that the frequency domain sounding at near zone of the line current source is possible and effective. [Key words]Directly iterative finite element algorithm (DIFE), Line current source, Electromagnetic response, Whole range apparent resistivity.

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