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An inversion algorithms was suggested for array induction logging curves. Three parameters, true-formation resistivity, invasion depth and invasion-zone resistivity, were obtained simultaneously from the present inversion calculation using various forward algorithms. The inversed algorithm using numerical mode-matching method as forward calculation can yield stable and satisfied results. The inversion using geometry factor theory as forward calculation can save more computer time. Synthetic formation models and site applications illustrated the efficiency of the present algorithm. Introduction Saturation of hydrocarbon in a reservoir is an important parameter during oil exploration; but it can not be measured directly in site and it relates to formation resistivity [1]. Formation resistivity is usually determined from resistivity logging devices such as traditional dual-induction tool that can only provide two responses with mediumand deepdetective depths [1] ; or new array induction tool [2], which can record five logging data with five investigating depths simultaneously [3-5]. However, the resistivity measurement was influenced by environmental effects such as borehole, mud, invasion, shoulder bed, and so on. Hence, the resistivity logging data would deviate from the true-formation resistivity. Inversion technique is an efficient method to eliminate environmental influence as far as possible and obtain the true-formation resistivity from logging data. AIT measurement is influenced by environmental effects also, so that the logging responses deviate from the true-formation resistivity. In order to eliminate the environmental influence and obtain the true-formation resistivity, a fast inversion technique was suggested in the present study. Since the inversion calculation was dependent on the forward algorithm, two forward calculation algorithms, geometry factor (GF) theory [1] and numerical mode-matching (NMM) method[6-8], were used in the inversion calculation respectively. Forward Algorithms for AIT Response A two-dimensional axisymmetric formation model was considered in the present calculation. In the direction of vertical to the ground, the formation was divided into N layers. After the porous formation was opened, the drilling-mud filtrate pours. The native fluids, water and hydrocarbon, were displaced by the filtrate under the pressure differential between borehole and formation. Thus, an invasion zone, with radius rh and mud resistivity Rm, formed near to the borehole. The present model included borehole with radius rh and mud resistivity Rm, formation with thickness H and true-formation resistivity Rt . There was a shoulder with resistivity Rs between reservoirs. AIT apparent responses Ra can be obtained from geometry factor theory:

AIT Inversion and Site Applications Using Various Forward Algorithms