Real geological formations may exhibit resistivity anisotropy in two ways: micro-anisotropy and macro-anisotropy. Micro-anisotropy is intrinsically anisotropic because of the microstructure of the formation. However, macro-anisotropy is often due to electrical and electromagnetic -well logging methods can achieve only limited resolution of the resistivity layering, for example, we often have to consider a collection of many thin layers as one composite layer, which is then macro-anisotropic. Macro-anisotropy is also found in cases of a fractured formation. In this paper, it is assumed mat the resistivity is the same in all horizontal directions, but is different in the vertical direction, i.e. a transversely isotropic layered model. The determination of resistivity anisotropy is desirables as it may indicate the presence of otherwise unresolved thin layers and fractured formations, From a hydrogeological point of view, these may severely influence me hydraulic flow pattern in the ground. Thin clay layers in an otherwise sandy formation will lower the vertical hydraulic conductivity considerably and will deflect infiltration, and thin sand and gravel layers in an otherwise clayey formation may be serve as fast hydraulic conduction channels for polluted water. Three-component induction well logging may be the best method to determine the resistivity anisotropy. However, the tool has still not used in China. All data are typical lateral and dual induction logs. Neither lateral well logging methods nor inductive well logging methods alone can resolve the anisotropy of the formation. However, a joint inversion of lateral and inductive data makes that anisotropy be taken into account and it can also resolve the coefficient of anisotropy, thus contributing to a more detailed description of the formation resistivity. In this paper, an analysis of the importance of taking anisotropy into account in inverse modeling is presented, and it is shown how the combined use of lateral and inductive logs can resolve the coefficient of anisotropy of a formation. Through a synthetic 2D model, we show that inductive methods will only sensitive to the horizontal resistivity of a layer, while the thickness is undistorted. That is to say, we can determine the horizontal resistivity and formation thickness by inductive methods, but we can not determine the vertical resistivity by inductive methods, thus we cannot determine the coefficient of anisotropy. However, apparent resistivity of lateral methods can be approximate as the geometry mean resistivity of horizontal and vertical resistivities. And apparent thickness of lateral methods is the multiplication of anisotropy coefficient and real thickness. Therefore we can not determine any parameters alone by lateral logs. However, a joint inversion of data from lateral and inductive logs may determine three parameters: the coefficient of anisotropy, horizontal resistivity and formation thickness. Synthetic data show that the joint inversion method is feasible.
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