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How to correctly obtain the structure parameters of earth is the precondition of grounding system design. Based on the local searching algorithm, an efficient method for estimating earth structure with any number of layers from the test data by Wenner configuration method is presented. In each iterative procedure, earth apparent resistivities are calculated by complex image method. A new method to directly obtain the partial derivatives of the earth apparent resistivities with respect to the earth parameters from the results of the complex image method is presented, which is more efficient than the conventional methods that arduously calculate the improper integrals with a Bessel's function and an infinite limit of integration. At the same time, a method to determine suitable initial earth parameters for the local searching algorithm is also proposed. The validity of the method is shown by several applications.