Brute electric force method

We denote by the element of the vector corresponding to the r, -th receiver position. It can be treated as an electric field, generated by an electric source Sae, located in the cell Vq. The Frechet derivative matrix, F, is formed by the components 6ej /6a. Therefore, the direct, brute force method of computing the Frechet matrix would require Nm forward modeling solutions for each inversion iteration. 

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