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Inversion Transforming the apparent to « real » resistivity. Find a numerical model that explains the field measurment.

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Presentation on theme: "Inversion Transforming the apparent to « real » resistivity. Find a numerical model that explains the field measurment."— Presentation transcript:

1 Inversion Transforming the apparent to « real » resistivity. Find a numerical model that explains the field measurment.

2 Direct prob. / Inverse problem Measure or data Parameters or model d m

3 Problem to solve Error on measurment, sub sampling Field constraints Miss choose of relevant parameters Over simplified physical model or « law » Non unicity of the solution A priori knowledge to be included Cost in time, money …..

4 Search for solution Minimise the error (y) between the measred data (d) and the reproduction of these data ( đ ) from a synthetic model (m): Least square (norm L 2 ) : Robust inversion (norm L 1 ) : (filtering ouliers)

5 The mean square approach  Linear case : solution : Non linear case : Gauss-Newton method J = jacobian matrix : Initial model :

6 Gauss-Newton / Quasi Gauss-Newton

7 Damped inversion « damped LS » (Marquardt-Levenberg or Ridge regression) : « Smoothness constraint » : C : « smoothing matrix »

8 Damping factors

9 Initial damping factor : – minimum damping factor : (valeurs par défaut) Initial damping factor : – minimum damping factor : (inversion non amortie)

10 Smoothness constraint NO YES

11 « Blocky » inversion Taking into account sharpe changes of resistivity in the model : R m and R d : matrix ginving an independant weigth of data and model in the inversion processus.

12

13 Initial model

14 Model discretization for forward modelling

15 Model discretization

16 1&2 1&

17 Topographical correction


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