Method for estimating elastic parameters of subsoil

1. A method for estimating elastic parameters in a subsoil region, the method comprising:

representing the subsoil region using a grid having columns of cells arranged in an array of horizontal positions, wherein cells of a column are delineated by geological layering of the subsoil region

obtaining a seismic trace associated with said horizontal positions;

performing a seismic inversion for each of a plurality of said columns of cells,

wherein the seismic inversion for a column of cells at a horizontal position of the array comprises selecting a set of elastic parameter values that minimizes a cost function regardless of statistical distribution of the elastic parameter values, wherein the set of elastic parameter values is selected among a plurality of candidate sets of elastic parameter values each having values of elastic parameters for the cells of the column, wherein the cost function has a component given by a difference between the seismic trace associated with said horizontal position of the array and a synthetic trace resulting from a candidate set of elastic parameter values of said plurality of candidate sets of elastic parameter values.

2. The method of claim 1 , wherein the seismic inversion is carried out by running a global minimization algorithm.

3. The method of claim 2 , wherein the global minimization algorithm comprises a covariance matrix adaptation—evolution strategy (CMA-ES) algorithm.

4. The method of claim 3 , comprising:

selecting an uncertainty value for a group of candidate sets of elastic parameter values, wherein the group comprises a number of candidate sets of elastic parameters equal to, or greater than, 4+E[3*ln(n)], E being a floor function, ln a natural logarithm function and n a total number of elastic parameters in a column.

5. The method of claims 3 , comprising:

selecting an uncertainty value for a group of candidate sets of elastic parameter values, wherein the group comprises between ten and fifty candidate sets of elastic parameters.

6. The method of claim 1 , further comprising:

selecting each column of cells randomly;

selecting the set of elastic parameter values by taking into account any available set of

elastic parameter values for other cells of the grid.

7. The method of claim 2 , further comprising:

selecting an initial position of the array;

determining a boundary consisting of horizontal positions of the array which are adjacent to the initial position;

implementing the global minimization algorithm for at least one position of the boundary;

calculating a value of the cost function for each of said at least one position of the boundary;

selecting a propagation position in the boundary, a value of the cost function at said propagation position having a smallest value, then removing the selected propagation position from the boundary;

updating the boundary by adding positions of the array which are adjacent to the propagation position.

8. The method of claim 1 , wherein the elastic parameters comprise acoustic impedances and Poisson ratios.

9. The method of claim 1 , wherein the geological layering of the subsoil region comprises both permeable layers and non-permeable layers.

10. The method of claim 1 , wherein the geological layering of the subsoil region defines more than one hundred layers in the subsoil region.

11. The method of claim 10 , wherein each layer in the subsoil region corresponds to soil thickness between 0.5 meters and 2 meters.

12. A non-transitory computer readable storage medium having stored thereon a computer program comprising a program instructions, the computer program being loadable into a data-processing unit and adapted to carry out a method for estimating elastic parameters in a subsoil region, wherein the subsoil region is represented using a grid having columns of cells arranged in an array of horizontal positions, wherein the cells of a column are delineated by geological layering of the subsoil region, the instructions comprising, when run in at least one processor unit:

obtaining seismic traces associated with said horizontal positions;

performing seismic inversion for each of a plurality of said columns of cells,

wherein the seismic inversion for a column of cells at a horizontal position of the array comprises selecting a set of elastic parameter values that minimizes a cost function regardless of statistical distribution of the elastic parameter values, wherein the set of elastic parameter values is selected among a plurality of candidate sets of elastic parameter values each having values of elastic parameters for the cells of the column, wherein the cost function has a component given by a difference between the seismic trace associated with said horizontal position of the array and a synthetic trace resulting from a candidate set of elastic parameter values of said plurality of candidate sets of elastic parameter values.