3.2.5. Inversion Input File

The inverse problem is solved using the executable program e3d.exe. The lines of input file are as follows:

Line #	Description	Description
1	OcTree Mesh	path to octree mesh file
2	Observation File	path to observations/survey file
3	Initial/FWD Model	initial/forward model
4	Reference Model	reference model
5	Susceptibility Model	background susceptibility
6	Active Topography Cells	topography
7	Active Model Cells	active model cells
8	Cell Weights	additional cell weights
9	Face Weights	additional face weights
10	beta_max beta_min beta_factor	cooling schedule for beta parameter
11	alpha_s alpha_x alpha_y alpha_z	weighting constants for smallness and smoothness constraints
12	Chi Factor	stopping criteria for inversion
13	tol_nl mindm iter_per_beta	set the number of Gauss-Newton iteration for each beta value
14	tol_ipcg max_iter_ipcg	set the tolerance and number of iterations for Gauss-Newton solve
15	Reference Model Update	reference model
16	Hard Constraints	use SMOOTH_MOD or SMOOTH_MOD_DIFF
17	Bounds	upper and lower bounds for recovered model
18	Field options	model total or secondary field
19	Memory options	memory options for factorizations
20	Solver options	direct or iterative solver options

Fig. 3.5 Example input file for the inversion program (Download ). Example input file for forward modeling only (Download ).

3.2.5.1. Line Descriptions

• OcTree Mesh: file path to the OcTree mesh file

• Observation File: file path to the observed data file or a survey file (forward modeling only).

• Initial/Forward Model: On this line we specify either the starting model for the inversion or the conductivity model for the forward modeling. On this line, there are 3 possible options:

  • If the program is being used to forward model data, the flag ‘FWDMODEL’ is entered followed by the path to the conductivity model.

  • If the program is being used to invert data, only the path to a conductivity model is required; e.g. inversion is assumed unless otherwise specified.

  • If a homogeneous conductivity value is being used as the starting model for an inversion, the user can enter “VALUE” followed by a space and a numerical value; example “VALUE 0.01”.

Important

If data are only being forward modeled, only the active topography cells and tol_ipcg max_iter_ipcg fields are relevant. However, the remaining fields must not be empty and must have correct syntax for the code to run.

• Reference Model: The user may supply the file path to a reference conductivity model. If a homogeneous conductivity value is being used for all active cells, the user can enter “VALUE” followed by a space and a numerical value; example “VALUE 0.01”.

• Susceptibility Model: The user may supply the file path to a background susceptibility model. If the Earth is non-susceptible, the user enters the flag NO_SUS.

• Active Topography Cells: Here, the user can choose to specify the cells which lie below the surface topography. To do this, the user may supply the file path to an active cells model file or type “ALL_ACTIVE”. The active cells model has values 1 for cells lying below the surface topography and values 0 for cells lying above.

• Active Model Cells: Here, the user can choose to specify the model cells which are active during the inversion. To do this, the user may supply the file path to an active cells model file or type “ALL_ACTIVE”. The active cells model has values 1 for cells lying below the surface topography and values 0 for cells lying above. Values for inactive cells are provided by the background conductivity model.

• Cell Weights: Here, the user specifies whether cell weights are supplied. If so, the user provides the file path to a cell weights file If no additional cell weights are supplied, the user enters “NO_WEIGHT”.

• Face Weights: Here, the user specifies whether face weights are supplied. If so, the user provides the file path to a face weights file cell weights file. If no additional cell weights are supplied, the user enters “NO_FACE_WEIGHT”. The user may also enter “EKBLOM” for 1-norm approximation to recover sharper edges.

• beta_max beta_min beta_factor: Here, the user specifies protocols for the trade-off parameter (beta). beta_max is the initial value of beta, beta_min is the minimum allowable beta the program can use before quitting and beta_factor defines the factor by which beta is decreased at each iteration; example “1E4 10 0.2”. The user may also enter “DEFAULT” if they wish to have beta calculated automatically.

• alpha_s alpha_x alpha_y alpha_z: Alpha parameters . Here, the user specifies the relative weighting between the smallness and smoothness component penalties on the recovered models.

• Chi Factor: The chi factor defines the target misfit for the inversion. A chi factor of 1 means the target misfit is equal to the total number of data observations.

• tol_nl mindm iter_per_beta: Here, the user specifies the number of Newton iterations. tol_nl is the Newton iteration tolerance (how close the gradient is to zero), mindm is the minimum model perturbation \(\delta m\) allowed and iter_per_beta is the number of iterations per beta value.

• tol_ipcg max_iter_ipcg: Here, the user specifies solver parameters. tol_ipcg defines how well the iterative solver does when solving for \(\delta m\) and max_iter_ipcg is the maximum iterations of incomplete-preconditioned-conjugate gradient.

• Reference Model Update: Here, the user specifies whether the reference model is updated at each inversion step result. If so, enter “CHANGE_MREF”. If not, enter “NOT_CHANGE_MREF”.

• Hard Constraints: SMOOTH_MOD runs the inversion without implementing a reference model (essential \(m_{ref}=0\)). “SMOOTH_MOD_DIF” constrains the inversion in the smallness and smoothness terms using a reference model.

• Bounds: Bound constraints on the recovered model. Choose “BOUNDS_CONST” and enter the values of the minimum and maximum model conductivity; example “BOUNDS_CONST 1E-6 0.1”. Enter “BOUNDS_NONE” if the inversion is unbounded, or if there is no a-prior information about the subsurface model.

• Field Options: The user can model the total field or the secondary field. In the latter case, the user may choose whether the primary field is computed analytically or numerically for a homogeneous background conductivity.

  • Use the flag TOTAL_FIELD to model the total field.

  • Use the flag SECONDARY_ANALYTIC followed by a value for the background conductivity to model the secondary field. In this case, the code will compute the total field for the conductivity model provided, then subtract the analytic total field using the homogeneous background conductivity provided. To subtract the free-space primary field, let the background conductivity be 1e-8 S/m.

  • Use the flag SECONDARY_NUMERIC followed by a value for the background conductivity to model the secondary field. In this case, the code will compute the total field for the conductivity model provided, then subtract the numerically computed total field using the homogeneous background conductivity provided. To subtract the free-space primary field, let the background conductivity be 1e-8 S/m.

• Memory options: This code uses a factorization to solve the forward system at each frequency. These factorizations must be stored. By using the flag ‘FACTOR_IC’ (in cpu), factorizations are stored within a computer’s RAM. Although this is faster, larger problems cannot be solved if insufficient temporary memory is available. The factorizations are stored in permanent memory (disk) if the flag ‘FACTOR_OOC’ (out of cpu) is used followed by the path to a directory. This is slower because the program must read these files many times. The second options is ill-advised if files are being transferred over a network.

• Direct or iterative solver options: Here the user chooses whether the forward problem is solved using a direct or iterative solver.

  • For Pardiso solver, the flag ‘USE_DIRECT_PARDISO’ is used.

  • For the BICG iterative solver, the flag ‘USE_ITER’ is used followed by values for the parameters tol_bicg, tol_ipcg_bicg and max_it_bicg.

    • tol_bicg: relative tolerance (stopping criteria) when solver is used during forward modeling. Ideally, this number is very small (default = 1e-10).

    • tol_ipcg_bicg: relative tolerance (stopping criteria) when solver needed in computation of \(\delta m\) during Gauss Newton iteration. This value does not need to be as large as the previous parameter (default = 1e-5).

    • max_it_bicg: maximum number of BICG iterations (default = 100)