Forward modeling

fm.csem computes the 3D CSEM response of a conductivity model at a single frequency, discretizing the electric field with Nédélec elements of order 1 to 6 on an unstructured tetrahedral mesh. The formulation is described in Numerical formulation.

Workflow

  1. Generate or import a mesh (see Mesh generation).

  2. Define the conductivity model in sigmas.txt.

  3. Run utils/preprocess.py -mode fm to produce the input bundle and the parameter file.

  4. Run fm.csem.

  5. Post-process the responses.

Kernel options

fm.csem takes its options from a PETSc options file (-options_file params.txt) or directly on the command line.

Required

  • -input_filename - the input bundle (HDF5).

  • -output_dir - output directory (created if absent).

  • -output_filename - output stem; writes <output_dir>/<stem>.h5.

Optional

  • -order <1..6> - override the bundle’s /order.

  • -mms - run the method-of-manufactured-solutions verification instead of a CSEM simulation (see Testing).

Everything else in the options file is passed to PETSc (solver, preconditioner, matrix type); see Solver options.

-help intro prints a usage summary and exits; -help prints the full PETSc option database; --version prints the version.

Parameter file

The parameter file emitted by utils/preprocess.py -mode fm is:

-input_filename <case_dir>/input.h5
-dm_mat_type is
-ksp_type fgmres
-pc_type bddc
-pc_bddc_use_deluxe_scaling 1
-pc_bddc_coarse_pc_type lu
-output_dir <case_dir>/
-output_filename responses

The polynomial order is not written here - the kernel reads it from the bundle’s /order dataset.

Input files

Source file

-source_filename describes the transmitters. Two layouts are accepted and auto-detected (see Data formats). The shipped examples use the second:

# 8 fields per row (canonical)
freq x_pos y_pos z_pos current length dip_angle azimuth_angle

# a lone frequency line, then 7-field rows
freq
x_pos y_pos z_pos current length dip_angle azimuth_angle

Fields are: frequency (Hz), dipole position x y z (m), current (A), length (m), dip_angle and azimuth_angle (degrees).

A forward run is single-frequency: the kernel’s transmitter set carries one operating frequency shared by all transmitters. Inverse modeling uses a multi-frequency source file (see Inverse modeling).

Receiver file

-receiver_filename lists receiver positions, one Cartesian point x y z per row. Whitespace- or comma-separated; # comments and blank lines are allowed.

Running

mpirun -n 4 build/fm.csem -options_file path/to/params.txt

or through the dispatcher:

mpirun -n 4 build/petgem fm -options_file path/to/params.txt

Output

The kernel writes a single HDF5 file, <output_dir>/<output_filename>.h5, holding all six field components (Ex, Ey, Ez, Hx, Hy, Hz) at the receivers, for every transmitter. The layout is documented in Data formats.

A runnable walkthrough is given in Forward modeling examples.