3.2.1. Create OcTree Mesh Input File

OcTree meshes used in the e3d code are created using the program create_octree_mesh_e3d_v2_tiled.exe. This includes a global mesh (the mesh for which the inverse problem is solved) and a set of local meshes (OcTree meshes use to solve the set of forward problems). Parameters necessary for creating all of the OcTree meshes are set in the input file. The lines within the input file are as follows:

Important

This code has parameters which define the global mesh used in the inversion (where the recovered model lives) and parameters defining the local meshes where the forward problem is solved for each transmitter. The former will be referred to as the global inversion mesh and the latter will be referred to as local forward meshes.

Line #	Parameter	Descriptions
1	dx dy dz	min. cell widths in x, y and z for base mesh
2	min_cell_fact min_cell_size_fwd max_topo_cell	additional cell size parameters
3	x_pad y_pad down_pad up_pad	sets the extent of mesh in x, y and z direction
4	dist_inv_1 dist_inv_2 dist_inv_3	sets core mesh discretization for the inverse mesh
5	dist_fwd_1 dist_fwd_2 dist_fwd_3	sets core mesh discretization for local forward meshes
6	n1 n2 n3	sets thickness of cells of finest discretization near receivers
7	locFile	the file containing observation locations
8	txFile	the file defining all transmitters
9	rxFile	the file defining all receivers
10	freqFile	the file containing the frequencies being measured
11	topoFile	sets topography
12	polygon edge width	sets horizontal extent of core region for the inversion mesh
13	read/create mesh	read in or create global inversion mesh

Fig. 3.1 Example input file for creating octree mesh (Download )

3.2.1.1. Line Descriptions

• dx dy dz: Minimum cell widths in x, y and z for the base mesh.

• min_cell_fact min_cell_size_fwd max_topo_cell: These parameters determine the rate of cell expansion for regions near topography and for the local forward meshes.

  • min_cell_fact: Defines the rate of topography-based cell size increase on the global inversion mesh with respect to depth. After each layer of N cells, the cell size will increase by a factor of 2 until a maximum cell size (max_topo_cell) is reached. N must be an integer value that is a power of 2.

  • min_cell_size_fwd: This sets the minimum cell size for the local forward meshes. A value of 2 means the minimum cell size in the local mesh has a side width of 2 times the base mesh cell size. This parameter must be an integer value that is a power of 2.

  • max_topo_cell: This determines the maximum cell size for which topography-based cell size increase is used on the global inversion mesh; after which typical OcTree cell expansion is used. This parameter must be an integer value that is a power of 2.

• x_pad y_pad down_pad up_pad: Distance from the core mesh region in the x, y, downward and upward directions, respectively, that the global inversion mesh extends.

• dist_inv_1 dist_inv_2 dist_inv_3: For the global inversion mesh, these parameters set the discretization of the core mesh region (i.e. the region near the transmitters and receivers) in terms of depth. Up to a depth of dist_inv_1 from the surface, the smallest cell size is used (set by dx, dy, dz). For the following dist_inv_2 metres, a cell width 2 times large is used. For the following dist_inv_3 metres, the cell width is doubled again. Below the third depth region, the cells widths increase by a factor of 2 for every additional layer (see the figure below).

• dist_fwd_1 dist_fwd_2 dist_fwd_3: For the local forward meshes, these parameters set the discretization of the core mesh region (i.e. the region near the transmitter and receivers) in terms of depth. Up to a depth of dist_fwd_1 from the surface, the smallest cell size is used (set by dx, dy, dz). For the following dist_fwd_2 metres, a cell width 2 times large is used. For the following dist_fwd_3 metres, the cell width is doubled again. Below the third depth region, the cells widths increase by a factor of 2 for every additional layer (see the figure below).

Note

These values must be entered. However, they are only relevant for the e3dinv_ver2_tiled code.

• n1 n2 n3: This sets the thicknesses of layers of finest discretization near the receivers. n1 = 4 means that around each receiver, there is a layer 4 cells thick that uses the finest discretization. This is followed by a layer which is n2 cells thick, where the cell dimensions are increased by a factor of 2. Likewise for the 3rd layer.

• locFile: Path to the file containing the survey information. This can be either an observed data file, or a survey index file.

• txFile: Path to the file defining the transmitters; i.e. the transmitter file.

• rxFile: Path to the file defining the receivers; i.e. the receiver file.

• freqFile: Path to the file defining the frequencies used in the survey; i.e. the frequencies file.

• topoFile: If a topography file is available, the file path to the topography file is entered; see topography file for format. In the case of flat topography, the user instead enter “TOPO_CONST”, followed by a space, then the elevation of the surface topography; for example “TOPO_CONST 125.5”.

• polygon edge width: Here we define the horizontal extent of the core inversion mesh region. The user may do this by providing the path to a file containing the points for a polygon. The user may also set the horizontal extent of the core mesh region based on transmitter and receiver locations. The set of transmitter and receiver locations can be used to create a convex hull. For this option the user types “MAKE_POLYGON d”, where d is the distance outside the convex hull the user want to extend to core mesh region.

• read/create mesh: If the global inversion mesh has already been created, then it may be loaded by typing “READ_LARGE_MESH filepath”. In this case, the global inversion mesh is used to define the local forward meshes. If the global inversion mesh needs to be created, the user types “CREATE_LARGE_MESH filename”, where the global inversion mesh is output to the file filename.