E4D User Guide
E4D User Guide
Dictionary of Terms
| Term | Definition |
|---|---|
| Blocky constraints | Encourages solutions in the inversion that contain sharp internal boundaries |
| Boundary flag | An integer value that defines where a control point is located within the mesh. Surface point=1 Surface boundary point=2 Internal points=0 This is defined within the Control Block of the mesh configuration file. |
| bx | A utility program written in fortran that is provided in the E4D installation. This utility creates exodusII format files to facilitate visualization in VisIt. Separate compilation is required. |
| Velocity file | A file that contains a velocity value for each element in the mesh in the same order as the .ele file. |
| Control points | Points necessary to define mesh geometry, surface topography, internal boundaries, sensor locations. Control points can also be referred to as nodes. |
| fmm.inp | The required name of the run configuration file which contains the run mode and associated input files and options. |
| fmm.log | The name of the log file output by E4D that records the date and time of execution, all information required to reproduce the simulation, summary statistics and convergence information (for inversion modes). |
| E4D mesh | The files created by tetgen and E4D that define the mesh, the translation and the boundaries for a particular problem. These have file extensions of .ele, .node, .edge, .face and .trn |
| Element | A single tetrahedron with the mesh. An E4D mesh is comprised of tetrahedral elements. |
| Forward modeling | The process of simulating and reporting the subsurface travel-time data generated, given the subsurface velocity distribution specified in the velocity file listed in <fmm.inp>. |
| Hole | A zone that is defined as a void or hole in a mesh. |
| Inversion Options |
The information provided by a travel-time survey is typically insufficient to uniquely determine the subsurface velocity distribution at the scale of the computational mesh. In order to produce a reasonable representation of the true subsurface velocity, the inverse solution must be constrained by a priori information in addition to the information provided by the data. The a priori solution constraints are provided in the inversion options file. |
| Inverse Modeling | The process of using simulated travel-time measurements within an optimization algorithm to estimate the velocity distribution of the subsurface. The results of an inversion are typically visualized as an image of subsurface velocity. The terms inversion, inverse solution, inverse model, and image, are used synonymous in this guide. |
| Mesh configuration file | A file with a .cfg extension that defines all nodes, PLCs, holes, zones and boundaries of an E4D mesh |
| Mesh Generation | Refers to the process E4D uses to generate the unstructured tetrahedral computational mesh used in forward and inverse simulations. |
| mpirun | A message passing interface (MPI) executable that allows for parallel processing within E4D. |
| nodes | See control points |
| Travel-time file | Defines a travel-time value for each node in the mesh in the same order as the .node file. These files can be visualized in VisIt using the utility program bx or px. |
| px | A utility python script provided in the E4D installation that creates xdmf/h5 format files to facilitate visualization in VisIt . Separate compilation to create an executable is recommended. |
| Reference model | For an inversion or time lapse inversion, the reference model is the velocity distribution used to constrain the inversion in the structural metric equation. |
| run mode | A keyword entered on the first line in fmm.inp that defines the execution mode. |
| Smoothness constraint | Encourages the inversion to enforce similarity between neighboring elements in the mesh. |
| Starting model | For an inversion, the starting model is the velocity distribution used to determine the initial model fit of the data. |
| Structural metric codes | In inversion run modes, an equation which imposes a desired structure by defining a relationship between an element and 1) a specified reference value; 2) a neighboring element or 3) a reference model. Typically applied for a zone, the defined relationship is applied to every element. |
| tetrahedron | The 4-sided element used by E4D to create a mesh. Different maximum element volumes can be defined for different zones, creating an unstructured tetrahedral mesh |
| VisIt | VisIt is an open Source, interactive visualization tool that allows E4D solutions to be visualized. |
| Weighting function codes | In inversion run modes, determines the condition under which the corresponding structural metric should be applied to constrain the inversion. |
| Zone | A watertight region within an E4D mesh. Defined by internal boundaries (also called piecewise linear complexes (PLCs) in tetgen) in a configuration (.cfg) file |