Dispersion curve inversion
- 1 Getting ready
- 2 Running the inversion
- 3 Viewing the results
- 4 Checking the robustness
Start Dinver with Surface Wave Inversion module.
At first glance, the interface might look a little bit messy. If it is the first time you start dinver, you'd better close all windows until having an empty workspace. The various tools will be displayed one by one and explained in this tutorial, in a logical order.
Importing the dispersion curve to fit
- Activate the Target panel in menu Tools.
- Select the Dispersion option. Leave the Misfit weigth and the Min. misfit to their default values, 1 and 0, respectively.
- Click on Set to load the dispersion curve, the Dispersion curve target is displayed.
Load a dispersion curve and fix its frequency sampling:
- Load the experimental dispersion curve from a text file (e.g. Test_Rayeigh_2modes.disp, a theoretical curve computed using tutorial Computing a theoretical dispersion curve).
- Select the fundamental curve
- Use Curve data scroll bar and Visible button to identify it.
- In this example (Test_Rayleigh_2modes) it is the curve defined over the complete frequency range and with higher slowness.
- Re-sample it from 2 to 20 Hz on log scale with 50 samples (menu Actions/Resample).
- Cut it from 2 to 20 Hz (menu Actions/Cut).
- Select the first higher mode curve
- Use Curve data scroll bar and Visible button to identify it.
- This curve is not defined over the complete frequency range and it has a lower slowness.
- Remove it (menu Actions/Remove).
Achieving a good frequency sampling is an art, for a better understanding see Curve sampling.
Defining the parameter space
Defining the parameter is the key point of the inversion. At this step you have to figure out what information you already know about the ground structure and information you would like to extract. More details on how to achieve a suitable parameterization.
- Activate the Parameter panel in menu Tools.
- Add two layers for Vp profile by clicking two times on Add button in Compressional velocity profile.
- Add one layer for Nu profile by clicking once on Add button in Poisson's ratio profile.
- Add two layers for Vs profile by clicking two times on Add button in Shear velocity profile.
- Add one layer for Rho profile by clicking once on Add button in Density profile.
- Link Vp interface to Vs interface by selecting Vs0 in Linked to combo box.
At this step, the parameterization describes a ground structure with one layer over a half-space with uniform Vp and Vs in the top and bottom layer. The default range for values is relatively large. Vp contrast is forced to be at the same depth of Vs contrast, and only one parameter for depth is kept. Poisson's ratio is not a parameter but a condition imposed to Vp and Vs values: computed Poisson's ratios must remain between 0.2 and 0.5 (usual values for soils and rocks). The parameter panel should look like this one:
Running the inversion
Before going any further, it is wise to save the current environment, i.e. the target and parameter space definitions into a .dinver file. Select menu File/Save as .... Target and parameterization defined here above are available within file Dinverdc_tutorial-1.dinver.
Add a new Run by using menu Runs/Add. All the created runs are listed in Runs tab with their NA tuning parameters: itmax, ns0, ns and nr. All actions of menu Runs apply only to the selected rows this table.
Upon run initialization the parameter space specified previously with the graphical interface is translated into parameters ranges and explicit conditions. A report is presented in Log tab. Each run has an individual dedicated log.
---------------------- List of parameters and conditions Dimension of parameter space = 5 200 < TopVp0 < 5000 m/s(323 bins) TopVp0 < TopVp1 Poisson's ratio checked 200 < TopVp1 < 5000 m/s(323 bins) TopVp0 < TopVp1 Poisson's ratio checked 150 < TopVs0 < 3500 m/s(316 bins) TopVs0 < TopVs1 Poisson's ratio checked 1 < DVs0 < 100 m(462 bins) 150 < TopVs1 < 3500 m/s(316 bins) TopVs0 < TopVs1 Poisson's ratio checked TopRho0=2000 kg/m3
The number of bins for each parameter is calculated to ensure a minimum relative precision of 1% (discrete values on a log scale).
Starting the inversion and status
Select run in Runs table and hit Start in menu Run. The Status pane shows the current advance of the inversion process. The minimum misfit achieved as a function of the number of generated models is plotted on the left. The table on the right displays among other information the minimum misfit and the number of model generated.
Viewing the results
There are three ways of plotting inversion of dispersion curves within Dinver. They are available in menu View, only the selected runs in Runs table are included in the plots.
This is probably the first thing to look at when running the first inversion on dispersion curve. You can estimate whether the best models have a dispersion close to the inverted curve. Select the correct slowness (phase or group) and polarization (Rayleigh or Love). All models with a misfit lower or equal to Maximum misfit are displayed. Maximum number of modes is used only when several modes (fundamental and higher) are inverted together. In this case, the dispersion curve has no standard deviation, hence the misfit is normalized by the slowness value. A 10% fit of the dispersion correspond to a misfit of approximately 0.1.
A similar dialog box pops up when viewing the ground profiles. Maximum number of profiles must be set to 2 for Vp and Vs. This must not be confused with the number of models displayed. This later is controlled only by the maximum misfit (suggested value: 0.1, see previous section).
Obviously, the dispersion curve is not perfectly matched at high frequency due to the too simplistic parameterization used in this tutorial. To go further, more complex parameterizations are required. Moreover, if the true model for Vp (vp.layer) and Vs (vs.layer) is pasted on top of inversion results, one can notice that the estimation of depth is not correct.
Parameter space projections
Under menu View, option Parameter space displays various 2D projections of the models. The default plots are parameter versus misfit values. Existing plots can be customized through the property editor or new plots can be inserted (menu Insert/Parameter plot).
Command line tools
Models generated during inversions are directly written to the file system under .report files. If there is no environment saved, everything is saved in .dinvertemp in your home directory. After saving, a directory with _report extension is created together with .dinver files. Those .report files can be analyzed with various command line tools.
Checking the robustness
The inversion process is a random process based on a pseudo-random generator. The exploration paths covered may slightly vary from run to run (keeping everything else constant), especially in complex parameter spaces with a high number of dimensions. This is good practice to check the robustness of the exploration by starting the inversion with another seed. Add a new Run, select it in the Runs table and start it.
In this too simple parameterization (5 dimensions) the solution region is pretty well explored and a very similar misfit is achieved in all cases. At this scale (1e-7), the misfits can be identical due to the discrete nature of the inversion algorithm. A complete and reliable solution to the inversion problem require testing more complex parameterizations.