Getting started with TEM IP inversion
SPIA includes the possibiltiy to invert for IP effects in the TEM data. There are two inversion options: Cole-Cole and Maximum Phase Angle (MPA) which is a re-parameterization of the Cole-Cole that gives less correlation between the inversion parameters and is the recommended inversion type to use.
There are several articles on these two inversion methods and it is recommended to read the following articles:
Fiandaca, G., E. Auken, A. Gazoty, and A. V. Christiansen, 2012, Time-domain induced polarization: Full-decay forward modeling and 1D laterally constrained inversion of Cole-Cole parameters, Geophysics, 77, 3,E213-E225. Download
Fiandaca, G., L. M. Madsen, and P. K. Maurya, 2018, Re-parameterizations of the Cole-Cole model for improved spectral inversion of induced polarization da-ta, Near Surface Geophysics, 16,385-399. Download
It often takes several tries to get an inversion model that fits the data and there are several things that needs to be taken into account:
- Processing of the data. It is very important to have a well defined error around the sign-change if a sign-change exists. If the data imported into SPIA is unstacked, the error will be calculated from the stacking. Otherwise it is important to set a high error on the timegates around the sign-change.
- The starting model is very important and it is needed to change this to give the inversion a better possibility to fit the data. The starting model can be change within SPIA for each layer, and the default parameter values can be change in the SPIACONFIG.INi file located in the SPIA folder:
- The two inversion parameters Tau and C is fixed in the inversion for the first number of iterations. As default the setting is
NIteTau = 3 %Number of iterations before inverting for Tau/T2* (model type specific)
NIteC = 1 %Number of iterations before inverting for C (model type specific)
If needed, these can be changed in the configuration file for the AarhusInv inversion code. They are located in the TEM_inversion_IP.con file located in the SPIA/AarhusInv installation folder. +The two settings can be found in line 30 and 31. These settings should only be changed after trying to change the starting model, as this is the most important part to get a good inversion model fit.