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Question about geometric factor for an ERT survey with submerged and terrestrial electrodes

amehriban opened this issue · comments

commented

Dear PyGimli team,

I have a question regarding the geometric factor for an ERT survey with a combination of submerged electrodes (i.e., on the seafloor) and terrestrial electrodes. In RES2DINV, we can indicate that the apparent resistivities that we import are calculated with the normal surface geometric factor. When we import these data into PyGIMLi, does the program also take this into account?

Thank you in advance!

commented

This is actually up to the user to a certain extent. If the geometry is flat (no matter if surface or subsurface electrodes) like in https://www.pygimli.org/_tutorials_auto/3_inversion/plot_8-regionWise.html then one can use the classical geometric factor, which is always using mirror electrodes. If the terrestial part contains topography, it will be computed numerically as in this example https://www.pygimli.org/_examples_auto/3_ert/plot_02_ert_field_data.html

commented

Hi! Thank you for the reply. I think I didn't properly elaborate in my question the first time, so let me re-iterate:

Our profiles involve a combination of underwater electrodes and terrestrial electrodes. To convert measured resistances to apparent resistivity, the normal surface geometric factor was used. This geometric factor assumes a half-space beneath the electrodes. This geometric factor does not consider that some electrodes have a water layer on top. In RES2DINV, the user can specify that the imported apparent resistivities were calculated using a normal surface geometric factor. Does pyGIMLi take this into account when we import the data, or should we be using an exact geometric factor? To further clarify this issue, I copy and paste an image out of the RES2DINV manual.

image

commented

pyGIMLi is either computing the geometric factor

  • numerically taking the whole geometry into account, OR
  • analytically, but always taking the electrode depths into account by mirror electrode

So as long as there is no topography on land and the water level is zero, the geometric factors are correct. At the end, it is just a factor to scale the measurements, so even using a wrong geometric factor will not much change the result.