while processing schlumberger sounding data in IX1D software, we need to input values of layer resistivities, depths (or thickness), for an inital model. In some references, I found the depth is chosen 1/2 of the AB/2 value at inflection point while in some it is AB/2.
Which one is the correct?
The maximum current electrode spacing used is 120m (AB/2=60m) (pic is attached for reference) and the expected depth is 40m-60m but the output depth after inversion is between 15m to 25m.
Please share your ideas and some papers/links related to asymptote/inflection point method or any technique to define layers and depth on a sounding curve.
Dear Maurya, there is no hard and fast rule of getting over electrical resistivity modeling. What happens is that the software you are using was written with codes. You can as well guess your initial model depth to be 1/2 of AB/2 at inflection points and perform iterations as the case may be to get your final model. But the 'rule of thumb' which I think was used in writing the code is that the depth at inflection points is 1/6 of AB or 1/3 of AB/2 for Sclumberger electrode configuration. That is why for AB of 120 m the final depth after inversion will be ~20 m. Note that it is always advisable to constrain your models with 'ground truth' information such as lithologic description or core samples from nearby boreholes. This will also guide your choice of depth values for your initial model.
Dear Maurya,
I agree with Dr. Ebong that the best way to estimate an initial model is based in 'ground truth' information such as lithologic description or core samples from nearby boreholes. But when these data are not available what I use is the "Estimate Layered Model" option within IX1D in the "Calculate" Menu. This algorithm is based in Koefoed (1976). You can read a little more about it in the IX1D manual. I've have used this tool a lot and I think it is quite better than to estimate a model manually. I believe it would work well with the examples you showed.
Hope this will help.
Koefoed, O., 1976, Progress in the direct interpretation of resistivity soundings: an
algorithm: Geophys. Prospect., 24, pp. 233-240.
Besides the assumption that one third of AB/2 gives you the vertical depth of penetration as assumed in the software codes, ground truthing information (Borehole data) is the best way of defining the layers, depth and even the layer resistivity. In an event where lithological information is not available, you can use a simple inverse slope method (ISM), which will distinctly differentiate between the layers. Constraining models from software program with this manual method (ISM), can remove the ambiguity and the layer overlap that masks interpretation in electrical resistivity methods.
To answer these questions you must first know the lecture of
Resistivity curves
1) Two-layer case (A & Q resistivity types)
2) Three Layer Case (AA, QQ, K & H resistivity types)
3) four Layer Case
4) Multilayer case
After that, with some training, you will be perfect in identification
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