Basins having high drainage density seem to have remarkable lateral in-homogeneity. Electrical resistivity sounding is a highly promising tool for correlation and subsequently undertake basin modeling. However, if at some of the points on a profile, proper calibration is not done, we can not expect good quality interpretation. While calibrating, a major decision involves in the choice of 'prominent surfaces'. For example, if within a thick sand, a number of clay lenses are there, (1) should we just consider the sand top and the bottom as the major surfaces, or (2) some of the clays should also be considered? In giving 'prominence' to certain boundaries, the style of separating essential from non-essential introduces subjectivity. Is there any standardized format for calibration to minimize subjectivity?
Hi Siddhartha,
this is a complex question, pretty complicated as geology itself. Some very general principless exist, but their importance is useful for educational purposes mostly.You can made some calibration of ressistivity data only in this case if you have enough drilling works. But achieved results do not meed valid for other localities even in the same basin. Heterogeneity (e.g. various number of clay lenses in a one sandstone layer) and anisotrophy of resistivity of studied lithofacies, as vell as multiplicity and directions/dipping of faults, character of overbunden from resistivity point of view are the best tools to deformate your skill. Moreover many other geological factors influence of resistivity values. My personal experieces (40 years) is, when I come to the poit that everything in some region was clear for me, i have obtained suprisingly quite different result (e.g. from the boreholes) and I had to find a reason why. My opinion is that geologist has to be modest in his in his belief about own knowledge, because the nature is always at least one step before your thinking or scientific reasoning. Despite of this fact it is our duty to compose of mosaic various results for setting up our skill on high level, but it is still inadequate, although in some cases an expected result may be achieved. I am very sorry, if I have dissapointed you.
Best regards
Ludovit Kucharic
Thank you Ludovit for sharing your experience. While putting the question, I was interested to know in specific terms whether certain new research works are there or not to guide the workers to minimize subjectivity in interpretation. Otherwise, the methodology might be treated by workers of other disciplines as a 'wish fulfilling tool'. There is no question of disappointment because theoretical foundation of this tool is quite strong. The onus goes to the researchers to refine this tool further. There is no doubt about it that this low cost device is the life line for quality research works for the scientists engaged in shallow subsurface investigations in the poor third world countries.
Warm regards
The question is interesting and as said Ludovit due to the complexity of the question we could conclude that is without answer. This question has many similarity with another that I have found in Q&A regarding the seismic "When are seismic boundaries the same as geologic boundaries?" You can read my answer to that to have my general idea to attempt an answer.
In your question you use the term "calibrating" that could have a different meaning in terms of geophysical interpretation and of geological interpretation.
For me the "calibration" not is the "the geological interpretation", as well as the two interpretative levels, geophysical and geological, are complementar but should be not confused.
Electrical resistivity method gives a resistivity-stratigraphy (resistivity model) that is the result of a geophysical acquisition , processing, and model reconstruction procedures. In general the resolution and errors of the reconstructed resitivity stratigraphy, should be taken in to account when we pass to geological interpretation.
In general these procedures applied to the "real resistivity stratigraphy" constitute a smoothing, or generate an averaged stratigraphy which is dependent also by the local etherogeneity and layer morphology of the "real layers" (see the complexity claims in the Ludovit answer). In other words the geophysical resitivity stratigraphy is a local averaged model of the real resistivity stratigraphy. It depends of our experimetal procedure (spacing, measurement errors, software inversion etc.)
The geo-stratigraphy obtained from boreholes data generally have a resolution and error different with respect geophysical resitivity stratigraphy, generally higher resolutions and lesser errors in the layer definition (reconstruction). For this reason, the geophysical model, several time, does not fit the borehole data but generally constitutes a raw but useful interpolation of the geologic data.
So if we have only the geologic stratigraphy, the "calibration" of geophysical stratigraphy, in strict sense, is not possible but we can attempt an interpretation of geophysical results in geologic sense.
In my opinion a method (unfortunately to expensive) to calibrate the resitivity stratigraphy or the resitivity model is to acquire resistivity logs during the core drilling, or alternately to perform laboratory resistivity measurements on the drilled cores.
If are available these data, you can compare the borehole data with geologic stratigraphy and you can extract several degree of correlations, and averages, in order to construct an interpretative model, in terms of the resistivity layer boundary and of the facies resistivity, that we can use to construct a conceptual model for the interpretation of the indirect surface results, resitivity stratigraphy obtained with VES, ERT tomography procedures. This could be a standard of calibration but I said before is expensive. For oil research this is a standard. The "while drilling" technology have reached a high degree of development for this reason: to calibrate and to interpretate the results of seismic and other geophysical surveys. Neverthless this is not exaustive for a complete calibration but is surely better in terms of the mining risk reduction.
Unfortunately, now, the funding for water resource research and water protection are not comparable with the oil exploration, but probably when the water resource will become “the transparent gold” like “the black gold” for oil this will be the standard.
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