Resistivity Tomography - Science method

Wael Fathi Galal, I tried Grouwt, but I have to try gel or Silica gel. Thanks for your answers.

Electrical Resistivity Tomography (ERT) is a geophysical technique that measures subsurface electrical resistivity properties to produce images of the subsurface. The Root Mean Square (RMS) value is an important parameter used to assess the quality of ERT data. It represents the goodness of fit between the observed data and the predicted data from the inversion process. However, there is no universal standard range of RMS values that can be regarded as a good quality of results because the acceptable RMS value can vary depending on several factors such as the complexity of the subsurface, the data quality, the inversion algorithm used, and the objective of the survey.

In general, a lower RMS value indicates a better fit between the observed and predicted data, which is desirable. However, the acceptable range of RMS values can vary depending on the specific goals of the ERT survey. For example, if the goal is to detect the presence of a specific geological feature, such as a fault or a mineral deposit, then a lower RMS value may be required to accurately image the feature. On the other hand, if the goal is to estimate the bulk resistivity of the subsurface, then a slightly higher RMS value may be acceptable.

In conclusion, there is no standard range of RMS values for ERT data to be regarded as good quality of results. The acceptable RMS value depends on various factors, and it is usually determined based on the specific objectives of the survey and the complexity of the subsurface. However, it is generally accepted that a lower RMS value indicates a better fit between the observed and predicted data, which is desirable.

for me I see there is no significant differences between the obtained models, and to answer your question about (How to choose optimum model between many models) you have to integrate your models with another tool to achieve integration and see if both techniques provide good matching results for the problem at hand, if this is not available, thus, I can guarantee you that there nothing to do except studying the geological setting of the study area very well especially outcrops at or near your study area.

First of all, I experienced such problem before and I highly recommend you explore (ResIPy) for better error-analysis link below: https://github.com/hkexgroup/resipy/releases/download/v3.4.2/ResIPy-windows.exe

secondly, the problem maybe that the bad data points you exterminated to get lower inversion RMS error is the problem, it may eliminate the anomalous IP measurements.

finally, to better understand the problem I have to see IP measurements.

Mariana Gómez-Nicolás

Hello Gomez,

So many thanks for helping to answer my doubt!! I just figured out the problem.. Since I am using an extended large monitor, the software does not detects the new screen resolution size and remain the same size as my small main monitor on the second larger monitor. This problem could be overcome by process the data in the small screen.

Anyway, thank you for your kindness!

Dear;

for more information please check the following documents :

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

Dear Mostofa,

Also, to conduct ERT survey in an urban environment, you can use the normal electrodes. BUT you have to use a suitable electric drill to make holes for electrodes along the necessary positions in the ERT line. This was conducted in several areas in Malaysia to collect 2D ERT data along the roads. It considered a practical solution for such cases.

Dear Navneet Kaur,

1. If data acquired is in plain area (less topographic variations) then curve matching will yield more frutifull results, if topographically vairated area then you have to closely monitor the surfacial changes otherwise curve matching may yield abnormal results

2. Resisitivty is an indirect method so your resisitivty section may not match with bore data as bore gives detail even of small beds which are not detected by resisitivty measurements

3. One thing you can do carry out curve matching and then adjust your curve according to borehole but for that you will be needing more then one point and bore as well, also the subsurface geological strata must be kept in mind

4. Apparent resistivity depth section is not a good idea because of surface spacing adjustments, also bed thicknesses will not be known, so it is not recommended

Pelase check this instrument https://www.campbellsci.com/tdr100 and this article:

Without more information on your surface geology, this is a guess, but granites gnerally weather along joints formed as the overlying cover is eroded. In areas of deep weathering, clay could form along joints. It is also possible that a mafic dike of some variety could be causing this phenomenon. Sulfide mineralization is also a possiblity.

First of all, if you mean the volume fraction of one component of the outlined earth materials, you should 3D-ERT data to work with.

You are not giving me much to work with here. What are the materials? Are they layered, graduated or uniformly distributed? What is the geometry? What are the dimensions?

Ms. Coutinho,

Lineaments tend to be fairly useful in general and can indicate preferential flow paths, potential flow barriers, or recharge characteristics unrelated to fractures. I have attached a few articles. Hope they help!

James

I'd recommend you the link that Danney has sent. Also a close look at the manual would be greatly helpful as well:

Well,

The attached article explains a geophysical study performed east of Rutba town due to vertical electrical sounding in a net of forty points between Dhalaa and Dhabaa valleys. Geophysical electrical model applicated using Winsev 6 program to determine the geo-electrical layers.

https://www.researchgate.net/publication/303933182_GeoElectrical_ Investigation_of_Mullusi_Aquifer_Rutba_Iraq

Sincerely

yes that can be possible, depend of the size of the pyrite vein and the configuration of the electrodes. If the pyrite veins not is too small the method can delimit a area where there is.

There are a variety of commercially avaialble existing VLF-EM  processing algorithms, here's one that I have used previously:

Hi

Yes, by using Van der Pauw method you can measure small resistivities and pls. find the original paper of Van der Pauw's.

//chapter-4

//Example-4-2-2 page no.-147

//program_to_find_all_the_TE(n,p)_and_TM(n,p)modes_for_which_energy_transmisssion_is_possible.

radius=.02; //Given. Here_radius_is_in_metres.

uo=(4*(%pi))*(10^-7); eo=8.85*(10^(-12)); //scientific_values_of_permeability_and_permittivity_in_free_space

f=(10^10); //guide_is_operating_at_the_frequency_of_10GHZ

wc=(2*%pi*f); //since, wc=(2*%pi*f)

kc=wc*sqrt(uo*eo); //kc_is_cut-off_wave_number

X=kc*radius ; //the product X=(kc*radius) for_a_given_mode_is_constant

disp(kc*radius,'The_value_of_the_product X=(kc*radius)is = '); //display_the_product_X=(kc*a)

disp('Any mode having a product (kc*radius) less than or equal to 4.18 will propagate the wave with a frequency of 10 GHZ .This is (kc*radius)<=4.18');

syms i j //Defining_the_variables

p=[3.832 1.841 3.054 4.201 5.317 6.416;7.016 5.331 6.706 8.015 9.282 10.520 ; 10.173 8.536 9.969 11.346 12.682 13.987]//represent_the_values_of X_for_ different_modes_in_a_form_of_matrix. Where_columns_represent the_n_values_of_mode_and_rows_represent_the_m_values_of_mode.

for i=1:1:3 //value_of_i_traverse_across_the_rows

for j=1:1:6 //value_of_j_traverse_across_the_columns

if(X >=p(i,j)) //check_if_the_value_in(n,p)_matrix_is_less_than_or_equal_to_X

disp(p(i,j),i,j-1,'TE mode(n,p) and corresponding value of X='); //display_TE_mode_for_which_value_in [(n,p)matrix] <= X and print corresponding_value_of_X

end //end if

end //end for

end //end for

m=[2.405 3.832 5.136 6.380 7.588 ; 5.520 7.106 8.417 9.761 11.065 ; //represent_the_values_of_X_for_different_modes_in_a_form_of_matrix.Where columns_represent_the_n_values_of_mode_and_rows_represent_the_m_values_of_mode.

8.645 10.173 11.620 13.015 14.372]

for i=1:1:3 //value_of_i_traverse_across_the_rows_in [(n,p)matrix].

for j=1:1:5 //value_of_j_traverse_across_the_columns in [(n,p)matrix].

if(X >=m(i,j)) //check_if_the_value_in(n,p)_matrix_is_less_than_or_equal_to_X

disp(m(i,j),i,j-1,'TM mode(n,p) and corresponding value of X='); //display_TM_mode_for_which_value in [(n,p)matrix] <= X and_print corresponding_value_of_X.

end //end if

end //end for

end //end for

Depending upon your geometry of ERT survey, you may need some depth-weighting. However, in most cases such as pole-dipole or dipole-dipole, we usually do not use depth-weighting, because data itself has sensitivity about depth, and inversion could resolve targets in different depths. 

So, what kind of ERT survey are you trying?

Besides Dale's tips I you advice to adopt a multiscale approach carrying out several  profiles sampling, along the same line,  with different electrode spacings. Without a priori information the best practice is to perform the acquisition along a grid of  lines. This approach could guarantee the reconstruction of more resolved electrical stratigraphies and to resolve thin layers which could characterize the paleochannel. The  long scale acquisitions would allow the preliminar location of paleochannel area (better the corresponding anomaly area where it could be placed) and short scale its characterization.

I am grateful to all contributors for their suggestions. I thank everyone for their input and I am looking forward to apply your scientific achievements into uncovering the hidden mysteries at this location

The most reliable is covering opposite edges with silver paint, to force the current to go throughout the sample only in a way strictly parallel to the surface. Then force a small current to flow, measure the corresponding voltage drop with a micro or nanovoltmeter, and measure accurately the distance between the silver paint borders and the thickness of the sample. Then apply Ohm's law. That simple.

Alain

It depends on the target and its depth. the GPR method is good but it requires corrections for the "cultural noise. I am proposing a new method. You can use an electromagnetic device called PL2000 (made in Japen). I've successfully used it ( with Minimum error). that is very better of GPR but only use iron target for example Gas iron pipe , power lines, Conductive cables and ......

Generally, substrate are highly insulating materials and their conductivity is negligible as compared to material of the films. In four probe method current flows through the material of the film rather then substrate. Hence , the voltage developed in other two probes is due to the film and not the substrate. This way we measure the resistivity of the thin film and not the substrate.

It is a possibility that you can check with some simple 1D forward models computed with your best guess resistivity and thickness values as initial input.