Stage terrain 3A Heissenstein. Electrical surveying

Transcription

1 Stage terrain 3A Heissenstein Electrical surveying

2 Introduction The principle method for measuring the surface resistivity is always the same. An electric current is sent into the subsurface through two electrodes embedded into the ground. Then, the potential difference, resulting from the spread of the electric current, is measured within the medium. The devices used in the field differ by the spatial distribution of electrodes.

3 Introduction From these measurements, i.e. the electric current I and potential difference ΔV, we can deduce the apparent resistivity of the subsurface. We can then determine the electrical structure of the subsurface, in terms of resistivity. Since dry rocks are generally poor conductors, low electrical resistivity is often associated with the presence of water and/or clay in the subsurface, or, more rarely, the presence of conductive minerals in the rock or soil.

4 Introduction

5 Theory A homogeneous medium has a resistivity ρ (or conductivity σ = 1/ρ). Ohm's Law tells us that J = -σ V, where J is the vector current density and V the electric potential. By solving this equation in spherical coordinates in a homogeneous halfspace, we see that the potential V at distance r from a point current source is given by: V = A/r where A is a constant.

6 Theory When the electrode placed on the surface of the Earth (C 1 in the figure) delivers a current I, we can show that the constant A is ρi/(2π) Hence we can express the resistivity ρ as a function of measured quantities: ρ = 2π(V/I)r

7 Theory In practice, the current I is injected into the subsurface through two electrodes (C 1 and C 2 ) fed by a current generator. Similarly, two measuring electrodes (P 1 and P 2 ) permit the measurement of the potential difference ΔV between the measuring points. The equipotentials and current lines for two injection electrodes are shown in the figure opposite.

8 Examples

9 Examples

10 Types of array Schlumberger Dipole-Dipole Wenner Pole-Dipole Pole-Pole

11 Types of array

12 Types of array

13 Schlumberger Spacing between measuring electrodes (M and N) is fixed at the centre of the survey and is small in comparison to the current electrodes (A and B). Measurements are made for different positions of current injection electrodes, as long as the measured signal is strong enough to be measured.

14 Dipole-Dipole Spacing between measuring electrodes (M and N) and the current electrodes (A and B) is fixed. Measurements are made by changing the spacing between the two sets of electrodes. Gives better resolution than the Schlumberger, but does not penetrate as deep.

15 SYSCAL R1+ SYSCAL R1+ is a new all-in-one multi-node resistivity imaging system. 48 electrodes 200 W resistivity meter 2 channels

16 The data The data was extracted from the SYSCAL using the serial port using a USB connection. The programs you need to look at and interpret the data are: PROSYS II PROSYS II allows you to see the data in a line-by-line format, rather like Microsoft Excel. RES2DINV 2D resistivity and IP inversion (to display and invert the data). PROSYS II and RES2DINV are available for FREE download: The other program, used to make the different array programs, is: ELECTRE II (you don t need to download this) Sequence management for multi-electrode units (to define the acquisition setup). Used the programs Schlum (Schlumberger) and DD (dipole-dipole).

17 PROSYS II changing electrode spacing PROSYS II is used to manipulate the data before it is converted into a RES2DINV file. For example, the electrode spacing can be easily changed. Processing Modify spacing... The programs you used were optimised for the correct electrode spacing, you will not need to change the electrode spacing.

18 PROSYS II eliminating bad data Your dataset may contain some bad values. The number in the column on the far left is the calculation step number. In the example shown here, calculation step #94 recorded a negative Rho. The data can be eliminated by unchecking the box next to the calculation step number.

19 PROSYS II eliminating bad data The easiest way to find strange data is to look at Processing Exterminate bad data point You will see a graph of Rho against depth. In the example, I have highlighted two odd data points. To delete them click on them so that they change colour from blue to red. When you click OK the data will be excluded.

20 PROSYS II adding topography You can add your relative topographic information using PROSYS II (you will need the Dongle to introduce your topography to the inversion however!). Processing Insert topography... Add in the relative Z-spacing for each electrode. Then press OK REMEMBER that the units are in metres!

21 PROSYS II - exporting When you are happy with your data, you then need to save the data as a RES2DINV file. Select the data you would like to export (the ticked boxes on the left hand side). Export each survey separately. File Export and save Res2dinv/Res3dinv... Make sure you select Rho as the type of measurement After you have saved the file you can open it using RES2DINV.

22 RES2DINV The RES2DINV program uses the smoothness-constrained least-squares method inversion technique (Sasaki, 1992) to produce a 2D model of the subsurface from the apparent resistivity data. The program will automatically choose the optimum inversion parameters for a data set. However, the inversion parameters can be modified by the user. To load your data, click File Read data file The data will load and the program will tell you whether there are still some negative or strange data points. You can either go back to PROSYS II and try to find the strange data, or you can try to remove them using the RES2DINV program.

23 RES2DINV managing the data If there are problems with very high or very low resistivity, you can manipulate the data. Click on Edit Exterminate bad data points The data should look like a series of sub-parallel lines (as shown in the example opposite). Strange data points can be identified as those that deviate from the series of sub-parallel lines. To remove them, click on the data points (they will turn red). Then click Exit Quit edit window and agree to the changes. You will now have to save the updated file and reopen it in RES2DINV.

24 RES2DINV To perform the inversion, you need to first read your RES2DINV file. Then select Inversion Least squares inversion

25 RES2DINV the inversion absolute error in the iteration

26 RES2DINV the inversion If there is a problem with your inversion o perhaps the program has warned you about negative or high/low values, o or your inversion is a single colour with a very high absolute error. Exit RES2DINV and reopen the data in PROSYS II to recheck the data for strange values.

27 RES2DINV the inversion

28 RES2DINV display topography You can check whether your topography data was inputted/saved correctly

29 RES2DINV include topography Finally, you can include the topography into the inversion. You will need the Dongle to perform this step (ask Pascal Sailhac for the Dongle).

30 RES2DINV The sections with and without topography can be quite different

31 RES2DINV error analysis If you like, you can perform an error analysis

32 RES2DINV manual FREE download available from: Available in either English or Japanese!

33 Interpretation Performing the inversion does not count as the interpretation! However, as with many subsurface geophysical surveying, there is no perfect, unique solution. WHAT WE KNOW: Low electrical resistivity is often associated with the presence of water and/or clay in the subsurface From the geology we should expect granite in the subsurface. In parts of the path the granite was at the surface. Perhaps the granite is closer to the surface near the path? From the table:

34 Interpretation Where do we see the granite in the seismic profiles? We could use this to constrain our 1 m spacing profiles (that reach a depth of about 7 m). Do we expect to see the granite at 7 m depth? Do you think the subsurface will be saturated? Perhaps there is a clay-rich layer? Can topographic features provide information regarding the subsurface? (break in slope, hill, trees...). How does this profile fit in with any of your other profiles?

35 For your final report Theory (electric fields, Ohm s law, the types of survey...) Methods (how did you perform the survey? which survey? what orientation? what equipment did you use? how did you perform the inversion?...) Results (show the final profiles) Interpretation (what can you say about the subsurface? what are the differences between Schlumberger and dipole-dipole?, how useful are these measurements? is 7 m deep enough to see anything interesting? what would you do differently?...)