OPERATIVE GUIDE V.E.S. VERTICAL ELECTRIC SURVEY

Transcription

1 OPERATIVE GUIDE V.E.S. VERTICAL ELECTRIC SURVEY 1

2 Quadrupole geoelectric procedure (V.E.S.) Generals V.E.S. (Vertical Electric Survey) geoelectric prospection method consists in investigating a specific territory resistivity variation with depth. It belongs to most ancient indirect geognostic surveys, non-invasive, which allows to analyze long soil profiles at great depth, with low costs and without disturbing existing buildings or underground water flowing. V.E.S. (Vertical Electric Survey) technique includes a series of resistivity determinations performed with increasing distance between current electrodes (A-B) and potential electrodes (M-N), according to 4 electrodes array outlined as follows. Through two external electrodes A and B a continuous current supplied by dry batteries or a generator is input into the ground; through central electrodes M and N potential difference generated into the ground when current flow between A and B is measured. Potential difference measures (pd) and current intensity (I) are performer with precision instruments, equipped with spontaneous potentials canceler existing into the ground. From these measures apparent resistivity of the soil is detected (R) according to formula: where K indicates array constant geometry. Array Schlumberger K= (AMxAN/MN) Array Wenner K= 2MN R = K ddp/i We talk about apparent resistivity because a whole rocky volume is investigated. Subsoil investigated volume is related (indicatively) at a parallelepiped of L =3(AB/2) length, l =AB/2 thickness and H=AB/4 depth. ELECTRODE DISPOSITION Wenner quadrupole (constant spacing) K factor computing facility Schlumberger quadrupole (MN measure electrodes distance small compared to AB) advantage is increasing of depth of investigation. Tripole or pole-dipole (is an asymmetric quadrupole with a current electrode very distant from the other three). Advantage of this device is possibility to reach high depths of investigation with arrays relatively reduced and high sensitivity to lateral disturbs. 2

3 To extend resistivity measures at layers more and more deep, distance between electrodes A-B and M-N is increased; in this way, current lines cross subsoil layers more and more in depth. Transferring apparent resistivity values on a bilogarithmic diagram, apparent resistivity curves are generated in which values AB/2 on abscissa are expressed in meters, while those of resistivity, on ordinate axis, are expressed in ohm*meter. From so obtained resistivity curves it is possible to detect real resistivity values and layers thickness of different resistivity, with the help of a mathematic model. V.E.S. single interpretations will be eventually visualized in a graphic in the form of geoelectric profiles which allow to visualize elaborated sections underlying correlations with available stratigraphical data. Moreover, thematic charts at level curves can be elaborated to underline the trend of surfaces and/or thickness of particularly interesting units, such as water layers or water tight layers. Acquisition mode Given that electrodes must be moved manually between one recording and the other, it is recommended to first set instrument for the analysis so as to avoid possible problems 3

4 during the job. Operating steps are described below. At instrument starting, after few seconds necessary for operating system uploading and peripherals starting, Activate section VERTICAL ELECTRIC SURVEY and upload a configuration table (file with.sev extension) which, as shown in picture, includes semi-distances between current electrodes (A-B) and those between potential electrodes (M-N). Related K geometric factor is automatically calculated. Electric current (I) is applied to electrodes A and B and potential (V) is measured between electrodes M and N. Resistivity ρ is calculated with the formula: 4

5 File.sev in table form includes semi-distances between current electrodes A-B and those between potential electrodes (M-N). Before starting with single measure it is recommended to check configuration window and parameters set on instrument according to the type of investigation. Place electrodes at fixed distance (AB/2 and MN/2 indicated in the first line of the table) which is for the first measure 0.5 m between M,N and P and 2 m between A, B and P, then connect electrodes to instrument through the reels. 5

6 Now we are ready to perform measurement. Electrodes must be in line, along an arbitrary linear direction, possibly preferring the direction of minimum slope. Measures refer to the vertical crossing the symmetry center P of electrodes, which must remain the same during all measures set. Electrodes must be in line long a linear direction, preferring directions with minimum slopes so as to reduce errors caused by rough topography. Generally speaking current density increases in dumps and decreases in correspondence of hills. For quadrupole measures it is recommended to plan array in order to easily position cables while proceeding with measurements. In particular if user wants to perform survey with arrays of even hundreds of meter in length. It is recommended that measures are related to the vertical passing for symmetry centre of electrodes. Once visualized table with distances, it is possible to start measure set by fixing electrodes at predetermined distance. If the box "measure only spontaneous potentials" is selected, no current will be generated and only spontaneous potentials values will be registered, measured for each electrode configuration. 6

7 If user has previous information about investigation site, such as presence of a lithology characterized by relatively high resistivity, it could be necessary to increase input current intensity. So it is possible to proceed with all single measures moving every time quadrupole at different distances from central point as shown in table AB/2 and MN/2. Here is a sum up of all operations to perform a measure once fixed a reference point P on the ground and a direction d: - select corresponding line in table; - connect electrodes through cable reels according to provided scheme; - place electrodes respectively at distances AB/2 and MN/2 indicated in line, selected according to P point and along d direction. For example, for the measure selected in picture 9, electrodes M and N must be placed each at 0.5 m from P and A and B each at 2 m from P. - press the button Single measure and wait for the measure to be completed. Pressing the button 'Start sequence' instrument will perform current measure and will stop waiting for confirmation that electrodes displacement is done. Therefore it will proceed with immediately following measure and so on until the end. To ensure user and all operators safety, before moving electrodes, always pay attention to instrument being in rest and with high voltage generator deactivated. At measures conclusion, it is possible to save data in.tsv and also in.dat format. File saved in the last format can be used directly for processing with freeware program RES1D, downloadable at link: which includes also instructions for its use. It is possible to perform measures with spacing different from that written in the example table Schlumberger.sev. For this aim it is necessary to prepare a different table. Table consists simply in a list of values couples, which, as indicated in heading, represent respectively semi-distances between current and potential electrodes. Computing of K geometric factor, necessary to determine apparent resistivity, will be automatically calculated from that distances when performing measures. User shall pay attention to place 4 electrodes in proper way, according to prospection scheme of Schlumberger. Same table can be also used for a quadrupolar array of Wenner type. For filling table on PC it is possible to use a text editor, such as Notepad, or a computing paper such as Microsoft Excel. Below there is procedure, in which at each line corresponds a measure. 2.0; , ; , ;0.5 or: 6.0;0.5 7

8 10.0; ; ; ;1.0 Save file as text document with.sev extension (for a type). With Notepad is necessary to specify All files in the box Save as:, to avoid that.txt extension will be automatically added to file name. V.E.S. interpretation Without added information about lithology or piezometric level, interpretation is actually inaccurate. Knowledge through geognostic surveys on site will help improving resolution of final model. INTERPRETATION BUILDING A GEOLOGICAL MODEL DETERMINATION OF THEORETICAL RESISTIVITY CURVES COMPARISON WITH EXPERIMENTAL DATA 8

9 Resistivity diagrams represents tool resistivity-depth based on subsoil lithologic situation. In quantitative interpretation, inverse process must be done, that is to detect soil model in terms of thickness and layers resistivity according to resistivity trend with depth. This procedure is made through a computing program that applies a proper mathematics model. Elaboration softwares provide immediately computed resistivity curve (based on a model which has been entered) overlapping that measured on ground, allowing to evaluate correspondence between two curves. Through some tools on correspondence between interpretative solutions (properly varying thickness and resistivity of a layer, its electric response does not relevantly change), it is possible to elaborate a physical model of the subsoil which takes into consideration interpretations of all V.E.S., compared one another for each area. Curves elaborated and detected on ground can be transferred on a single graphic in which it is possible to visually check their correspondence. Interpretation of single V.E.S. is done with the hypothesis that subsoil layers, below station point, are homogeneous, horizontal and with plane-parallel separation surfaces. Typical of electric prospection, there is a possibility of error in determining resistivity values and thickness of each single layer not lower than +/ %, referred mainly to limits from above-mentioned equivalence principles. 9

10 SIZE RANGE OF WATER AND ROCKS RESISTIVITY (from Astier, 1971 mod). 1. Good conductors(ρ < 10 7 Ωm); 2. Medium conductors (1< ρ < 10 7 Ωm); 3. Bad conductors or insulators (ρ >10 7 Ωm); Sea water <0,2 Water with 20% of salt 3*10-2 Pure water 102 3*103 Limestones and dolomites Sandstones Clays (fresh water) Clays (salt water) 1 10 Marls 2 50 Sand (from wet to dry) Granites and more Basalts Quartzites 2*104 and more Pyrite Galena Graphite Bauxite

11 Below there are results of vertical electric survey in which it is possible to observe how resistivity curve changes according to present stratigraphic attitude (from course Applied General Geophysics of Roma Tre AA12/13 - Antonio Meloni). VES for a 2 layers case 11

12 VES for a 3 layers case 12

13 VES for a 4 layers case 13

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