OPERATIVE GUIDE MULTIELECTRODE GEOELECTRICS

Transcription

1 OPERATIVE GUIDE MULTIELECTRODE GEOELECTRICS 1

2 geoelectrics procedure - tomography Generals System of electrical surface profiles with multi-array (multielectrode) instrument represent an innovative methodology of classic geoelectric measurements. Electrical resistivity tomography (acronym ERT) consist in geoelectric and dimensional characterization, with high detail, of structures presenta long bidimensional sections. It allows to reconstruct spatial distribution in two dimensions of real resistività into subsurface with a resolution depending on electrodes distance and type of array. Arise of this methodology during 90s allowed to perform ERT which give 2D and 3D images of resistivity distribution into subsurface through different numbers of electrodes placed along a defined geometry arranged according to the kind of instrument used (array). Innovation compared to resistivity profiles consist in the possibility to perform a great number of measures in short time and in the following data processing through bidimensional inversion programs. This technique allows also to operate in presence of irregular soils (topography), after detecting altitude between each electrode; that information will be then taken into account when processing data. Measured resistivity value is an apparent resistivity because current lines which allow to evaluate that parameter have crossed many rocky structures: that is an anomalous value compared to value measured in homogeneous and isotropic subsurface. This datum is not simply average encountered electric layers resistivity, but it is defined by a more complex function depending both on resistivity of various crossed layers both on their thickness. Due to the fact that apparent resistivity distribution is related to thickness, localization, shape and resistivity of layers crossed by current, it is possible to obtain information on investigated soil. All electrodes are connected, through a proper multi-conductor cable of low impedance, to recording instrument. Current is applied to a couple of electrode measuring then potential difference between all other electrode couples available in selected configuration. Then move to a second transmitting couple and so on, until maximum number of independent measures on available poles and dipoles is reached. 2

3 To resistivity measures recording phase, their interpretation follows, and it is technically named inversion phase; it consists in the simultaneous use of numerical modeling algorithms to finite elements (or finite differences) and minimum squares optimization methods. Iterative procedure of resolution allows to estimate real resistivity distribution which turns into a graphical image that must be interpreted. Recording modality Theory mention For multielectrode measurements instrument shall be a digital resistivity meter able to perform through software the following main operations: - measure and record of contact resistance between electrodes; - measure, record and reset of spontaneous potentials; - execution of repeated measurement cycles and compute of standard deviation ; - possibility to set measurement cycles of different duration; - resolution of 30 nv measures; - measures recording: resistivity, dv, I, Stand. Dev. And electrodes geometry; - control unit and electrodes management; - multichannel cable equipped with electrodes so called intelligent (smart electrodes) because they have an inner electronic power that allows to use both as current electrodes and as potential electrodes, or equipped with multichannel cable with common stainless steel, corten or brass electrodes, for instruments with electronics totally embedded. Power generated by transmitter shall be proportional to maximum depth to reach; below we provide indicative examples: up to 200 m => 18 W min, 0.5 ma min, ± 200 V min; for array > 200m length => 100 W min, 1 ma min, ± 400 V min. Anyway, normally error between set up stacks (min 3) shall not overcome 1%. Finally, instrument shall allow to configure at least 4 temporal windows to measure chargeability (PI). Acquisition of dv and I data on each single quadrupole is basical to geoelectric measures. Given that there are other sources of potential difference beside I current we inject, it is recommended to adopt a measurement methodology which deletes from dv measures any other component not related to I. Other causes of dv into the soil can be: spontaneous potentials (due to sources inside ground such as mineralized corps, or water flow into porous mean in this case we talk about streaming potential), telluric flows (that is soil currents caused by electromagnetic induction of ions fluxes in Ionic sphere) and polarization effects to electrodes themselves (when non-polarizing electrodes are not used, as it often happens). 3

4 In continuous current measurements, it is not important to distinguish cause for potential difference in MN dipole, just remove them. Measurement procedure consists in injecting current I with commuted verse for a certain period, leaving pause period where current is not injected (a square wave): in this way zero, that is potential not caused by +I injection, -I at AB is easily identified and removed. Please notice that using square wave in this way, we are not properly working with continuous current. However, procedure is adequate because current is kept continuous for long periods (from hundreds of ms to few seconds) compared to system pausing periods, therefore transitional potential differences can be observed if necessary but don t affect continuous current measurement. Representative scheme of square wave and induced polarization effects. Before performing measurement it is important to establish reason and goals of the survey, so as to properly set what kind of instrument to use, according to site background, available space and logistics. 4

5 Most common arrays main features are sum up below. Each type of array has its proper use in particular geological conditions, according to its sensitivity. Sensitivity gives indication on how a resistivity change into soil affects potential measurement, to which it is directly related. Normally Wenner and Wenner-Schlumberger configurations are more indicated for vertical variations detection, while dipole-dipole is better to identify lateral variations, but has a less favorable signal/noise ratio; pole-dipole configuration has a better signal/noise ratio and allows to go deeper, while multiple gradient configuration is a good compromise between resolution capacity both in lateral and vertical sense and its signal / noise ratio is comparable to Wenner and Schlumberger configurations. More in detail, Wenner array allows to get same results with a lower number of measurements compared to other geometries, ensuring a good horizontal coverage. Sensitivity is distributed mainly horizontally through profile centre, this is why it is indicated to detect vertical variations instead of horizontal variations. Signal intensity is inversely proportional to geometric factor, 2πa (almost small), so given that signal is strong, it can go deeper than other geometries. Investigation depth is about half of spacing a. Dipole-dipole is sensitive to horizontal variations, so it detects horizontal structures such as sedimentary levels. Although ensuring horizontal resolution better than Wenner array, investigation depth is lower. Another disadvantage is weakness of signal for high values of n, because voltage is inversely proportional to n 3. This means that for same current, 5

6 measured resistivity decreases of about 200 times while n increases from 1 to 6. A method to reduce this potential fall, is to increase a spacing. Therefore, to use this array in a proper manner, instrument must present an high sensitivity and a good circuit of noise refusal and during measurement phase it is recommended to check electrodes-soil coupling, which shall be optimal. A configuration such as " Wenner-Shlumberger is an hybrid and it is indicated in areas where there are both vertical than horizontal structures; it represents a good compromise between Wenner and dipole-dipole. In addition, there are arrays such as Pole-Pole which allow to go deeper but losing in terms of resolution, which remains too low. Also pole-dipole array presents a good horizontal coverage, but signal intensity is higher than dipole-dipole and it is not sensitive to noise as pole-pole array. Being asymmetrical, it produces asymmetrical anomalies also on symmetrical structures. To avoid this effect, measures are repeated according to an inverse disposition, so they are combined. EMLab software To allow an adequate control and improve preparatory phase of multielectrode survey, EMLab software was developed, in order to set parameters necessary for a correct configuration of the sequence to be used. For a detailed treatment of software features, please refer to user manual. Software allows to generate different types of array in 2D and 3D, layers number so as to control investigation depth. Once generated sequence, a.sem file is exported, which can be uploaded on instrument and recalled to perform measurement. Once executed measurement and saved file in tsv format, software can be used also as first phase of data processing through filtering operations such as deleting errors > 10% or delete resistivity measures < 1. Entering topography for each electrode and geometry setting are fundamental for inversion process; through its interface, software allows to enter coordinates for each electrode in different referential systems. 6

7 In this way file can be exported in.dat format and then opened and processed through proper inversion programs. EMLab allows to manage customers so as to facilitate operations of report creation and which eventually can also be statistics. On field On field acquisition phase consists in the use of arrays made of minimum 32 electrodes with a regular distance along investigated profile, or in case of 3D arrays, along X and Y directions of the area. Once placed and connected electrodes to multipolar cables through proper clamps, it is possible to turn on instrument. Before setting it, it is important to perform and save electrodes TEST through which checking of electrodes connection and computing of contact resistance between one another is performed; test generates a current flow and measures contact resistance between electrodes. During test, electrodes correctly connected are colored in blue, while those not communicating are colored in red. In the example in picture 3 electrode n. 7 is isolated, while all the other have contact resistance of about 1 kohm. Test result. 7

8 If it is necessary to reconnect an electrode, please check in advance if test is concluded, to avoid the risk of electric discharge absorption. Once concluded test, it is possible to save result in a file, for any later reference, by pushing Save button. CONNECTING SCHEME FOR MEASURES THROUGH GEOELECTRIC PROSPECTION CABLE Electrodes 1-24 Electrodes Connection up to 48 electrodes. prolunga interlink BOX n Electrodes 1-24 Electrodes Electrodes Electrodes Connection with expansion boxes and 96 electrodes. 8

9 Once performed electrodes TEST, it is possible to set instrument. First of all, in field Channels to be used select number of electrodes of which instrument is composed. Each measure is performed by current injection first with a polarity and then with the opposite, in order to reduce at minimum electrodes polarization phenomenon. In 'energization diagram' it is possible to define current input time and pause between one energization and the other. To change it, position cursor on one of the vertical lines of the diagram and move it towards left or right. Displayed times are those of default, which can fit most situations. Selection of field Symmetric wave ensures that times of current input with both polarities are equal. Moreover, for each electrode configuration, it is possible to iterate measure from 2 to 10 times and get average value as result. Set parameter 'Average on n iterations' on chosen value. If at least 2 iterations are required, at each measures conclusion a standard deviation indication will be displayed. If field 'measure only spontaneous potentials' no current will be generated and only values of spontaneous potentials for each electrode configuration will be measured. Select field measure chargeability' to perform simultaneously resistivity and chargeability measures (or induced polarization). Because determining this parameter requires sampling of potential discharge curve after an energization, times of energization diagram will be automatically increased, if necessary. This will cause naturally an increase of total duration of the survey. Parameter 'minimum current (ma)' determines value of the current which must be reached by instrument, gradually increasing voltage between electrodes A and B, before registering measure. Values most commonly used are in the range of 50 and 300 ma. To get more intense current, when dealing with sites characterized by high resistivities, such as igneous or metamorphic rocks, an external generator may be necessary, because product of voltage generated between A and B and current between electrodes (DV*I) must not overcome generator nominal power. In some situations limit maximum energization voltage can be useful to prevent generator protective fuse activation. 9

10 In field Maximum voltage it is possible to select values in the range of 50 Volt and 800 Volt. Take into account that setting of a too low voltage values can prevent to reach minimum current required for the measurement. In this case, measurement itself is anyway recorded with the highest value obtained. Normally instrument uses an absolute value for potential difference measured between electrodes M and N to compute resistivity. Once configured instrument, select array to use (file.sem) and set spacing between electrodes. Please notice that if user does not generate sequence, instrument already embeds different arrays which can be used with proper name according to type of array and electrodes number. By clicking on button 'Open', instrument uploads selected table and ask to enter distance between electrodes. During uploading, a series of test on configurations to check formal correctness is performed. To compute automatically geometric coefficient, used to determine apparent resistivity, it is assumed that electrodes are equidistant and aligned (except for those at infinite in pole-pole and pole-dipole configurations). 10

11 Measurement table At each measure a progressive number is assigned and used electrodes are displayed. Execution will give following values: o Input current (I in ma); o Measured potential between M and N (V in mv); o Spontaneous potential, measured immediately before energization and already detracted by V measure; o o o Apparent resistivity (r in Ohm x meter) computed; Sampling standard deviation; chargeability (M in msec) (if activated in configuration). It is recommended to perform some measure before starting entire sequence. Touch line corresponding in table and arrow on the left of the screen will indicate selected measure. Then click on button 'Single measure'. In fields labeled I: and V: current values and potential measured in each instant are displayed respectively, while below indicator turns red any time voltage generator is 11

12 active. After few seconds selected line is updated with the result. It is possible to estimate value of the acquisition by comparing percentage of standard deviation with resistivity measure. Ratio in the range of 5-10% can be considered normal. If low values of V are read (around 1-2 mv or less) it can be necessary to go back to configuration (by clicking Esc button) and increase of few tens of milliampère minimum current required. Once concluded setting checks, select first measurement not performed yet and click on button 'Start sequence'. After confirmation request, instrument performs one after the other all measures included in the table. If necessary, it is possible to stop sequence by clicking Stop button. In this case, instrument stops measure eventually ongoing and waits for new operations. After sequence interruption or when it is finished, it is necessary to save recorded data in instrument internal memory and/or on provided USB pen-drive. Then click Save button. As spacing between electrodes, that already specified at starting will be considered, while configuration type must be selected manually. This is particularly important when user wants to export data directly in.dat format used by inversion software RES2DINV by Geotomo software / ZONDRES by Zond software, because internal codification of format varies considerably according to each configuration type. By clicking on button OK screen to select file destination folder is displayed, which can be named and saved into specific format. It is possible to select 5 different formats. In any case, it is recommended to save in format 'values separated by tabulation (extension.tsv), which preserves all values visualized in table and so allows to open file again on same instrument. It is important to remember that.tsv file can be open again through EMLab software to perform proper analysis such as filtering some data or entering topography. To operate on generated file directly through RES2DINV / ZONDRES softwares, enter again saving tool and select DAT format. It preserves only apparent resistivity values and measurement points coordinates. 12

13 Elaboration principles To recording of measurements of apparent resistivity phase, follows interpretation phase, so called inversion process, which consists in the simultaneous use of numerical modeling algorithms to finite elements (or finite differences) and minimum squares optimization methods. Iterative procedure of resolution allows to estimate real resistivity distribution which turns into a graphical 2D or 3D image of easy comprehension such as the one displayed below. Through this process is then possible to make proper considerations. Data processing has two phases: Reconstruction of resistivity pseudosections / chargeability, previous filtering / clearing through proper software use; Computing of real resistivity values through bidimensional inversion and develop of an adequate distributive model of underground resistivity through inversion software at finite and / or distinguished elements, which shall be able to apply eventual topographical correction. As first approximation, pseudosection visualizes data distribution with depth. A valid support in interpretation of electric tomography can be given by inversion programs such as Loke s, but user must know limits and take results not as tout court" but with critical spirit. Therefore we want to underline that if many models are possible, given a series of resistivity values, inversion programs provide one of the possible solutions and not the solution. In conclusion, we can say there is no absolute correspondence between detected resistivity value of the model and that of the material into the soil and subsoil: only interpretation which takes into account geological, archeological and physical characteristics of investigated site can solve the issue of resistivity values variability. 13

14 REFERENCE RESISTIVITY Litotype Resistivity (Ωm) Fresh water Sea water Dried loose sands ~1000 Loose saturated sands in fresh water Saturated loam in fresh water Saturated clay in fresh water 5-20 Dried gravels >1000 Saturated gravels in fresh water Metallic rocks and minerals Resistivity (Ωm) Sedimentary Chalk limestone Clay Gravel Limestone Marl Quartzite Shale Sand Sandstone Igneous and metamorphic Basalt Gabbro Granite Marble Schist Slate Minerals and metallic minerals Silver 1, Graphite (massive deposit) Galena (Pbs) Deposit magnetite Sphalerite (ZnS) Pyrite 10 2 Chalcopyrite ,3 Quartz Rock salt Water and effect of salt content Pure water 10 6 Natural water Sea water 0,2 Salt 20% Granite 0% of water Granite 0,19% of water 10 6 Granite 0,31% of water

15 MATERIAL RESISTIVITY (Ω m) CONDUCTIVITY (Siemen/m) IGNEOUS AND METAMORPHIC ROCKS Granite 5x Basalt Slate 6x10 2-4x x x10-3 Marble x10 8 4x Quartzite x10 8 5x SEDIMENTARY ROCKS Sandstone 8-4x x Clay 20-2x10 3 5x Limestone 50-4x x SOILS AND WATER Clay Alluvial x Ground water Sea water OTHER MATERIALS Iron 9.074x x10 7 KCl 0,01M NaCl 0,01M Acetic acid 0,01 M Xylene 6.998x x

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