OPERATIVE GUIDE INDUCED POLARIZATION

Transcription

1 OPERATIVE GUIDE INDUCED POLARIZATION 1

2 Induced polarization geoelectric procedure Generals Induced polarization (capacitive effect) represents attitude of a material to collect electric charges of opposite polarity (charge as an electric condenser) along discontinuity surface between means of different resistivity, during flowing of current into subsoil; it allows discharge in instants immediately following current interruption. Soils with high capacitive effects (high chargeability) are those where there is simultaneous presence of very reductive parts and very resistive parts, without continuity of the first. Induced polarization is a phenomenon generated by electric current stimulation observed after voltage application into the ground. Generally speaking, a curve of potential decay is observed after interruption of input current. Induced polarization method is based on the existence of electro-chemical phenomena generated by contact between conductive means of different nature. Polarization is performed through a circuit made of two electrodes which induce polarization into the ground, and a measurement circuit made of other two electrodes connected to a potentiometer. This method is mostly used in mining field. Induced polarization sources can be: - oxidation-reduction process along interface between metallic grains and interstitial fluids (electrode polarization) - moving electrolyte ions accumulation after movement variations along path (electrokinetics polarization). To have electro-kinetics polarization, there shall be fibrous or lamellar silicate minerals negatively charged on surface which attract cations. Clay (fillosilicates) is necessary to have polarization. Use of induced polarization (also known as chargeability) is very useful in environmental field (waste dumps) and cultural heritage field. As for resistivity measures, also for induced polarization (chargeability) there is a current input and potential difference when using quadrupolar systems is detected. More in detail, current is input for a longer period, so as to have a more stable signal. 2

3 Residual potential difference will be detected after some time (few milliseconds) until current decays to zero. Potential difference decay it is not detected in continuous but at short temporal steps so as to allow reconstruction of charge damping. Once created damping curve, area under it is calculated and, dividing for potential difference (initial V final V), chargeability or means charge capacity is obtained. Chargeability represents concentration of a conductor near multielectrode array. When stopping current flow at electrodes A and B, potential difference between M and N does not interrupt instantaneously, but it is discharged in a determined time according to a specific trend. It can be expressed both in time domain, with relaxing tensions at electric current interruption at step, and in frequency domain, with an exact law of electric resistivity dispersion at energizing alternate current flow frequency variation. Required time to restore balance conditions is related to soil polarization grade. If a metallic or similar corp is included into the ground, there will be anomalies in electric decay depending on time. Same phenomenon is observed also for polarization of organic contaminants which may eventually be into the ground of a polluted site. Typical configuration for this kind of data acquisition varies according to many variables. Depth of investigation depends on poles distance (n x a). Usually more values for n are used, allowing to perform pseudosections of ground polarization. For an homogeneous soil of infinite dimension, decay time depends on soil resistivity and does not change at changing of two points between whom potential difference is measured..field experience suggests that IP survey can often detect buried metallic objects of quite small dimensions, difficult or impossible to find with resistivity method. 3

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5 Acquisition mode Measures acquisition modalities is similar to electric resistivity, therefore similar configurations can be used. Generally speaking, there are three ways to measure polarization effects: In time domain, in frequency domain, in spectral analysis. Depth of investigation depends on electrodes distance and the kind of array chosen for measure. More in detail, measures are performed after a delay time of few milliseconds, preferably one second (M Delay Time), so as to allow effects of cables pairing and other parasites effects vanish or at least are negligible. Measures of partial apparent chargeability (Mi) and deducted global average (Ma) provide information on soil capacity to be charged by current flows. Field operations first of all require placing of electrodes in line with sequency (array) that will be used for measurement. Once properly placed and connected electrodes, instrument can be configured after testing of electrodes connection through Electrodes test section. In "configuration" screen, select chargeability measure box to perform resistivity and chargeability measures simultaneously (or induced polarization). As determining this parameter requires sampling of potential discharge curve after energization, times of energization diagram will be automatically increased. A value of 1200 ms is acceptable. As a consequence, total duration of the survey increases. 5

6 (ms) M Tomographic inversion of field recorded data gives as output chargeability profile (M in mv/v or msec) from which it is possible to make related interpretations. Chargeability is in general proportional to the charge stored into litotype and represents concentration of a conductor near multielectrode array. Practically speaking, soils which present best capacitive effects, that is an high chargeability, are those in which there is contemporary presence of very conductive parts 6

7 together with very resistive parts, without marked continuity between them. Among these situations, areas with metallic conductive mineralizations (eg. Sulfides) spread in compact and resistant rocks, rough granulometry floody soils with a matrix of fine elements and also areas such as garbage dumps or polluted sites where leaking effluents, generally very conductive, can be locally confined in areas electrically isolated or can contain insulating barriers which, although easily eludible, act as areas of momentary stack of electric charges. Materials and minerals Chargeability (msec) Pyrite 13.4 Galena (lead sulphide) 3.7 Rocks with 20% sulphides Rocks with 8-20% sulphides Rocks with 2-8% sulphides Magnetite 2.2 Sandstones and silts Tuffs Marnes and clays Limestones and dolomites Compact vulcanic rocks Granites Underground waters 0 Alluvial material 1-4 Gravel 3-9 7

8 Induced Polarization Method White measure Uncontaminated water level Clay minerals presence Anomalies of contaminated water level chargeability (anthropization) Pianvallico contaminated water levels??? Clay minerals presence Contamination * Degraded clays **Marnes **Sandstones ** Underground waters 8

9 Induced Polarization Method Analysis of soil response that once crossed by continuous current input into the ground shows different discharge times Target Individuation of chargeability anomalies related to chemical characteristics alteration of ground water in the area which probably is contamination source Non-polarizing probe Single probe commercial cost: 100 Removal of electrolytic concentration dissymmetry on the electrode, copper sulphate (CuSO4) allows a good probesoil contact. 9

10 * Degraded clays **Marnes **Sandstones ** Underground waters 10

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