Slide 1

Given these dipole-dipole data, fill-in the missing column, and create a pseudo-section. What might be going on in this example? Slide 2 Pseudo-sections A pseudo-section is not a true resistivity section it is only a way of plotting the data. The data from a scale model illustrate the limitations note the typical inverted-Vee shape of the result Slide 3 Model study of pseudo-section Slide 4 Design of resistivity surveys 1.Survey objectives 2.Depth of target 3.Adequate signal strength 4.Sensitivity to target parameters 5.Symmetry of the response 6.Minimize manual repositioning of electrodes 7.Compatibility with Induced Polarization (see later) Slide 5 Design of resistivity surveys Minimize manual re-positioning of electrodes! Slide 6 Induced polarization (IP method) Discovered by accident by observation of resistivity meters slow decay of voltage in some areas Widely used in mineral exploration More recently found application in groundwater and environmental studies Major advantage: can detect disseminated deposits Slide 7 Induced polarization (IP method) IP Effect: Switch off current, voltage decays Switch on current, voltage builds up This is time-domain IP at t o current is switched off, the IP effect is measured by measuring the area A under the decay curve. Alternative is frequency-domain IP see later Slide 8 Mechanisms causing the IP effect Membrane polarization: in clays, electrolytic current flow is impeded by positive ions (attracted to negatively charged clay particles) charges accumulate, voltage builds up on release of current, charges drift back to equilibrium, voltage decays Slide 9 Mechanisms causing the IP effect Electrode polarization: metallic mineral grains conduct electronically, electrolytic ions accumulate at pore restrictions causing a buildup of voltage on current release, ions drift to equilibrium positions, leading to a voltage decay Slide 10 IP Effect groundwater and environmental applications Membrane polarization observed in clay minerals can be used as an indicator of clays Applications in aquifer studies, monitoring of clay-organic processes in organic contamination Salinity affects the strength of the effect Metal remnants, galvanic sludge, glazed ceramics are also chargeable Slide 11 IP Effect mineral exploration The greater the exposed metallic surface area, the stronger the effect Enhanced effect for disseminated mineral grains Often these are cases in which standard resistivity response is weak In mineral exploration, the effect of near-surface clays complicates the measurement Slide 12 IP Effect factors Slide 13 Time-domain IP measurement Use electronics to measure the area (i.e., the integral) A: This is called the chargeability (measured in ms) Typically t 1 ~0.5 s and t 2 ~1.0 s Slide 14 A square wave current source is switched at two frequencies The AC output voltage is measured for each input frequency At low frequency, the output voltage stabilizes to the DC response before the voltage is switched At high frequency the output voltage is lower, as the input switches before the voltage stabilizes Frequency domain measurement of Induced Polarization Slide 15 Frequency-domain IP measurement At very low switching frequencies, the voltage has time to build up to a level V DC At high switching rates, the voltage does not build up to V DC Instead we measure a lower voltage, V AC For each we can define an apparent resitivity: The apparent resisitivity thus decreases with frequency in the presence of an IP effect Slide 16 Frequency-domain IP measurement The apparent resisitivity thus decreases with frequency in the presence of an IP effect We define the Frequency-effect as In conductive areas the charge buildup is partially short- circuited, reducing the FE This is compensated for in the Metal Factor: Slide 17 Pseudo-sections Top: a pseudo-section from a groundwater application. Bottom: a real-section result using the same data. Slide 18 Pseudo-sections vs Real Sections Pseudo-sections: an inaccurate image of the sub-surface nothing more than a representation of the field data There are computer modelling methods for generating synthetic data, for a given 2D model of the resisitivity There are automatic methods for updating the model to be consistent with the data generally known as inverse modelling methods in resistivity/IP methods, the results are often referred to as real sections Slide 19 Model study of inversion Psuedo section Inversion result, or real section Slide 20 Model study of inversion Psuedo section Inversion result, or real section Slide 21 Next lecture: Electrical methods case studies