Electrical Surveying Flashcards
What is resistivity? What is its reciprocal?
Resistivity is the resistance (ohms) between the opposite faces of a unit cube of a material
Its reciprocal is conductivity (siemens/m)
How are resistivity and porosity related?
Most rock-forming minerals are insulators, so electrical current is carried mainly by the passage of electrons in pore waters, making porosity the dominant control on resistivity (resistivity increases as porosity decreases)
How and why is resistivity measured?
Resistivity is measured through artificially generated electric currents that are introduced into the ground and their resulting potential differences measured at the surface (using a resistivity metre). This can be used to determine the composition of rocks and distribution of subsurface minerals
How does vertical electrical sounding work and what is it used for?
Current and potential electrodes are maintained at the same relative spacing, with the entire spread progressively expanded about a fixed centre and readings taken as the current reaches greater depths
How does constant separation traversing work and what is it used for?
Current and potential electrodes maintain at a fixed separation and progressively move along the profile
What are the 2 common electrode spreads and what are their advantages?
The Wenner configuration keeps current and potential electrodes at equal spacing, so that spacing is gradually increased about a fixed central point for VES and the entire spread is moved at a fixed value for CST
The Schlumberger configuration contains inner potential electrodes at a smaller distance apart than outer current electrodes, with electrodes systematically expanded for VES and several movements of potential electrodes accommodated for for a given current electrode position in CST
What is induced polarisation and how is measured?
Induced polarisation occurs when polarisable materials are placed inside a magnetic field and retain magnetism temporarily. It is measured by injecting a current into the ground and then switching it off, measuring voltage decay as a function of time
What are the 2 ways in which electrical energy is stored in rocks through electrochemical processes?
Membrane polarisation: negative charge on the surface of minerals in contact with pore fluid attracts positive ions, inhibiting ion movement in the fluid so that negative and positive ions build up as a blockage in the presence of a current and return to original locations when voltage is removed
Electrode polarisation: a voltage is applied to either side of a pore space blocked by a metallic mineral, imposing charges on opposite sides of the grain and causing accumulation of ions. Charge builds up due to rate of conduction and removal of the voltage causes ions to slowly diffuse back to original locations
How are time-domain and frequency-domain IP measurements collected?
Time: The decay of voltage after a current is switched off is monitored for mineral changeability
Frequency: apparent resistivity is measured at multiple frequencies
What causes noise in IP surveying?
Telluric currents, barren rocks (membrane position), and electromagnetic coupling in measuring equipment
What sort of currents does the self potential method utilise and what information do they produce?
Natural electrical currents generated in the earth due to subsurface material electrochemical processes. These are associated with groundwater flow and mineralisation, and are useful for detecting base metal deposits and groundwater
What is required for a self potential anomaly to occur?
A causative body must lie partially in a zone of oxidation, as electrolytes in pore fluids below the water table undergo oxidation and release electrons upwards through the ore body, causing the reduction of electrolytes at the top to create a circuit
How are self potential surveys conducted?
Data is gathered by fixing 1 non polarising electrode in barren ground and another (connected via high-impedance millivoltmeters) over the survey area. Anomaly minimums and symmetry are interpreted
What is true of all self potential anomalies?
They are invariably negative and undergo no chemical reactions, creating potential differences at the surface. Steep slope lies downdip
What are the 3 different electrical surveying methods used for?
Resistivity measures the resistance of the subsurface to an electrical flow to provide information on bulk electrical properties, which can be used to map geological layers and locate groundwater or lithological changes
Induced polarisation measures time dependent response of polarisable materials to an applied electrical field to provide information about subsurface materials and their ability to store a charge. This is used to detect (magnetic) minerals and ore bodies
Self potential measures natural electrical potentials generated due to electrochemical processes associated with groundwater movement and mineralisation, making them useful for groundwater monitoring and mineral exploration
