Properties - Electrical
Example plot showing the resistivity and phase of a desert soil sample. The sample is extremely resistive due to the lack of water, and exhibits non-linear behavior.
Sample prepared for a 4-electrode electrical property measurement.
A number of different electrical properties can be measured in PetLab, including resistivity, phase, dielectric permittivity and magnetic permeability.
Resistivity and Phase
Resistivity and phase are the most common properties measured. Resistivity is the measure of how strongly a material resists the flow of electric current. Resistivity units are ohm-meters. Resistivity measurements are made by transmitting electric current through a sample and measuring the voltage across the sample. The ratio of the measured voltage to the transmitted current is proportional to the sample's resistivity. Phase is a measure of a material's ability to store electric charge. For our measurements, phase is the lag, or lead, of the received voltage relative to the transmitted current. Phase is reported as an angle, typically in milli-radians or degrees.
Dielectric Permittivity and Magnetic Permeability
Dielectric permittivity and magnetic permeability are properties that affect the velocity of electromagnetic wave propagation within a material. Dielectric permittivity is a measure of the materials capacity to store electric charge when an electric field is applied. Relative dielectric permittivity is the ratio of a material's permittivity to that of free space. Magnetic permeability is the ratio of the magnetic induction within a material to the inducing magnetic field strength. Permeability is closely related to magnetic susceptibility. Relative magnetic permeability is the ratio of a material's permeability to that of free space.
Factors That Affect Electrical Properties
Electrical properties are highly dependent on a sample's water content and mineral composition. Typically, the amount and quality of pore water are the major factors controlling the observed resistivity of rocks and sediments. All other things being equal, a sample with a high water content will have a lower resistivity than a sample with a low water content; and a sample containing high total dissolved solid (TDS) water will have a lower resistivity than a sample containing low TDS water. Samples composed of metallic minerals (i.e. pyrite) can have a very low resistivity. Clay minerals, such as montmorillonite and illite, are moderately conductive and produce low resistivity measurements. Quartz and feldspars are non-conductive minerals, producing high resistivity measurements.
Many earth materials have non-linear electrical properties, where the observed properties vary as a function of measurement frequency and or current density. Therefore, to characterize the range of properties it is important to make measurements at different frequencies or at different current levels. PetLab is equipped with different instrumentation capable of making frequency dependant measurements from dc (direct current) to the gigahertz range (0 – 3 GHz), and can vary the transmitted current to produce current densities consistent with those used in field measurements.
Types of Electrical Measurements
Direct current (dc) meaurements
Direct current measurements are made by transmitting a constant current through the sample and measuring the voltage drop across the sample. Direct current measurements are typically made as 4-electrode measurements to avoid electrode impedance effects. Instrumentation varies, but typically includes a constant current power supply as a current transmitter and a digital voltmeter as a receiver.
LCR system making a 2-electrode, mid-frequency (100Hz to 1MHz) electrical property measurement on a sample of drill core.
Network Analyzer system used for making high frequency (30 kHz to 3 GHz) electrical property measurements. Note the sample holder, a General Radio GR-900 airline, is the brass tube inline with the cables. An unassembled GR-900 airline, showing the center conductor, end-caps and sample on a watch glass, are also shown.
Low-frequency measurements, 0.001 to 1,000 Hz
Low-frequency measurements are made using the lab's non-linear complex resistivity (NLCR) instrumentation (Olhoeft, 1985). Low-frequency measurements are usually made with 4-electrode sample holders to minimize electrode impedance effects. Electrical properties determined from the NLCR system include resistivity, phase and measures of linearity including Hilbert distortion and total harmonic distortion.
Mid-frequency measurements, 100 Hz to 1 MHz
Mid-frequency measurements are made using inductance-capacitance-resistance (LCR) meters. Mid-frequency measurements are typically made using 2- or 3-electrode sample holders. Electrical properties determined from the LCR measurements include resistivity and phase; relative dielectric permittivity can also be determined from capacitance measurements.
High-frequency Measurements, 30 kHz to 3 GHz
Example of high-frequency resistivity data from the Network Analyzer measurements.
High-frequency electrical properties are determined from scattering parameters (S-parameters) measured using a 2-port network analyzer (NA). The equations for deriving permittivity and permeability from S-parameter measurements are given by Patitz (1995). For the high-frequency measurements, coaxial air lines are used for sample holders. Electrical properties determined from the S-parameter measurements include dielectric permittivity, magnetic permeability and resistivity. Electric and magnetic loss tangents can also be determined from the S-parameter measurements.
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