resistivity Flashcards
What is Ohm’s Law?
The relationship between applied current and the resulting potential
–ΔV = IR
–(ΔV / L) = ρ (I / A)
What is resistivity?
A material’s ability to oppose electric flow
- inverse of conductivity
What is the conservation of electrical charge?
a physical law that states the change in the amount of charge in a fixed volume is equal to the amount that flows into the volume less the amount that flows out of that volume
What is the flow pattern for current flow lines
Electric current flow through a material from a source (+) electrode to a sink (-) electrode.
What are equipotential surfaces
the potential field associated with current flow lines
- intersect flow lines at 90 degrees
What types of rocks have low resistivity?
Rocks with metallic minerals, porous rocks/water saturated rocks
Describe electrical conduction
Free electrons that exist in metals and graphite act as the charge carriers. These electrons move very easily when a voltage difference is applied to these materials.
Rocks containing metallic ores or graphite can have very low resistivity due to electronic conduction.
Describe ionic conduction (electrolytic conduction)
In porous rock or soil containing water, electrical current can be transported through the pore space using the dissolved ions in the water as charge carriers.
Depends on concentration & mobility of dissolved ions, porosity & water saturation
Describe interfacial conduction (surface conduction)
Electrical structures called diffuse double layers occur along solid/pore water interfaces. Electrical current is conducted along the surface in these structures.
- independent of electrolytic conduction component
What is the least resistive rock type?
Sedimentary
(due to lower porosity and lack of pore connectivity in igneous/meta)
Are unconsolidated or consolidated rocks more resistive?
Consolidated.
Does fracturing and other permeability enhancing processes (e.g., jointing, weathering, dissolution) increase or decrease resistivity?
Decrease
How do surface resistivity methods work? (i.e., general 4 electrode array)
measurement of electrical potential/voltage at points along the Earth’s surface which result from the electric flow between the current electrodes
Source and sink A B
Potential electrodes M N
A M N B
What is apparent resistivity pa
Apparent resistivity is calculated under the assumption that Earth is homogenous. Actual resistivity can vary from this as Earth is heterogeneous
How can we determine the resistivity of the individual layers from surface resistivity measurments
Inversion.
If ρ1 > ρ2, then θ1 < θ2, which way do current flow lines bend?
Away from the normal
If ρ1 < ρ2, then θ1 > θ2, which way do current flow lines bend?
Towards the normal
Describe vertical electrical sounding (VES)
- vertical variation where bedding is horizontal
- performed by systematically expanding array dimensions while the array remains centered at a fixed midpoint
- As the array expands, current flow lines penetrate deeper into the Earth.
Describe electric profiling / constant separation transversing (CST)
- horizontal variations
- systematically moving an array with fixed electrode separations along a profile line
Describe electrical resistivity tomography (ERT)
- horizontal and vertical data
- regularly spaced electrodes
What is the Wenner array?
equally spaced electrode where the inner electrodes are the potential electrodes; the outer electrodes are the current electrodes.
A M N B
What is the Schlumberger array?
The inner electrodes are the potential electrodes; the outer electrodes are the current electrodes.
- Spacing of inner is small
A. M N. B
What is the Dipole-dipole array
the current and potential are paired to form a current dipole and a potential dipole at the ends of the array
B. A M N
What is the principle of equivalence
A basin shaped curve would cause it to appear that there is a relatively more/less resistive layer at depth
What is the the principle of suppression
inability to detect intermediate layers due to sounding curve indicating a progressive increasing or decreasing resistivity with depth
Applications of resistivity methods
Near-surface stratigraphy and aggregate exploration
Bedrock surface mapping
Engineering & geotechnical issues (e.g., landslide monitoring & characterization)
Groundwater resource management
Groundwater salinity & contamination
Agricultural application (e.g., monitoring soil moisture, soil salinity)
Archaeology
