What is the geoelectric method used for?
Determining the distribution of electrical resistivity in the subsurface
It interprets resistivity variations in terms of geology, groundwater, or ore bodies.
The geoelectric method can be used to a depth of up to _______.
300 metres
This depth allows for significant subsurface investigation.
What law is the geoelectric method based on?
Ohm’s Law and Poisson’s equation,
It describes how electric current flows through the Earth.
In Ohm’s Law, what does (R) represent?
Measured resistance
It is a key component in calculating resistivity.
Materials with low resistivity conduct electricity well. Name two examples.
- Clays
- Brines
These materials are typically saturated and allow for better conductivity.
What is the goal of the geoelectric method?
Inject current into the ground and measure resulting voltages
This helps infer the resistivity distribution.
What does the geometric factor (K) depend on?
Electrode spacing and configuration
It is crucial for calculating apparent resistivity.
Name a typical electrode configuration used in geoelectric measurements.
- Wenner
- Schlumberger
- Dipole–Dipole
Each configuration has its own strengths and weaknesses.
What is a pseudo-section in geoelectric measurements?
A plot of apparent resistivity values vs. electrode separation and location
It visually suggests resistivity changes with depth and along the profile.
True or false: A pseudo-section is a true geological cross-section.
FALSE
It represents where the measurements are sensitive, not an actual geological profile.
To increase vertical sensitivity, what should you do?
- Increase spacing between current electrodes
- Expand array symmetrically in profile direction
- Use arrays suited for depth
These actions enhance the depth of investigation.
To increase lateral sensitivity, what should you do?
- Move array along profile while keeping spacing smaller
- Use arrays suited for lateral mapping
This helps highlight near-surface variations.
Ohm’s Law
V=R·I
Modstanden R er uafhængig af spændingen V.
Sammenhængen mellem spændingen V over og strømmen I igennem en modstand R er lineær.
Calculating resistance (R)
R=ρ*L/A
(R) = measured resistance (ρ) = resistivity of subsurface material (L) = length of current path (A) = cross-sectional area of flow
Materials with low resistivity
clays, brines, saturated soils
Materials with high resistivity
dry sand, bedrock, granite
How is appearent resistivity found?
Using known current (I) and measured voltage (V)
ρa=K· V/I
v: voltage
I: current
K: The geometric factor determined by electrode
What do we often assume with a geometric factor k?
Assuming a homogeneous and isotropic half space
