Magnetometry Flashcards
Magnetometry
- Uses the existing magnetic field in the ground from the
Earth - Measures small changes due to Iron Materials
- Has a sensor - the height of the sensor above ground is
varied depending on surface contamination - Accuracy in position = 50 cm
- Maximum depth = 2 m
- Like a metal detector , highly sensitive to iron metal
Key Quantities and Units
Current = flow of charges
- defined as 1 Amp (A) = 1 coulomb per second = 6.24x10^19 electrons per second
Magnetic Field = force of currents
- defined as 1 Tesla (T) = 1 Newton per amp per metre
- 1 Tesla = 10,000 Gauss (G)
Magnetic Field
- Generated by the earth, magnetic material and current carrying coils.
- Denoted by either B or H
STRENGTH:
- Earths magnetic field = roughly 0.5 Oersted
- Anomaly in Earths Magnetic field = 1 gamma = 10^-5
Oersted
DIRECTION:
- compass needle points roughly towards North Pole
- Orientation specified - map heading and vertical
direction
Effect of Soil and Rock types
- Average iron oxide concentration = 6.8% (earths crust)
- Magnetic irons oxides = Fe2O3 (maghemite) and
Fe3O4 (magnetite) - Most soils/rocks contain iron oxides and are weakly
magnetic:
Magnetic field varies over different regions - Low magnetic susceptibility = Granite, Gravel and
sedimentary rock - High magnetic susceptibility = Iron-containing rocks and
Igneous rocks - CHALK = no iron oxides
Define Magnetic susceptibility
- This is how magnetic a material is
- Iron (Fe) has the highest magnetic susceptibility, and therefore magnetic materials contain a lot of iron
Effect of Soil disturbances
- Cause small-scale magnetic anomalies
-
Types of Magnetometers
The uses of Magnetometers
- Take sensitive measurements of magnetic fields
- Field strength measured along parallel line; can
measure total field or just vertical component
1. Proton Magnetometers
2. Gas Magnetometers (Alkali gas or optical pumping)
3. Fluxgate Magnetometers
USE: - Carried along lines, and data is recorded on a computer
- Results plotted on a contour map
- Sensitivity of 1 gamma required
What is Nuclear Precision?
- Provides basis for sensitively measuring magnetic field
- Based on spinning of the nucleus
- Initially upright, then gravity makes the top wobble
- Wobbling frequency is proportional to the strength of
gravity
Proton Magnetometer
- Protons present in form of a liquid
- Spins are initially aligned using a large coil and batter
- Spins then start precessing (wobbling) due to earths
magnetic field - The coil of the magnetometer detects the precession
- The frequency is converted to sound for the operator
- Changes in frequency are used to map out magnetic
anomalies
Proton Magnetometer
- Measure total Magnetic field
- Protons present in form of a liquid
- Spins are initially aligned using a large coil and batter
- Spins then start precessing (wobbling) due to earths
magnetic field - The coil of the magnetometer detects the precession
- The frequency is converted to sound for the operator
- Changes in frequency are used to map out magnetic
anomalies
Gas Magnetometers
- Measure total Magnetic field
- Same principle as Proton Magnetometer
- However uses the nuclei of rubidium Rb (or cesium Cs
or helium He) instead of protons - These are present as gas - only small volume required
- light is used to speed up gas atom relaxation
- Gas atoms move quickly in a container so can deal with
moderate gradients
Drawbacks of Gas Magnetometers
- More complicated in construction
- More expensive
Gradiometers
- These can be used to highlight the general Earths magnetic field variations
- Two detectors placed on a vertical pole, and the background variations effect both pole similarly
- An anomaly in the background will be stronger at the lower detector
Fluxgate Magnetometers
- Usually measures the vertical component of a magnetic
field - Doesn’t use the principle of nuclear precession
- Uses two pieces of strongly magnetic metal, called
“metglas” - The two opposing magnetic field = balanced
- Any external field = balance upset, this can be
measured accurately
-
Drawbacks of Fluxgate Magnetometers
- Affected by Tilting of the apparatus
