Magnetic surveying Flashcards
From where do you get the expected magnetic value for a location?
The expected value of the field at any place is that of International Geomagnetic Reference Field (IGRF). The difference between the observed and expected values gives the magnetic anomaly
Which two magnetometers are comonally used?
Proton precision magnetometer: Measures the total strength of the magnetic field but not its direction and so shows a total field anomaly (total intensity anomaly).
Does not drift with time but is best kept stationary.
Fluxgate magnetometer: Measures the component of the field along the axis of the sensor, ususally align vertical which gives the vertical component anomaly. Best for continuous readings, gives faster and higher precision.
How can magnetic measurements be taken and what is important to think about when doing measurements?
From land, sea or air. Air is the most efficient way.
Important to keep the magnetometer away from the impact of metals in the ships or aircrafts.
What is meant by diurnal variations?
It is variations believed to be caused by electric currents induced in the Earth from an external source. In this case, the external source is believed to be electric currents in the upper atmosphere. These electric currents are driven by solar activities.
What is meant by magnetic storms?
Occasionally, magnetic activity in the ionosphere (a region of the upper atmosphere) will abruptly increase. The occurrence of storms correlates with enhanced sunspot activity. The magnetic field observed during such times is highly irregular and unpredictable. Exploration magnetic surveys should not be conducted during magnetic storms.
Reduction to the pole is done to magnetic data, what does it mean?
Removing the dependence of magnetic data on the magnetic inclination, i.e., converting data that have been recorded in the inclined Earth’s magnetic field to what they would have looked like if the magnetic field had been vertical.
What is meant by upward continuation?
The method effectively attenuates high-wavenumber anomalies due to near-surface features, thus providing a powerful method for examining deeper structures. In magnetic surveys it is particularly useful in tying together aeromagnetic surveys made from different flying altitudes.
The upward continuation is done and then subtracted from the total field to show deeper structures
What is meant by downward continuation?
Can be used to emphasize the anomalies of near surface bodies, but it needs to be used with caution because unwanted anomalies will also be emphasized.
Why is Earth’s global field often subtracted from measurements?
Due to the magnetic field being twice as strong at Earth’s poles than at the equator.
Vad är enheten för magnetisk flödestäthet?
Symbol B, enhet Tesla
Vad är den magnetiska flödestätheten i Sverige?
Ungefär 50 mikroTesla
What’s the advantage of aeromagnetic surveys?
arge and perhaps inaccessible areas can be surveyed rapidly and more cheaply than by land-based surveys.
It won’t provide the detail of a ground survey. However, this may be an advantage if a regional survey of the deeper rocks is required, preferably without the effects of near-surface rocks.
