Airborne magnetics

Airborne magnetics in the Antarctic Source: BGR

Sketch of the BGR airborne magnetics system Source: BGR

Airborne magnetic methods are used to determine the Earth's magnetic field and its anomalies by measurements from an aircraft. Since crustal rocks show different magnetisations, magnetic measurements can reveal information on the crustal structure. Surveys of low altitude flights are used to explore near-surface and locally restricted magnetic bodies such as ore deposits or artificial structures such as waste dumps. In somewhat greater flight heights and in flights over ice or water, the main focus is the exploration of geologic-tectonic structures on a regional scale. Results of the measurements are presented in the form of maps and profiles showing the anomalies of the magnetic field. Advanced data analysis methods include the modelling of shape and properties of the identified magnetic bodies. In BGR, airborne magnetics is done using the BGR helicopter, as well as using fixed-wing aircrafts in polar research.

In most cases, the sensors used in airborne magnetics are Caesium magnetometers measuring the total magnetic field. The output of Caesium magnetometers is a frequency that is directly proportional to the strength of the outer magnetic field (Larmor frequency) and can be measured with high precision. High resolution measurements are possible with such sensors. Vector magnetometers (Fluxgate sensors) are used for applications where not only the strength but also the spatial orientation of the magnetic field is of interest.

When using fixed-wing aircrafts the sensors are normally mounted in the wing tips ("Stingers"). In order to reduce the disturbing influence of the aircraft's magnetic field as much as possible, the magnetic sensors can be installed in tow-bodies ("birds") that are towed at some distance below and behind the plane or helicopter. Remaining signals from the aircraft can be identified through certain manoeuvres and be eliminated during data processing or using compensation systems. In applications where data of high spatial resolution are required, the sensor should be flown near to the Earth's surface. Therefore, in such surveys, the aircraft is flown following the topography at a constant ground clearance.

Besides magnetic variations caused by different magnetisations of crustal rocks, also the Earth's main dipole field and temporal variations due to effects in the ionosphere and the magnetosphere are measured. In order to identify temporal variations, magnetic base stations located at fixed positions near or inside the survey areas are operated during magnetic surveys. These records can then be used to correct the data of the airborne survey. The Earth's main dipole field is determined using a reference field and used for reduction of the survey data. The final result after application of the different corrections and reductions to the data is the values of the magnetic anomalies that give insight in the structure of the Earth's crust.