Airborne Magnetic Surveys

Analytical Signal The analytical signal in aeromagnetic data integrates vertical and horizontal magnetic field gradients into a single value (Nabighian et al., 2005) and is useful for interpreting geological structures with remanent magnetisation. It provides a position-independent measure of the magnetic field, with signal amplitude unaffected by source magnetisation direction.

Remanent magnetic anomalies appearing as strong magnetic lows in the TMI or RTP imagery will be resolved as positive anomalies in the analytical signal. The analytical signal filter has poor spatial resolution, often producing nebulous-looking anomalies. It is typically unsuitable for structural interpretation, however is very effective for mapping the edges of magnetic bodies and in areas with low magnetic field inclinations (Roest et al., 1992).

Vertical Derivatives

The first vertical derivative (1VD) is a high-pass filter that yields interpretable data related to geology. Conceptually, the 1VD can be visualised as the vertical magnetic gradient measured by two magnetometers positioned one above the other, capturing subtle changes in magnetic field strength at different elevations (Hood, 1965; Cooper and Cowan, 2004). An advantage of the 1VD is that it has no directional biases, the location of each anomaly peak is retained, and the anomaly becomes narrower, becoming closer to the width of the causative body (Isles and Rankin, 2013). Calculating the second vertical derivative (2VD) determines the rate of change of the first vertical derivative. The positions of peaks are retained in the second vertical derivative; anomalies are even narrower, closely reflecting the width of the source body. Enhancing short wavelength anomalies can often enhance noise, so derivatives higher than the second vertical derivative are rarely used except in cases of exceptional data quality.

Reduction to the Magnetic Pole

For TMI data, the shape of the anomaly is influenced by a combination of the shape of the source body and the inclination and declination of the magnetic field, which varies with location. Strong magnetic responses occur when the main and induced magnetic field vectors align. This results in a high-amplitude anomaly at the Earth’s poles directly above the magnetic source. At the equator, these anomalies appear on either side of the source and are offset when the field is inclined. Reduction to the pole processing recalibrates magnetic data to simulate collection at the magnetic poles, where the magnetic field is vertical. This aligns magnetic anomalies directly above their sources, ensuring the shapes more accurately reflect the positions and geometries of the causative magnetic bodies by minimising distortions from the Earth’s magnetic field inclination and declination.