Magnetic filters and derivatives: Tilt, Analytic Signal and Horizontal Gradient

The raw magnetic field is only the starting point. To extract structural information, you apply filters and derivatives that enhance different features of the signal. Each one answers a specific question about the geology. Here are the ones most used in mineral exploration.

Residual magnetic anomaly

Before any filter, the regional field is removed to isolate the residual anomaly — the local variations caused by nearby rocks. It is on this residual that the other products are calculated.

Analytic Signal (Total Gradient)

The Analytic Signal combines the field derivatives in all directions and has a valuable property: its maxima tend to sit over the edges of magnetic bodies, regardless of the magnetization direction. It is one of the best products to delineate body boundaries and shallow sources.

Tilt Derivative

The Tilt Derivative normalizes the signal and has the advantage of enhancing both strong and weak sources on the same scale. It is particularly effective for mapping contacts and structures, because its zero crossing usually coincides with source edges. This makes it one of the favorite filters for structural mapping.

Horizontal Gradient Amplitude (HGA)

The Horizontal Gradient measures the lateral rate of change of the field. Its amplitude (HGA) produces narrow ridges over contacts, making it excellent for tracing lineaments and lithological boundaries with precision. Many interpreters use HGA and Tilt together to validate structures.

Vertical derivative (dZ)

The vertical derivative enhances high-frequency detail and shallow sources, sharpening edges that the raw field smooths out. It is useful to highlight surface structures and small bodies.

How to combine everything

No filter is "the best" on its own — they are complementary:

• Want to delineate a body's edge? Start with the Analytic Signal.
• Want to map contacts and faults? Tilt and Horizontal Gradient.
• Want to detail shallow structures? Vertical derivative.

The experienced interpreter alternates between these products, looking for coincidences: when several derivatives point to the same feature, confidence in the interpretation increases.

Processing automation

Generating these filters one by one in specialized software is laborious. Automation applies the entire processing chain in a standardized, reproducible way, delivering the complete set of derivatives ready for analysis — reducing weeks of work to a single run.

How Lithogeo helps

This is exactly the kind of work the Lithogeo platform automates. From an ANM process number or a polygon of your area, it processes and integrates the public data — magnetometry, gamma-ray spectrometry, geology, terrain and mineral occurrences — and delivers the products described here ready for analysis, in minutes and right in the browser.

Educational content. The choice and parameterization of filters depend on the objective and geological context of each project.