4.3 Magnetic surveying and data interpretation

Magnetic Surveying Principles and Techniques

Principles and Instrumentation

Magnetic surveying measures variations in Earth's magnetic field to detect subsurface geological features and mineral deposits. Because the magnetic field is a vector quantity with both magnitude and direction, these variations carry information about what lies beneath the surface.

Ground magnetic surveys use portable magnetometers carried along traverses or across a grid pattern. The two most common instrument types are:

• Proton precession magnetometers measure total field intensity by detecting the precession frequency of protons in a hydrogen-rich fluid. They're robust and require no orientation but sample discretely rather than continuously.
• Fluxgate magnetometers measure individual components of the field vector. They can sample continuously and are sensitive to field direction, but they need careful orientation.

Airborne magnetic surveys mount magnetometers on fixed-wing aircraft or helicopters, allowing rapid coverage of large areas. These surveys are typically flown along parallel lines at a constant elevation above terrain.

Survey Design and Data Collection

Magnetic data are recorded as total magnetic intensity (TMI) or as individual components of the field vector (horizontal, vertical, or total). Several design parameters control the quality and cost of a survey:

• Line spacing and sampling interval determine spatial resolution. Closer spacing gives finer detail but increases time and cost.
• Survey altitude controls sensitivity to shallow sources. Lower altitudes enhance detection of near-surface features, while higher altitudes smooth out shallow noise and emphasize deeper, broader anomalies.

The right balance depends on target depth and the scale of features you're trying to resolve.

Magnetic Anomalies and Subsurface Geology

Magnetic Properties of Rocks and Minerals

Magnetic anomalies are local deviations from the expected background field, caused by magnetic minerals in the subsurface. The most important of these minerals is magnetite ($Fe_3O_4$). Anomalies can be:

• Positive: magnetic intensity higher than the background field
• Negative: magnetic intensity lower than the background field

The key physical property controlling anomaly strength is magnetic susceptibility, which describes how easily a material becomes magnetized in an external field. Ferromagnetic minerals like magnetite have very high susceptibility and are the primary sources of magnetic anomalies.

As a general rule, igneous and metamorphic rocks tend to have higher magnetic susceptibility than sedimentary rocks, making them more likely to generate detectable anomalies. This is because crystallization and metamorphic processes concentrate magnetite and other iron-oxide minerals.

Geologic Structures and Mineral Deposits

The shape, amplitude, and wavelength of a magnetic anomaly encode information about the geometry, depth, and magnetic properties of the source body.

• Mineral deposits containing magnetic minerals (e.g., iron ore with abundant magnetite) produce distinct anomalies that aid detection and delineation.
• Faults juxtapose rocks with different magnetic properties and often appear as linear features or sharp offsets in the magnetic data.
• Folds can produce characteristic "bulls-eye" patterns (for domes or basins) or arcuate anomalies that trace the fold axis.
• Intrusions of mafic or ultramafic composition typically stand out as strong positive anomalies against less magnetic country rock.

Data Processing for Magnetic Surveys

Raw magnetic data contain signals from both the geology you care about and several sources of noise. Processing strips away the noise and reshapes the data to make interpretation more straightforward.

Diurnal Correction

Earth's external magnetic field fluctuates throughout the day due to ionospheric currents driven by solar activity. These diurnal variations can reach tens of nanoteslas and must be removed.

1. Set up a base station magnetometer at a fixed location within or near the survey area.
2. Record the field continuously at the base station throughout the survey day.
3. Subtract the time-varying base station signal from each survey reading, matched by timestamp.

This isolates the spatial variations caused by geology from the temporal variations caused by solar activity.

Reduction-to-Pole (RTP)

At most latitudes, the Earth's magnetic field is inclined rather than vertical. This inclination causes magnetic anomalies to be asymmetric and shifted away from directly above their source bodies. Reduction-to-pole is a mathematical transformation that recalculates what the anomaly pattern would look like if the field were perfectly vertical (as at the magnetic pole).

After RTP, anomalies are centered over their causative bodies, which makes interpretation much more intuitive. RTP works well at mid to high latitudes but becomes unstable near the magnetic equator where inclination approaches zero.

Additional Processing Techniques

• Leveling removes systematic differences between adjacent flight lines caused by heading errors or drift.
• Gridding interpolates irregularly spaced data onto a regular grid for map display.
• Filtering enhances or suppresses features at different spatial scales:
  • Low-pass filters smooth the data, emphasizing broad, deep sources.
  • High-pass filters sharpen the data, highlighting shallow or small-scale features.
  • Band-pass filters isolate anomalies within a specific wavelength range.

Magnetic data are often integrated with other geophysical datasets (gravity, electromagnetic) and geological information such as drill hole logs, geologic maps, and seismic sections. This integration constrains the interpretation and reduces the inherent ambiguity of potential field methods.

Interpreting Magnetic Data for Geology and Minerals

Magnetic Anomaly Maps and Profiles

Anomaly maps display the spatial distribution of magnetic field variations across the survey area using color scales or contour lines. They let you identify regional trends, locate discrete anomalies, and recognize geological patterns like fault networks or lithological contacts.

Magnetic profiles plot field intensity along a single survey line, giving a cross-sectional view. Profiles are especially useful for analyzing the shape, amplitude, and wavelength of individual anomalies, which feed directly into depth and geometry estimates.

Interpretation Techniques and Considerations

Interpreting anomalies involves linking their characteristics to plausible geological sources:

• Positive anomalies often indicate highly magnetic lithologies such as mafic intrusions (e.g., gabbro) or iron-rich ore bodies.
• Negative anomalies may indicate less magnetic rocks (e.g., sedimentary units) or hydrothermal alteration zones where magnetite has been destroyed.

Anomaly shape carries geometric information:

• Symmetric, circular anomalies suggest vertical or steeply dipping bodies, such as kimberlite pipes or vertical plugs.
• Elongated or asymmetric anomalies point to dipping structures like inclined dikes or fault-bounded blocks.

Depth estimation techniques relate anomaly shape to source depth:

• The half-width rule uses the horizontal distance from the anomaly peak to the point where intensity drops to half its maximum value. For a simple sphere, the depth to center is approximately $z \approx 1.3 \times x_{1/2}$, where $x_{1/2}$ is the half-width.
• Euler deconvolution is an automated method that solves for source location and depth using the spatial derivatives of the field, with a "structural index" chosen to match the expected source geometry (e.g., 0 for a contact, 1 for a dike, 2 for a horizontal cylinder, 3 for a sphere).

As with all potential field methods, magnetic interpretation suffers from non-uniqueness: multiple subsurface configurations can produce the same surface anomaly. Integrating magnetic results with gravity data, geological mapping, and drill hole information is the best way to narrow down the most geologically reasonable model.