Aeromagnetic Survey

Key Takeaways

• Caesium vapour (optically pumped) magnetometers are the current standard for production aeromagnetic surveys, achieving sensitivity of 0.001-0.01 nT (1-10 picotesla). The measurement relies on the Zeeman splitting of atomic energy levels in an alkali vapour cell illuminated by a circularly polarised laser beam at a frequency resonant with the caesium hyperfine transition; the cell's optical absorption varies with the component of the ambient magnetic field along the laser axis, and feedback electronics maintain the resonance condition, providing a continuous, dead-time-free measurement proportional to total field intensity. Proton precession magnetometers, the previous standard through the 1980s, measure the Larmor precession frequency of protons in a hydrocarbon fluid after a polarising pulse, achieving 0.1 nT sensitivity but with a dead time of 0.5-1 second between measurements, limiting the spatial sampling density at normal survey speeds of 220-370 km/h. Dual-sensor caesium systems with sensors fore and aft of the aircraft compute the vertical gradient of TMI in real time (dTMI/dz), which enhances resolution of shallow magnetic sources and allows separation of near-surface noise from deeper geological signals. Superconducting quantum interference devices (SQUIDs) offer theoretical sub-picotesla sensitivity but require liquid helium cryogenic cooling and have limited practical deployment in airborne configurations.
• Aeromagnetic survey design parameters control which geological features can be resolved and at what depth. Line spacing determines the highest spatial frequency (shortest wavelength anomaly) that can be reliably reconstructed from the data: Nyquist-equivalent sampling requires at least two lines per anomaly wavelength, so a 200 m line spacing can resolve anomalies with half-wavelengths as short as 400 m, corresponding to depth-to-source of approximately 200-400 m for compact bodies. Flight altitude above ground level (AGL) is a low-pass filter on the anomaly spectrum: each additional 100 m of altitude attenuates anomalies from a 1 km deep source by approximately 10% while attenuating anomalies from a 100 m shallow source by 60%, so low-altitude flight (30-80 m AGL for detail surveys, 60-150 m AGL for standard surveys) is essential for resolving near-surface features while remaining unambiguous about deeper basement structure. Tie lines orthogonal to production lines at 5-10 times the production line spacing provide redundant crossings for levelling: at each crossover point the TMI readings from the two orthogonal lines should be identical, and systematic discrepancies reveal levelling errors or heading-dependent sensor bias that are removed in the tie-line levelling step of processing. In the WCSB, the Geological Survey of Canada (GSC) maintains an open-access national aeromagnetic database with coverage at 200-400 m line spacing across Alberta and British Columbia, downloadable from Natural Resources Canada's Geoscience Data Repository.
• Processing of raw aeromagnetic data converts TMI measurements to interpreted geological products through a sequence of mathematical transformations. After IGRF removal and diurnal correction, the anomaly grid undergoes reduction to pole (RTP), which mathematically transforms the data to the anomaly shape that would be observed at the magnetic pole (where the inducing field is vertical), removing the characteristic asymmetric positive-negative dipolar shape of anomalies measured at non-polar latitudes and aligning anomaly centres over their causative sources. The total horizontal derivative (THD = sqrt((dA/dx)2 + (dA/dy)2)) highlights the edges of geological contacts and is used to extract fault and boundary positions automatically from the gradient maxima, providing map-resolution fault lineament data comparable in utility to a regional structure contour map. The tilt angle (TILT = atan(VD/THD), where VD is the vertical derivative) normalises anomaly amplitude and is particularly effective at mapping geological contacts in areas with large susceptibility contrasts. Upward continuation convolved with the TMI grid produces a smoothed anomaly image that suppresses shallow noise and enhances deeper crustal sources: upward continuation to 5 km AGL shows only basement and deep crustal features, while the original near-ground data captures shallow detail including dykes and sills within the sedimentary section.
• Depth to magnetic source estimation is the primary quantitative product of aeromagnetic interpretation for petroleum basin analysis. Euler deconvolution calculates a structural index N (0 for contacts, 1 for dykes and cylinders, 2 for spheres) and a depth estimate at each point in a moving window by solving the Euler homogeneity equation: (x-x0)dT/dx + (y-y0)dT/dy + (z-z0)dT/dz = -N(T - B), where (x0,y0,z0) is the source location, B is the regional field, and T is the anomaly. Solutions are filtered by structural index consistency and solution cluster coherence to produce a point cloud of depth estimates that is then contoured into a basement depth map. Euler deconvolution depths carry uncertainties of 10-30% of the estimated depth in areas of complex geology or overlapping source contributions; in a WCSB setting with 2-6 km of sediment over Precambrian basement, the basement depth estimate from Euler solutions is typically accurate to within 300-600 m, sufficient to identify structural highs with basement relief of 1 km or more and to prioritise areas for follow-up seismic acquisition. Werner deconvolution provides an alternative depth estimate for linear features such as dykes and basement contacts, fitting two-parameter models to short-wavelength anomaly segments and yielding consistent results with Euler for simple source geometries.
• In the Western Canada Sedimentary Basin, aeromagnetic surveys provide critical pre-drill information for two distinct geological problems. First, basement depth mapping by the Alberta Geological Survey (AGS) and GSC has established that Precambrian basement lies at 2.5-4 km depth across the central Alberta plains, deepening to 5-8 km in the Peace River Embayment and rising to near-surface in the Peace River Arch basement high, which exerts first-order control on the preserved thickness of Devonian and Mississippian carbonate reservoirs including Swan Hills, Leduc, and Slave Point formations. Second, aeromagnetic lineament mapping reveals northeast-trending basement faults that propagate upward as Precambrian-rooted fault systems and control the distribution of Devonian reef complexes, Cretaceous channel sands, and Triassic Montney formation compartmentalisation in the Deep Basin. The Buffalo Head Hills kimberlite field in northern Alberta was first delineated by aeromagnetic anomalies (kimberlites carry titanomagnetite xenocrysts and produce distinctive circular positive anomalies of 500-3,000 nT magnitude) prior to any drilling, demonstrating that aeromagnetic surveys can guide the drill bit toward structurally and lithologically constrained targets even at depths of 400-900 m. GSC open-data aeromagnetic coverage of WCSB at 200-400 m line spacing is accessible at no cost, making it a standard input to all Alberta and BC play fairway analyses.