Question 6 of 8: Magnetic Anomaly Interpretation and Instrument Comparison

Question 6: Magnetic Anomaly Interpretation and Instrument Comparison (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

[Figure not reproduced: The two magnetic anomaly profiles from the exam figure, redrawn: Anomaly A is narrow and high-amplitude; Anomaly B is broad and lower-amplitude. See the official exam paper.]

(a)(i) Shallower source. Anomaly A (the narrow, sharply-peaked anomaly). For two identical sources (same shape, size, magnetization direction and intensity) differing only in depth, a shallower source produces an anomaly that is spatially NARROWER and has a shorter horizontal wavelength — the anomaly's characteristic width scales with the source-to-observation distance, exactly as a nearer light source casts a smaller, sharper-edged shadow than a farther one of the same size. Anomaly A's much tighter, more sharply-peaked shape is therefore diagnostic of the shallower body.

(a)(ii) Stronger magnetization. Distinguishing this from depth alone requires comparing anomaly SHAPE (width, which fixes depth) against PEAK AMPLITUDE independently. Because the two sources share identical size/shape and the profile shows Anomaly A both narrower AND of comparable-or-greater peak amplitude to the broader Anomaly B, the fact that B still reaches a substantial amplitude despite being from a deeper (more attenuated) source implies the source under B has the stronger magnetization — a deep source needs a stronger dipole moment to still produce a comparable surface amplitude, since amplitude falls off with a high power of depth ($\propto 1/z^2$ to $1/z^3$ depending on source geometry) while width only grows roughly linearly with depth.

(a)(iii) Estimating depth from profile shape alone. The standard technique is a half-width method: measure the anomaly's half-width $x_{1/2}$ (the horizontal distance from the peak to the point where the anomaly falls to half its peak amplitude) and multiply by a shape-dependent depth factor determined by the assumed source geometry (e.g. for a point dipole depth $\approx x_{1/2}$; for a thin horizontal sheet or a 2-D horizontal cylinder, a different tabulated multiplier applies) — exactly the same half-width principle used quantitatively for the gravity cylinder in Question 7(b), where $z=x_{1/2}$ for that specific source geometry. Alternatively, characteristic-point methods (using the horizontal distance between the anomaly's maximum and its inflection points, or between the peak and the zero-crossing) give equivalent depth estimates for a chosen source model.

(b) Magnetic instrument comparison.

• (i) Proton precession magnetometer. Advantages: measures absolute TOTAL field with high absolute accuracy (~0.1–1 nT) and no orientation sensitivity (reads the same regardless of sensor tilt, unlike a fluxgate), simple, robust, low drift, and needs no regular calibration — the standard base-station and reconnaissance instrument. Disadvantages: relatively slow cycle time (a proton-precession "polarize then measure" cycle takes roughly 1–3 seconds, limiting survey speed/spatial sampling density) and it measures only the TOTAL field magnitude, not individual vector components.
• (ii) Fluxgate magnetometer. Advantages: measures a VECTOR component continuously and very rapidly (fast enough for airborne/continuous-tow surveys and for gradiometer construction), compact and low power. Disadvantages: is orientation-sensitive (must be actively stabilized/oriented, since it reads only the field component along its own axis) and has poorer absolute accuracy and more drift/temperature sensitivity than a proton-precession or optically-pumped sensor, so it needs frequent calibration/base-station correction.
• (iii) Magnetic gradiometer. (Typically a pair of proton-precession or fluxgate sensors at a fixed vertical or horizontal separation, differenced.) Advantages: the DIFFERENCE between two closely-spaced sensors cancels the diurnal/regional field variation and much of the far-source regional field automatically (no separate diurnal correction needed), and it dramatically enhances the signal from small, shallow, near-surface targets (utilities, UXO, shallow archaeology) by emphasizing the field's spatial GRADIENT. Disadvantages: the gradient signal falls off even faster with depth than the total field itself, so gradiometers are poor at detecting deep/regional targets, and precise, stable sensor separation/orientation must be maintained (a small relative sensor-tilt error becomes a large fractional error in a small gradient measurement).