04-Geol-B10 · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-B10-1 Gravity and Magnetic Fields, 2017-Dec. Closed book; no calculator permitted. All ten questions require an answer in essay format, with diagrams used wherever appropriate. The exam instructs "choose six (6) of the following ten (10) questions, the first six as they appear in the answer book will be marked, each of equal value".
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics, 2nd ed. (physical properties ch.2 & 5; gravimeters, gravity reduction and terrain correction ch.2; magnetometers and magnetic surveying ch.4–5; anomaly interpretation throughout); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration, 3rd ed. (survey design, diurnal correction, case-history applications ch.6 & 7); Blakely, Potential Theory in Gravity and Magnetic Applications (potential-field theory, Fourier-domain filters, reduction-to-pole, non-uniqueness ch.2, 5, 9 & 12).
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.
The sensor is a bottle of a hydrogen-rich fluid (kerosene, decane, or water) wound with a coil. A strong DC polarizing current is passed through the coil for a second or two, creating a magnetic field far stronger than the Earth's field and aligning most of the fluid's proton (hydrogen nucleus) spins along it. The current is then abruptly switched off. The protons, now spinning (with angular momentum), no longer aligned with any applied field, precess about the ambient (Earth's) field direction like a spinning top precessing about gravity, at the Larmor frequency:
$$f = \frac{\gamma_p}{2\pi}\,F$$
where γp/2π = 0.042577 Hz/nT is the proton gyromagnetic ratio (a fixed physical constant) and F is the total ambient field strength. This precession induces a small, decaying AC signal in the same coil, whose frequency is measured very precisely by a frequency counter; because f depends on nothing but the physical constant γp and the field F, the instrument gives an absolute measurement of the total field magnitude without requiring calibration against a reference standard. For a representative mid-latitude field of F = 50,000 nT, this gives f ≈ 2,128.9 Hz.
| Quantity | Value |
|---|---|
| Proton gyromagnetic ratio, γp/2π | 0.042577 Hz/nT |
| Example field strength, F | 50,000 nT |
| Precession (Larmor) frequency, f | 2,128.9 Hz |
Because f is fixed by a known physical constant, the proton-precession magnetometer needs no periodic recalibration and gives the same absolute reading regardless of manufacturing tolerances between individual sensors. It measures the TOTAL field magnitude independent of the sensor's orientation (only a coarse pointing, avoiding the narrow cone around the field's own axis, is needed — unlike a fluxgate, which must be precisely oriented along each axis it measures), making it fast and simple to operate in the field, and it is mechanically robust with no moving parts, tolerating rough handling on a ground survey.
The instrument measures only the scalar TOTAL field, not vector components, so directional information about the source must be inferred indirectly (from the anomaly's shape across a grid, or from a gradiometer pair) rather than read directly, unlike a fluxgate. The polarize–precess–measure cycle takes roughly one to a few seconds per reading, appreciably slower than continuously-sampling optically pumped (caesium/potassium vapour) magnetometers, which limits its use in fast-moving airborne surveys. It also has a dead zone: near the magnetic equator and poles, where the ambient field lies close to perpendicular (or parallel) to the sensor coil's axis over part of the flight/traverse, the induced precession signal becomes too weak to detect reliably, a limitation optically pumped sensors do not share.
