Question 5 of 10: The Magnetic Method for Reconnaissance, Property-Scale and Borehole Surveys

Question 5: The Magnetic Method for Reconnaissance, Property-Scale and Borehole Surveys (16.66% of paper)

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.

A magnetic anomaly can arise from induced magnetization (proportional to susceptibility $\kappa$ and the ambient field), remanent magnetization (a rock's own frozen-in magnetic history, independent of the present field), or both — which is why the same total-field anomaly shape can, in principle, be produced by very different source geometries, and why interpretation must always be constrained by independent geological knowledge wherever possible.

1) Reconnaissance (area selection). Setup. An airborne total-field magnetometer (caesium-vapour sensor, wide dynamic range, high sensitivity) is towed in a "bird" or fixed-wing-mounted, with a fixed ground base station recording the diurnal field continuously through the survey for later subtraction. Specifications. Wide line spacing (200–400 m), constant terrain-clearance flying, sampling at 5–10 Hz along-line (giving very dense along-line but coarse across-line sampling). Interpretation. Regional/residual separation and pattern recognition (linear trends = structure/dykes; broad highs/lows = basement or alteration) to select prospective sub-areas for follow-up, not quantitative source modelling. Case history. Regional airborne magnetic surveys over the Abitibi greenstone belt have long been used to trace magnetite-bearing iron-formation horizons that host gold mineralization, guiding where property-scale follow-up is warranted.

2) Property-scale (target delineation). Setup. A ground proton-precession or fluxgate magnetometer surveyed on foot along cut/GPS-guided lines, again with a base station (or a second, continuously-logging magnetometer left stationary) recording diurnal variation for correction. Specifications. Tight station spacing (5–10 m) and line spacing (25–50 m), chosen to resolve the specific target's expected width; data are reduced-to-pole (RTP) to remove the dipolar skew imposed by the ambient field's inclination, centring each anomaly directly over its source. Interpretation. Quantitative source modelling (e.g. Euler deconvolution or 2-D forward/inverse modelling) to estimate the source's depth, width and dip for drill targeting. Case history. Ground magnetic surveys are routinely used to trace a steeply dipping, magnetite-bearing skarn contact beneath cover, directly guiding drill-hole collar placement along strike.

3) Borehole (target delineation). Setup. A three-component downhole magnetometer/susceptibility tool is logged along the hole; no separate "base station" diurnal correction is normally needed over the short logging run, though a nearby continuously-recording ground station is still good practice for a long logging day. Specifications. Continuous logging at a fine depth sample interval (cm-scale), oriented relative to the hole's own deviation survey so vector components can be rotated into true geographic coordinates. Interpretation. The susceptibility/vector-field log directly identifies which intersected lithological units are magnetite-bearing and, via 3-D inversion combining several nearby holes, refines the geometry of a magnetic body already located by the property-scale survey. Case history. Downhole magnetic susceptibility logging is used routinely to confirm and refine the true dip and thickness of a magnetite skarn intersected in a drill hole, resolving ambiguity left by the surface survey's non-unique inversion.