Question 9 of 9: The Geophysical Project Workflow – A Case History

Question 9: The Geophysical Project Workflow – A Case History (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.

The case history chosen here is a multi-method exploration program for a volcanogenic massive sulphide (VMS) deposit within a Precambrian volcanic-sedimentary belt of the kind mined across the Canadian Shield (e.g. the Bathurst Mining Camp, New Brunswick, or the Abitibi belt, Ontario/Quebec) — a classic and heavily documented integration of gravity, magnetics, EM and IP.

a) What is the problem? Locate and delineate buried VMS sulphide lenses within a favourable volcanic-sedimentary sequence, under cover, well enough to rank drill targets and estimate approximate tonnage/depth before committing to expensive diamond drilling.

b) Important physical property contrasts. Massive sulphide is markedly CONDUCTIVE (often <1 Ω·m) against a resistive volcanic/sedimentary host (hundreds to thousands of Ω·m); pyrrhotite-rich VMS lenses are also strongly MAGNETIC against a comparatively non-magnetic felsic/mafic host; sulphide is somewhat DENSER (∼4.0–4.5 g/cm³) than typical volcanic host rock (∼2.7–2.9 g/cm³); and disseminated (as opposed to massive) sulphide produces a distinct CHARGEABILITY response in induced polarization even where the conductivity contrast alone is weak.

c) Best survey method(s). No single method is definitive, so an integrated suite is used: airborne electromagnetics (e.g. a time-domain system) and airborne magnetics FIRST, for rapid, low-cost regional coverage and first-pass conductor/magnetic-high identification, followed by ground follow-up — ground magnetics, ground EM (moving-loop TDEM or fixed-loop), and IP/resistivity — over the most promising airborne anomalies, and finally gravity over any confirmed, drill-tested target to help estimate its approximate size/tonnage.

d) Mode of data collection. Airborne EM/magnetics is flown FIRST at wide (100–200 m) line spacing for reconnaissance over the whole property, since it is far cheaper per unit area than ground work; ground follow-up is then acquired only over the resulting anomalies, on a tighter grid (25–50 m station spacing, lines perpendicular to the interpreted geological/EM strike); once a target is drill-tested, downhole (borehole) EM and magnetic surveys are run inside and around the drill hole to vector toward any off-hole conductor the hole itself did not intersect, closing the loop between surface geophysics and the actual drill result.

e) Processing. Airborne EM channels are converted to apparent conductivity or conductance, and anomalies are modelled (plate or layered-earth inversion) for approximate depth, dip and conductance; magnetics data are reduced to the pole, upward-continued to separate near-surface (overburden/culture) noise from bedrock signal, and derivative-filtered to sharpen contacts; IP/resistivity data are inverted (2-D pseudosection inversion) for chargeability and resistivity sections along each ground line.

f) Interpretation methodology. Individual property maps/sections are INTEGRATED rather than read in isolation: a genuine VMS target should show a coincident (or closely associated) EM conductor, magnetic high, and IP chargeability high, all located consistently with the geological setting (a favourable felsic-mafic volcanic contact or exhalative horizon mapped from surface geology/drilling); this cross-checking is essential because a conductor alone could equally be a graphite- or sulphide-poor, water-saturated fault/shear zone (conductive but not necessarily magnetic or dense), and distinguishing the two from a single EM survey alone is often impossible.

g) Conclusions. Targets are ranked by the NUMBER and STRENGTH of independent, spatially coincident property anomalies (a target with all three — EM, magnetic, IP — support is prioritized well above a target defined by only one), tied back to the geological model, and the highest-ranked targets are recommended for diamond drilling; drill results (sulphide mineralogy, off-hole EM response, downhole assay) then feed back into steps (b) and (f), refining the property-contrast model and interpretation for the next round of targets — the explicit "repeat all steps if necessary" instruction in the question.

Consistent with the question's own caveat, these steps are not always followed in a clean, explicit sequence in real exploration programs: it is common, for instance, for a specific survey type to be acquired on a new claim simply because a neighbouring company had recent success with that method on the adjacent property, rather than because a fresh a)–g) analysis was formally documented — property-scale exploration decisions are frequently precedent-driven as much as first-principles-driven, and a competent geophysicist should recognize when that has happened and retroactively check that the chosen method still suits the actual property contrasts and problem at hand.