Question 7 of 10: Airborne, Ground and Borehole Electromagnetic Systems

Question 7: Airborne, Ground and Borehole Electromagnetic Systems (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.

i) Airborne — helicopter time-domain EM (e.g. VTEM-class system). Transmitter waveform: a large, near-horizontal transmitter loop towed below the helicopter is driven with a repeating trapezoidal/ramped current pulse that is switched abruptly to zero; the decaying secondary (eddy-current) field is sampled at a series of discrete time gates during this transmitter off-time. Receiver: a coincident or closely nested receiver coil, inside or immediately adjacent to the transmitter loop, measures $dB/dt$ at each gate. Geometry: horizontal coplanar, coincident-loop configuration. Normalization: gate amplitudes are normalized to the transmitter's dipole moment (current × loop area) so results from different flights/loop sizes are comparable, and reported as apparent conductance or resistivity per gate. Advantages: rapid, low-cost-per-area regional coverage, large transmitter moment giving good depth penetration for a portable system, and inherent depth information from a single decay curve (early gates shallow, late gates deep). Disadvantages: expensive per survey, a conductive near-surface overburden or lake can mask a deeper bedrock conductor, and airborne noise (power lines, cultural EM) requires careful line planning. Most appropriate: first-pass regional reconnaissance over a large, remote base-metal exploration property to identify conductor targets for ground follow-up.

ii) Ground — moving-loop time-domain EM (e.g. a Crone PEM or Geonics PROTEM-class system). Transmitter waveform: a square-wave current, repeatedly switched fully on then abruptly off, is driven through a large (commonly 100–300 m side) transmitter loop laid on the ground. Receiver: a separate, smaller multi-turn receiver coil measures the decaying secondary voltage at a series of stations, either inside the moving loop (moving-loop configuration, both loops advance together down the line) or at fixed stations around one large stationary transmitter loop (fixed-loop configuration). Geometry: both moving-loop and fixed-loop are used depending on whether lateral coverage or maximum depth of investigation over one target is the priority. Normalization: as with the airborne system, the decay curve is normalized to transmitter current and loop area, and the late-time decay rate/amplitude is fitted to a plate or half-space conductance model. Advantages: larger, more controllable transmitter moment than a typical airborne system (deeper investigation for a well-defined target), and much finer station control for detailed anomaly definition. Disadvantages: slow and labour-intensive (large loops must be physically laid out and moved), impractical over very large areas. Most appropriate: detailed ground follow-up over a specific airborne conductor anomaly, to refine its location, depth, dip and conductance ahead of a drill decision.

iii) Borehole — surface-loop borehole EM (e.g. a Crone Borehole PEM probe). Transmitter waveform: the same square-wave time-domain waveform as the ground system, driven through a large loop laid on the surface around or near the drill collar. Receiver: a three-component (X, Y, Z) coil probe is lowered down the borehole and records the amplitude and orientation of the decaying secondary field at closely spaced depth stations. Geometry: surface-loop-to-downhole-probe (most common) or borehole-to-borehole (crosshole) for greater sensitivity between two known holes. Reduction: the three vector components are combined into a total-field profile and their directional information is inverted (typically via plate/Maxwell-type modelling) for the strike, dip, and off-hole distance of any conductor detected. Advantages: directly detects and vectors toward a conductor that lies OFF to the side of a drill hole that itself failed to intersect it, using the vector information to guide the very next hole. Disadvantages: only investigates the rock volume around one existing hole, so it requires drilling to already have occurred and cannot substitute for a surface reconnaissance survey. Most appropriate: immediately after a drill hole intersects weak or barren mineralization near a strong surface EM anomaly, to determine whether the real conductor lies just off to one side of the hole and, if so, in which direction to step out the next hole.