Question 9 of 10: Airborne, Ground and Borehole Electromagnetic Systems

Question 9: 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 — VTEM (Versatile Time-domain EM). A large transmitter loop is towed on a rigid frame below a helicopter, with a receiver coil mounted coaxially at (or near) the loop's centre. Waveform: a bipolar, half-sine (or trapezoidal) current pulse, ramped to peak amperage and then rapidly turned off each half-cycle, alternating polarity to cancel DC drift. Receiver: measures $dB/dt$ (the time derivative of the decaying secondary field) in a series of logarithmically-spaced time gates immediately after each turn-off, stacking many transmitter cycles per flight-metre for signal-to-noise. Normalization/reduction: decay-voltage readings are normalized by the transmitter dipole moment and typically transformed to apparent conductivity via a half-space or layered-earth transform for each gate, producing a conductivity-depth profile along the flight line. Advantages: extremely rapid coverage of large, often inaccessible areas (hundreds of line-km/day), good depth of investigation for a moving platform. Disadvantages: lower spatial resolution and shallower effective depth than a comparable ground TDEM survey, expensive to mobilize, altitude/terrain-clearance sensitive. Most appropriate: regional reconnaissance mapping of massive-sulphide conductors over large, remote, poorly-accessible exploration ground.

ii) Ground — Geonics EM-31/EM-34 (frequency-domain slingram). Transmitter and receiver coils are held at a fixed separation (EM-31: 3.7 m rigid boom, one operator; EM-34: 10/20/40 m variable separation, two operators/cable). Waveform: a continuous sinusoidal current at a single fixed frequency (EM-31: 9.8 kHz) or a selectable frequency depending on coil spacing (EM-34). Receiver: continuously measures both in-phase and quadrature secondary-field components relative to the primary field while walking. Normalization/reduction: the quadrature component is converted directly to apparent ground conductivity via the low-induction-number approximation (valid because coil spacing is small relative to skin depth), while the in-phase component is used separately to flag compact, highly-conductive metallic targets (drums, pipes, ordnance). Advantages: fast, continuous walking-speed reconnaissance, simple field operation, good for shallow (roughly 0–6 m for EM-31, up to ~60 m for EM-34 depending on coil orientation/spacing) conductivity mapping. Disadvantages: limited depth range compared to TDEM, sensitive to cultural EM noise (fences, buried pipes, vehicles). Most appropriate: rapid detailed mapping of a landfill boundary, groundwater conductivity/contamination plume, or buried utility corridor.

iii) Borehole — downhole (borehole) pulse-EM probe. A large transmitter loop is laid out on the surface (or in an adjacent borehole) around the target zone, while a small three-component receiver coil is run down the borehole of interest on a wireline, logging continuously as it is lowered/raised. Waveform: the same bipolar time-domain step/decay waveform as ground TDEM. Receiver: the moving downhole coil records the decaying secondary field, in multiple time gates, as a function of depth in the hole, rather than as a function of surface position. Normalization/reduction: readings are normalized by transmitter moment as in surface TDEM, and the resulting depth-vs-time-gate decay profile is examined for a localized anomaly peak that indicates the borehole has passed near (though not necessarily through) a conductor. Advantages: can detect and estimate the off-hole distance/direction of a conductor that the drill hole itself missed by tens of metres, resolving 3-D target geometry far better than a single surface EM anomaly alone. Disadvantages: only investigates the volume immediately around one existing hole, requires an active drill program (cannot be used pre-drilling), and per-hole logging adds cost to the drilling program. Most appropriate: following up an ambiguous surface EM/magnetic anomaly once a hole has already been drilled near it, to determine whether the true conductor was missed and in which direction to step out the next hole.