18-Geol-A7 Applied Geophysics · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 04-Geol-A7 Applied Geophysics. Three-hour, closed-book exam; approved Casio or Sharp calculator permitted. The paper offers a choice of six of the following ten questions, each worth 16.66% of the total mark, and every question requires an essay-format answer — this is a genuinely all-essay sitting with no numeric data, formula sheet, or figure supplied. All ten questions are answered below so the set stands as a complete study resource.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method touched in this paper (density/rock physics, seismic refraction, magnetotellurics, resistivity, induced polarization, magnetics, data enhancement, well logging, EM systems, forward/inverse modelling); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey planning, array geometry, data display; Simpson & Bahr, Practical Magnetotellurics — MT acquisition/processing (Q3); Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field forward/inverse modelling (Q6, Q10); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging tool context (Q8).
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.
System chosen: a ground-based, fixed-loop time-domain EM (TDEM) system, used routinely for conductive-target (massive sulphide, groundwater/clay-boundary) mapping.
Survey configuration. A large, fixed transmitter loop (e.g. 100 m × 100 m, tens of turns of wire) is laid out and left in a single position on the ground; a small receiver coil is then moved to a series of stations inside and around the transmitter loop, recording a full decay-curve response at each station before moving to the next. Source and sensor geometry therefore stays fixed relative to each other only within one station reading, but the receiver's position relative to the fixed transmitter changes systematically station to station, which is the defining feature of the fixed-loop configuration (as opposed to a moving-loop system, where transmitter and receiver move together at constant offset).
Transmitter waveform. A bipolar square-wave current (steady DC "on" period, then abrupt shut-off, alternating polarity each half-cycle to cancel any DC drift/self-potential bias) is driven through the transmitter loop, building a static primary magnetic field during the "on" time.
Receiver sampling. Immediately after each current shut-off, the receiver coil records the decaying secondary voltage (induced by eddy currents set up in any conductive ground) in a series of logarithmically-spaced time gates (early gates sample the fast-decaying near-surface/shallow response, late gates the slower-decaying deep response), stacking many transmitter on/off cycles to improve signal-to-noise before moving to the next station.
Data normalization/reduction. Raw decay-voltage readings are normalized by the transmitter moment (current × loop area × turns) so that data collected at different current/loop settings are directly comparable, and are typically converted to apparent conductivity or apparent resistivity at each time gate using a half-space or layered-earth transform, producing a decay curve (voltage or conductivity vs. time gate) at each station.
Advantages vs. other systems. Because the receiver senses only the secondary (ground-induced) field after the primary field is already off, TDEM has no primary-field coupling problem to correct for (unlike frequency-domain EM, which must separate a much larger primary field from the target's secondary response); the wide range of time gates gives an inherent range of depths of investigation from one sounding, similar in spirit to a resistivity VES. Disadvantages. Fixed-loop surveys are logistically slower than airborne or moving-loop systems (the large transmitter loop must be physically laid out and left in place); weak (low-conductance) conductors decay so quickly that much of their response is gone before the first off-time gate, where frequency-domain EM can still detect them, and deep targets need a large transmitter moment (big loop, high current, heavy generator).
Example profile and interpretation. A receiver-station profile crossing a steeply-dipping, well-conductive massive-sulphide body typically shows, in the vertical (Hz) component, a crossover: the late-time response swings from one polarity to the other with the zero crossing directly over the conductor's top edge (the eddy currents concentrate along that edge and act like a line current), the flank on the down-dip side being the larger, so the asymmetry indicates dip direction; the horizontal (Hx) component peaks over the same point. The anomalous decay persists to much later time gates over the conductor than over background ground (background decays quickly to noise level, a true conductor's decay remains above noise for many more gates). The interpreter picks the crossover to locate the conductor, reads where the anomalous late-time response appears/disappears along the profile to bracket its extent, and uses the decay's time constant (rate of decay) as a rough conductance ($\sigma t$, conductivity-thickness product) indicator — a slowly-decaying response indicates a high-conductance target (a strong exploration target such as massive sulphide), while a rapidly-decaying response is more consistent with a weak conductor (e.g. conductive overburden or a graphitic/shear-zone response) that is a lower exploration priority.
