18-Geol-A7 Applied Geophysics · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2019 — 18-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 in the source. All ten questions are answered below so the set stands as a complete study resource for choose-N-of-M exams.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method touched in this paper (physical properties, gravity, magnetics, electrical/EM methods, seismic refraction/reflection, radiometrics, well logging, magnetotellurics); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey design, array geometry, data acquisition, processing and display; Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field survey design and reduction (Q2); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging tool context (Q7).
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.
Passive electrical method — self-potential (SP). No current is injected; the survey instead measures naturally occurring DC voltage differences that already exist in the ground. These arise from several mechanisms: electrochemical (mineralization) potentials at the boundary between a conductive sulphide/graphite body and the surrounding electrolyte-bearing groundwater, where the body acts like a giant natural battery driven by an oxidation-reduction gradient across the water table; streaming (electrokinetic) potentials generated by groundwater flow through porous/fractured rock, where fluid movement past charged pore-wall surfaces produces a measurable voltage; and thermoelectric or bioelectric potentials (generally much smaller). Field generation and measurement: the "field" is generated entirely by the subsurface process itself; the survey simply measures potential difference between a fixed reference electrode and a roving electrode (or between two roving electrodes moved along a line) using non-polarizing porous-pot electrodes (copper/copper-sulphate or silver/silver-chloride) and a high-input-impedance millivoltmeter, because ordinary metal stakes would introduce their own electrode potential and swamp the tiny natural signal (tens to hundreds of mV). Applications: mapping sulphide/graphite mineralization (strong negative SP anomalies, hundreds of mV, over the oxidized top of a conductive body), tracing groundwater seepage through or beneath a dam (streaming-potential anomalies locate leakage paths), and geothermal exploration.
Active electrical method — DC resistivity. A known current $I$ is deliberately injected into the ground through a pair of current electrodes and the resulting potential difference $\Delta V$ is measured at a separate pair of potential electrodes (e.g. Wenner or Schlumberger array); the apparent resistivity is computed from $\rho_a=K\Delta V/I$, where $K$ is a purely geometric array factor. Field generation and measurement: the field is actively excited by the surveyor at a controlled, known current, so signal strength (and hence depth of investigation and signal-to-noise) is under the operator's control by increasing current or electrode spacing, unlike the passive SP method where signal amplitude is whatever nature provides. Applications: depth-to-bedrock and groundwater/aquifer delineation (vertical electrical sounding), lateral mapping of contamination plumes or lithology change (profiling), and geotechnical site characterization, yielding a quantitative resistivity-versus-depth or resistivity-versus-position section rather than the qualitative anomaly map typical of SP.
Contrast. The passive method is simpler and cheaper to run (no current source/transmitter needed) and directly flags mineralization or seepage, but gives only a qualitative anomaly with limited depth control; the active method requires more equipment (transmitter, current electrodes, more power) but returns a quantitative, invertible resistivity model with controllable depth of investigation.
