18-Geol-A7 Applied Geophysics · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 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 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 (magnetics, seismic reflection, radiometrics, downhole resistivity, EM/IP, filtering, well logging, forward/inverse modelling); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey planning, array geometry, data display; Blakely, Potential Theory in Gravity and Magnetic Applications — potential-field filtering and forward/inverse modelling (Q7, 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.
Source/sensor layout. A reflection profile is shot with a linear (2-D) or gridded (3-D) array of receivers — a string of geophones on land or a towed hydrophone streamer at sea — recording each shot from a source (vibroseis truck, dynamite, weight-drop on land; an airgun array at sea) moved progressively along the line ("roll-along" shooting). Every source-receiver pair with a common midpoint (CMP) between them, regardless of offset, images the same subsurface reflection point (for a flat reflector); by shooting many overlapping source positions into a long receiver spread, the same midpoint is sampled by many different offsets, giving redundancy ("fold") that is central to noise attenuation by stacking.
Generating a zero-offset section. The recorded field data are first sorted from shot-receiver order into common-midpoint (CMP) gathers — every trace that shares the same midpoint is grouped together, regardless of which shot recorded it. Velocity analysis is then performed on each CMP gather (semblance/velocity-spectrum analysis) to pick the stacking velocity that best flattens the reflection hyperbola. Normal moveout (NMO) correction is applied using the picked velocity, shifting each trace's arrival time to remove the offset-dependent delay predicted by
$$t(x)=\sqrt{t_0^2+\dfrac{x^2}{v^2}}$$
so that after correction every trace in the gather reads the same two-way zero-offset time $t_0$ for a given reflector. The far-offset, shallow-reflector portion of each NMO-corrected trace is stretched by the correction and is muted (top-muted) before stacking to avoid injecting distorted low-frequency energy. Finally, all NMO-corrected, muted traces within each CMP gather are summed (stacked) into a single output trace, producing a section that simulates what a single coincident source-receiver pair (zero offset) would have recorded at every midpoint — the "zero-offset" or "stacked" section. Migration is typically applied afterward to collapse diffractions and reposition dipping reflectors to their true subsurface location.
Artifacts and their removal. Multiples (energy that has reflected more than once, e.g. water-bottom or peg-leg multiples) mimic real primary reflections at a longer travel time; removed by predictive deconvolution or Radon-transform-based demultiple, which exploit the multiple's periodic timing or its different moveout velocity from the primary. NMO stretch distorts shallow, far-offset waveforms (lowers their apparent frequency); removed by top-muting the stretched zone before stacking. Diffractions from point scatterers or fault terminations spread energy into hyperbolic tails that clutter the section; collapsed by migration. Ground roll (low-velocity, low-frequency surface (Rayleigh) wave noise on land data) swamps shallow reflections; attenuated by f-k (frequency-wavenumber) filtering, which separates it from reflection energy by its distinctly low apparent velocity. Migration artifacts (uncollapsed diffraction "frowns" left by under-migration with too low a velocity, "smiles" produced by over-migration with too high a velocity) arise from an incorrect migration velocity and are corrected by iterating the velocity model.
