Question 10 of 10: Case History — the Seismic Reflection Method

Question 10: Case History — the Seismic Reflection Method (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.

Case history: shallow reflection survey for a proposed dam foundation. A high-resolution shallow seismic reflection survey was run along a proposed embankment-dam alignment to map bedrock depth and geometry, including detecting any buried channel or fault zone that could compromise the foundation or provide a preferential seepage path.

Survey objective. Determine depth-to-bedrock and identify any structural discontinuity (buried channel, fault, karst void) along the alignment before final foundation design.

Expected physical property contrast. Acoustic impedance $Z=\rho V_p$ jumps sharply at the overburden-bedrock contact — unconsolidated overburden has $V_p\approx400$–$1500\text{ m/s}$ and $\rho\approx1800$–$2000\text{ kg/m}^3$, while competent bedrock has $V_p\gtrsim3000$–$5000\text{ m/s}$ and $\rho\approx2600\text{ kg/m}^3$, producing a strong, high-amplitude, positive-polarity reflection coefficient precisely at the contact.

Survey design. A short, dense geophone spread (2–5 m geophone spacing, 24–48 channels) with a high-frequency source (sledgehammer/weight-drop or a small accelerated-weight vibrator) chosen to deliver a broad, high-frequency bandwidth needed to resolve a shallow target; multiple overlapping shot points along the line gave the fold (multiplicity) needed for a useable common-midpoint (CMP) stack.

Data processing. The most critical steps for this shallow target were ground-roll/noise suppression (f–k or bandpass filtering to remove the low-velocity, high-amplitude surface wave that otherwise swamps the much weaker, higher-frequency shallow reflection) and static corrections (removing near-surface weathering-layer travel-time distortions, which are proportionally much larger relative to a shallow target's own short two-way time than for a deep target); careful velocity analysis and NMO correction before CMP stacking then produced the final time section, with migration applied to correctly reposition any dipping bedrock segment.

Analysis and conclusions. The final section showed a continuous, strong reflector at 40–120 ms two-way time along most of the alignment (bedrock depth increasing smoothly from one abutment toward the valley centre), but a discrete offset and diffraction pattern in that reflector at one location was interpreted as a buried bedrock channel or fault, confirmed by a follow-up borehole that intersected substantially deeper, more fractured rock at exactly that station — directing the final design toward either avoiding that section or adding deep grouting/cutoff treatment there.

Improvements with a larger budget. A denser 3-D (rather than 2-D line) survey would resolve the true lateral extent and orientation of the anomalous zone rather than only its intersection with the single 2-D line; additional geophysical constraint (a companion electrical resistivity or GPR line, since a fault/channel infill often also carries a resistivity/permittivity contrast) would independently corroborate the seismic interpretation; and a higher-fold, broader-bandwidth source/receiver combination would improve both vertical resolution (thinner features resolvable) and the signal-to-noise ratio of the deeper part of the section.