Question 2 of 10: Seismic Reflection — Physical Properties, Application and Processing

Question 2: Seismic Reflection — Physical Properties, Application and Processing (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.

Physical properties governing wave propagation. Seismic wave propagation is governed by the elastic moduli of the medium — the bulk modulus $K$ and shear modulus $\mu$ — together with bulk density $\rho$, which combine to give the compressional- and shear-wave velocities $V_p=\sqrt{(K+\tfrac{4}{3}\mu)/\rho}$ and $V_s=\sqrt{\mu/\rho}$. These properties in turn depend on lithology, degree of cementation/consolidation, porosity, confining pressure and the degree of fluid saturation (a fluid-saturated unconsolidated sediment has a markedly higher $V_p$ than the same sediment dry, because water raises the effective bulk modulus while barely affecting shear rigidity, which is why $V_p$ is such a sensitive water-table indicator while $V_s$ is comparatively insensitive to saturation).

How contrasts produce reflections. A seismic reflection is generated wherever acoustic impedance $Z=\rho V_p$ changes across an interface; the normal-incidence reflection coefficient is $R=(Z_2-Z_1)/(Z_2+Z_1)$. No reflection is produced across a boundary with no impedance contrast, however important that boundary may be geologically — the method is entirely a mapping of impedance discontinuities, not of geological units per se.

Geotechnical/engineering example. A shallow, high-resolution seismic reflection survey used to map the depth and geometry of bedrock beneath a proposed dam foundation, including detecting an infilled buried channel or fault zone that could compromise the foundation. The physical property contrast exploited is the strong impedance jump between unconsolidated overburden (low $\rho V_p$, typically $V_p\approx400$–$1500$ m/s) and competent bedrock (high $\rho V_p$, $V_p\gtrsim3000$–$5000$ m/s), which produces a strong, high-amplitude reflection precisely at the overburden-bedrock contact.

Most important processing steps and why. For this shallow engineering-scale target, static corrections and ground-roll/noise suppression (f–k or bandpass filtering to remove the low-velocity, high-amplitude surface (Rayleigh) wave that otherwise swamps the much weaker, higher-frequency shallow reflection) are the most critical steps, together with careful velocity analysis and NMO correction before stacking. These are more important here than in a deep exploration-seismic survey because at shallow depth (a) the reflection of interest arrives at very short two-way time, in the same time window the ground roll and direct/refracted arrivals dominate, so removing that near-surface noise is what makes the reflection visible at all, and (b) near-surface velocity heterogeneity (variable weathering-layer thickness/velocity) produces travel-time distortions that are a LARGER fraction of the shallow target's own two-way time than the equivalent distortion would be for a deep reflector, so an uncorrected static error can shift or destroy the shallow reflection entirely during stacking, whereas the same absolute static error is comparatively minor for a deep target. Deconvolution to sharpen the wavelet and migration to reposition any dipping segment of the bedrock surface are also important but are secondary refinements that only matter once the reflection has first been recovered from the noise and correctly time-aligned.