Acoustic Impedance and Seismic Reflection: 24-Pet-A1 Principles of Stratigraphy and Sedimentation, Undated paper, Q14

Question 14 of 19: Acoustic Impedance and Seismic Reflection

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Petroleum Engineering, 17-Pet-A1 Principles of Stratigraphy & Sedimentation, 2019-May. 3 hours duration; closed book, approved Sharp/Casio calculator permitted. The paper has two parts: Part A (Questions 1–10, Sedimentology and Sedimentary Processes) – Questions 1 and 2 are mandatory (10 marks each, 20 marks), plus any five of the remaining eight (3–10) at 6 marks each (30 marks), for a Part A total of 50 marks; and Part B (Questions 11–19, Stratigraphy and Sedimentary Basin Analysis) – answer any six of the nine at 6 marks each, for a Part B total of 36 marks – an 86-mark maximum (50 for Part A + 36 for Part B).

Check: the mark values and question count used throughout this solution (6 marks each for Questions 3–19, Part A = 50, Part B = 36, maximum = 86) are as printed on the paper.

Reference texts: Boggs, S. Jr., Principles of Sedimentology and Stratigraphy, 5th ed., Pearson (grain texture, sediment transport, bedforms, carbonate/evaporite systems, sequence stratigraphy, unconformities, stratigraphic principles); Tucker, M.E., Sedimentary Petrology, 3rd ed., Blackwell (sandstone/carbonate classification, diagenesis, porosity); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial/deltaic/deep-marine systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, alluvial fans, deltas, deep-marine systems); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (basin analysis, well-log correlation, seismic/acoustic impedance); International Commission on Stratigraphy, International Chronostratigraphic Chart (geological time scale).

Question 14: Acoustic Impedance and Seismic Reflection (6 marks)

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.

Acoustic impedance is defined as $Z=\rho V$, the product of a rock's bulk density ($\rho$) and its seismic (P-wave) velocity ($V$) – a fundamental material property that controls how seismic energy interacts with a boundary between two rock layers.

At normal incidence on the boundary between two layers of impedance Z₁ and Z₂, the reflected-wave amplitude is set by the reflection coefficient R = (Z₂−Z₁)/(Z₂+Z₁); the rest of the energy is transmitted downward.

Relationship to seismic reflection. At the (normal-incidence) boundary between an upper layer of impedance $Z_1$ and a lower layer of impedance $Z_2$, the fraction of incident wave amplitude reflected back is the reflection coefficient:

$$R=\frac{Z_2-Z_1}{Z_2+Z_1}$$

If $Z_2>Z_1$ (impedance increases downward – e.g., shale over a denser, faster-velocity cemented limestone), $R$ is positive, a "hard" reflection of the same polarity as the incident wave. If $Z_2no reflection is generated at all, even where a nominal lithologic boundary exists. This is the essential reason seismic reflection profiling images impedance contrasts (bedding planes, unconformities, fluid contacts), not lithology directly, and why subsurface stratigraphy is interpreted from reflection geometry and amplitude rather than read as a direct rock log.

Topic. Acoustic impedance and the seismic reflection coefficient.

Key relations. $Z=\rho V$; $R=(Z_2-Z_1)/(Z_2+Z_1)$; $Z_1=Z_2 \Rightarrow R=0$, no reflection.

Why this works. A seismic wave partitions its energy at a boundary in proportion to how abruptly the medium's density-velocity product changes; a truly gradual or absent impedance change transmits the wave with essentially no reflected energy, regardless of whether the rock "type" nominally changes.

Common pitfall. Assuming every lithologic contact produces a seismic reflection – two different lithologies with coincidentally similar $\rho V$ products (e.g., a well-cemented sandstone and an adjacent limestone) can be seismically invisible, while a fluid contact within a single lithology (oil/water contact) can produce a strong reflection.

Source. Selley & Sonnenberg, Elements of Petroleum Geology, 3rd ed., ch.9 (seismic stratigraphy); Boggs, Principles of Sedimentology and Stratigraphy, 5th ed., ch.16.