Seismic Reflection Basics: How We See Rock Layers with Sound
Key takeaways
- Seismic is like an echo: we send a wave into the ground and record when its reflections return and how strong they are.
- A reflection occurs where acoustic impedance Z = ρ × V changes between layers.
- Reflection coefficient R = (Z2 − Z1) ÷ (Z2 + Z1); its sign tells us whether the lower layer is harder or softer.
- Depth = velocity × two-way time ÷ 2. Example: 1.6 s at 3,000 m/s is about 2,400 m.
- A gas sand can give a strong negative reflection called a bright spot, but not every bright spot is gas.
Educational content only: procedures and operating values vary between companies and sites. Always follow your company’s approved procedures, your supervisor’s instructions and the permits in force at your site.
The idea: an echo underground
Shout in front of a cliff and your voice comes back a moment later; from the delay you know how far away the cliff is. Seismic reflection works the same way, inside the earth. A wave is generated at surface by a vibrator truck (vibroseis) or a charge in a shot hole, and it travels down through the layers. At every boundary between two different layers, part of the energy bounces back up and the rest continues down.
At surface, small sensors called geophones pick up these reflections and record them for a few seconds. The record from one receiver position is a seismic trace. Thousands of traces side by side form an image of the layers, much like a medical ultrasound scan.
Acoustic impedance: what makes a reflection?
Every rock has two properties that matter to the wave: density ρ and wave velocity V. Their product is the acoustic impedance:
Z = ρ × V
If two layers have similar impedance, the wave passes with little reflection; if the difference is large, the reflection is strong. At normal incidence, the reflection coefficient is:
R = (Z2 − Z1) ÷ (Z2 + Z1)
where Z1 is the upper layer and Z2 the lower. A positive R means the lower layer is acoustically harder; a negative R means it is softer.
A shale lies on top of a sand. Shale: ρ = 2.40 g/cm³ and V = 2,800 m/s, so Z1 = 6,720.
Gas-saturated sand: ρ = 2.10 and V = 2,400, so Z2 = 5,040:
R = (5,040 − 6,720) ÷ (5,040 + 6,720) ≈ −0.14
Same sand, water-saturated: ρ = 2.25 and V = 3,000, so Z2 = 6,750:
R = (6,750 − 6,720) ÷ (6,750 + 6,720) ≈ +0.002
The water sand is almost invisible, while the gas sand gives a strong negative reflection. This is the origin of the bright spot, one of the direct hydrocarbon indicators (DHI). But coal and very porous sands can do the same, so it is never used alone.
From time to depth
What we record is time, not depth. The wave goes down and comes back, so the recorded time is the two-way time (TWT). To convert it to depth we need the average velocity of the layers above the reflector:
Depth = V × TWT ÷ 2
Example: a reflection at 1.6 s with an average velocity of 3,000 m/s:
Depth = 3,000 × 1.6 ÷ 2 = 2,400 m
A small velocity error gives a large depth error. If the true velocity were 3,100 m/s, the depth would be 2,480 m, 80 m deeper. That is why velocities are calibrated with wells: sonic logs, checkshots and VSPs.
Resolution: how thin a layer can we see?
A seismic wave is not a sharp spike but a wavelet with a frequency and a wavelength:
λ = V ÷ f
At 3,000 m/s and a dominant frequency of 30 Hz, the wavelength is 100 m. A common rule is that the thinnest layer whose top and base can be separated is about a quarter wavelength, here about 25 m. Thinner layers still show, but their top and base merge into one reflection. And because high frequencies fade with depth, resolution gets worse in deep layers.
Where is seismic used?
- Exploration: mapping geological structures and looking for traps before any well is drilled.
- Well placement: choosing the drilling location and well path to avoid faults and shallow hazards.
- Field development: estimating reservoir size and planning well spacing.
- Time-lapse (4D): repeating the survey over the same field years later to see water or gas moving in the reservoir with production.
Takeaway for the technician
Seismic measures two things: when a reflection comes back, and how strong it is. Time gives depth once velocity is known; strength tells us the impedance contrast between layers, and sometimes the fluid they contain. In the next two lessons we see how data are acquired in the desert and how they become a reservoir map.
Test your understanding
Related lessons
- Log interpretation & correlation
- Huge Pressure and Trapped Oil: The Answer in Darcy's Law
- Land seismic acquisition: vibroseis and fold
Sources
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