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Fundamentals of Seismic Data Acquisition, Processing, and Interpretation

An introduction to seismic principles covering wave generation, reflection, data processing, acoustic impedance, wavelets, polarity conventions, and their impact on geological interpretation.

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M O D U L E 0 1 · F U N D A M E N TA L S Seismic in one sentence Module 01 · What is seismic? 6 Seismic is a controlled echo experiment: we make a sound, and we listen for where the rock changes. 1 We make a wave Airgun offshore, vibroseis or dynamite onshore. Energy travels down as an elastic wave. 2 Rock changes reflect it Wherever acoustic impedance changes, part of the energy bounces back. No contrast, no reflection. 3 We record arrival time Geophones or hydrophones record amplitude against two- way travel time — not depth. 4 We process it into an image Stacking and migration turn thousands of traces into a volume that looks like a cross- section. The raw measurement is TIME and AMPLITUDE. Everything geological you get from seismic is an interpretation on top of those two numbers.
M O D U L E 0 1 · F U N D A M E N TA L S From field to volume — and why it matters to you Module 01 · What is seismic? 7 1 Acquisition Geometry, fold, offsets, azimuths. Sets the maximum resolution you will ever have — nothing downstream adds it back. 2 Processing Noise removal, deconvolution, statics, velocity analysis. Every step involves choices made by a person. 3 Migration Moves reflections to their true position and collapses diffractions. Time or depth migrated — ask which. 4 Interpretation Picking events, mapping surfaces, extracting attributes. Where geology finally re- enters the story. Three questions to ask about any volume 1. Is it pre-stack or post-stack migrated? 2. Time or depth migrated? 3. What is the vintage, and has it been reprocessed since? Why it matters A 1998 volume and a 2020 reprocessing of the same field can differ by tens of milliseconds on the same event — which becomes tens of metres on your map.
M O D U L E 0 1 · F U N D A M E N TA L S What the data actually looks like Module 01 · What is seismic? 8 I M AG E P L AC E H O L D E R Raw shot gather from your survey Drop in a single shot record. Point out the direct arrival, the refraction, the hyperbolic moveout of the reflections and the noise cone. This is what the processor starts with — nothing here looks like geology yet. I M AG E P L AC E H O L D E R Migrated stack section through a well Drop in an inline through a key well in your asset. Same rock, same physics, after processing. Ask the room what changed — and how many of those changes were decisions made by a person. Between these two pictures sit hundreds of processing choices. That is why vintage and processing history matter.
M O D U L E 0 1 · F U N D A M E N TA L S Acoustic impedance and the reflection coefficient Module 01 · What is seismic? 9 The only two equations today AI = × V ρ RC = (AI − AI ) / (AI + AI ) ₂ ₁ ₂ ₁ • Impedance is a rock property — density times velocity. • The reflection is a CONTRAST, not a rock. Same shale above and below = no reflection. • Positive RC = harder rock below = a peak on normal polarity. • Typical RC values are small: 0.05 to 0.20. Coal or a tight streak can reach 0.3+. Worked example Interface AI above AI below RC Shale / Sand (gas) 8.4 6.1 −0.16 Shale / Sand (brine) 8.4 8.9 +0.03 Sand / Limestone 8.9 13.5 +0.21 Shale / Coal 8.4 4.5 −0.30 AI in 10⁶ kg·m⁻²·s⁻¹. Note how the brine sand almost disappears — a real reservoir you cannot see. A strong reflector is not the same as a good reservoir — and a good reservoir is not always a strong reflector.
M O D U L E 0 1 · F U N D A M E N TA L S The wavelet — why one boundary makes several loops Module 01 · What is seismic? 10 -60 -40 -20 0 20 40 60 -0.6 -0.4 -0.2 5.55111512312578E-17 0.2 0.4 0.6 0.8 1 Zero-phase Ricker wavelet, 30 Hz dominant frequency Time (ms) Amplitude • The earth returns a spike; the wavelet is what we actually record. • Dominant frequency sets resolution — high f, sharp wavelet. • Bandwidth matters more than peak frequency. Broad band = fewer side lobes. • Side lobes are NOT geology. Do not pick them. • Zero-phase: energy symmetric about the boundary, peak on the interface. • Minimum-phase: energy trails after the boundary — onset is the interface. One geological boundary produces a trough–peak–trough pattern. Three loops, one interface.
M O D U L E 0 1 · F U N D A M E N TA L S The convolutional model Module 01 · What is seismic? 11 Earth reflectivity Wavelet + Noise = Seismic trace ⊛ S e r i e s 1 -0.3 -0.2 -0.1 2.77555756156289E-17 0.1 0.2 0.3 1. Reflectivity series from logs -60 -40 -20 0 20 40 60 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 2. Wavelet (extracted or theoretical) S e r i e s 1 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 3. Synthetic seismogram Five clean spikes become a smeared, overlapping trace. Interference between nearby beds is not noise — it is the physics.
M O D U L E 0 1 · F U N D A M E N TA L S Polarity and phase — the classic silent error Module 01 · What is seismic? 12 SEG Normal polarity • Increase in impedance = PEAK • Hard rock below a soft rock plots as a positive loop • Most modern European / Asian datasets SEG Reverse polarity • Increase in impedance = TROUGH • Common in older US Gulf Coast convention • Flip it and every pick moves half a loop How to check in 30 seconds • Find the seabed or a known hard event (basalt, limestone, tight streak) — is it a peak or a trough? • Check the polarity of your synthetic against the seismic before you trust the tie, not after. • Write the convention in the project notes. Every mixed-vintage project has at least one flipped volume.
M O D U L E 0 1 · F U N D A M E N TA L S Polarity in practice — the same boundary, two conventions Module 01 · What is seismic? 13 -60 -40 -20 0 20 40 60 -1.1 -0.6 -0.1 0.4 0.9 SEG Normal — hard rock below plots as a PEAK Time (ms) -60 -40 -20 0 20 40 60 -1.1 -0.6 -0.1 0.4 0.9 SEG Reverse — the same boundary as a TROUGH Time (ms) I M AG E P LA C E H O LD E R Seabed or a known hard event from your own survey Insert a shallow display showing the seabed reflection, or a strong limestone/basalt top. Whichever way that event plots is your convention — establish it before anyone picks anything.