2️⃣ First Arrival Picking & Time–Depth Calibration
First Break Picking
Pick Downgoing P-wave arrival at each depth Accuracy here is everything.
Checkshot / Time-Depth Curve
From picked first arrivals Compute cumulative travel time vs depth and Derive:
- Interval velocity
- RMS velocity
📌 This is the backbone of VSP value to surface seismic.
Align downgoing arrivals vertically often done by flattening first breaks Purpose:
- Makes wavefield separation easier
- Improves S/N
3️⃣ Wavefield Separation & Deconvolution
Downgoing / Upgoing Separation
In VSP data (especially zero-offset VSP), separating downgoing and upgoing wavefields is a fundamental preprocessing step before corridor stacking, deconvolution, and migration.
Methods:
1) Median filter in depth
Advantages
✔ Very simple
✔ No FFT required
✔ Good for quick QC
Limitations
✖ Window-size dependent
✖ Lower accuracy
✖ Not robust for complex wavefields
2) FK (f-kz) filtering
Advantages
✔ Simple
✔ Fast
✔ Works well when dips are clearly separated
Limitations
✖ Fails if wavefields overlap
✖ Requires regular depth sampling
✖ Sensitive to aliasing
3) Model-based separation like Tau-pi filtering
Advantages
✔ Better separation than FK
✔ Handles multiple dips
✔ More robust for complex data
Limitations
✖ Higher computational cost
✖ Needs regular depth sampling
Outputs:
- Downgoing wavefield (direct arrivals)
- Upgoing wavefield (reflections)
📌 Upgoing wavefield ≈ “reflection seismic recorded in the well”
Deconvolution (Often Corridor Decon)
VSP deconvolution is a signal processing step applied to Vertical Seismic Profile (VSP) data to:
- Compress the source wavelet
- Improve temporal resolution
- Remove source signature and reverberations
- Enhance primary reflections
- Stabilize amplitude and phase
In VSP, especially zero-offset VSP, downgoing waves contain strong source signature and multiples. Deconvolution helps convert the recorded wavelet into a sharper spike-like response, making interpretation and inversion more reliable. Downgoing waves are used to extract VSP deconvolution operator. This operator is convolved on flattend upgoing waved.
4️⃣ Corridor Stack
What is Corridor Stack?
- Stack of upgoing reflections
- Taken in a time window just after first arrivals
- Produces a 1D seismic trace at the well
📌 This is the best seismic–well tie trace you can get.
Why Corridor Stack Matters
- Ties synthetic seismograms
- Validates seismic polarity
- Confirms wavelet phase
- Reduces multiples
👉 Many interpreters trust corridor stack more than surface seismic at the well.