Source taxonomy: impulsive vs extended

Seismic sources come in two families. Impulsive sources release their energy in milliseconds — dynamite detonations, air-gun bubbles, weight drops. Extended sources release the same energy over several seconds — vibroseis sweeps, marine vibrators, harmonic sources. The two families share the same physics downstream but demand very different fieldwork and signal-processing pipelines.

Why the pilot correlation matters

An impulsive source is already near-ideal: a short pulse with broadband spectrum. Extended sources look nothing like that — in the raw recording, every reflection shows up as a smeared-out chirp several seconds long, and consecutive reflections overlap each other mercilessly.

The trick is to record the pilot sweep (the exact waveform sent into the ground) and cross-correlate the recording with the pilot. The auto-correlation of a sweep is the Klauder wavelet — short, symmetric, with bandwidth equal to the sweep band. After correlation, each reflection collapses to a Klauder wavelet at its correct two-way time. The data becomes processable.

When to pick each

• Dynamite: the most impulsive source available. Used in land surveys where permitting allows, shot in deep holes for good coupling and surface-ghost suppression. Banned or limited in many regions.
• Air-gun array: the standard marine impulsive. High peak pressure; environmental-impact concerns drive current research.
• Vibroseis: standard land extended source. Safer than dynamite, reusable, with precise force control. Required for urban/sensitive areas.
• Marine vibrator: emerging; halves peak SPL at similar total energy (§1.7).

References

• Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
• Yilmaz, Ö. (2001). Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data (2 vols.). SEG Investigations in Geophysics 10.