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.