Source radiation patterns
Learning objectives
- Identify the four canonical source radiation patterns: monopole, vertical dipole, horizontal dipole, double-couple
- Read a polar amplitude plot |A(θ)| for each
- Connect the radiation pattern of each real-world source to its canonical idealisation
- Anticipate where nulls will appear in the wavefield from a given source type
No real source puts out the same amplitude in every direction. Different source mechanisms radiate with different angular patterns — and those patterns are what tells you whether your targets are getting illuminated or sitting in a null.
The canonical four
- Monopole — an explosion. An isotropic pressure burst, radiating equally in all directions. Dynamite in a shot hole or an air-gun submerged in water comes close to a pure monopole.
- Vertical dipole — a downward-pushing point force. A vibroseis truck on the ground or a vertical-impact weight drop both look like vertical dipoles to first order. Their P-radiation is strong straight down and zero to the sides.
- Horizontal dipole — a sideways point force, used when you want to generate S-wave-rich wavefields (shear vibrators, horizontal impulse).
- Double-couple — a pair of offset, opposing force couples. This is the P-wave radiation of an earthquake slip on a fault, and relevant when you’re designing microseismic acquisition where the “source” is natural.
Why this matters for geometry
A vertical dipole has nulls at 90° from vertical. If you fire one and record at far offsets, those receivers will catch only S-wave-converted arrivals, not direct P from the source. Knowing the radiation pattern tells you what each receiver is realistically going to record before you’ve deployed a single geophone.
References
- Aki, K., Richards, P. G. (2002). Quantitative Seismology (2nd ed.). University Science Books.
- Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
- Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.