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.