Dynamite: charge, depth, coupling
Learning objectives
- Explain why shot depth, not charge size, sets the ghost-notch frequency
- Calculate the ghost notch f_n = V/(2h) for given h and near-surface V
- Describe the trade-off between coupling (deeper is better) and logistics (shallower is faster)
- Interpret the charge-depth combination's effect on wavelet amplitude AND shape
A dynamite shot in a hole of depth h sends energy two ways: downward (the primary wavefield we want) and upward to the free surface, where it reflects back inverted (the ghost). The ghost combines with the primary with a delay of 2h/V — the round-trip time from shot depth to surface and back — creating a comb-filter notch in the spectrum at:
and its integer multiples. Place the shot at 12 m in V = 600 m/s near-surface rock and the first notch sits at 25 Hz — right in the middle of the useful seismic band. Go to 30 m in the same rock and the notch moves to 10 Hz, well below most targets.
What charge size controls
More charge = more amplitude, period. It does not shift the notch, does not change the wavelet shape, does not fix bad coupling. Doubling the charge buys you +6 dB of SNR at best; going from 100 g to 200 g rarely justifies the extra cost if the hole isn’t deep enough to put the notch below your target bandwidth.
Coupling
Coupling is how efficiently the shot transfers energy into the rock rather than heating air or venting up the hole. Deeper holes with tamped cuttings above them couple better. Insufficient tamping → blow-out; too shallow → crater formation and energy loss. Every extra metre of depth is more signal and a lower ghost notch. That’s why land crews drill as deep as budget and geology allow.
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.
- Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.