Shot-hole dynamite deployment

Shot-hole dynamite is the oldest land seismic source and still the cleanest. A charge detonated below the weathering layer produces a near-minimum-phase impulse, couples cleanly into the bedrock, and avoids the ground-roll mess generated by surface sources. In mountainous, forested, or sensitive terrain where vibrator trucks cannot reach, dynamite is still the only viable source.

The cycle

A single shot-hole runs through six phases. Drill: a truck- or track-mounted rig bores a 75-125 mm hole to the target depth at roughly 2 m/min, a 15 m hole takes 7-8 min. Load: the crew lowers the explosive charge (seismic gelatin, pentolite, or packaged emulsion) on a weighted line, then a blasting cap on a shooting wire; 2-3 min. Seal: tamping material (gravel, bentonite, water) is poured on top of the charge to confine the shot, preventing blow-out of energy upward; 1-2 min. Clear: the crew backs off 30-50 m to safe standoff; 1 min. Fire: the shot is initiated via shooting wire or radio signal; 2 s. Move: drill rig tracks to the next hole; 2-3 min. Total ≈ 15-20 min per hole in open terrain.

Charge sizing

Rule of thumb: 0.5-1 kg for shallow targets (<1 km), 1-2 kg for typical exploration (1-3 km), 2-5 kg for deep basin work (>3 km). Above 5 kg the hole is at risk of surface crater and regulatory limits often bite. Charge size scales source amplitude roughly as mass^0.6 (the source is not a point monopole at these sizes), so doubling the charge buys ~1.5× amplitude, or ~3.5 dB.

Below the weathering

The weathering (low-velocity) layer is typically 5-20 m thick and acts as an impedance mismatch: a charge fired within it wastes energy as surface waves and heat. Placing the charge below the weathering base lets the direct P-wave couple into the bedrock substrate with much lower Rayleigh conversion. Uphole surveys (a vertical geophone string in a pilot hole) are used early in a survey to confirm the base of weathering, and then the routine shooting depth is set 3-5 m below that.

Pattern and productivity

When a single hole does not give enough amplitude, crews use patterns: a pair of holes 10 m apart fired simultaneously, or a cluster of 3-4 holes. The pattern acts as a small source array, boosting low-frequency output and attenuating surface-wave radiation. But patterns multiply drill and load work, a 4-hole pattern at 15 m depth is 60-80 min per shot. With 2 drill rigs working in parallel, a 10-hour day is roughly 30-40 holes, or 10-20 shot-patterns depending on cluster size. Compare this to 4,000+ VPs/day for vibroseis: dynamite is never chosen for speed.

References

• Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
• Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.