Underwater or waterlogged blastholes put two demands on a blast design: the explosive column must not be washed out or weakened by standing water, and the firing system must not risk misfires or accidents from stray electric current in wet ground. Checking each option against these two demands settles the question.
- ANFO and electric detonator: ANFO is ammonium nitrate prills soaked in fuel oil, and ammonium nitrate dissolves readily in water, so a wet ANFO charge quickly turns into a weak, unreliable slurry. Pairing it with an electric detonator also puts a live circuit into wet rock, risking stray current initiation.
- emulsion and non-electric detonator: an emulsion explosive traps its oxidiser solution as microscopic droplets inside a continuous wax and oil coating, water in oil rather than oil in water, so outside water cannot reach and dissolve the oxidiser. It stays fully sensitive even packed under water. The non-electric detonator fires through a shock wave travelling down a plastic tube, with no metal wire and no electric current, so wet ground cannot cause premature or missed firing.
- HANFO and safety fuse: heavy ANFO gains some water resistance by blending in emulsion, but it is still well short of a pure emulsion in wet holes. A plain safety fuse burns from the outside in open air; if water reaches it, the flame can be doused or the burn rate thrown off, which is unacceptable for underwater timing.
- PETN and electronic detonator: PETN is normally the core of detonating cord or boosters, not a bulk column charge, so it is not really comparable as a standalone underwater charge. Electronic detonators bring precise timing but are delicate and costly, and are not the routine underwater choice compared to a rugged non-electric shock tube system.
Only the emulsion and non-electric detonator combination satisfies both the water resistance and the electrical safety needs of underwater blasting, so option (B) is correct.
Let's summarize:
- Water resistance rules out ANFO and favors emulsion, whose water in oil structure keeps water away from the oxidiser.
- Electrical safety in wet ground favors non-electric (shock tube) firing over any electric or electronic circuit.
So the best combination for underwater blasting is emulsion explosive with a non-electric detonator.