Some information about shockwaves

On TV3 last night John Campbell, alleged that the video of the blast shockwave from Pike river Mine had been suppressed. This merely highlights how bad a researcher he is.

The duration of the mine shockwave has been known on the Westcoast shortly after the blast and even known by Whales spies as far afield as Dunedin.

One thing that needs to be understood is the magnitude of the explosion. The blast shockwave lasts some 52 seconds. This tells anyone who has played with explosives or combustible materials just how large the explosion was. For example a typical 2000kg gravity bomb has a blast shockwave of about 3-5 seconds. It has been suggested to me that the 50 seconds can be compared to a bombs which is in the order of 3 – 5 seconds. This just isn’t the case. This explosion is far larger than a simple high explosive detonation.

The Chapman–Jouguet condition holds approximately in detonation waves in high explosives. It states that the detonation propagates at a velocity at which the reacting gases just reach sonic velocity (in the frame of the lead shock wave) as the reaction ceases.

In other words the blast is dependent on the chemical reaction that caused it.

And 50 seconds is a huge blast by any definition. From Wikipedia

Detonation wave
A detonation wave is essentially a shock supported by a trailing exothermic reaction. It involves a wave traveling through a highly combustible or chemically unstable medium, such as an oxygen-methane mixture or a high explosive. The chemical reaction of the medium occurs following the shock wave, and the chemical energy of the reaction drives the wave forward.

A detonation wave follows slightly different rules from an ordinary shock since it is driven by the chemical reaction occurring behind the shock wave front. In the simplest theory for detonations, an unsupported, self-propagating detonation wave proceeds at the Chapman-Jouguet velocity. A detonation will also cause a shock of type 1, above to propagate into the surrounding air due to the overpressure induced by the explosion.

When a shockwave is created by high explosives such as TNT (which has a detonation velocity 6,900 m/s), it will always travel at high, supersonic velocity from its point of origin.

The chemical reaction of a methane/oxygen explosion that generates a detonation wave of 52 seconds is massive. The force shown at the mine entrance some 2.5km from the actual blast site shows just how massive the blast was. To have survived, in the confined area of a mine and less than 500 metres from the source of the explosion, would be a miracle.

The Commission of Inquiry will no doubt calculate the magnitude of the explosion and when we see the findings I think we will all be shocked.

UPDATE: Karyn Hay and Andrew Fagan on Radio Live last night got a call from Jeremy, a former explosives technician with the Australian Army. He shares his knowledge of exploding through rock and using drills.


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As much at home writing editorials as being the subject of them, Cam has won awards, including the Canon Media Award for his work on the Len Brown/Bevan Chuang story. When he’s not creating the news, he tends to be in it, with protagonists using the courts, media and social media to deliver financial as well as death threats.

They say that news is something that someone, somewhere, wants kept quiet. Cam Slater doesn’t do quiet and, as a result, he is a polarising, controversial but highly effective journalist who takes no prisoners.

He is fearless in his pursuit of a story.

Love him or loathe him, you can’t ignore him.

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