Atmospheric conditions can, on occasions, concentrate or focus air vibration in certain directions and distances from the blast. Weather conditions that include an ‘inversion’ or a layer of warm air between colder air layers, such as exists on smog pollution days, can cause an increase of up to 10 or more decibels at distances from 2-5 km from the blast.
Similar effects may be caused by increasing wind speed with altitude, especially when accompanied with a change of wind direction or wind shear. For meteorology to have a significant influence on air vibration levels, the inversion layer or wind shear must be at levels less than about 200-250 metres above the blast.
The prediction of meteorological conditions requires accurate local data that is generally not freely available for use, however progress is being made with the development of SODAR and RASS systems for local area installation (i.e. near a particular minesite). The practical effect of meteorological reinforcement is that, on occasions, blasts may be noticed in locations distant from the operation where they are normally imperceptible. Elevated airblast levels due to meteorology are usually below regulatory limits because at the distance from the blast at which they characteristically occur (>2 km), the basic emission levels are low following natural attenuation.
An assessment of the degree to which air vibration levels will be affected by the effects of meteorology requires that temperature, wind speed and wind direction be measured or evaluated at the surface and at various levels above the ground. Experience to date has shown that airblast levels can be influenced and increased by meteorology at distances of up to 20km from mine sites.Terrock has developed the Envmet system to predict the effect of local meteorological conditions on overpressure from blasting.
Envmet requires data from a predictive meteorological model. To provide this Terrock can produce a customised model based on the WRF model to provide multi-day forecasts about a specific region. This output can be used for many other purposes. Terrock has partnered with a number of other companies and researchers to provide dust modelling and noise modelling services using the same weather forecasts that Envmet relies upon.
The weather model data is input to an atmospheric refraction model that enables the effect of meteorology on airblast overpressure levels in the area surrounding the blast site to be evaluated. Graphical outputs from the model, which are superimposed over a scaled airphoto of the mine and surrounding area, show the extent that airblast levels will be increased.
When combined with inputs that quantify the effect of blasting specifications, the airblast levels that will result in the surrounding area from the combined effect of meteorology and basic blast emissions are determined.
The model permits those responsible for blasting to schedule blasts to avoid unfavourable weather conditions and better conform with applicable airblast limits.
Envmet modelling is made in a number of ways. Three day forecasts of the likely meteorological effects are made available daily for subscribers via the Envmet website. Modelling output can also be made available via a web based service so that Envmet forecasts can be included on internal websites. For specific investigations, Terrock will utlilise Envmet modelling to determine the likely effects of local meteorology.