Coal mining generates significant adverse environmental impacts. Historically, coal mining has been a very dangerous activity and the list of historical coal mining disasters is a long one. Underground mining hazards include suffocation, gas poisoning, roof collapse and gas explosions.
The term 'OVD' (Optically Variable Device) is used to describe both laser generated hologram images and electron beam generated images. These optically variable devices are created in highly-secure facilities and are at the forefront of overt asset and brand protection programmes.
OVA and OVF: The Differences. OVF is not only the name of the packaging format standard, but it also refers to the package when distributed as a group of files. An OVA (open virtual appliance or application) is merely a single file distribution of the same file package, stored in the TAR format. DMTF: OVF Standard.