Gas probes, also known as perimeter or migration probes, are used for subsurface monitoring and detect gas concentrations in the local environment around the probe. Sometimes multiple probes are used at different depths at a single point. Probes typically form a ring around a landfill. The distance between probes varies but rarely exceeds 300 metres. The typical regulatory limit of methane here is 50,000 parts per million (ppm) by volume, or 1% methane and 1.5% carbon dioxide above geological background levels in the UK (see " Guidance on the monitoring of Landfill Gas " LFTGN03, EA 2004).