Common Midpoint (CMP)
The numbering system used across the top of a seismic profile is the CMP number. Each CMP is a vertical wiggle trace and has its own sequential number and x,y co-ordinate so that it can be located on an ordinance survey map. The distance between each CMP on a given seismic profile is the same, and a tradition has arisen in the seismic industry where 12.5m, 25m and 50m are the most common intervals. The zero crossings are important on the CMP wiggle trace, they represent the point of change, where the properties of one seismic layer are different to the next. The resulting change in velocity is seen on the seismic profile as a change of polarity (peak to trough, or trough to peak).