A system of long distance, horizontal strain lines has been established in Wilmington Oil Field, Long Beach, California. The length of these lines, which ranges from about 2,600 to 18,000 feet (729 to 5,486 m), is established twice yearly using electro-optical, line-of-sight ranging devices. These lines are used to measure changes in compression and tension over the Wilmington Oil Field subsidence bowl. The ground surface is unstable owing to water injection operations which slowly raise and lower the ground surface.
Most of the strain lines that are shorter than about 5,000 feet (1,524 m) lengthen as the surface is raised and shorten as it is lowered. The amount of length change roughly duplicates elevation change. Average strains over the total line lengths are in the order of 10−4 to 10−5, which as a general rule do not harm surface structures.