The technology used varies by the service required and the design of the pig; each pigging service provider may have unique and proprietary technologies to accomplish the service. Surface pitting and corrosion, as well as cracks and weld defects in steel/ferrous pipelines are often detected using magnetic flux leakage (MFL) pigs. Other "smart" pigs use electromagnetic acoustic transducers to detect pipe defects. Caliper pigs can measure the roundness of the pipeline to determine areas of crushing or other deformations. Some smart pigs use a combination of technologies, such as providing MFL and caliper functions in a single tool. Trials of pigs using acoustic resonance technology have been reported.