Ludger Mintrop (1880-1956), a German scientist, was one of the founders of refraction seismic exploration for oil and natural gas (1919). His initial work in seismology was done in World War I, when he used a portable seismograph to locate the position of Allied heavy artillery pieces. After the war, Mintrop reversed the process, setting off explosions at a known distance from the seismograph and measuring the return time of subsurface shock wave reflections to estimate the depths of geologic formations. Mintrop set up Seismos in 1921, the oldest geophysical contracting company in the world. In June of 1924, a Seismos crew discovered the Orchard salt dome in Fort Bend County, Texas. This was the first salt dome to be found in the U.S. by the seismic method.