Geologists Day is a professional holiday of geologists, geophysicists and geochemists. Seismic waves are studied by geophysicists called seismologists. A significant portion of the Saudi Aramco workforce consists of geophysicists and geologists. The work of geologists and geophysicists is complex and multifaceted. However, around three quarters of fellows are professional astronomers or geophysicists. GPlates is used by geophysicists, students and researchers in academic institutions, government departments and industry. It is used by geophysicists in hydrocarbon exploration. This topographic surface is generally the concern of topographers, hydrographers, and geophysicists. Geophysicists use VLF-electromagnetic receivers to measure conductivity in the near surface of the Earth. It was established in 1936 and is published by the Society of Exploration Geophysicists. A worldwide group of geophysicists collaborated with the observations made from Spacelab 2. According to Nature these programs had been "unofficially known to Western geophysicists for several years". Julia Lezhneva was born in Yuzhno-Sakhalinsk (Sakhalin island), Russia, into a family of geophysicists. Bullard became one of the most important geophysicists of his day. Some geophysicists fear the world's helium could be gone in a generation.