: velocity of seismic
S-waves in the Earth near the surface in three tectonic provinces: TNA = Tectonic North America SNA = Shield North America and ATL = North Atlantic. In 1925, Lehmann was assigned to be the assistant of seismologist
Niels Erik Nørlund. She took an interest in his field, and she began studying it on her own. She was chosen as a delegate for Denmark to attend the
International Union of Geodesy and Geophysics in 1927—a role she filled another eight times over the next forty years. By 1928, Lehmann obtained a
magister scientiarum in seismology, and she was appointed head of the
Geodætisk Institut's seismological department the same year. In this position, she was responsible for overseeing the operation of three seismographic observatories, two of which were in Greenland. She personally operated the one in Copenhagen, producing reports based on its readings. Though it was not part of her job, Lehmann also engaged in research at the facility. In 1929, Lehmann studied the
Murchison earthquake which struck on the South Island of New Zealand. She analyzed the seismic data from the earthquake and noticed that there were waves of significant amplitude recorded in the Russian cities of
Sverdlovsk and
Irkutsk, both unexpected locations. Lehmann noticed after reviewing the data from the earthquakes that there were waves emanating from the earthquakes, which were called seismic waves. They were unexpected locations due to the theory that S-waves and some P-waves are deflected by the core creating a shadow area in which waves are not able to pass through. The waves seemed to pass through that area to reach Russia. This led to her discovering that there is a spherical core of solid material at the Earth's centre. . Lehmann was the first to interpret
P-wave arrivals as reflections from an inner core. Lehmann observed seismic waves from earthquakes, leading her to hypothesize that the Earth's core consisted of two parts: "a solid metal core surrounded by an outer liquid core, overturning the accepted theory of an entirely liquid core". She published these findings in a paper titled '''' (1936). Prior to 1936, scientists believed that the Earth's core was a single, massive molten sphere. However, many global observations did not analytically add up until Lehmann reached the heart of the issue. The theory she developed was that the Earth consisted of 3 shells: the mantle, outer core and inner core. Lehmann inferred that the core wasn't homogeneous; rather, there is a smaller core that exists that is surrounded by the outer core. She deduced that waves travel faster in the smaller core, but the waves can be reflected off if it arrived tangentially. Her theory allows for another wave deflection at the extra boundary and this accounts for the direction and location in which the waves emerge. Other leading seismologists of the time, such as
Beno Gutenberg,
Charles Richter, and
Harold Jeffreys, adopted this interpretation within two or three years, but it took until 1971 for the interpretation to be shown correct by computer calculations. She continued her work during
World War II, though international collaboration was limited. When American geologist
Maurice Ewing visited her station in 1951, he invited Lehmann to work at the
Lamont Geological Observatory, now called the
Lamont–Doherty Earth Observatory, that he ran at
Columbia University. She was invited there to study the seismic wave 'Lg', which was a new seismic wave being researched by Maurice Ewing and Professor
Frank Press. She studied there for a few months in 1952, after her retirement from the Royal Geodetic Institute. She retired from her position as head of the Geodætisk Institut's seismological department in 1953, giving her more time to conduct research over the following decades. Throughout the 1950s and 1960s, Lehmann traveled to North America several times and visited different seismological observatories throughout the United States and Canada. She became a prominent member of the community at the
University of California, Berkeley, one of her most frequent stops. During the 1960s, Lehmann was able to explore more of the Earth using new technologies made specifically for detecting nuclear bombs during the Cold War. While in the United States, Lehmann collaborated with
Maurice Ewing and
Frank Press on investigations of the
Earth's crust and
upper mantle. During this work, she discovered another seismic discontinuity, which is a step-change increase in the speed of seismic waves at depths between 190 and 250 km. This discontinuity was named the
Lehmann discontinuity after her. Lehmann was also involved in the creation of the
International Seismological Centre from 1961 to 1967. ==Awards, honours, and legacy==