Hello. In this sequence, we shall discover the structure of our planet from the first ideas of Eratosthenes to more recent interpretations. For the ancients, the Earth was totally flat. It was a circular disc with a very simple structure and only three continents. How has the knowledge of the Earth's shape and structure evolved? During the 3rd century before Christ, Eratosthenes compared the sun rays between two different locations in Egypt, Aswan, and Alexandria. The distance between both cities was known as d. In Aswan, the sun rays in summertime, were known to directly light the bottom of a deep well. Conversely, in Alexandria, Eratosthenes measured the shadow of the obelisk erected in front of the library. He then found that the angle between Alexandria and Aswan, was approximately seven degrees. The distance, d, between both cities being around 790 kilometers. The Earth radius could be derived as r equals d over Alpha, which is approximately 6,500 kilometers to be compared to an actual average of nearly 6,400 kilometers. Eratosthenes made an excellent approximation. In the 18th century, detailed measurements of the meridian were performed by Mechain and Delambre, starting in 1792. They have determined the length of the meridian from Dunkirk to Barcelona using a triangulation method shown on this map. It was a risky task during the French Revolution. Their idea was to choose the Earth as a standard to define the whole metric system. The meter was thus defined as the 10 millionth fraction of a quarter of the meridian. The shape and size being known, what is the internal structure of the Earth? At the end of the 17th century, the astronomer, Lord Halley, already tried to describe the earth's structure, accounting for variations of the magnetic field. He assumed that the Earth was composed of three different shells with a magnetic core rotating with various velocities. The dimensions of these internal spheres corresponding to the radii of Mercury, Mars, and Venus. What is the actual Earth structure? In 1906, Richard Oldham, proposed the first Seismological Earth model. He noticed that some seismic waves propagate slowly in a given azimuth and assumed the central part of lower wave velocity. He thought he evidenced a major seismological discontinuity at a depth of 3,800 kilometers. In 1909, Mohorovicic observed an increase of the seismic wave velocity at 50 kilometers beneath Croatia. It was interpreted as the interface between the crust and the mantle. It is currently named the Moho, in memory of Mohorovicic. A few years later, in 1912, Beno Gutenberg relocated the Oldham discontinuity at only 2,900 kilometers from the surface. This depth remains a reference for the location of the Earth core. One additional contribution was still missing to characterize the Earth metallic core. In 1928, a Danish seismologist called Inge Lehmann, analyzed seismic data due to an earthquake in Mexico. The seismograms recorded at various stations from Oxford to Wien allowed her to identify the various peaks and start refining the characterization of the metallic core. In 1936, in a very famous paper, she proposed a detailed analysis of the various seismic rays crossing different regions of the Earth interior. Let's follow Lehmann's analysis starting from the detailed structure she proposed. The Earth core is now decomposing to an outer core and an inner core. Considering a first seismic ray propagating only in the mantle, she simply identified a straight path in a homogeneous medium. Changing the wave incidence, we get a straight grazing path at the mantle core interface. Inclining the incident ray, it is now refracted at the mantle core interface and then at the core-mantle interface. The location reached on the right-hand side is very far from the previous one due to both refraction steps. For larger incidence, the refraction process now leads to a much closer location. This is due to the shape of the mantle core interface acting as an antenna. Increasing the incidence a bit more, the ray path is nearly straight and it grazes the outer inner core interface. What will be observed next? If we exceed the grazing incidence, the seismic ray appears strongly deviated at the outer inner core interface. Lehmann thus identified the transition between the outer liquid core and the inner solid core. Increasing the incidence a bit more leads to nearly straight or perfectly straight seismic rays in the various areas. To sum up, the earth structure is now composed of four areas with different properties. These wave velocities plots have been proposed in the early '80s. The blue curve corresponds to pressure waves, the red one to shear waves. They will be studied in details in the following sequences. The important thing here is to identify the Moho interface between the crust and the mantle, the Gutenberg interface between the mantle and the outer core showing a huge variation in the wave velocities at 2,900 kilometers. There are no shear waves in the liquid outer core as shown by the zero shear wave velocity in red. Finally, at 5,200 kilometers, the Lehmann interface between the outer liquid and the inner solid magnetic core. The actual Earth structure is finally not so different from Lord Halley's model. Three shells with a magnetic core.
