Basic Geophysics The Birth of Seismics

A 4000-kilogram steel ball is attached to a 14-meter high frame and pulled upwards. It pauses briefly, becomes disengaged, falls and impacts after a second on the forest floor. The vibrations are clearly noticeable for bystanders. Using this construction, Ludger Mintrop attempted this drop test for the first time in 1908. This is the original recording of it made at a distance of 120 m. What kind of seismic waves are radiated as a result of such artificial impacts and how can they be measured? These were the questions that interested Ludger Mintrop. His measuring equipment and observations formed an important basis for the later use of artificial vibration sources in oil exploration. The frame and the Mintrop-ball named after its experimenter are still exhibited at their original venue on the grounds of the Wiechert Earthquake Station in Göttingen. Even today, drop tests are carried out regularly for interested audiences. Hello and welcome. At the beginning of the 20th century crude oil production began to boom. An important part of this boom was played by applied seismics. With its help, salt domes, at the flanks of which crude oil and natural gas deposits often lie, can be found and their location can be described. In this video I would like to introduce to you three people who have significantly influenced this progress. Alongside many others they have driven crude oil exploration through new instrumental and methodological developments. Such developments were patented and promised a great deal of success. I will introduce some of these patents and achievements in this video. Here, I will show you how the development of portable seismometers has contributed to the success of seismics in oil exploration. The methodological progress of reflection and refraction seismics has made it possible to detect oil deposits in the subsurface better and to determine their location. Mintrop's drop tests in Göttingen were the start of a meteoric career. He received his doctorate in Göttingen from Emil Wiechert and was a lecturer in mine surveying in Bochum. During the First World War, he developed seismographs designed to locate enemy artillery, but because the war had ended they were never put into service. In 1917 Ludger Mintrop applied for his first patents for portable seismographs and their application to explore rock layers. Here you can see an image of a portable vertical pendulum from a later Mintrop patent from 1923. During a vibration, the reference system of the frame moves relative to the inertial mass here below. The mass is suspended with a return spring that has a natural frequency of approximately 10 Hertz. A clamped hair or small metal linker transfers the movement to a rotating mirror. This then reflects light through a lens into the recording unit in a distance of approximately half a meter, where the signal is recorded on light-sensitive photographic paper. The gain of the signal is therefore only modulated by mechanical and optical means. On this drawing from a Mintrop patent from 1926, you can see a refracted wave and the associated travel time diagram. For short source-receiver distances, the travel time of the first impact increases linearly with the distance, that is the direct wave. From the critical distance, the first impacts systematically arrive earlier, indicating deeper lying rock layers with a faster propagation velocity. In his memoirs, Ludger Mintrop wrote that he discovered the head wave, which propagates along layer boundaries in the subsurface, in 1919. This discovery was the dawn of refraction seismics. For this reason, the head wave is still named after its discoverer today as the Mintrop wave. In 1921 Mintrop founded the company Seismos and already in the same year he carried out his first investigations in Northern Germany, the Netherlands, and some other European countries with the new method. Here you can see pictures of typical exploration teams of Seismos, who worked later in the United States and also in Mexico. Vibrations were no longer initiated with falling weights, but rather with dynamite explosions, and the seismic waves were registered with previously mounted seismographs. When doing this it was necessary to keep the instruments in tents, so that the optical transfer would not be interfered with by direct sunlight. On the last photo you can see the elongated vertical pendulum on the right and the recording apparatus on the left. The breakthrough came in 1924 in the USA near Houston, Texas, where the Seismos team detected the Orchard salt dome. This discovery is hailed as the first find of crude oil using seismic methods. The procedure worked, and in the following years it was also successfully used at other locations in Texas and Mexico. In the 1920's, Ludger Mintrop, by developing portable measuring instruments and utilising refracted head waves was able to discover salt domes and subsequently crude oil deposits. Reflection seismics, on the other hand, was first used by others. John Clarence Karcher used seismic methods for subsurface exploration with reflected waves since 1919. In addition, he also developed portable electromagnetic instruments, that were much more sensitive than the previously used mechanical instruments. Here you can see a portable electromagnetic vertical seismometer from Karcher from the year 1929. A solenoid is suspended swinging freely in a permanent magnet or electromagnet. When a vibration occurs, the magnet moves relative to the inertial coil and induces a voltage that can be measured directly. The induced voltage is proportional to the velocity of the movement. This basic principle is still used today in most of the geophones used in seismics. In 1921 Karcher at Oklahoma recorded clear reflections from a shale-limestone boundary. Here you can see the corresponding depth profile and the recording of the seismic waves. The arcs and dotted lines indicate the graphical migration of the signals. At that time, it was already correctly assumed that with inclined layer boundaries the seismic waves in the zero offset case run orthogonally to the layer boundary. Despite this methodological success, John Clarence Karcher was unable to make any financial gain, as the price of oil had slumped massively due to oversupply. Later in May 1925, Karcher and his partners founded the Geophysical Research Corporation - GRC in brief. After some initial work with refracted rays they were shortly also to use reflection seismics successfully for crude oil exploration. In 1928 they made a find in Oklahoma. The Mauld crude oil field was the first to have been discovered by reflection seismics. This was followed by further finds and reflection seismics replaced the refraction method more and more. Although it was possible using refraction seismics to find shallow salt domes, reflected rays could also be used to explore deeper or inclined layer boundaries. Help arrived for Karcher in the form of a patent granted in 1917 to Reginald Fessenden. Originally designed for detecting gold deposits, Fessenden was the first to patent exploration with seismic waves. Here he envisioned equipping water-filled boreholes with seismic sources and receivers to locate intermittently located gold deposits. The chances of success for this type of approach at this point in time was doubted, and indeed he found no success in this undertaking. In 1925 however, Karcher obtained the patent and the support of Fessenden. He put the idea with seismic reflection profiles in different directions into practice and successfully located crude oil deposits. In the following years, the Great Depression and the Second World War put dampers on the development of applied seismics. Greater methodological progress then followed with the publication of the Common Midpoint gather in 1950, which only truly gained the upper hand in the mid-1960s. From the end of the 1950s, seismic waveforms were recorded in analogue form on magnetic tape. This form of storage allowed the filtering and processing of data repeatedly. At the beginning of the 1970s, digital recording procedures became established and allowed even greater quantities of data to be processed. This paved the way for large 3D explorations on land and at sea. Today, the consistently larger computer processing capabilities allow for steadily increasing amounts of data and therefore, an even more detailed description of the three-dimensional subsurface of the Earth. In this video I have shown you how the development of instruments and methods brought great success to applied seismics at the beginning of the 20th century in the exploration of crude oil. Since 1908 Ludger Mintrop developed the portable seismometer, which from the beginning of the 1920s he used commercially for detecting salt domes. The Mintrop wave - a wave refracted at the layer boundary - was named after him and allowed him to determine shallow layer boundaries. At the end of the 1920's John Clarence Karcher and colleagues established the technique of reflection seismics. Using sensitive electromagnetic seismometers, they could also detect deeper structures in the search for crude oil. Both examples show how the development of methodology and instruments goes hand-in-hand and how it enables the future development of seismic methods.