1.2 Geophysical Exploration Methods… and Some History


Robert Mallet (1810–1881)


Carl August von Schmidt (1840–1929)


the US for most of his life, making significant contributions to the development of alternating currents (AC). He patented an AC induction motor – and today Tesla is possibly best known as a brand of electric car. Magnetometry is commonly used both for mineral and hydrocarbon exploration. In physics, the weber (Wb) is the SI unit of magnetic flux. A flux density of one Wb/m2


is one Tesla. Gravity is a force of nature that we experience every day. It


John Clarence Karcher (1894–1978)


Figure 1.6


Ludger Mintrop (1880–1956)


is produced by all matter in the universe and attracts all bodies of matter, regardless of type. Te principle of the gravitational force of attraction F between two mass bodies (m andM) separated by a distance r was introduced by Isaac Newton (1642–1726) in his Principia from 1687: Here, G = 6.673x10-11


m3 kg-1 s-2 is the universal gravity


Geophysicists commonly use four surface methods to explore the subsurface: seismics, electromagnetics, gravity and magnetics. Most of the money spent on exploration today is used for seismic acquisition. However, there has been an increased focus on electromagnetic methods over the past decade, and also on how to combine the four methods in more efficient ways.


1.2.1 Four Basic Methods


One of the first studies within seismology was undertaken by the ‘father’ of seismology Robert Mallet in 1846, who studied the damage after an earthquake in Italy by visual inspection and use of photos. In 1851, he used dynamite explosions to measure subsurface sound propagation velocities. In 1888 August Schmidt estimated seismic velocities from plots showing seismic travel time versus distance. In the 1920s both John Clarence Karcher and Ludger Mintrop made significant contributions to seismology by using reflection and refraction seismic for exploration purposes, and by the end of this decennium seismic exploration had become a common tool. Te first magnetometer (an instrument used to measure


the variation in the Earth’s magnetic field as a function of position) was developed by Carl Friedrich Gauss in 1833, who is mostly known for his significant contributions within mathematics. Together with Wilhelm Weber, he spent time studying and measuring the Earth’s magnetic field. Te unit for the magnetic field was therefore named the ‘gauss’ to honour his achievements. In 1960 the international committee for units (SI) incorporated the unit into a new metric system, and the unit for magnetic field strength was changed to Tesla (1 Tesla = 10,000 gauss). Tis was in honour of Nikola Tesla (1856–1943), a Serbian physicist who worked for Tomas Edison, and lived in


6


constant. However, what we today refer to as Newton’s gravitational law has a somewhat turbulent history. Robert Hooke (1635–1703) claimed that he had suggested the idea of the gravitational behaviour in 1660. Historians are still discussing this issue, and a common understanding seems to be that Newton’s contribution of demonstrating the accuracy of the law is important. At the time that Newton was working on his masterpiece, the Principia, several scientists had suggested a -law, and hence Newton acknowledges Wren, Hooke and Halley in his famous book. Today, physicists are struggling to unify the four forces


known in the universe: the strong force (acting between quarks), the weak force (responsible for natural radioactivity), the electromagnetic force (acting between electrically charged particles) and gravity (acting between mass particles). It is evident that gravity is the weakest force, but it is also the force that is still lacking the interaction particle associated to it.


Isaac Newton (1642–1726)


Carl Friedrich Gauss (1777–1855)


Wilhelm Eduard Weber (1804–1891)


Figure 1.7


Nikola Tesla (1856–1943)


Gerhard Keppner


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