الکتریسیته زمین - Geoelectricity
Electromagnetic phenomena and electric currents, mostly of natural origin, that are associated with the Earth. Geophysical methods utilize natural and artificial electric currents to explore the properties of the Earth's interior and to search for natural resources (for example, petroleum, water, and minerals). Geoelectricity is sometimes known as terrestrial electricity. All electric currents (natural or artificial, local or worldwide) in the solid Earth are characterized as earth currents. The term telluric currents is reserved for the natural, worldwide electric currents whose origins are almost entirely outside the atmosphere. Geoelectromagnetism is a more comprehensive term than geoelectricity. Time variations of any magnetic field are associated with an electric field that induces electric currents in conducting media such as the Earth.
Fig. 1 Time variations of the horizontal orthogonal components of the natural (a) magnetic and (b) electric fields, simultaneously measured at one site at the surface.
Magnetic fields, electric fields, and electric currents are the constituents of electromagnetism, and are related by Maxwell's equations. For instance, Fig. 1 shows the time variations of the natural magnetic and electric fields simultaneously measured at one location at the surface of the Earth. These two traces are related to each other, not only by Maxwell's equations but also by the physical properties of the subsurface rocks in the vicinity of the measuring site. Either one of the two traces may be computed synthetically from the other if the properties of the subsurface rocks are known. Conversely, the two traces together can yield geologic information; this is a form of geophysical exploration or prospecting. Thus, the terms geoelectricity, geomagnetism, and geoelectromagnetism are essentially interchangeable, although each one may have a somewhat different emphasis. For example, the term geomagnetism is sometimes used for the study of the Earth's quasi-stationary main magnetic field. See also: Geomagnetism
Measurements of electric and magnetic fields
A component of the electric field in a desired direction is measured by planting two electrodes (for example, metal stakes or special nonpolarizable electrodes) aligned in that direction. The electrodes are connected by an insulated wire, and voltage difference between them is measured with a voltmeter of high input impedance (for example, 10 megohms). The average electric field between the electrodes is expressed in units of volts per meter. Since this unit is very cumbersome for measuring the Earth's field, it is customary to use millivolts per kilometer. (Figure 1b and Fig. 2 show the time variations of such components.) To obtain the total horizontal electric field, two orthogonal components, north-south (N-S) and east-west (E-W), are measured by means of an L-shaped electrode array. The trajectory of the head of the electric field vector is traced by feeding the two components into an oscilloscope or a paper X-Y recorder (Fig. 3). The magnetic field is measured by magnetometers. The cryogenic magnetometer has a resolution of better than 1 picotesla, 1 part in 50,000,000 of the Earth's total magnetic field. The nanotesla (nT) or gamma γ is used in practice. Figure 1a shows the time variations of one horizontal component of the Earth's natural magnetic field, measured with a coil-core magnetometer whose output is the time derivative of the magnetic field, with the scale given in terms of nanoteslas times frequency. Worldwide studies of natural electromagnetic phenomena are made by monitoring primarily the magnetic field rather than the electric field, which is much more affected by local geology.
Fig. 2 Two tellurograms (stations 1 and 2, San Joaquin Valley, California) representing the time variations of the natural electric field (micropulsations), N60°E components, simultaneously recorded over a time interval of 30 min. The recording sites are separated by 27 mi (43 km) in the direction of the components. PST = Pacific Standard Time.
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Fig. 3 A vectogram representing a few minutes of recording of the natural electric field vector. The band-pass filter peaked at the 20-s period (0.05 Hz).
Electric earth currents
These may be local or worldwide.
Fig. 4 Approximate and schematic frequencies and origins of the natural electromagnetic fields.
Such currents can be natural or caused by human activities. The latter (called stray, industrial, or cultural currents) may be caused by electric trains, rural water pumps, and pipelines. Natural local currents represent the phenomena of spontaneous potentials or self-potentials. Some deposits in the Earth, such as certain metallic sulfides and graphite, constitute buried natural electric cells because of their high electrical conductivity and also because of oxidation and reduction processes associated with ground water. Thus, a hidden ore body, such as a copper ore deposit, can be discovered by measuring the electric field at the surface of the Earth, which may be as large as 1 V over a distance of 300 ft (100 m). Two other sources of spontaneous potentials are ground water movements and topographic elevation changes.
Telluric currents are of natural origin. There are various types, sources, and frequencies (or periods) of the worldwide natural electromagnetic fields which are associated with electric currents in the Earth (Fig. 4). The time variations of these electromagnetic fields are simply called variations.
The Earth's main magnetic field is thought to be caused by motions in the electrically conducting fluid core of the Earth, which acts as a kind of dynamo, creating electric currents which in turn create the magnetic field. This field is not stationary, but has time variations with periods ranging from about 30 to 300 years per cycle, which are the secular variations. Electric currents at the surface of the Earth associated with the main field and its secular variations have not been monitored effectively because of the difficulties involved in separating them from other effects, such as electrode potentials and tidal potentials. See also: Geomagnetic variations
Diurnal (daily) variations
The air layers of the ionosphere, from a height of about 60-200 mi (100-300 km), are ionized by solar radiation, while air below the ionosphere is practically nonconducting. The ionization (electrical conductivity) in the ionosphere is renewed daily. Tidal oscillations of the ionosphere in the presence of the Earth's main magnetic field constitute an atmospheric dynamo, inducing electric currents in the Earth. They are thought to be driven primarily by the thermal effects of the Sun and partially by the attraction of the Moon (Fig. 5). See also: Ionosphere
Fig. 5 Diurnal variations (solar plus lunar) of the horizontal component of the magnetic field, December 21, 1933, Huancayo, Peru. (After S. Chapman and J. Bartels, Geomagnetism. 2 vols., Oxford University Press, 1962)
Exospheric-origin variations, or micropulsations
Shorttime fluctuations of the Earth's magnetic field (micropulsations) that fall within the approximate period range of 0.2-600 s per cycle (5-0.0017 Hz) occur almost continuously as a background noise. Amplitudes depend on latitude, solar activity, frequency, local time, season, and local geology, with worldwide and long-term statistical amplitudes of the order of a few millivolts per kilometer and a few tenths of a nanotesla. While the mechanism of their generation is not completely understood, it appears that micropulsations are generated by the magnetohydrodynamic effect through the interaction of the solar wind with the main magnetic field and atmosphere of the Earth. Study of the exospheric-origin electromagnetic phenomena constitutes a branch of geophysics called aeronomy. Figure 1 is a record of micropulsations measured at stations located in a sedimentary basin. The magnetic field trace (Fig. 1a) is called a magnetogram; the electric field trace (Fig. 1b) a tellurogram. Figure 2 shows two tellurograms simultaneously measured at two stations 27 mi (43 km) apart and in the direction of station separation. These measurements represent normal, usual activity on a quiet day. Figure 6 shows the amplitude spectra of the tellurograms shown in Fig. 2. The differences between the two tellurograms, and consequently between the two spectra, are almost totally due to the differences in the geologic conditions at the two measuring sites (stations). Such measurements can be used for geologic exploration of the subsurface. See also: Magnetohydrodynamics; Seismic stratigraphy; Solar wind; Upper-atmosphere dynamics
Fig. 6 Amplitude spectra of the tellurograms shown in Fig. 2.
Magnetic storms are very intense disturbances of long duration that occur about once a month on the average. Caused by large-scale bursts of solar wind associated with sunspots and solar flares, they usually commence suddenly and almost instantaneously (within about 0.5 min) throughout the world. Their amplitudes may reach hundreds of nanoteslas and hundreds of millivolts per kilometer, disrupting radio and telegraph communications. It is interesting to note that they cause fish to migrate into deeper waters. Figure 7 shows the records of a magnetic storm. Magnetic storms are frequently associated with aurorae polares (northern or southern lights), which are seen as spectacular luminous formations at ionospheric heights. See also: Aurora
Fig. 7 Three components of the magnetic field of a magnetic storm of May 14, 1921, Potsdam, Germany. (After S. Chapman and J. Bartels, Geomagnetism, 2 vols., Oxford University Press, 1962)
The major cause of the variations within the frequency range of about 5-10 kHz is the lightning occurring almost continuously in Central Africa and in the Amazon region. While audio-frequency variations are included in atmospherics, lightning itself is a concern of meteorology. See also: Lightning; Sferics
Above the frequency of 30 MHz, these originate predominantly from the direct radiation of electromagnetic waves propagated by the Sun.
Subsurface geophysical exploration
Electrical methods, more properly called electromagnetic methods, are used to explore the subsurface from depths of a few inches (for example, popular coin detectors or mine detectors) down to depths of hundreds of miles. In general, these methods require an input into the Earth, either an artificial direct or alternating electric current, or a natural electromagnetic field, such as micropulsations or diurnal variations. This input is a source signal coupled with the Earth, which behaves as a filter whose response is measured in terms of electric or magnetic fields. It is analogous to measuring the input and output of an electronic filter to determine its characteristics, which in this case is the geologic information sought. These methods supply only the electrical properties of the subsurface (mainly the electrical conductivity). Different rocks have, in general, different conductivities. For instance, limestones usually have much lower conductivities than clay-rich shales. A knowledge of the conductivity distribution in the subsurface, combined with other geologic information, allows interpretation of the rock-type distribution. Artificial direct-current methods involve feeding a current into the Earth with a pair of electrodes and measuring the resulting electric field with another pair of electrodes. The alternating-current methods use magnetometers to measure the magnetic field created by inducing currents in the Earth. The two most popular methods employing the natural electromagnetic fields are the magnetotelluric and telluric methods. The magnetotelluric method requires simultaneous measurements of the electric and magnetic fields at one site. Figure 1 represents such a data set. The telluric method requires only the measurements of the electric field, made simultaneously at two or more sites (Fig. 2). These electromagnetic (or electrical) methods are unlike the magnetic methods of geophysical exploration, which deal only with the magnetization of rocks due to the main magnetic field of the Earth. See also: Geophysical exploration
- G. V. Keller and F. C. Frischknecht, Electrical Methods in Geophysical Prospecting, 1966
- Ali Fazeli = egeology.blogfa.com
- S. H. Yungul, The telluric methods in the study of sedimentary structures: A survey, Geoexploration, 15:207-238, 1977
- Ali Fazeli = egeology.blogfa.com
- M. S. Zhdanov and G. V. Keller, The Geoelectrical Methods in Geophysical Exploration, 1993