Method and apparatus for detecting electromagnetic waves generated by the earth&#39;s crust under strain
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for detecting electromagnetic waves which are generated when pressure and strain are exerted on the earth&#39;s crust, and more particularly, to a method and apparatus for detecting electromagnetic waves generated as a premonitory symptom of earthquakes in such manner as to discriminate these electromagnetic waves from man-made noise, static and other radio or electromagnetic waves. 
     2. Prior Art Statement 
     It is known that large changes in pressure and strain occur in the earth&#39;s crust in and around a seismic region prior to the occurrence of an earthquake. It is also known that earthquake-prone regions of the earth crust are constituted mainly of rock or base rock consisting of ionic crystal which generates electricity (piezoelectricity) under pressure and strain, and that electromagnetic waves are generated when changes arise in the generated electricity. These electromagnetic waves have frequencies ranging from a direct current component to higher harmonics, and the waves having a frequency above about 3 kHz are greatly attenuated while traveling through the earth. Thus, when the seismic region lies deep within the earth, there is little possibility of being able to detect the 3 kHz and higher frequency electromagnetic waves produced by the seismic region at the surface of the earth. However, for reasons that will be explained below, electromagnetic waves generated by changes in pressure and strain can be detected at positions near the surface of the earth which are remote from the seismic region. As one example, there will be considered the case in which electromagnetic waves are produced by piezoelectricity. The amount of electromagnetic waves per unit volume of the rock producing piezoelectricity is proportional to the square of the amount of change in pressure or strain exerted on the rock. On the other hand, the amount of change in earth crust pressure or strain is inversely proportional to approximately the square of the distance from the seismic region to the point at which the electromagnetic waves are detected. In other words, the amount of electromagnetic waves generated at a point located a given distance from the seismic region is inversely proportional to approximately the fourth power of the distance. Within the earth, therefore, the electromagnetic waves produced at the seismic region appear to be attenuated in proportion to approximately the fourth power of the distance. An attenuation proportional to the fourth power of the distance is the same rate of attenuation as that of scattered radar waves. This means that it is possible to detect the 3 kHz and higher frequency electromagnetic waves produced as a premonitory symptom of an earthquake. However, when the electromagnetic waves are actually received by an antenna, they are received together with man-made noise, static and other extraneous radio waves. Thus it is possible to use the pre-earthquake electromagnetic waves for the prediction of earthquakes only if these waves can be discriminated from other radio waves constituting noise. 
     As far as is known, only the following two methods have been used for discriminating pre-earthquake electromagnetic waves from other radio or electromagnetic waves: 
     (1) The method as described in Japanese Patent Public Disclosure No. SHO 62(1987)-103596 wherein discrimination is carried out on the basis of the difference in dynamic spectra between the pre-earthquake electromagnetic waves and other radio waves; and 
     (2) The method which utilizes the fact that man-made noise and static are attenuated at points deep within the earth. In this method, the discrimination is carried out within abandoned mines or underground caverns at a depth of 500 m or more below the earth&#39;s surface, at which depth the difference in strength between the pre-earthquake electromagnetic waves and the other radio waves is large. 
     However, for carrying out the first method based on the difference between the dynamic spectra of the electromagnetic and radio waves, it is necessary to have a large amount of actually measured seismic data. As the amount of such data accumulated to date is not sufficient for the purpose, this method cannot be put into immediate use. 
     While the technology for implementing the second method of subterranean observation and discrimination has already been established, there is no assurance that an appropriate mine or cavern will be available in the region in which it is desired to carry out earthquake prediction. Nor is it always possible to bore an appropriate hole to such a depth. The areas in which this method can be applied are thus limited. 
     OBJECT AND SUMMARY OF THE INVENTION 
     An object of this invention is to provide a method and apparatus for detecting pre-earthquake electromagnetic waves which overcome the aforesaid shortcomings of the prior art. 
     Another object of this invention is to provide a method and apparatus for easily detecting pre-earthquake electromagnetic waves at any desired point on land or sea with high accuracy. 
     To accomplish the object described above according to this invention, there is provided a method for detecting pre-earthquake electromagnetic waves, which comprises providing one or more linear elements constituting an antenna, each having a length equal to approximately one quarter of the wavelength of the electromagnetic waves to be received. 
     When the invention is applied on land, the steel pipe casing of a deep well or a conductive cable inserted within such a casing is used as the linear element of a monopole antenna and conductive plates or wires are laid on the surface of the earth with the linear element at the center to provide the monopole antenna with a reflector for preventing radio waves above the earth from propagating into the earth. In this way, reception of the pre-earthquake electromagnetic waves from the earth which are relatively free from man-made noise can be carried out reliably. 
     When the invention is applied offshore, a dipole or loop antenna is positioned on or near the sea bottom and the sea water inhibits the propagation of radio waves above the sea surface to the sea bottom. In this way, the difference in strength between the pre-earthquake electromagnetic waves traveling through the earth and the other radio waves becomes great at the point of reception, whereby the pre-earthquake electromagnetic waves can be accurately detected. 
    
    
     The above and other objects and features of the invention will become more apparent from the following detailed description with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory view of the manner of receiving pre-earthquake electromagnetic waves by the method of one embodiment of the present invention as applied on land. 
     FIG. 2 is an explanatory view showing one example of the manner of receiving pre-earthquake electromagnetic waves by the method of another embodiment of the present invention as applied offshore. 
     FIG. 3 is an explanatory view showing still another embodiment of the present invention as applied offshore. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the case where the observation for earthquake prediction is carried out on land. In this case, a linear element 1 of a monopole antenna for receiving electromagnetic waves generated from the earth&#39;s crust under pressure and strain is embedded in the earth and a reflector 2 for the monopole antenna is provided with the linear element 1 at the center thereof. 
     It is preferable for the conductive cable to have a length equal to approximately one quarter of the wavelength of the electromagnetic wave to be received. For instance, an electromagnetic wave having a frequency of about 1 kHz has a wavelength of about 3.16 km because a refractive index of wet soil is about 94.8 at 1 kHz, and therefore, the wavelength of an electromagnetic wave propagating through the soil becomes 1/94.8 of that in the air. Therefore, in the case that an electromagnetic wave of about 1 kHz is to be detected, it is sufficient for the linear element used as an antenna in this invention to have a length of about 1,000 m, though, to be exact, a quarter wavelength of the 1 kHz wave would measure 790 m. 
     The linear element 1 embedded in the earth can be obtained by inserting a conductive cable into the casing 9 of a deep well. If the deep well casing 9 is a steel pipe, the steel pipe can itself be used as the linear element. If the deep well does not have a casing, the conductive cable is inserted directly into the well. 
     The cable can be with or without an insulation cladding. Where the casing of a deep well is to be used as the linear element, it should preferably extend to a depth of at least 1,000 m and have a diameter of at least 10 cm. Where a cable is to be inserted into a deep well as the linear element, it is preferable to use a helical cable since this increases the capacitance of the antenna. 
     The reflector 2 for the monopole antenna is typically constituted by laying a plurality of conductive wires to extend radially from the linear element 1 at the center. Alternatively the reflector 2 may be an annular plate. The members of the reflector 2 can be with or without an insulation cladding. Part or all of the reflective surface of the reflector 2 can be formed using existing underground water or gas pipes. In a case where no measurement is to be made of the direct current component of the electromagnetic waves, if both the linear element 1 and the reflector 2 are uninsulated, direct current will flow between them, which will have an effect on the received electromagnetic waves. In this case, therefore, it is necessary to clad one or the other with an insulating material. The diameter of the reflecting surface of the reflector 2 as measured from the linear element 1 should not be less than the length of the conductive cable embedded in the earth so that external noise such as man-made noise propagating in the air toward the earth can be completely reflected. When the reflector 2 is constituted of wires or rods, the wires or rods should preferably be laid both radially from and concentrically with the linear element 1, with the crossing points of the radial and concentric wires electrically connected with each other. While the spacing between adjacent wires will depend on the frequency of the electomagnetic waves to be received, in view of the fact that the frequency of the electromagnetic waves to be received is generally not higher than 80 kHz, a spacing of about 10 m is ordinarily appropriate. 
     As will be understood from FIG. 1, the purpose of the reflector 2 is to reflect atmospheric radio waves 6 and subterranean electromagnetic waves 5. While it therefore suffices to provide the reflector 2 on the surface 8 of the earth, it may be better to bury it to a certain depth in cases where there is a danger of its being damaged by humans or animals. Where the reflector is to be installed or agricultural land, it is important that it be buried deep enough to be safe from being cut by cultivators and the like. 
     With the aforesaid arrangement, when subterranean electromagnetic waves 5 (i.e. pre-earthquake electromagnetic waves) are produced from a seismic region 4, the electromagnetic waves 5 propagate through the earth 7 and reach the linear element 1 both directly and after being reflected by the reflector 2 installed at the surface of the earth, whereafter they are detected by a receiver 3. On the other hand, a large part of such atmospheric radio waves 6 as man-made noise and static is reflected by the reflector 2, while the small portion thereof that enters the ground passes through the earth 7 and reaches the linear element 1 in an attenuated state. 
     The pre-earthquake electromagnetic waves received by the receiver 3 include the wave component reflected by the reflector in addition to that directly received by the antenna, and therefore, are about twice as strong as those received by conventional methods, while the other radio waves (noise) received are weaker. The intensity ratio between the two types of electromagnetic waves is thus increased, making it possible to discriminate them on the basis of the difference in strength. As a result, the pre-earthquake electromagnetic waves can be reliably detected. 
     FIG. 2 shows an example of the observation for earthquake prediction being carried out offshore. In this case, two mooring lines 10 anchored at one end to the sea bottom are attached at the other end to a mooring buoy 11, and an antenna 1&#39; is attached therebetween. The antenna 1&#39; in this embodiment is composed of two or more linear elements connected to each other through a receiver 3 so as to form a dipole antenna. For the antenna, there may be used a loop antenna. The mooring buoy 11 should preferably be located at a position where the depth of the sea is 200 m or more so that atmospheric radio waves 6 entering into the sea are absorbed by the sea water 13 and completely disappear before they reach the sea bottom. Thus, in this embodiment, the sea water 13 functions as the reflector used in the former embodiment. The receiver portion should be at a depth of 100 m or more. All the elements constituting the antenna have to be made from insulated conductive cables, which are grounded at their ends. Where a dipole antenna structure is used, it is not necessary for the linear elements of the antenna to be linearly attached to the mooring lines 10. In the case of the loop antenna, it is in fact preferably for them to be made in the form of coils so as to increase the inductance. The mooring lines 10 may be either conductive or non-conductive. 
     With the aforesaid arrangement, when subterranean electromagnetic waves 5 (i.e. pre-earthquake electromagnetic waves) are produced in the vicinity of a seismic region 4, the electromagnetic waves 5 propagate through the earth 7 and reach the dipole or loop antenna 1&#39;, whereafter they are detected by the receiver 3. On the other hand, part of the atmospheric radio waves 6 is reflected by the sea surface 12 and the remainder thereof enters the sea and is attenuated while propagating through the sea water 13 on the way to the antenna 1&#39;. Therefore the pre-earthquake electromagnetic waves are strongly received by the receiver 3, while reception of the other radio waves is weak. The intensity ratio between the two types of radio waves is thus increased, making it possible to discriminate them on the basis of the difference in strength. 
     FIG. 3 illustrates another embodiment of the invention as applied to offshore seismic observation. In this embodiment, a sensor 14 equipped with a dipole or loop antenna and a receiver is appropriately positioned on the floor of the ocean, and the sensor 14 is connected with a land-based observation system 17 by a cable 15 that passes through one or more repeaters 16. The repeaters 16 are designed to maintain the strength of the signal from the sensor 14 within a prescribed range by amplifying it by an amount sufficient to compensate for the attenuation thereof during transmission. 
     In the case where the sensor 14 is equipped with a loop antenna, if the pressure-resistant vessel incorporating the sensor 14 is not a good conductor, the aforesaid insulated conductive cable constituting the antenna element can be located inside the pressure-resistant vessel. When a dipole antenna is used, the insulated conductive cables of the linear elements should preferably be disposed to extend across the ocean floor in four directions and are grounded at their ends so that two perpendicularly intersecting dipole antennas are formed. 
     The sensor 14 can be easily disposed at a great depth and, when it is, the antenna thereof receives substantially only the pre-earthquake electromagnetic waves 5 propagated through the earth 7 from the seismic region 4 since the part of the noise radio waves 6 from the atmosphere entering the sea 13 is greatly attenuated while traveling to the sensor 14 at the sea bottom. As a result, only the required pre-earthquake waves can be detected with high precision. 
     The computation for determining the seismic region and for estimating the magnitude of the impending earthquake can be accomplished using existing techniques. More specifically, the direction from which the radio waves arrive is detected at two or more observation points and the seismic region is found from the point of intersection of these directions. In the case of land observation, this is accomplished by the disposal on the surface of the earth of one or more an auxiliary loop antennas or other type of directional antennas. In the case of offshore observation according to the second embodiment shown in FIG. 2 it is accomplished by attaching to the mooring lines 10 two perpendicularly intersecting dipole antennas, while in the case of offshore observation according to the third embodiment shown in FIG. 3 it is accomplished by providing the sensor 14 with two perpendicularly intersecting dipole antennas. Where three or more observation points are provided, it is possible to determine the location of the seismic region from two points of intersection of hyperbolic curves, whose foci are the points at which an observation is conducted. Where two or more observation points are provided, the location of the seismic region can be determined from the intersections of the hyperbolic curves and the direction of arrival of the electromagnetic waves detected by a directional antenna. 
     The intensity of the pre-earthquake electromagnetic waves is approximately proportional to intensity of the earthquake which is to occur and is approximately inversely proportional to the fourth power of the distance between the seismic region and the point of reception. It is therefore possible to estimate the magnitude of the impending earthquake from the intensity of the received pre-earthquake electromagnetic waves. 
     Pre-earthquake electromagnetic waves are produced between two weeks and two hours prior to the occurrence of the earthquake and have a frequency of 80 kHz or less, but as explained in the foregoing, it has been difficult to discriminate these electromagnetic waves from other noise radio waves. In accordance with this invention, however, pre-earthquake electromagnetic waves can be detected at a much greater strength than other noise radio waves, whereby it becomes easy to discriminate the pre-earthquake waves. As there are distinct differences between pre-earthquake or principal-shock electromagnetic waves and aftershock electromagnetic waves (namely, the aftershock waves are weaker in the frequency component below 3 kHz in comparison with the pre-earthquake or principal-shock waves), the present invention makes it possible to predict an earthquake two hours before the principal shock happens.