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'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'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'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.