JPH0236197B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention is based on the ULF band (ultra-ultra low frequency band).
This relates to an underground structure exploration system that utilizes naturally occurring electromagnetic waves in the VLF band (very low frequency band) to efficiently conduct underground exploration from near the surface to 10km above ground.

The exploration method that uses electromagnetic waves that exist in nature has been known as the magnetotelluric method, and has already been put into practical use. However, the conventional magnetotelluric method (hereinafter abbreviated as MT) requires long-time measurements because it uses extremely long-period electromagnetic wave data, and also requires complex data analysis. Therefore, it was difficult to survey a wide area in sufficient detail.

The present invention was developed in view of the above matters, and
2-component (1 magnetic field component and 1 electric field component) ELF band (extremely low frequency band) receiving type MT that allows exploration in a short time
Using the exploration method, we obtain detailed apparent resistivity distributions at many measurement points throughout the exploration area, and based on this, we select particularly important points within the exploration area, and only at those points. By measuring 5 components (3 magnetic field components and 2 electric field components) using a multi-frequency reception type MT exploration method and analyzing the data, we clarified the resistivity distribution in the depth direction at that point. The objective is to provide an exploration method that can efficiently obtain detailed three-dimensional resistivity distribution over the entire exploration area by interpolating and using data from the two-component ELF band reception type MT exploration method. It is.

Next, examples of the present invention will be described.

First, in the desired exploration area, a lightning discharge receives the extremely low frequency band electromagnetic waves propagated by the Schumann resonance phenomenon and the extremely low frequency band electromagnetic waves transmitted from the Omega station, and generates one magnetic field component and one electric field component. Two-component ELF band reception type MT exploration 1 will be carried out over the entire exploration area to measure the intensity of The spacing between exploration points varies depending on the exploration target and topographical conditions, but is approximately 200 to 500 m.
In principle, the measuring arrangement is in the form of a grid. The equipment used for this exploration is compact and easy to carry, and the measurement time for each exploration point is approximately 30 minutes, meaning that measurements can be completed in a short time at many locations. Also, with this device, the magnetic field and electric field can be directly read and recorded on site, so the apparent resistivity value of the exploration result can be obtained immediately.

In this way, the apparent resistivity distribution map 2 is created, more important points are selected from these exploration points, and these points are successively carried out using the five-component multi-frequency band reception method.
Determine the MT exploration point.

Five-component multi-frequency band reception type MT exploration 4 is similar to the conventional MT method, in which the magnetic field is used to detect extremely low frequencies caused by geomagnetic pulsations, extremely low frequencies caused by lightning discharges, and extremely low frequencies transmitted from the Omega station. This is an exploration method that calculates the apparent resistivity over a wide frequency band by memorizing the waveforms of the three components and the two components of the electric field and analyzing them.

Exploration 4 was conducted at the above points, and the waveforms of 3 magnetic field components (2 horizontal components and 1 vertical component) and 2 electric field components (2 horizontal components) were simultaneously recorded on a data recorder for electromagnetic waves of 0.01Hz to 2KHz at these points. Record. This waveform record is analyzed by a computer or the like, and the relationship between frequency and apparent resistivity is determined for the frequency range in the two principal axis directions of the anisotropy of the underground structure.

Next, a resistivity columnar diagram 5 showing the relationship between depth and resistivity at that point was obtained by layer structure analysis using an electronic computer, and then the underground structure for the analytical cross section that matched the exploration purpose was obtained. Estimation 6 is performed, and at this point, by comparing the apparent resistivity distribution 7 determined by the model calculation for the estimated underground structure with the actually measured apparent resistivity distribution, it is possible to determine whether the underground structure estimated so far is correct. If it is not correct, consider it again to make it correct, repeat the process until you obtain the correct cross section, and then perform the interpretation 8 of the desired underground structure.

In addition, at specific points in the area where the two-component extremely low frequency band receiving type MT survey 1 was carried out, five-component multi-frequency band receiving type MT survey 4 was continuously measured over a long period of time, and based on changes in the underground resistivity distribution, underground It is used to explore changes in the earth's crust.

Since the present invention uses the above method, unlike conventional exploration methods, it is possible to determine important points to be explored in detail in a very appropriate manner in a short time, and the entire exploration area is covered with extremely low frequency waves. Since exploration can be performed by receiving electromagnetic waves in the ultra-low frequency band and the ultra-low frequency band, the entire underground structure can be grasped quickly and accurately, and important points are taken into account with data for a wide range of frequencies, making detailed and accurate comparisons possible. It is possible to determine the resistance distribution, and is extremely effective in exploring geothermal areas and fault areas with complex underground structures.

Furthermore, by conducting these surveys at a specific point over a long period of time, it is possible to obtain temporal changes in the resistivity distribution at that point and observe changes in the earth's crust, which is extremely effective.

[Brief explanation of drawings]

The drawing is a flowchart showing one embodiment of the present invention. 1...Two-component ELF band reception type MT exploration, 4...5
Component multi-frequency band reception type MT exploration, 8... Interpretation of underground structure.

Claims (1)

[Claims] 1. At each point in a desired area, electromagnetic waves of the extremely low frequency band propagated by the Schumann resonance phenomenon and the extremely low frequency band transmitted from the Omega station are received, and the electromagnetic waves at each point are received. By directly reading the strength of one component of the magnetic field and one component of the electric field, we conducted a wide area survey within a short period of time.Subsequently, at important points within the area, we detected extremely low frequency bands caused by geomagnetic pulsations and lightning. Receive electromagnetic waves in the extremely low frequency band caused by electric discharge and the extremely low frequency band emitted from the Omega station, record the waveforms of the three magnetic field components and the two electric field components on a data recorder, and record these waveforms using an electronic computer. Process the data, calculate the strength of the electric field and magnetic field for many frequency components, calculate the tensor apparent resistivity for each frequency component, and calculate the underground resistivity distribution in the depth direction as well as the anisotropy of the underground structure. An underground structure exploration method that is characterized by the ability to investigate a wide area in detail by performing an analysis to determine the 2. At specific points in the area where two-component extremely low frequency band reception magnetotelluric method exploration was carried out to receive the extremely low frequency band and the very low frequency band and determine the strength of one component of the magnetic field and one component of the electric field. , a long-term, continuous 5-component multi-frequency band reception type magnetotelluric survey that receives electromagnetic waves in the extremely low frequency band, very low frequency band, and extremely low frequency band and determines the three components of the magnetic field and the two components of the electric field. This is an underground structure exploration method that is characterized by measuring and exploring the changes in the earth's crust during that period.