Correction method for eliminating daily variation influence in natural electric field frequency selection method based on ratio method

Abstract

The invention discloses a correction method for eliminating daily variation influence in a natural electric field frequency selection method based on a ratio method. According to the method, firstly, natural electric field signals collected in the field are converted from a time domain to a frequency domain, then the ratio of the real part to the imaginary part of the natural electric field serves as observation data, the influence of daily variation is eliminated through the ratio, and meanwhile effective natural electric field signals reflecting the underground structure are not suppressed. The method can guarantee the accuracy of the exploration result of the natural electric field frequency selection method in the measurement area, and improves the interpretation precision.

Description

Technical Field

[0001] The present invention relates to the technical field of geophysical exploration, and in particular to a correction method for eliminating the influence of diurnal variation in a natural electric field frequency selection method based on a ratio method. Background Art

[0002] The Natural Electric Field Frequency Selection Method (NEFMS) is a passive geophysical exploration technique based on the magnetotelluric field. It is primarily used in mineral resource exploration, groundwater detection, and geological structure surveys. Natural electromagnetic fields are primarily generated on the Earth's surface by natural phenomena such as solar wind and lightning activity (especially thunderstorms near the equator). Their frequencies typically range from 0.1 Hz to 10 kHz.

[0003] Interaction between electromagnetic fields and geological bodies: Electromagnetic waves of different frequencies penetrate to different depths (low frequencies penetrate deeper). When electromagnetic waves encounter electrical differences (such as ore bodies, aquifers, and faults), secondary electromagnetic fields are induced. By measuring these responses, the underground electrical structure can be inferred.

[0004] The natural electric field frequency selection method conducts geological surveys by measuring the natural electric field of the earth. It has the advantages of simple layout (only two electrodes are required), low cost and high efficiency. However, the earth's electromagnetic field is not constant, but constantly changing, that is, the intensity of the natural electric field data obtained at different times is different. The traditional method directly uses the electric field modulus as the observation data, ignoring the impact of the daily variation (fluctuation over time) of the natural electromagnetic field on the data, resulting in deviations in the electric field intensity measured at different times.

[0005] Based on the current problems in natural electric field exploration, in order to improve exploration accuracy, it is worthwhile to further study the correction method for natural electric field diurnal variations to provide more reliable data support for the detailed characterization of geological results and accurate prediction of resource distribution. Summary of the Invention

[0006] The purpose of the present invention is to solve the problem of poor exploration results caused by different natural electric field signal intensities at different times in the existing natural electric field frequency selection method, and to propose a diurnal variation correction method for the natural electric field frequency selection method that does not rely on external calibration data. When the spatial variation and temporal fluctuation of the field source are superimposed, a high stability can still be maintained, thereby improving the exploration accuracy of the frequency selection method.

[0007] A correction method for eliminating the influence of daily variation in a natural electric field frequency selection method based on a ratio method comprises the following steps:

[0008] (1) Select the exploration area and arrange data collection points: Based on the geological structure matching requirements, terrain adaptability requirements, and interference source avoidance requirements, select areas with less human interference as the exploration area, and arrange data collection points for the natural electric field frequency selection method in the exploration area, and configure the corresponding natural electric field collection instruments and supporting measurement equipment;

[0009] (2) Collect the potential difference of each measuring point over time and calculate the natural electric field value: In the selected exploration area, arrange multiple data collection points, deploy natural electric field collection instruments, and arrange two measuring electrodes in the north-south direction and the east-west direction respectively, and record their pole distance l x and l y , collect the potential difference data of each measuring point that changes with time as V x and V y , through the potential difference V x With pole distance l x The ratio of the natural electric field value E x , through the potential difference V y With pole distance l y The ratio of the natural electric field value E y , record the coordinates of each collection point as (X n ,Y n ,Z n ), where 2 represents the number of different collection points;

[0010] (3) Using Fourier transform to convert the electric field signal into the frequency domain: According to the working frequency range set by the exploration target, the time series data at different frequencies are recorded respectively, and the electric field signal of the collection point is collected and stored in real time, and then the electric field signal is converted into the frequency domain using Fourier transform;

[0011] (4) Use ratio correction method to eliminate the influence of diurnal variation: Assuming that there is no diurnal variation in the ideal case, the horizontal components of the natural electric field in the frequency domain are and In order to maintain generality, it is assumed that the daily influence factors in the two directions are and The electric fields measured in two directions after being affected by diurnal variations can be expressed as and

[0012] The ratio method is used to eliminate the influence of diurnal variation. By calculating the real part (Re) and the imaginary part (Im) of the two electric fields and taking their ratio, the interference caused by diurnal variation can be effectively weakened. The calculation formula is:

[0013]

[0014] In the above calculation formula, the diurnal variation factor and When the numerator and denominator appear at the same time, the ratio operation is performed to obtain the value without the influence of daily changes. and

[0015] Preferably, in step (1), the supporting measuring equipment includes a non-polarizable electrode, a copper wire and a lithium battery.

[0016] Preferably, in step (2), the data collection time can be adjusted according to actual needs. If deep information needs to be understood, a longer data collection time is required according to the skin depth principle.

[0017] Preferably, in step (4), the value of the daily influence factor fluctuates periodically with changes in measurement time and measurement frequency.

[0018] Beneficial effects

[0019] Compared with the existing technology, the beneficial effects of the present invention are: the present invention can effectively eliminate the influence of daily changes in natural electromagnetic fields on observation data, can ensure the accuracy of the exploration results of the natural electric field frequency selection method in the survey area, and provide more accurate data support for subsequent geological interpretation, thereby obtaining higher-precision electric field exploration results. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a flow chart of a correction method for eliminating the influence of daily variations in a natural electric field frequency selection method based on a ratio method according to the present invention;

[0021] Figure 2 This is a flow chart of the conventional exploration method of the existing natural electric field frequency selection method;

[0022] Figure 3 Schematic diagram of the field arrangement of a natural electric field diurnal variation correction method of the present invention;

[0023] Figure 3 The numbers 1 to 49 represent the locations of 49 measuring points, the solid circle symbols represent the locations of each measuring point, and the squares represent the scope of the measuring area. DETAILED DESCRIPTION

[0024] The following references Figure 1 、 Figure 2 、 Figure 3 The present invention will be further described with reference to specific embodiments.

[0025] (1) Select appropriate natural electric field acquisition instruments and related supporting non-polarized electrodes, copper wires, lithium batteries, etc. Non-polarized electrodes are used to reduce the polarization effect of the electrodes themselves. According to the geological task, taking into account factors such as topography, geological structure, and potential interference sources, select areas with little human interference to deploy data acquisition points for the natural electric field frequency selection method. Areas near high-voltage power lines, communication base stations, factories, and areas prone to lightning may generate significant electromagnetic interference, thereby affecting the measurement results of the natural electric field.

[0026] (2) Arrange multiple measuring points in the selected exploration area, set up natural electric field collection instruments, set up two measuring electrodes in the north-south direction and the east-west direction respectively, and record their pole distance l x and l y , collect the potential difference data of each measuring point that changes with time as V x and V y , used to calculate the natural electric field value E x and E y The data collection time can be adjusted according to actual needs. If you need to understand deep information, according to the skin depth principle, you need to collect data for a longer time, and vice versa. The coordinates of each collection point are recorded as (X n ,Y n ,Z n ), where n represents the number of different collection points; X n 、Y n and Z n The values represent the horizontal, vertical, and elevation coordinates of the acquisition point in three-dimensional space, respectively, with n being the number of the acquisition point. The skin depth principle states that lower frequencies lead to longer acquisition times and deeper detection, while higher frequencies lead to shorter acquisition times and shallower detection.

[0027] (3) Set the operating frequency range according to the exploration target (assuming the operating frequency range is 0.01 Hz to 1000 Hz), record the time series data at different frequencies, and collect and store the electric field signals of the collection points in real time to ensure the integrity of the data; use Fourier transform to convert the signal to the frequency domain; during the collection process, ensure that the collection equipment used can obtain and save the electric field signals of each collection point in real time.

[0028] (4) Assuming that there is no diurnal variation in the ideal case, the horizontal components of the natural electric field in the frequency domain are and In order to maintain generality, we assume that the diurnal influence factors in these two directions are and Therefore, the electric fields measured in the two directions after being affected by diurnal variations can be expressed as and in, and The value of the diurnal influence factor will fluctuate periodically due to factors such as measurement time and frequency;

[0029] According to the above steps, the ratio method is used to eliminate the influence of diurnal variation. By calculating the real part (Re) and the imaginary part (Im) of the two electric fields and taking their ratio, the interference caused by diurnal variation can be effectively weakened. The calculation formula is:

[0030]

[0031] In the above calculation formula, the diurnal variation factor and When the numerator and denominator appear at the same time, the ratio operation is performed to obtain the value without the influence of daily changes. and The natural electric field data of all measuring points at various frequencies are used to conduct mapping analysis and infer the underground geological conditions of the exploration area.

[0032] The present invention first converts natural electric field signals collected in the field from the time domain to the frequency domain. The ratio of the real and imaginary parts of the natural electric field is then used as observation data. This ratio eliminates the effects of diurnal variations while simultaneously maintaining the effective natural electric field signals reflecting underground structure. This method ensures the accuracy of natural electric field frequency selection exploration results within the survey area and improves the precision of interpretation.

[0033] like Figure 3 As shown, it is necessary to Figure 3 The new diurnal variation correction method proposed in this invention was used to conduct natural electric field exploration at 49 measuring points in the exploration area. The specific steps are as follows:

[0034] (1) Natural electric field acquisition instruments were deployed in areas with minimal human interference. Consistency tests were conducted during use to ensure comparability of measurement results between different devices. To improve signal stability, copper electrodes were selected as measurement electrodes. Reasonable acquisition points were arranged within the survey area based on the selected survey area and exploration objectives. A 50m×50m grid was used, with 49 acquisition points numbered 1 to 49 set within the survey area.

[0035] (2) Two measuring electrodes are buried in each direction at each collection point to form an electric dipole for measuring the horizontal component of the electric field. The x direction represents the east-west direction, and the y direction represents the north-south direction. The potential difference data of each measuring point that changes with time is collected to calculate the natural electric field value. The electrode is buried at a depth of 20-50 cm and the electrode spacing is 20 m, which can be adjusted according to the on-site conditions. The operating frequency range is set according to the exploration target, and the time series data at different frequencies are recorded separately. When the burial depth of the target body is large, the operating frequency should be low; vice versa.

[0036] (3) Since the natural electric field signal collected by the natural frequency selector is a time-domain signal, the time-domain signal is converted into a frequency-domain signal using Fourier transform, thereby obtaining a complex frequency-domain signal containing real and imaginary parts. The present invention uses the ratio method to effectively eliminate the influence of daily variations on the observed data.

[0037] (4) Mark out the abnormal electric field areas based on the mapping analysis, make a preliminary judgment on the nature of the abnormal areas based on geological background knowledge and experience, and conduct a comprehensive interpretation and analysis of the underground geological conditions in the exploration area.

[0038] The foregoing is merely a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, or technical improvements made within the technical spirit and principles of the present invention shall be deemed part of the scope of protection of the present invention.

Claims

1. A correction method for eliminating the influence of daily variation in natural electric field frequency selection method based on ratio method, characterized in that: The following steps are involved: (1) Select the exploration area and arrange data collection points: Based on the geological structure matching requirements, terrain adaptability requirements, and interference source avoidance requirements, select areas with less human interference as the exploration area, and arrange data collection points for the natural electric field frequency selection method in the exploration area, and configure the corresponding natural electric field collection instruments and supporting measurement equipment; (2) Collect the potential difference of each measuring point over time and calculate the natural electric field value: In the selected exploration area, arrange multiple data collection points, deploy natural electric field collection instruments, and arrange two measuring electrodes in the north-south direction and the east-west direction respectively, and record their pole distance l x and l y , collect the potential difference data of each measuring point that changes with time as V x and V y , through the potential difference V x With pole distance l x The ratio of the natural electric field value E x , through the potential difference V y With pole distance l y The ratio of the natural electric field value E y , record the coordinates of each collection point as (X n ,Y n ,Z n ), where n represents the number of different collection points; (3) Using Fourier transform to convert the electric field signal into the frequency domain: According to the working frequency range set by the exploration target, the time series data at different frequencies are recorded respectively, and the electric field signal of the collection point is collected and stored in real time, and then the electric field signal is converted into the frequency domain using Fourier transform; (4) Use ratio correction method to eliminate the influence of diurnal variation: Assuming that there is no diurnal variation in the ideal case, the horizontal components of the natural electric field in the frequency domain are and In order to maintain generality, it is assumed that the daily influence factors in the two directions are and The electric fields measured in two directions after being affected by diurnal variations can be expressed as and The ratio method is used to eliminate the influence of diurnal variation. By calculating the real part (Re) and the imaginary part (Im) of the two electric fields and taking their ratio, the interference caused by diurnal variation can be effectively weakened. The calculation formula is: In the above calculation formula, the diurnal variation factor and When the numerator and denominator appear at the same time, the ratio operation is performed to obtain the value without the influence of daily changes. and 2. The correction method for eliminating the influence of daily variation in the natural electric field frequency selection method based on the ratio method according to claim 1 is characterized in that: In step (1), the supporting measuring equipment includes a non-polarizable electrode, a copper wire and a lithium battery.

3. The correction method for eliminating the influence of daily variation in the natural electric field frequency selection method based on the ratio method according to claim 1 is characterized in that: In step (2), the data collection time can be adjusted according to actual needs. If deep information is needed, according to the skin depth principle, a longer data collection time is required.

4. The correction method for eliminating the influence of daily variation in the natural electric field frequency selection method based on the ratio method according to claim 1 is characterized in that: In step (4), the value of the daily influence factor fluctuates periodically with the change of measurement time and measurement frequency.

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