A method and system for suppressing strong magnetotelluric noise during a period of time

Abstract

The present invention discloses a method and system for suppressing a period of strong magnetotelluric noise. The method comprises: obtaining a magnetotelluric data array; constructing a complex channel sequence based on the first channel data and the second channel data in the magnetotelluric data array; performing Hilbert transform on the complex channel sequence, and obtaining an odd sequence and an even sequence of the complex channel sequence; performing short-time Fourier transform on the odd sequence and the even sequence to obtain an instantaneous spectrum; calculating the polarization ellipticity, polarization direction, and phase difference of orthogonal electromagnetic components based on the instantaneous spectrum; determining the polarization property time spectrum based on the polarization ellipticity, polarization direction, and phase difference of orthogonal electromagnetic components; determining a strong noise period based on the polarization property time spectrum, and deleting it to obtain a denoised magnetotelluric data array. The present invention can specifically remove noise interference in the magnetotelluric time series, and at the same time has strong applicability, is suitable for suppressing most human noise, and is not restricted by complex noise waveforms.

Description

Technical Field

[0001] The present invention relates to the technical field of magnetotelluric signal processing, and in particular to a method and system for suppressing magnetotelluric strong noise during a period of time. Background Art

[0002] Magnetotelluric (MT) is an important geophysical exploration method. This method can effectively obtain resistivity information deep underground by observing natural electromagnetic signals on the surface. It is widely used in mineral exploration, oil and gas reservoir exploration, and structural research. However, the natural electromagnetic field is weak. Under the interference of human noise, the MT response curve often shows obvious distortion, which seriously affects the inversion and interpretation of underground structures. Therefore, it is of great significance to select effective methods to suppress the noise interference in the MT measurement results to ensure the application effect of MT.

[0003] With the advancement of modernization, the noise interference intensity of magnetotelluric signals is getting stronger and stronger, and the noise types are becoming more complex. When processing low signal-to-noise ratio data with complex noise, the traditional frequency domain method is blind because the spectrum processed is the weighted average result of the signal in the frequency domain of the whole period, rather than directly processing the noise. It is easy to lose effective signals and leave a lot of noise. When the noise intensity in the magnetotelluric signal is very large, the results obtained by the frequency domain method are still obviously distorted. The traditional time domain method can suppress the abnormal waveform in the noise period and avoid the loss of effective signals caused by the processing of non-noise periods by identifying specific waveforms, so it has strong pertinence. However, the assumptions of this type of method on the noise waveform are too ideal. It is difficult to suppress the complex noise in the magnetotelluric signal by filtering out only one or several abnormal waveforms in the measured signal. At the same time, there are a few noises that do not have special waveforms and only appear abnormal in the spectrum. Summary of the invention

[0004] Based on this, the purpose of the present invention is to provide a method and system for suppressing strong noise periods in magnetotelluric signals, so as to overcome the defects of traditional denoising methods and realize the correction of magnetotelluric distortion curves by suppressing strong noise periods.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A method for suppressing strong magnetotelluric noise during a period of time, comprising:

[0007] Acquire a magnetotelluric data array; the magnetotelluric data array includes 5 data, the first data is a north-south electric track measurement value, the second data is an east-west electric track measurement value, the third data is a north-south magnetic track measurement value, the fourth data is an east-west magnetic track measurement value, and the fifth data is a vertical magnetic track measurement value;

[0008] constructing a complex track sequence based on the first track data and the second track data;

[0009] Performing Hilbert transform on the complex channel sequence, and obtaining an odd sequence and an even sequence of the complex channel sequence;

[0010] Performing short-time Fourier transform on the odd sequence and the even sequence to obtain an instantaneous spectrum;

[0011] Calculating polarization ellipticity, polarization direction, and phase difference of orthogonal electromagnetic components based on the instantaneous spectrum;

[0012] Based on the polarization ellipticity, the polarization direction and the phase difference of the orthogonal electromagnetic components, a polarization property time spectrum is determined; the polarization property time spectrum includes a polarization ellipticity time spectrum, a polarization direction time spectrum and a orthogonal electromagnetic field component phase difference time spectrum;

[0013] Based on the polarization property time spectrum, a strong noise period is determined and deleted to obtain a denoised magnetotelluric data array.

[0014] Optionally, the construction formula of the complex channel sequence is as follows:

[0015] C(t)=e x (t)+ie y (t)

[0016] Where C(t) is the complex channel sequence at time t, i is the imaginary unit, e x is the first data, e y This is the second data.

[0017] Optionally, the calculation formula of the instantaneous spectrum is as follows:

[0018]

[0019]

[0020] in, For even sequence C + The corresponding instantaneous spectrum is For even sequence C －The corresponding instantaneous spectrum, t=1,2,3,...,N, is a different time point, m is the time factor of the instantaneous spectrum, n is the frequency factor of the instantaneous spectrum, g* is the conjugate of the window function g, and i is the imaginary unit.

[0021] Optionally, the calculation formula of the polarization ellipticity ε is as follows:

[0022]

[0023] Optionally, the calculation formula of the polarization direction θ is as follows:

[0024]

[0025] Optionally, the calculation formula of the phase difference Δφ of the orthogonal electromagnetic components is as follows:

[0026]

[0027] The present invention also provides a system for suppressing strong magnetotelluric noise during a period of time, comprising:

[0028] A magnetotelluric data array acquisition module is used to acquire a magnetotelluric data array; the magnetotelluric data array includes 5 data tracks, the first track is a north-south electric track measurement value, the second track is an east-west electric track measurement value, the third track is a north-south magnetic track measurement value, the fourth track is an east-west magnetic track measurement value, and the fifth track is a vertical magnetic track measurement value;

[0029] A complex track sequence construction module, used for constructing a complex track sequence based on the first track data and the second track data;

[0030] An odd sequence and an even sequence acquisition module, used for performing Hilbert transform on the complex channel sequence and acquiring an odd sequence and an even sequence of the complex channel sequence;

[0031] An instantaneous spectrum acquisition module is used to perform short-time Fourier transform on the odd sequence and the even sequence to acquire an instantaneous spectrum;

[0032] A calculation module, used for calculating the polarization ellipticity, polarization direction and phase difference of orthogonal electromagnetic components based on the instantaneous spectrum;

[0033] A polarization property time-frequency spectrum determination module, used to determine the polarization property time-frequency spectrum based on the polarization ellipticity, the polarization direction and the phase difference of the orthogonal electromagnetic components; the polarization property time-frequency spectrum includes the polarization ellipticity time-frequency spectrum, the polarization direction time-frequency spectrum and the orthogonal electromagnetic field component phase difference time-frequency spectrum;

[0034] The denoising module is used to determine and delete the strong noise period based on the polarization property time spectrum to obtain a denoised magnetotelluric data array.

[0035] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0036] The present invention obtains the instantaneous spectrum of the magnetotelluric time series through short-time Fourier transform, and obtains the polarization properties of the magnetotelluric signal in different time periods and different frequency bands based on the obtained instantaneous spectrum, and finally identifies and screens out the strong noise period according to the polarization properties of the signal in different time periods. Since the instantaneous spectrum is a concept in the time-frequency domain, it has both frequency resolution and time resolution, so it can reflect the noise content of the magnetotelluric signal in different time periods, thereby realizing targeted filtering of noise. In addition, the method of the present invention is based on the different polarization properties of the signal and the noise rather than the special noise form, so it can still be effectively identified and suppressed when the noise form is complex. The present invention can overcome the defects of the traditional denoising method, and realize the correction of the magnetotelluric distortion curve by suppressing the strong noise period. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0038] Figure 1 This is a flow chart of the method for suppressing strong magnetotelluric noise during a period of time provided by the present invention. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] The purpose of the present invention is to provide a method and system for suppressing a period of strong magnetotelluric noise, so as to overcome the defects of the traditional denoising method and realize the correction of the magnetotelluric distortion curve by suppressing the period of strong noise.

[0041] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] like Figure 1As shown, the method for suppressing strong magnetotelluric noise during a period of time provided by the present invention comprises the following steps:

[0043] Step 101: Acquire a magnetotelluric data array.

[0044] Import the magnetotelluric data into the computer to obtain the data matrix X; X is an N×5 matrix, N is the data length recorded by the instrument, each column is a data, and a total of 5 data are collected by magnetotelluric sounding, of which the 1st and 2nd tracks are the north-south and east-west electric track measurement values, the 3rd and 4th tracks are the north-south and east-west magnetic track measurement values, and the 5th track is the vertical magnetic track measurement value.

[0045] Step 102: construct a complex track sequence based on the first track data and the second track data.

[0046] Take the first and second data in the data matrix X, and record them as e x With e y (Electrical tracks are more sensitive to noise than magnetic tracks, so orthogonal electrical tracks are selected for noise period identification) and based on e x With e y Construct a complex channel sequence C:

[0047] C(t)=e x (t)+ie y (t) (1)

[0048] Here t = 1, 2, 3, ..., N, represents different time points, and i is an imaginary unit.

[0049] Step 103: Perform Hilbert transform on the complex channel sequence, and obtain an odd sequence and an even sequence of the complex channel sequence.

[0050]

[0051]

[0052]

[0053] in is the Hilbert transform result of C, C + With C － are the even and odd sequences of C respectively.

[0054] Step 104: Perform short-time Fourier transform on the odd sequence and the even sequence to obtain an instantaneous spectrum.

[0055]

[0056]

[0057] in and C + With C － The corresponding instantaneous spectrum, m is the time factor of the instantaneous spectrum, n is the frequency factor of the instantaneous spectrum, g* is the conjugate of the window function g, and the Hanning window is selected as the window function in the present invention and is defined as follows:

[0058] g(t)=0.5*[1-cos[2*π*(t-1) / (N-1)]] (7)

[0059] Step 105: Calculate the polarization ellipticity ε, polarization direction θ, and phase difference Δφ of orthogonal electromagnetic components based on the instantaneous spectrum.

[0060]

[0061]

[0062]

[0063] in for The conjugate spectrum of , arg is the inverse tangent function (inverse tangent processing can be implemented in Matlab).

[0064] Step 106: Determine a polarization property time spectrum based on the polarization ellipticity, the polarization direction and the phase difference of the orthogonal electromagnetic components; the polarization property time spectrum includes a polarization ellipticity time spectrum, a polarization direction time spectrum and a phase difference time spectrum of orthogonal electromagnetic field components.

[0065] Step 105 is taken to obtain the polarization ellipticity ε, polarization direction θ, and phase difference Δφ of orthogonal electromagnetic components, with time factor m as the horizontal axis and frequency factor n as the vertical axis. The depth of color is used to represent the value of the polarization property, thereby obtaining the polarization ellipticity time-frequency spectrum, polarization direction time-frequency spectrum, and orthogonal electromagnetic field component phase difference time-frequency spectrum, respectively.

[0066] Step 107: Determine and delete the strong noise period based on the polarization property time spectrum to obtain a denoised magnetotelluric data array.

[0067] Based on the polarization property time-frequency spectrum obtained in step 106, the strong noise period is screened out (the strong noise period is manifested as a band-shaped anomaly perpendicular to the time axis on the time-frequency spectrum), and the segment corresponding to the strong noise period is cut off in the magnetotelluric data time series. The result is the denoised magnetotelluric data:

[0068]

[0069] Among them, t1~t2 is the noise period divided according to the time spectrum, X d is the denoised data matrix, and j is the column of the data matrix.

[0070] The magnetotelluric noise suppression method proposed in the present invention has a strong noise recognition ability and can specifically remove the noise interference in the magnetotelluric time series. At the same time, it is highly applicable and suitable for most human noise suppression and is not restricted by the complex waveform of the noise.

[0071] The advantages of the present invention are as follows: 1. The proposed noise identification method is carried out in the time-frequency domain, and has both the time-domain resolution capability and the frequency-domain resolution capability. It can divide the noisy time period according to the change of polarization properties over time, and can determine the actual influence range of noise on the magnetotelluric response according to the change of polarization properties over frequency. Therefore, it has a strong noise identification capability, and can achieve targeted noise suppression based on this. 2. The present invention is noise suppression based on the polarization properties of the signal. Since linear polarization is a characteristic of almost all human noises, it is not restricted by the form and type of noise and has wide applicability.

[0072] The present invention also provides a system for suppressing strong magnetotelluric noise during a period of time, comprising:

[0073] A magnetotelluric data array acquisition module is used to acquire a magnetotelluric data array; the magnetotelluric data array includes 5 data tracks, the first track is a north-south electric track measurement value, the second track is an east-west electric track measurement value, the third track is a north-south magnetic track measurement value, the fourth track is an east-west magnetic track measurement value, and the fifth track is a vertical magnetic track measurement value;

[0074] A complex track sequence construction module, used for constructing a complex track sequence based on the first track data and the second track data;

[0075] An odd sequence and an even sequence acquisition module, used for performing Hilbert transform on the complex channel sequence and acquiring an odd sequence and an even sequence of the complex channel sequence;

[0076] An instantaneous spectrum acquisition module is used to perform short-time Fourier transform on the odd sequence and the even sequence to acquire an instantaneous spectrum;

[0077] A calculation module, used for calculating the polarization ellipticity, polarization direction and phase difference of orthogonal electromagnetic components based on the instantaneous spectrum;

[0078] A polarization property time-frequency spectrum determination module, used to determine the polarization property time-frequency spectrum based on the polarization ellipticity, the polarization direction and the phase difference of the orthogonal electromagnetic components; the polarization property time-frequency spectrum includes the polarization ellipticity time-frequency spectrum, the polarization direction time-frequency spectrum and the orthogonal electromagnetic field component phase difference time-frequency spectrum;

[0079] The denoising module is used to determine and delete the strong noise period based on the polarization property time spectrum to obtain a denoised magnetotelluric data array.

[0080] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0081] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for suppressing strong magnetotelluric noise during a period of time, characterized in that: include: Acquire a magnetotelluric data array; the magnetotelluric data array includes 5 data, the first data is a north-south electric track measurement value, the second data is an east-west electric track measurement value, the third data is a north-south magnetic track measurement value, the fourth data is an east-west magnetic track measurement value, and the fifth data is a vertical magnetic track measurement value; constructing a complex track sequence based on the first track data and the second track data; Performing Hilbert transform on the complex channel sequence, and obtaining an odd sequence and an even sequence of the complex channel sequence; Performing short-time Fourier transform on the odd sequence and the even sequence to obtain an instantaneous spectrum; Calculating polarization ellipticity, polarization direction, and phase difference of orthogonal electromagnetic components based on the instantaneous spectrum; Based on the polarization ellipticity, the polarization direction and the phase difference of the orthogonal electromagnetic components, a polarization property time spectrum is determined; the polarization property time spectrum includes a polarization ellipticity time spectrum, a polarization direction time spectrum and a orthogonal electromagnetic field component phase difference time spectrum; Based on the polarization property time spectrum, a strong noise period is determined and deleted to obtain a denoised magnetotelluric data array.

2. The method for suppressing strong magnetotelluric noise during a period of time according to claim 1, characterized in that: The construction formula of the complex channel sequence is as follows: C(t)=e x (t)+ie y (t) Where C(t) is the complex channel sequence at time t, i is the imaginary unit, e x is the first data, e y This is the second data.

3. The method for suppressing strong magnetotelluric noise during a period of time according to claim 1, characterized in that: The calculation formula of the instantaneous spectrum is as follows: in, For even sequence C + The corresponding instantaneous spectrum is For even sequence C － The corresponding instantaneous spectrum, t=1,2,3,...,N, is a different time point, m is the time factor of the instantaneous spectrum, n is the frequency factor of the instantaneous spectrum, g* is the conjugate of the window function g, and i is the imaginary unit.

4. The method for suppressing strong magnetotelluric noise during a period of time according to claim 3, characterized in that: The calculation formula of the polarization ellipticity ε is as follows:

5. The method for suppressing strong magnetotelluric noise during a period of time according to claim 3, characterized in that: The calculation formula of the polarization direction θ is as follows:

6. The method for suppressing strong magnetotelluric noise during a period of time according to claim 3, characterized in that: The calculation formula of the phase difference Δφ of the orthogonal electromagnetic components is as follows:

7. A system for suppressing strong magnetotelluric noise during a period of time, characterized in that: include: A magnetotelluric data array acquisition module is used to acquire a magnetotelluric data array; the magnetotelluric data array includes 5 data tracks, the first track is a north-south electric track measurement value, the second track is an east-west electric track measurement value, the third track is a north-south magnetic track measurement value, the fourth track is an east-west magnetic track measurement value, and the fifth track is a vertical magnetic track measurement value; A complex track sequence construction module, used for constructing a complex track sequence based on the first track data and the second track data; An odd sequence and an even sequence acquisition module, used for performing Hilbert transform on the complex channel sequence and acquiring an odd sequence and an even sequence of the complex channel sequence; An instantaneous spectrum acquisition module is used to perform short-time Fourier transform on the odd sequence and the even sequence to acquire an instantaneous spectrum; A calculation module, used for calculating the polarization ellipticity, polarization direction and phase difference of orthogonal electromagnetic components based on the instantaneous spectrum; A polarization property time-frequency spectrum determination module, used to determine the polarization property time-frequency spectrum based on the polarization ellipticity, the polarization direction and the phase difference of the orthogonal electromagnetic components; The polarization property time-frequency spectrum includes the polarization ellipticity time-frequency spectrum, the polarization direction time-frequency spectrum and the orthogonal electromagnetic field component phase difference time-frequency spectrum; The denoising module is used to determine and delete the strong noise period based on the polarization property time spectrum to obtain a denoised magnetotelluric data array.

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