CN114924328A - Urban artificial source electromagnetic exploration method and system with vertical magnetic field reference channel - Google Patents
Urban artificial source electromagnetic exploration method and system with vertical magnetic field reference channel Download PDFInfo
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Abstract
The invention relates to the technical field of geophysical electromagnetic exploration, and provides an urban artificial source electromagnetic exploration method with a vertical magnetic field reference channel. The method comprises the following steps: acquiring an observation signal and a noise reference channel signal of the same observation point, wherein the observation signal is an electric field horizontal component or a magnetic field horizontal component, and the noise reference channel signal is a magnetic field vertical component; acquiring time frequency spectrums of an observation signal and a noise reference channel signal, comparing similarity, and screening similar time frequency units; performing statistical analysis on noise reference channel signals in the similar time-frequency units, determining the time-frequency units with noise energy higher than a set threshold value, eliminating the time-frequency units at corresponding positions in the observation signals, and reconstructing time-domain signals of the observation signals; and performing signal-noise separation on the observation signal based on a least square inversion denoising method. The observation of the noise reference channel is not influenced by the surrounding environment, the implementation is convenient, and the effective signal-noise separation can be realized.
Description
Technical Field
The invention relates to the technical field of geophysical electromagnetic exploration, in particular to an urban artificial source electromagnetic exploration method and system with a vertical magnetic field reference channel.
Background
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
When the artificial source electromagnetic method is used for detection, the requirements of different field sources and observation components and the accuracy degree to be achieved for solving the actual problems are different, and the differences are often large.
Under the complex urban environment, the urban detection depth is usually not deep and is generally shallow below 500m, but the covered detection range is large, and the resolution requirement is high for ore exploration relatively deep. Aiming at a large detection range, a plurality of field sources are adopted, and when detection results of different field sources are spliced, the effect is often poor due to the existence of various effects, so that the full coverage of a measurement area can be realized by one field source through selecting a proper transceiving distance. In addition, in the urban strong interference environment, the quality of electromagnetic signals is greatly influenced by short acquisition time and strong electromagnetic interference. Different components of the electromagnetic field are interfered by different conditions and anti-interference energy are different, and the anti-interference capability of the electric field is better than that of the magnetic field in urban strong interference environment. Therefore, a single field source can be adopted, the observation region is arranged in a far region of an artificial source as much as possible for measurement, and a single-component electric field, namely an electric field horizontal component (E) is adopted x ) The observation mode of long-time collection is carried out, and convenience is provided for field observation.
However, analysis shows that the signal-to-noise ratio of the data is increased along with the increase of the observation time, and if a single-component electric field is adopted for long-time acquisition, the E-based data is supplemented x -E min Denoising method of noise reference channel, i.e. calculating the observed electric field as zero (the electric field value is recorded as E at this time) min ) Deflecting at an angle corresponding to the time observation direction, and utilizing the characteristic that the minimum magnetic field is orthogonal to the minimum electric field along the direction E min Arranging magnetic bars in the direction perpendicular to the observation direction, collecting magnetic field signals as a noise reference channel (see CN112083508B), and indeed obtaining electromagnetic data with high signal-to-noise ratio is possible. However, with the progress of research, the urban complex environment is limited by surrounding fields, and especially when the designed polar distance is too large, the angular cloth cannot be measured through rotationAnd (5) placing an observation device.
Disclosure of Invention
The invention provides an urban artificial source electromagnetic exploration method and system based on a noise reference channel, which are used for simultaneously collecting a magnetic field vertical component H at an observation point during observation in a' far zone z As a noise reference channel, according to H z The method hardly contains the characteristics of electromagnetic signals from an artificial source, and converts the denoising problem into a noise solving problem by analyzing the characteristics of the electromagnetic signals, so that effective signal-noise separation is realized, the signal-to-noise ratio of data is improved, and meanwhile, the observation of a noise reference channel is not influenced by the surrounding environment.
In order to achieve the above object, one or more embodiments of the present invention provide the following technical solutions:
a city artificial source electromagnetic exploration method with a vertical magnetic field reference channel comprises the following steps:
acquiring an observation signal and a noise reference channel signal of the same observation point, wherein the observation signal is an electric field horizontal component or a magnetic field horizontal component, and the noise reference channel signal is a magnetic field vertical component;
acquiring time frequency spectrums of observation signals and noise reference channel signals, performing similarity comparison, and screening similar time frequency units;
performing statistical analysis on noise reference channel signals in the similar time-frequency units, determining the time-frequency units with noise energy higher than a set threshold, eliminating the time-frequency units at corresponding positions in the observation signals, and reconstructing to obtain new time-domain signals;
and performing signal-noise separation on the new time domain signal based on a least square inversion denoising method.
Further, after the observation signal is obtained, if the observation signal is an electric field horizontal component, preprocessing is further performed:
acquiring the frequency spectrum of the electric field horizontal component, estimating the amplitude of the CSEM frequency position according to the noise amplitude of the adjacent frequency position to obtain the estimated noise frequency spectrum of the electric field horizontal component, and further obtain the time domain waveform of the estimated noise;
detecting a time domain waveform mutation point of the estimated noise, and dividing the time domain waveform and the electric field horizontal component into a plurality of continuous time sections according to the mutation point;
fitting the baseline noise of each time section, and removing the fitted baseline noise from the electric field horizontal component of the corresponding time section.
Further, the correcting the amplitude of the CSEM frequency location according to the amplitude of the adjacent frequency location includes:
and replacing the amplitude of each CSEM frequency position by the average value of the amplitudes of the left and right adjacent frequency positions or by the maximum value of the amplitudes of the left and right adjacent frequency positions.
Further, detecting the time domain waveform discontinuities of the estimated noise comprises:
detecting a plurality of maximum value points in the time domain waveform of the estimated noise by using a Haar wavelet;
and screening a plurality of maximum value points larger than a set threshold value from the plurality of maximum value points to serve as mutation points.
Further, fitting the baseline noise for each of the time segments comprises:
sequentially adopting Legendre polynomials with a plurality of orders to fit a base line in each time section, and subtracting the noise of the fitting base line from the estimated noise of the time section to obtain the residual noise energy corresponding to the different orders; and taking the corresponding order when the residual noise energy is the lowest as the optimal order of the time section, and fitting the baseline noise of the time section.
Further, the similarity comparison pair includes:
dividing time frequency spectrums of the observation signal and the noise reference channel signal into time sections;
and respectively calculating the hash values of the time frequency spectrums of the observation signal and the noise reference channel signal in the same time section and the same frequency band by using a perceptual hash algorithm, and screening similar time frequency units.
Furthermore, before a new time domain signal is obtained through reconstruction, a time frequency unit corresponding to a non-CESM frequency position in the observation signal is removed.
Further, the time frequency units with noise energy higher than a set threshold value or the time frequency units corresponding to non-CESM frequency positions are removed from the observation signals, and frequency domain coefficients of the time frequency units in the time frequency spectrum are set to be 0.
Further, the signal-noise separation based on the least square inversion denoising method comprises solving the following overdetermined equation set:
Ax＝b
a is a matrix composed of Fourier orthogonal bases, and elements corresponding to the removed time-frequency units in the matrix are set to be 0; x is the frequency domain coefficient to be solved; b represents the new time domain signal.
One or more embodiments provide an urban artificial source electromagnetic survey system with a vertical magnetic field reference track, comprising: the device comprises an observation signal acquisition device, a horizontal coil and a signal processing device, wherein the observation signal acquisition device and the horizontal coil are arranged at the same observation point; wherein the content of the first and second substances,
the horizontal coil is used for collecting a magnetic field horizontal component as a noise reference channel signal;
the signal processing device is used for acquiring observation signals and noise reference channel signals and processing the observation signals by adopting the exploration method.
The above one or more technical solutions have the following beneficial effects:
by simultaneously collecting the vertical component H of the magnetic field of 'pure' noise at an observation point under the complex environment of a city z As a noise reference track for normally observing electric or magnetic field signals, based on the vertical component H of the magnetic field z The high similarity with the normal observation signal noise and the periodic characteristics of the artificial source signal are subjected to signal-noise separation, the problem that other observation points cannot be arranged due to the limitation of a field is solved, and the method can be widely applied to electromagnetic exploration in urban environments.
Aiming at the baseline drift phenomenon of the electric field, dividing the electric field signal into a plurality of continuous time sections based on the mutation point, and performing baseline noise fitting on each section, thereby ensuring that the local characteristics of the noise can be accurately fitted; and for each subsection, a Legendre polynomial with a plurality of orders is adopted for fitting, the order with the optimal fitting effect is selected, the local characteristics of noise can be fitted more accurately, and a better denoising result is obtained.
According to H z The time frequency spectrum sets an abnormal unit with high noise energy in a normal observation signal as 0, sets an element at a corresponding position of the matrix A as 0, and the position of the abnormal unit is fragmented, so that all data in a certain time period is not required to be removed, only a certain abnormal frequency in the time period is required to be removed, and information corresponding to the effective frequency is retained to the maximum extent.
Perpendicular component H of the magnetic field z The acquisition device is a coil which is horizontally placed, does not need to be grounded, is convenient to construct and is not easy to be limited by surrounding field conditions. And, H z And the signal is collected at the same place as the normal observation signal, so that the method is not limited by surrounding fields and has wide application range.
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The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention.
FIG. 1 shows a noise reference channel H in one or more embodiments of the invention z A schematic diagram of the arrangement of (1);
FIG. 2 is a diagram illustrating a "far zone" based magnetic field perpendicular component H in one or more embodiments of the present invention z A signal processing flow chart of the urban artificial source electromagnetic surveying method for the noise reference channel;
FIG. 3 is a CSEM frequency diagram of the emission current;
FIG. 4 shows the frequency spectrum of observed data, which is E from left to right x Raw data, E x After removal of the baseline, H z Short-time Fourier transform time-frequency spectrum of original data (50 Hz power frequency is removed);
FIG. 5 is a graph based on the reference track H z The time-frequency unit reconstruction diagram of the STFT;
FIG. 6 is a comparison graph of the time-frequency spectrum before and after the reconstruction of the normal observation signal;
FIG. 7 is a comparison diagram of normalized electric field before and after removing noise from the electric field signal;
FIG. 8 is a graph of normalized magnetic field comparison before and after removing noise from the magnetic field signal.
Detailed Description
The invention is further described with reference to the following figures and examples.
It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention. As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The embodiments and features of the embodiments in the present application may be combined with each other without conflict.
In the artificial source electromagnetic method, an observation region can be divided into a near region, a transition region and a far region according to a transmitting-receiving distance, and the "far region" generally refers to a region with the transmitting-receiving distance being greater than 10 times of a detection depth, and the region is also generally called a "wave region". In the "far zone" observation, the magnetic field vertical component H z Observation is inversely proportional to the fourth power of the transmit-receive distance, and effective signals are weak especially in a strong interference area. Relative to the horizontal component H of the magnetic field y In other words, the noise level is the same for both, but in a strong interference environment, H is observed in the "far zone z The noise in the data is much larger than the signal, and the noise is absolutely dominant. Therefore, in the strong interference region H z Can be used as a reference for eliminating noise in other observation channels (electric field or magnetic field components). And, H can be measured by adopting a horizontal coil z The horizontal coil is convenient to construct, easy to accurately level, does not need to be grounded, and is suitable for working in areas with difficult grounding; the method does not need to change the observation direction for collection, and is easy to develop in cities.
Example one
This example discloses a magnetic field perpendicular component H based on "far zone z A method for electromagnetic prospecting of artificial source in city by using noise reference channel includes collecting H by horizontal coil z The (magnetic field vertical component) is used as an electric field or magnetic field noise reference track, and in a far zone in the electromagnetic exploration field, the noise is removed aiming at the artificial source electromagnetic exploration signal in the strong interference environment. As shown in fig. 1-2, the method comprises the steps of:
step 1: acquiring an observation signal and a noise reference channel signal of the same observation point, wherein the observation signal is an electric field horizontal component E x Or horizontal component H of the magnetic field y The noise reference track signal is a magnetic field vertical component H z 。
Horizontal component E of electric field x And the perpendicular component H of the magnetic field z The noise still has high homology when observed at the same position and at the same time, so the embodiment acquires the observed signal and the noise reference channel signal of the same observation point. Of course, the horizontal component H of the magnetic field is satisfied y Region of collection conditions of (1), H z Can also be used as H y The noise reference trace.
In order to avoid abnormal conditions such as numerical value saturation and the like when the magnetic bar is used for signal acquisition in a strong interference environment, a coil is used for acquiring H z And (4) components. Specifically, the observation device is arranged as shown in fig. 1, wherein,
the included angle between the direction of a field source and a connecting line between a central point of the field source and a measuring point is shown, X, Y represents two orthogonal coordinate axes, and A, B represents two poles of the field source. Laying a horizontal coil at the observation point, and simultaneously acquiring normal observation signals and the vertical component H of the magnetic field by adopting a plurality of channels of the same instrument z 。
Step 2: if the observation signal is the electric field horizontal component, processing the electric field horizontal component, eliminating potential drift, and then executing the step 3; and if the observation signal is a horizontal component of the magnetic field, directly executing the step 3.
Due to passing throughElectric field E x During exploration, compared with a magnetic field component, besides electromagnetic interference with the same frequency as that in the magnetic field, an electric potential drift phenomenon may exist in the electric field component, so that for an electric field signal, the electric potential drift in the signal needs to be eliminated firstly.
The step of processing the electric field horizontal component specifically comprises the following steps:
(1) and acquiring the frequency spectrum of the electric field horizontal component, correcting the amplitude of the CSEM frequency position according to the amplitude of the adjacent frequency position to obtain the estimated noise frequency spectrum of the electric field horizontal component, and further obtaining the time domain waveform of the estimated noise.
The emission spectrum of the electromagnetic signal of the artificial source is shown in fig. 3, the position of the sharp pulse is the CSEM frequency position, when no noise interference exists, the signal only has energy at the pulse position, and the amplitude value at other frequency positions is zero. In the actually measured received signal, each spike position, that is, both the left and right sides of the CSEM frequency position, are noise signals, so in this embodiment, to acquire accurate noise characteristics, first, the noise amplitude of each CSEM frequency position is estimated based on the amplitudes of adjacent frequency positions, and specifically, an average value of corresponding coefficients (complex numbers) of two adjacent left and right frequencies may be adopted.
Specifically, the frequency spectrum of the normally observed electric field signal is obtained through Fourier transform; and obtaining the time domain waveform of the estimated noise according to the estimated noise spectrum through inverse Fourier transform.
And when the amplitude of the CSEM frequency position is corrected by adopting the average value of the amplitudes of the left and right adjacent frequency positions, the amplitudes of other positions are unchanged, and the frequency spectrum correction formula is as follows:
where f is the frequency of the signal, f s For artificial source electromagnetic survey frequencies, S (f) is the frequency domain amplitude of the frequency of the original signal f.
(2) Utilizing Haar wavelet detection to estimate a mutation point of a time domain waveform of noise, and dividing the time domain waveform and an electric field horizontal component into a plurality of continuous time sections according to the mutation point;
the Haar wavelet can well identify the mutation position in the signal, therefore, the convolution function is constructed through the Haar wavelet to identify the mutation of the time domain waveform of the estimated noise in the step 2, the maximum value point calculated by the convolution function corresponds to the mutation point, the maximum value point which is larger than the threshold value in the signal is screened as the main mutation point through the given threshold value, and the estimated noise and the original E are selected as the main mutation points according to the position of the mutation point x The signal is divided into a plurality of consecutive time segments. The length of the Haar wavelet determines the resolution of the step identification, and generally, the shorter the Haar wavelet, the higher the resolution. The length of the Haar wavelet is 1/2 main period length, and when the length of a segment is less than 1/8 main period length, the segment is combined with the segments which are shorter before and after the segment.
(3) And performing polynomial fitting on the baseline noise (mainly concentrated at low frequency) in each time section, and removing the fitted baseline noise from the electric field horizontal component of the corresponding time section.
Specifically, under the influence of factors such as underground natural potential and electrode polarization effect, the electric field observation signal has obvious baseline drift on the waveform of the time domain relative to the magnetic field signal. Moreover, under the influence of strong random human noise of the surrounding environment, the baseline drift has a plurality of sudden changes and is discontinuous.
Legendre polynomials have orthogonality, high order term coefficients go to zero, and adding and deleting a term has no effect on other terms. After segmentation, Legendre polynomials of 0, 1 and … … 10 orders are sequentially adopted to fit a baseline in each time segment, the baseline is subtracted from the estimated noise of the segment, and when Legendre polynomials of different orders are adopted, the energy of the residual noise of the segment is obtained respectively, and the order corresponding to the lowest energy is the optimal order of the segment. And then adopting Legendre polynomial fitting E of the judged optimal order x Base line of each segment at E x And subtracting the baseline to remove the baseline drift noise.
By adopting Legendre polynomial piecewise fitting of dynamic orders, local characteristics of noise can be accurately fitted, and a better denoising result is obtained.
And step 3: acquiring time frequency spectrums of an observation signal and a noise reference channel signal, comparing similarity, and screening similar time frequency units; and performing statistical analysis on the noise reference channel signals in the similar time-frequency units, determining the time-frequency units with the noise energy higher than a set threshold value, rejecting the time-frequency units at corresponding positions in the observation signals, and reconstructing time-domain signals of the observation signals.
Wherein the observation signal is a processed electric field signal or a horizontal component of a magnetic field. The baseline drift noise is mainly present in the electric field signal, but not in the magnetic field signal. However, both the electric field signal from which the baseline wander noise is removed and the originally acquired magnetic field signal have some random strong interference noise (e.g., pulses, steps) that can have a large impact on the signal quality. Due to normal observation signal and noise reference channel H z The noise contained in the two is highly homologous and similar and can be collected at the same place in the same time period according to H z The energy distribution of the medium noise removes the influence of random strong interference noise. The electric field signal and the magnetic field signal after the potential drift is removed have similar characteristics on electromagnetic interference, so that the electric field signal and the magnetic field signal are processed in the same way.
The step 3 specifically includes:
step 3.1: acquisition of the observation signal and the magnetic field reference track H by means of a short-time Fourier transform (STFT) z The time-frequency spectrum of (2); specifically, the short-time fourier transform is a time-frequency localization analysis method, and represents the signal characteristics at a certain moment through a segment of signals in a time window. As shown in FIG. 2, the normal observed signal and the noise reference channel H are obtained by Short Time Fourier Transform (STFT) z The window function is a rectangular window, and the length of the window function is equal to the length of a main period.
Step 3.2: observation signal and magnetic field reference channel H by using perceptual hash algorithm z And performing similarity judgment on the time frequency spectrums, and removing dissimilar time frequency units. In this embodiment, according to the characteristics of the time-frequency spectrograms of the two spectrograms, the perceptual hash algorithm is used to calculate the hash values of the spectrograms in the same time segment and the same frequency band in a blocking mannerScreening out similar time-frequency units;
step 3.3: similar time frequency unit information is reserved, and H is screened out according to the reserved time frequency unit z The abnormal unit with high medium noise energy level eliminates the time frequency unit with the same position as the abnormal unit in the observation signal and eliminates the time frequency unit corresponding to the non-CSEM frequency position; that is, the time-frequency spectrum coefficient corresponding to the time-frequency unit positions in the observation signal is set to 0, and the time-frequency spectrum coefficient corresponding to the non-CSEM frequency position in the observation signal is also set to 0.
Specifically, the robust statistical analysis method is used to determine outliers (e.g., abnormal units with higher noise energy and significant outliers) of the same CESM frequency in the Hz signal.
Step 3.4: and reconstructing the modified time-frequency spectrum coefficient in the step 3.3 based on short-time inverse Fourier transform (ISTFT) to obtain a new time-domain signal.
And 4, step 4: and (3) expressing the new time domain signal obtained in the step (3), namely the CSEM effective exploration signal, by using a Fourier orthogonal base in the inverse discrete Fourier transform matrix, constructing a solving matrix A aiming at the normal observation signal, constructing an over-determined equation set, and obtaining a coefficient corresponding to the signal and the noise by using least square inversion, thereby realizing signal-noise separation.
Wherein, the least square inversion Denoising method (see the invention patent: ZL 201610410616.5; and journal articles: Yang Y, Li D., Tong T., "Denoising controlled-source electronic data using least square inversion", geophilcs, 2018.83(4), E229-E244.)
Ax＝b (2)
Specifically, different frequency components in the CSEM effective exploration signal are represented by Fourier orthogonal bases corresponding to different frequencies, A is a matrix formed by the Fourier orthogonal bases, and elements corresponding to the removed time-frequency units in the matrix in the step 3 are set to be 0; x is the frequency domain coefficient to be solved; b is a time domain signal, i.e. the time domain signal of the observed signal obtained in step 3.
Assuming that there are two survey frequencies, 2, 1 anomalous cells are eliminated as shown in FIG. 4, respectively
N is the total number of sample points, ω ═ e 2πi/N ，
c 1 、c 2 For the location of the CSEM frequencies in the frequency domain,
are respectively F (c) 1 )、F(c 2 ) Complex conjugation of (a).
And solving by a formula 2 to obtain a least square solution of the frequency domain coefficient corresponding to the exploration frequency, thereby realizing the signal-noise separation of the normal observation signal.
Example two
Based on the method of the embodiment, the embodiment provides an urban artificial source electromagnetic surveying system with a vertical magnetic field reference channel, which comprises: the device comprises an observation signal acquisition device, a horizontal coil and a signal processing device, wherein the observation signal acquisition device and the horizontal coil are arranged at the same observation point; wherein the content of the first and second substances,
the horizontal coil is used for collecting a magnetic field horizontal component as a noise reference channel signal;
the signal processing device is used for acquiring an observation signal and a noise reference channel signal and processing the observation signal by adopting the exploration method as the embodiment I.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the scope of the present invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive efforts by those skilled in the art based on the technical solution of the present invention.
Claims (10)
1. A city artificial source electromagnetic exploration method with a vertical magnetic field reference channel is characterized by comprising the following steps:
acquiring an observation signal and a noise reference channel signal of the same observation point, wherein the observation signal is an electric field horizontal component or a magnetic field horizontal component, and the noise reference channel signal is a magnetic field vertical component;
acquiring time frequency spectrums of observation signals and noise reference channel signals, performing similarity comparison, and screening similar time frequency units;
performing statistical analysis on noise reference channel signals in the similar time-frequency units, determining the time-frequency units with noise energy higher than a set threshold, eliminating the time-frequency units at corresponding positions in the observation signals, and reconstructing to obtain new time-domain signals;
and performing signal-noise separation on the new time domain signal based on a least square inversion denoising method.
2. The method of electromagnetic surveying of urban artificial sources according to claim 1, characterized in that after acquisition of the observation signal, if the observation signal is an electric field horizontal component, a preprocessing is also carried out:
acquiring the frequency spectrum of the electric field horizontal component, estimating the amplitude of the CSEM frequency position according to the noise amplitude of the adjacent frequency position to obtain the estimated noise frequency spectrum of the electric field horizontal component, and further obtain the time domain waveform of the estimated noise;
detecting a time domain waveform mutation point of the estimated noise, and dividing the time domain waveform and the electric field horizontal component into a plurality of continuous time sections according to the mutation point;
fitting the baseline noise of each time section, and removing the fitted baseline noise from the electric field horizontal component of the corresponding time section.
3. The method of urban artificial source electromagnetic surveying according to claim 2 wherein modifying the amplitudes of CSEM frequency locations according to the amplitudes of adjacent frequency locations comprises:
and replacing the amplitude of each CSEM frequency position by the average value of the amplitudes of the left and right adjacent frequency positions or by the maximum value of the amplitudes of the left and right adjacent frequency positions.
4. The method of urban artificial source electromagnetic surveying according to claim 2, wherein detecting a time domain waveform discontinuity of the estimated noise comprises:
detecting a plurality of maximum value points in the time domain waveform of the estimated noise by adopting a Haar wavelet;
and screening a plurality of maximum value points larger than a set threshold value from the plurality of maximum value points to serve as mutation points.
5. The method of urban artificial source electromagnetic surveying according to claim 2, wherein fitting the baseline noise for each of the time segments comprises:
sequentially adopting Legendre polynomials with a plurality of orders to fit a base line in each time section, and subtracting the noise of the fitting base line from the estimated noise of the time section to obtain the residual noise energy corresponding to the different orders; and taking the order corresponding to the lowest residual noise energy as the optimal order of the time section, and fitting the baseline noise of the time section.
6. The method of urban artificial source electromagnetic surveying of claim 1 wherein similarity comparison comprises:
dividing time frequency spectrums of an observation signal and a noise reference channel signal into time sections;
and respectively calculating the hash values of the time frequency spectrums of the observation signal and the noise reference channel signal in the same time section and the same frequency band by using a perceptual hash algorithm, and screening similar time frequency units.
7. The method of electromagnetic surveying of urban artificial sources according to claim 1, characterized in that time-frequency units corresponding to non-CESM frequency positions in the observed signal are also rejected before the reconstruction obtains a new time-domain signal.
8. The method for electromagnetic surveying of urban artificial sources according to claim 7, characterized in that the elimination of time-frequency units from the observed signal whose noise energy is higher than a set threshold, or the elimination of time-frequency units corresponding to non-CESM frequency locations, sets the frequency domain coefficients of these time-frequency units in the time spectrum to 0.
9. The method of electromagnetic surveying of urban artificial sources as defined in claim 1, wherein performing signal-to-noise separation based on a least squares inversion denoising method comprises solving the following overdetermined system of equations:
Ax＝b
a is a matrix composed of Fourier orthogonal bases, and elements corresponding to the removed time-frequency units in the matrix are set to be 0; x is the frequency domain coefficient to be solved; b represents the new time domain signal.
10. An urban artificial source electromagnetic exploration system with a vertical magnetic field reference channel is characterized by comprising: the device comprises an observation signal acquisition device, a horizontal coil and a signal processing device, wherein the observation signal acquisition device and the horizontal coil are arranged at the same observation point; wherein the content of the first and second substances,
the horizontal coil is used for collecting a magnetic field horizontal component as a noise reference track signal;
the signal processing device is used for acquiring an observation signal and a noise reference channel signal, and processing the observation signal by using the exploration method of any one of claims 1-9.
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