CN116449432A - Ocean controllable source frequency domain electromagnetic method observation method - Google Patents
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Abstract
The application belongs to the field of ocean controllable sources, in particular to an ocean controllable source frequency domain electromagnetic method observation system, which comprises the following steps: the transmitting system is fixed in position and transmits by adopting a multi-frequency excitation signal through a transmitting coil; the receiving system navigates along the survey line, and electromagnetic response signals of the target area are collected through the receiving coil; and extracting amplitude and phase parameters of a plurality of effective main frequency points of the electromagnetic response signals, and acquiring abnormal position and distribution information. According to the method, the amplitude and phase multi-parameter combined observation and the common interpretation mode are adopted, so that the abnormal identification capacity of the observation system can be improved on the premise that the anti-interference capacity of the observation system is guaranteed.
Description
Technical Field
The application belongs to the field of ocean controllable sources, and particularly relates to an ocean controllable source frequency domain electromagnetic method observation method.
Background
The controllable source electromagnetic method (Controlled Source Electromagnetic Method, CSEM) is an effective ocean exploration method and has been successfully applied to the exploration of submarine natural gas hydrates and oil and gas reservoirs. During detection, specific excitation signals are applied to the submarine target detection area, and amplitude and phase parameters of electromagnetic response signals are extracted and observed, so that accurate evaluation can be performed on the submarine abnormal body scale, burial depth and space spread. Because of easy implementation and strong anti-interference capability, electromagnetic response signal amplitude parameter observation is more common in the detection of the traditional electromagnetic method. In the submarine hydrothermal solution metal sulfide detection experiment, because the seawater, the silt and the metal sulfide are all low-resistance mediums and the resistivity is relatively close, the electromagnetic sounding method based on the amplitude parameter measurement is difficult to give accurate evaluation. Compared with seawater and silt, the metal sulfide has stronger polarization effect, in the electromagnetic detection of the frequency domain, the polarization effect is represented by the difference of the phases of the response signal and the excitation signal, the identification of the high-polarization abnormality can be realized through the phase parameter observation, but the phase parameter measurement requires the strict synchronization of the excitation signal and the response signal in time, but the traditional high-precision synchronization mode based on the line synchronization and the GPS system cannot be realized due to the strong electromagnetic attenuation characteristic of the seawater and the complex topography environment of the seabed, and a sonar system with lower synchronization precision is often adopted as a synchronization device. Therefore, the phase parameter observation is difficult to realize in marine electromagnetic method detection.
Research on frequency domain electromagnetic observation systems has been performed for many years. In 1959, the r.wait proposed a variable frequency excitation method, which transmits excitation currents of two frequencies, respectively, and performs anomaly identification based on amplitude parameter observation. In order to obtain more abnormal parameters to improve the resolution of abnormal bodies, a complex frequency spectrum method and an amplitude phase spectrum method are sequentially proposed by K.L.Zonge and the like in 1975 and W.H.Pelton and the like in 1978, and the observation of the phase parameters of the abnormal bodies is increased. According to the method, the excitation currents with different frequencies are emitted successively to observe parameters of all frequency points one by one, so that the detection efficiency is low, and the observation precision is reduced due to time-varying environmental noise interference. He Jishan the pseudo-random three-frequency excitation method and the pseudo-random multi-frequency excitation method are developed on the basis of the variable-frequency excitation method, the simultaneous measurement of a plurality of frequency point parameters can be completed through one-time transmission experiment, the detection efficiency is greatly improved, and the method is widely applied to the field of frequency domain electromagnetic detection. However, due to the inherent characteristic of the fixed waveform of the multi-frequency pseudo-random signal, the amplitude and phase parameters of each frequency point of the excitation current are uncontrollable. Under the suppression effect of the inductance of the transmitting coil, the observation precision of the high-frequency points is often lower, and because the amplitude measurement is not carried out under the condition of equal precision, the amplitude parameters of each frequency point of the response signal cannot be directly utilized, and the amplitude ratio parameters of the total field and the primary field of the measuring point are required to be further calculated for carrying out anomaly identification. The primary field is calculated according to the relative distance and azimuth of the transmitting system and the receiving system, so that the calculation is complicated, and the accuracy of a calculation result cannot be ensured due to complex terrain environment. In addition, when the number of frequency points exceeds 5, the amplitude of low-frequency harmonic waves near the main frequency of the multi-frequency pseudo-random signal is higher, interference is brought to parameter extraction, and the observation precision is reduced. In order to improve the observation effect, chinese patent CN202010314490.8 proposes a phase observation method based on the phase difference between the total field and the primary field, and although the phase parameter has strong capability of identifying the anomaly, the GPS system is required to ensure high-precision synchronization of the excitation signal and the response signal, which cannot be applied to the detection occasion of the metallic minerals at the sea bottom.
Disclosure of Invention
The technical problem to be solved by the application is to provide an ocean controllable source frequency domain electromagnetic method observation method, which solves the problems that the existing method has stronger identification capability on anomalies and cannot be suitable for the detection occasion of the submarine metal minerals.
The present application is so embodied that,
an observation method of ocean controllable source frequency domain electromagnetic method, comprising the following steps:
the transmitting system is fixed in position and transmits by adopting a multi-frequency excitation signal through a transmitting coil;
the receiving system navigates along the survey line, and electromagnetic response signals of the target area are collected through the receiving coil;
and extracting amplitude and phase parameters of a plurality of effective main frequency points of the electromagnetic response signals, and acquiring abnormal position and distribution information.
Further, obtaining the abnormal position and distribution information comprises drawing an abnormal curve of amplitude and phase parameters changing along with the receiving and transmitting distance under different frequency points to perform abnormal observation after measurement and extraction of the total field parameters of all set measuring points are completed.
Further, the expression of the multi-frequency SPWM excitation signal is:u out for transmitting voltage, E is DC power supply, M is modulation depth, A i 、f i 、φ i The amplitude, frequency, and phase of the ith sinusoidal component, respectively.
Further, the frequency f i ＝2 n f 0 ，n＝0,1,2,…,N-1，f 0 Is the fundamental frequency.
Further, the design of the amplitude includes:
and carrying out frequency domain analysis on the equivalent circuit of the transmitting coil to obtain:
wherein Z is tc (ω) is a transmit coil equivalent impedance expression, expressed as:
Z tc (ω)＝R tc +jωL tc
the time domain expression of the multi-frequency excitation current signal is:
wherein B is i And theta i The amplitude and phase of the ith main frequency point of the excitation current are respectively expressed as
Further, MEA i Is calculated as follows: make the current amplitude B of each main frequency point i All the main frequency point voltage amplitude values are equal, and the relation between the main frequency point voltage amplitude values and the fundamental frequency voltage amplitude values satisfies the following conditions:
note the high frequency angular frequency omega H ＝10R tc /L tc When omega i ≥ω H When the resistor Rtc is ignored; when omega i ＜ω H When neglecting inductive reactance omega i L tc Then:
the amplitude of each main frequency point of the excitation current is controlled to be equal by adjusting the voltage value of each main frequency point of the excitation signal, and the voltage amplitude of each main frequency point meets the law of conservation of energy:
further, the design of the phase includes: setting the initial phase of the first main frequency point to 0, and the phase difference among the frequency points is 2 pi/N in turn, and each time a transmitting period T passes base Each frequency point is phase-stepped by 2 pi/L.
Further, extracting amplitude and phase parameters of a plurality of effective dominant frequency points of the electromagnetic response signal includes: let the sampling sequence of the total field signal be b r (n) spectral analysis according to discrete fourier transform:
wherein N is S For sample sequence b r Length of (n), B r (k) As DFT conversion result, the ith main frequency point f of the total field signal i Amplitude B of (2) r,i Phase difference from total field relative to excitation currentExpressed as:
in phi t,i The i-th principal frequency point phase of the excitation current signal is a known quantity.
Further, the total field phaseDuring extraction of the excitation signal and the response signal, time delay between the excitation signal and the response signal is calculated, the excitation signal and the response signal are aligned in time domain by combining the low-precision synchronization signal, and total field phase is extracted through the synchronization of the excitation signal and the response signal in time>
Further, calculating the time delay between the excitation signal and the response signal includes: setting the phase sequences of the ith frequency point of the excitation signal and the response signal as phi respectively t,i (n) and phi r,i (n) expressed as:
in theta t,i (n) and θ r,i (n) is interference noise, d is the time delay between two phase sequences, and the cross-correlation function of the two phase sequences is expressed as:
if the interference noise is random noise, the phase sequence is independent of the noise, and the noise is independent of the noise, and the phase sequence is independent of the noise, and the noise is independent of the noise:
R tr (τ)＝R ss (τ-d)
when τ -d=0, R, depending on the nature of the autocorrelation function ss (τ -d) reaches a maximum value, the phase sequence φ r,i (n) and phi t,i The time delay between (n) is:
in combination with the low precision synchronization signal, the excitation signal is aligned with the response signal on the time axis.
Compared with the prior art, the application has the beneficial effects that:
the application provides a frequency domain electromagnetic method observation system based on a multi-frequency SPWM excitation field source. Based on the flexibility of amplitude and phase configuration of each frequency point of the multi-frequency SPWM excitation signal, the equal-precision observation of amplitude parameters of a plurality of frequency points is realized by setting the amplitude of each frequency point of the excitation current to be equal. Setting the phase of each frequency point of the excitation signal to change according to a specific rule, and calculating the time delay between the two signals according to the cross correlation of the excitation signal and the phase sequence of each frequency point of the total field response signal, so as to realize the synchronization between the signals. Because the set phase change period is far longer than the emission period, the requirement of the observation system on the synchronization precision can be reduced. The observation system can directly perform abnormality identification according to the detected total field amplitude and phase parameters, omits complex one-time field calculation of the measuring point, and has higher observation precision and detection efficiency. By adopting the mode of amplitude and phase multi-parameter combined observation and common interpretation, the abnormal identification capability of the observation system can be improved on the premise of ensuring the anti-interference capability of the observation system.
Drawings
FIG. 1 is a schematic diagram of ocean controllable source frequency domain electromagnetic detection;
FIG. 2 is a simplified model of electromagnetic response parameter measurement (EMF) graph (a) relative positional relationship; (b) an anomaly loop equivalent circuit model;
FIG. 3 is a schematic diagram of the principle of generating a multi-frequency SPWM excitation field source (a) a schematic diagram of the structure of a multi-frequency SPWM electromagnetic emission system (b) a schematic diagram of modulating a multi-frequency SPWM excitation signal;
FIG. 4 is a schematic diagram illustrating a phase configuration of each main frequency point of a multi-frequency SPWM excitation signal according to an embodiment of the present invention;
FIG. 5 is a graph of a spectrum analysis of five-frequency pseudorandom signal and five-frequency SPWM excitation signal provided by an embodiment of the invention (a) a voltage spectrum of the five-frequency pseudorandom signal; (b) five-frequency pseudo-random signal current spectrograms; (c) five-frequency SPWM signal voltage spectrograms; (d) five-frequency SPWM signal current spectrograms;
fig. 6 is a graph of phase change of each frequency point of the five-frequency SPWM excitation signal, (a) a graph of phase change of each frequency point of the five-frequency SPWM excitation signal (l=50); (b) Five-frequency SPWM excitation signal phase change plot for each frequency point (l=200).
Detailed Description
In order to make the objects, technical solutions and advantages of the present application more apparent, the present application will be further described in detail with reference to examples. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the present application.
The application discloses an ocean controllable source frequency domain electromagnetic detection method, wherein a detection system adopted by the method comprises three parts: the transmitting system, the receiving system and the later data processing and interpretation. Firstly, alternating excitation current is injected into a transmitting coil through a shipboard transmitting system through a high-voltage cable, and the excitation current establishes an alternating primary electromagnetic field in a marine space. Under the action of the alternating electromagnetic field, the abnormal conductive body at the sea bottom generates induced eddy current, and generates secondary induced electromagnetic field to radiate into space. The receiving coil converts the total field signal overlapped by the primary electromagnetic field and the secondary electromagnetic field into an induced electromotive force signal, the signal contains the ground electric response information, and the signal is collected and stored by the receiving system. In the marine experiment, the scientific investigation ship with the transmitting system is fixed in position, the scientific investigation ship with the receiving system sails along the set survey line, the electromagnetic response information of the target area is completely recorded and observed, the electrical information such as resistivity, polarizability and the like of different depths of the area to be detected is further obtained through data processing means such as forward and backward in the later period of the exploration experiment, and the position and distribution of the abnormal submarine body are explained. As a source of the frequency domain electromagnetic detection system, the performance of the emission system plays a vital role in the difficulty of data extraction and observation effect.
According to the theory of the frequency domain electromagnetic method, the electrical information of different detection frequencies corresponding to different depths can be obtained by the detection depth approximately given by the skin depth formula (1),
where δ represents the depth of investigation, ρ represents the measured area medium resistivity, and f represents the detection frequency.
In order to complete detection within the target depth range, the traditional sweep frequency emission mode needs to complete emission of a plurality of frequency points one by one, detection efficiency is low, environmental noise is inconsistent during testing of different frequency points, and observation accuracy is greatly reduced. The problem can be solved by adopting a multi-frequency excitation signal transmitting mode, and quick and high-precision investigation of stratum with different depths can be completed by extracting amplitude and phase parameters of a plurality of effective main frequency points of the response signals.
In order to analyze the identification capability of amplitude and phase parameters to anomalies, a simplified electromagnetic response parameter measurement model is established, the electromagnetic detection experimental process of the ocean controllable source is simulated, the relative positions of a transmitting coil, a receiving coil and an anomaly ring are shown as a figure 2 (a), the transmitting coil and the receiving coil are positioned in the same horizontal plane, and the anomaly ring is positioned below the horizontal plane. The transmitting coil and the abnormal ring are fixed in position, the receiving coil moves along the measuring line, and the total field signal is acquired. Mtr, mta, and Mar are mutual inductances between the transmit coil and the receive coil, between the transmit coil and the anomaly loop, and between the anomaly loop and the receive coil, respectively. As shown in fig. 2 (b), the abnormal ring equivalent circuit model is equivalent to a closed loop formed by connecting a resistor Ra and an inductor La in series.
Let the excitation current at a certain frequency point be expressed as:
I t (ω)＝A t sin(ωt) (2)
in which A t For the emission current amplitude, ω is the emission current angular frequency. The magnetic flux generated in the receiving coil and the anomaly ring are denoted as ψ, respectively tr (omega) and ψ ta (ω), then the expressions are respectively:
ψ tr (ω)＝M tr I t (ω)＝M tr A t sin(ωt) (3)
ψ ta (ω)＝M ta I t (ω)＝M ta A t sin(ωt) (4)
the induced current generated in the anomaly loop is expressed as:
in E a (ω) is an abnormal ring induced electromotive force, Z a (ω) is the abnormal loop impedance, expressed as:
Z a (ω)＝R a +jωL a (6)
then induce a current I a (ω) the magnetic flux generated in the receiving coil is expressed as:
then the exciting current I in the receiving coil t (omega) and anomaly Ring induced Current I a The total magnetic flux (total field) generated by (ω) is expressed as:
the total field induced electromotive force is expressed as:
when the trigonometric function in the expression (9) is expressed in complex form, the expression (9) is rewritten as:
in the formula, the real partAnd imaginary part->Induced electromotive forces of the secondary field and the primary field, respectively. The total field amplitude is expressed as:
the phase of the total field relative to the excitation current is expressed as:
as can be seen from equations (11) and (12), the total field amplitude, the phase parameter, and the anomaly loop parameter L a 、R a Excitation signal angular frequency omega and mutual inductance M influenced by relative position ar 、M ta 、M tr The product coupling terms of three influencing factors are contained in the amplitude and phase parameter expressions. According to the theory of a frequency domain electromagnetic method, the detection depth is related to the excitation frequency and the receiving-transmitting distance. Therefore, the abnormal position and the distribution information can be obtained by analyzing the change rule of the amplitude and the phase parameter curve under different receiving and transmitting distances of the same frequency point and the amplitude and the phase parameter difference among different frequency points of the total field signal of the same measuring point. For the amplitude parameter, the latter term under the root number contains a primary field, and the primary field amplitude is often stronger, so that the amplitude of the total field is relatively close to that of the primary field, and the amplitude parameter difference under each main frequency point is smaller. The phase parameter is greatly influenced by frequency when the phase parameter is near an abnormality, and the position of an abnormal ring is easier to directly obtain from an abnormal curve.
When the absolute amplitude parameter of the total field is adopted for carrying out anomaly analysis, the anomaly position can be directly identified according to the amplitude difference between different frequency points of the total field at the same measuring point, so that the amplitude of each main frequency point of the excitation signal is required to be equal. The amplitude comparison between different frequency points of the total field signal under the same measuring point loses meaning because the difference between the main frequency points of each current of the multi-frequency pseudo-random signal is larger, and the amplitude ratio of the total field to the primary field is required to be calculated for carrying out abnormal feature identification. The submarine environment is extremely complex, and the calculation accuracy of a primary field is difficult to ensure.
According to the formula (12), the measurement of the phase parameters requires time synchronization of the excitation signals and the response signals, and because seawater has strong attenuation property on signals and serious electromagnetic interference on the seabed, the traditional synchronization mode based on GPS and line synchronization cannot be applied, and only a sonar system with lower synchronization precision can be used, so that higher requirements are put forward on a phase parameter extraction method.
After the measurement and extraction of the total field parameters of all the set measuring points are completed, the positions and the distribution of the abnormal bodies can be analyzed and interpreted by drawing abnormal curves of amplitude and phase parameters changing along with the receiving and transmitting distances under different frequency points for abnormal observation.
In order to improve the electromagnetic detection efficiency of the ocean controllable source, the multi-parameter observation system needs to realize multi-frequency point and other precision measurement, and the requirement on the synchronization precision is reduced as much as possible. The multi-frequency SPWM excitation field source has the advantage of controllable amplitude and phase of each main frequency, and the optimal excitation signal waveform can be obtained through the optimal design of the amplitude and phase of each frequency point of the excitation signal, so that the adaptability of the multi-parameter observation system in a submarine complex environment is improved.
The application provides an ocean controllable source frequency domain electromagnetic method observation method, which comprises the following steps:
the transmitting system is fixed in position and transmits by adopting a multi-frequency excitation signal through a transmitting coil;
the receiving system navigates along the survey line, and electromagnetic response signals of the target area are collected through the receiving coil;
and extracting amplitude and phase parameters of a plurality of effective main frequency points of the electromagnetic response signals, and acquiring abnormal position and distribution information.
Obtaining the abnormal position and distribution information comprises drawing an abnormal curve of amplitude and phase parameters changing along with the receiving and transmitting distance under different frequency points to perform abnormal observation after measurement and extraction of the total field parameters of all set measuring points are completed.
The transmitting system is a multi-frequency SPWM electromagnetic transmitting system structure, and a schematic diagram is shown in fig. 3 (a), and comprises a shipborne direct current power supply, a drive control circuit, an H-bridge transmitting circuit and a towing transmitting coil. The direct current power supply E is obtained by rectification and filtering of an alternating current generator on a ship and is used as an energy source of a transmitting system. The H-bridge transmitting circuit converts direct current into bipolar alternating current, and the bipolar alternating current is applied to two ends of the transmitting coil through a high-voltage cable. The transmitting coil can be equivalent to an equivalent inductance L tc And equivalent resistance R tc Is a series of (a) and (b). Inside the drive control circuit, the control signal s is generated by comparing the multi-frequency sine modulation signal with the triangular carrier 1 Sum s 2 After passing through the driving circuit, the H-bridge transmitting circuit is controlled to output a multi-frequency SPWM excitation signal, the multi-frequency SPWM excitation signal is applied to two ends of the transmitting coil, and multi-frequency excitation current is generated in the transmitting coil. The multi-frequency excitation current excites an alternating primary electromagnetic field containing a plurality of main frequency components in space through a transmitting coil to serve as an excitation field source. Set the emission voltage u out And emission current i out All of which are marked as positive directions from U to V.
The modulation principle of the multi-frequency SPWM excitation signal is shown in FIG. 3 (b). Let the number of main frequency points required by detection be N, and the main frequency points be f respectively 0 ,f 1 ,…,f N-1 Then the multi-frequency sinusoidal modulation signal u m The expression of (2) is
Wherein A is i 、f i 、φ i 、ω i The amplitude, frequency, phase and angular frequency of the ith sinusoidal component, angular frequency omega i ＝2πf i M is modulation depth, and the value range is 0 to 1.
Triangular carrier u c Is a peak-to-peak value of 2 and a frequency of f c Is a bipolar isosceles triangle wave. To ensure multiple frequenciesExcitation current signal waveform quality, carrier angular frequency f c To satisfy f c ≥10f i Where i=0, 1,..n-1. If f c Taking 10kHz, the main frequency point of the excitation signal should be within the range of 0-1 kHz.
Based on regular sampling method, the carrier wave u is subjected to triangle c For multi-frequency modulation signal u m Generates a control signal s 1 Sum s 2 . The emission voltage is a PWM signal with pulse width changing according to the multi-frequency sine rule and has a modulation signal u with multi-frequency sine m The same dominant frequency bin. By modulating the signal u only by multiple frequencies m Can fit the multi-frequency SPWM excitation signal containing the desired dominant frequency information to the multi-frequency excitation current.
Analysis of spectral characteristics of a multi-frequency SPWM excitation signal based on Fourier transform, emission voltage u out The fourier series expression of (c) is:
the 1 st item on the right side of the equation is a main frequency component, and comprises all expected main frequency points; the 2 nd harmonic component is generated by modulating the triangular carrier with the multi-frequency sinusoidal modulation signal, and is denoted by the expression H. Unfolding H to obtain
According to the H-mode, the angular frequency of each high-frequency harmonic generated by modulating the triangular carrier by the multi-frequency sine modulation signal isAmplitude is +.>When n=1, 3,5, …, k i =0, 2,4, …; when n=2, 4,6, …, k i ＝1,3,5,…。
The multi-frequency SPWM excitation signals according to equation (14) and equation (15) can be seen to contain each of the desired dominant frequency points. In the effective frequency band corresponding to the detection target area, the amplitudes of the other frequency components are 0 except for each expected main frequency point, and low-order harmonic waves near each expected main frequency point are eliminated through multi-frequency sinusoidal modulation, so that the difficulty of data processing and parameter extraction can be effectively reduced. The high-frequency harmonic wave generated by modulating the carrier wave by the multi-frequency sinusoidal modulation signal is distributed on the upper and lower side frequency bands at and near the odd-multiple carrier frequency, and compared with the required main frequency point, the frequency of the high-frequency harmonic wave component is very high and the amplitude is very low. Because the transmitting coil is inductive, the high-frequency harmonic wave can be well restrained, and the influence caused by the high-frequency harmonic wave can be further reduced by increasing the carrier frequency during detection. When high-frequency harmonic waves are ignored, the expression of the multi-frequency SPWM excitation signal is as follows
The DC power supply E is generally determined prior to the electromagnetic detection experiment, and thus the signal parameter A is modulated by multiple frequencies i 、f i And phi i The design of the system can enable the multi-frequency SPWM excitation signal to have expected frequency spectrum, amplitude spectrum and phase spectrum, and realize any configuration of the number, distribution, amplitude and phase of main frequency points in a specific frequency band.
For frequency, traditional 2 n The frequency ratio between each main frequency point of the multi-frequency pseudo-random signal is fixed according to 2 n And the indexes are distributed regularly. According to the electromagnetic method exploration principle and actual measurement experience, the detection efficiency is higher when all main frequency points of the excitation signal are distributed according to an exponential law. The multi-frequency SPWM excitation signal is generated by a modulation method, and the number and distribution of the main frequencies can be arbitrarily configured. To ensure the detection efficiency and is 2 n The multi-frequency pseudo-random signal is compared and analyzed, and the distribution of main frequency points of the multi-frequency SPWM excitation signal is also 2 n And designing an exponential law. Let the fundamental frequency be f 0 The ith main frequency point of the N-frequency SPWM excitation signal is f i ＝2 n f 0 (n＝0,1,2,…,N-1)。
According to the skin depth formula, excitation signals with different frequencies can detect different depths of a target area. And during detection experiments, determining the required fundamental frequency and the number of frequency points according to the depth range of the investigation target and the requirement on the longitudinal detection resolution. In theory, the more the number of frequency points contained in the excitation signal is, the higher the detection efficiency is, but the detection efficiency is limited by the requirements of the acquisition precision and the signal-to-noise ratio of the receiving system, and the number of main frequency points contained in the multi-frequency excitation signal is generally not more than 11.
The amplitude value of the application performs frequency domain analysis on the equivalent circuit of the transmitting coil to obtain
Wherein Z is tc (ω) is a transmit coil equivalent impedance expression, expressed as:
Z tc (ω)＝R tc +jωL tc
the time domain expression of the multi-frequency excitation current signal is that
Wherein B is i And theta i The amplitude and the phase of the ith main frequency point of the excitation current are respectively expressed as:
as can be seen from equation (19), due to the inductive nature of the transmitting coil, the voltage amplitude MEA of each main frequency point in the multi-frequency SPWM excitation signal i At the same time, the high frequency component has a lower amplitude. In order to realize the equal-precision observation of each frequency point, the current amplitude B of each main frequency point i All are equal, and the relation between the voltage amplitude of each main frequency point and the voltage amplitude of the fundamental frequency is obtained by combining the formula (17)
Note the high frequency angular frequency omega H ＝10R tc /L tc When omega i ≥ω H When the impedance of the transmitting coil is equal to the inductive reactance omega i L tc Plays a main role, ignoring the resistance R tc The method comprises the steps of carrying out a first treatment on the surface of the When omega i ＜ω H Resistance R in the transmit coil impedance tc Plays a main role, ignoring inductive reactance omega i L tc . The formula (21) can therefore be simplified as:
therefore, the amplitude of each main frequency point of the excitation current can be controlled to be equal by adjusting the voltage value of each main frequency point of the excitation signal. Meanwhile, the voltage amplitude of each main frequency point should satisfy the law of conservation of energy:
on the premise of meeting the law of conservation of energy, according to formula (16), the parameter A of the multi-frequency sinusoidal modulation signal can be adjusted during detection i The modulation depth M and the direct current voltage E to improve the signal to noise ratio of the observation system.
For phase, the observation of the phase parameters requires that the excitation signal and the response signal are aligned on the time axis, so that the synchronization signal time delay should be lower than the excitation signal period, otherwise the extracted parameters would be erroneous. In order to reduce the requirement of synchronous precision, based on the flexibility of setting the phase of each main frequency point of the multi-frequency SPWM excitation signal, the phase of each frequency point is set to change along with time according to the approximate sawtooth wave law, as shown in figure 4, phi in the figure t,i (i=0, 1,2, …, N-1) is the phase of the i-th dominant frequency point of the multi-frequency SPWM excitation signal. Let the period of the excitation signal be T base Then the phase change period T L For L periods of excitation signal, i.e. T L ＝L·T base . And designing the parameter L according to the requirement of the detection experiment on the synchronization precision. The initial phase of the first main frequency point is 0, and the phases among the frequency points are in turn phaseDifference 2 pi/N. Every time a transmission period T passes base Each frequency point is phase-stepped by 2 pi/L. Since the phase change period is much larger than the excitation signal period, a longer synchronization signal time delay is allowed.
In order to extract the amplitude and phase parameter information of a plurality of frequency points of the response signal rapidly, a frequency spectrum analysis method based on discrete fourier transform (Discrete Fourier Transform, DFT) is adopted. Let the sampling sequence of the total field signal be b r (n) according to DFT:
wherein N is S For sample sequence b r Length of (n), B r (k) Is the DFT conversion result. The ith principal frequency point f of the total field signal i Amplitude B of (2) r,i Phase difference from total field relative to excitation currentExpressed as:
in phi t,i The i-th principal frequency point phase of the excitation current signal is a known quantity.
According to equation (26), the total field phaseThe extraction of (a) requires that the excitation signal and the response signal are synchronized in time. The traditional GPS synchronous mode uses the second pulse signal as a synchronous mark to make the periods of the excitation signal and the response signal correspond to each other one by one, so that the time delay of the synchronous signal is required to be smaller than the period length of the excitation signal, otherwise, the period is dislocated. Limited by severe experimental conditions on the sea floor, line synchronization and GPS (Global positioning System) are identicalThe step mode can not be realized, and the accuracy of a sonar synchronization system commonly used for ocean exploration is lower. Because the phases of all frequency points of the multi-frequency SPWM excitation signal are set to change along with the emission cycle number according to a specific rule, the time delay between the excitation signal and the response signal can be calculated by analyzing the cross correlation of the phase sequences of all main frequency points of the excitation signal and the response signal, and then the excitation signal and the response signal are aligned on the time domain by combining the low-precision synchronous signal.
Setting the phase sequences of the ith frequency point of the excitation signal and the response signal as phi respectively t,i (n) and phi r,i (n) can be expressed as:
in theta t,i (n) and θ r,i (n) is interference noise, d is the time delay between two phase sequences, then the cross-correlation function of the two phase sequences can be expressed as:
assuming that the interference noise is random noise, the phase sequence is independent of the noise, and the noise is rewritten as follows:
R tr (τ)＝R ss (τ-d) (30)
when τ -d=0, R, depending on the nature of the autocorrelation function ss (τ -d) reaches a maximum value, the phase sequence φ r,i (n) and phi t,i The time delay between (n) is:
the excitation signal and the response signal can be combined with the low-precision sonar synchronous signal in the time axisAlignment is performed under low-precision signals because of low precision of sonar signals. The sonar is added with the phase change period of T L ＝L·T base The allowed synchronization signal time delay is thus amplified by a factor of L. At this time, the extraction and observation of the amplitude and phase parameters of each frequency point of the electromagnetic response signal are completed by combining the formula (25) and the formula (26), and further explanation is given to the positioning and distribution of abnormal bodies.
In order to verify the feasibility and effectiveness of the provided multi-frequency excitation field source, a five-frequency SPWM transmission system simulation model and a five-frequency pseudo-random transmission system simulation model are built in Matlab/Simulink. The simulation parameters are as follows: equivalent inductance L of transmitting coil tc =2mh, the equivalent resistance R of the transmitting coil tc =0.1Ω, fundamental frequency f 0 128Hz, a main frequency point N of 5, and a carrier frequency f c 20kHz.
Fig. 5 (a) and 5 (b) are the spectra of the five-frequency pseudo-random excitation voltage signal and the current signal, respectively, and fig. 5 (c) and (d) are the spectra of the five-frequency SPWM excitation voltage signal and the current signal, respectively. As can be seen from fig. 5 (a), the amplitudes of the main frequency points of the pseudo-random voltage signal are relatively close, but the amplitudes of part of the low-frequency harmonic voltages reach the same order of magnitude as the main frequency points, which brings great difficulty to data processing and parameter extraction. According to the current spectrum of fig. 5 (b), the current amplitude is very low at 1024Hz and 2048Hz frequency points, subject to coil high frequency suppression. In order to improve the detection precision of the high-frequency components, the current amplitude of the high-frequency components can be improved by increasing the power supply voltage in the exploration experiment, but the current amplitude of the low-frequency components can be improved due to the fixed waveform of the pseudo-random excitation signal, and the equal-precision measurement can not be realized. According to fig. 5 (c), in order to achieve equal current amplitudes at each frequency point, the five-frequency SPWM excitation signal is set such that the voltage amplitude at each frequency point is approximately proportional to frequency. According to fig. 5 (d), the amplitudes of all frequency points of the current signal are equal, so that equal-precision observation can be realized, the harmonic content near the main frequency is low, and the frequency spectrum characteristic is obviously superior to that of a 5-frequency pseudo-random signal. The flexibility of setting the amplitude of each frequency point based on the multi-frequency SPWM excitation signal is proved, the current amplitude of each main frequency point can be set to be equal, and the equal-precision observation of each main frequency point is realized.
Fig. 6 (a) and fig. 6 (b) are graphs of phase change of each frequency point of the five-frequency SPWM excitation signal when the L value is set to 50 and 200 respectively, phases of each frequency point change according to the approximate sawtooth wave law in both cases, the phase change period is about 0.39s and 1.56s respectively, that is, the allowable time delay of the adopted phase change period is 50 times and 200 times of that of the adopted excitation signal period respectively, the phase difference among each frequency point is 2 pi/5 in turn, the set phase change law is met, the feasibility of random setting of the phase of each frequency point of the multi-frequency SPWM excitation signal is verified, and the requirement of an observation system on a synchronous device is reduced.
The foregoing description of the preferred embodiments of the present application is not intended to be limiting, but is intended to cover any and all modifications, equivalents, and alternatives falling within the spirit and principles of the present application.
Claims (10)
1. An observation method of ocean controllable source frequency domain electromagnetic method is characterized by comprising the following steps:
the transmitting system is fixed in position and transmits by adopting a multi-frequency excitation signal through a transmitting coil;
the receiving system navigates along the survey line, and electromagnetic response signals of the target area are collected through the receiving coil;
and extracting amplitude and phase parameters of a plurality of effective main frequency points of the electromagnetic response signals, and acquiring abnormal position and distribution information.
2. The method for observing ocean controllable source frequency domain electromagnetic method according to claim 1, wherein obtaining the abnormal position and distribution information comprises drawing an abnormal curve of amplitude and phase parameters along with the change of the receiving distance under different frequency points for abnormal observation after measurement and extraction of total field parameters of all set measuring points are completed.
3. The method for observing the ocean controllable source frequency domain electromagnetic method according to claim 1, wherein,
the expression of the multifrequency SPWM excitation signal is:u out for transmitting voltage, E is DC power supply, M is modulation depth, A i 、f i 、φ i The amplitude, frequency, and phase of the ith sinusoidal component, respectively.
4. A method of marine controllably source frequency domain electromagnetic method observation according to claim 3, wherein said frequency f i ＝2 n f 0 ，n＝0,1,2,…,N-1，f 0 Is the fundamental frequency.
5. A method of marine controllable source frequency domain electromagnetic method observation according to claim 3 wherein the designing of the amplitude comprises:
and carrying out frequency domain analysis on the equivalent circuit of the transmitting coil to obtain:
wherein Z is tc (ω) is a transmit coil equivalent impedance expression, expressed as:
Z tc (ω)＝R tc +jωL tc
the time domain expression of the multi-frequency excitation current signal is:
wherein B is i And theta i The amplitude and phase of the ith main frequency point of the excitation current are respectively expressed as
6. The method for observing the ocean controllable source frequency domain electromagnetic method according to claim 3, wherein,
MEA i is calculated as follows: make the current amplitude B of each main frequency point i All the main frequency point voltage amplitude values are equal, and the relation between the main frequency point voltage amplitude values and the fundamental frequency voltage amplitude values satisfies the following conditions:
note the high frequency angular frequency omega H ＝10R tc /L tc When omega i ≥ω H When the resistor Rtc is ignored; when omega i ＜ω H When neglecting inductive reactance omega i L tc Then:
the amplitude of each main frequency point of the excitation current is controlled to be equal by adjusting the voltage value of each main frequency point of the excitation signal, and the voltage amplitude of each main frequency point meets the law of conservation of energy:
7. a method of marine controllable source frequency domain electromagnetic method observation according to claim 3 wherein the design of the phase comprises: setting the initial phase of the first main frequency point to 0, and the phase difference among the frequency points is 2 pi/N in turn, and each time a transmitting period T passes base Each frequency point is phase-stepped by 2 pi/L.
8. The method for observing the ocean controllable source frequency domain electromagnetic method according to claim 1, wherein,
the steps of extracting the amplitude and phase parameters of a plurality of effective main frequency points of the electromagnetic response signal include: let the sampling sequence of the total field signal be b r (n) according toThe spectrum analysis method of the discrete Fourier transform comprises the following steps:
wherein N is S For sample sequence b r Length of (n), B r (k) As DFT conversion result, the ith main frequency point f of the total field signal i Amplitude B of (2) r,i Phase difference from total field relative to excitation currentExpressed as:
in phi t,i The i-th principal frequency point phase of the excitation current signal is a known quantity.
9. The method for observing ocean controllable source frequency domain electromagnetic method according to claim 8, wherein,
total field phaseDuring extraction of the excitation signal and the response signal, time delay between the excitation signal and the response signal is calculated, the excitation signal and the response signal are aligned in time domain by combining the low-precision synchronization signal, and total field phase is extracted through the synchronization of the excitation signal and the response signal in time>
10. The method for observing ocean controllable source frequency domain electromagnetic method according to claim 9, wherein,
calculating the time delay between the excitation signal and the response signal includes: setting the phase sequences of the ith frequency point of the excitation signal and the response signal as phi respectively t,i (n) and phi r,i (n) expressed as:
in theta t,i (n) and θ r,i (n) is interference noise, d is the time delay between two phase sequences, and the cross-correlation function of the two phase sequences is expressed as:
if the interference noise is random noise, the phase sequence is independent of the noise, and the noise is independent of the noise, and the phase sequence is independent of the noise, and the noise is independent of the noise:
R tr (τ)＝R ss (τ-d)
when τ -d=0, R, depending on the nature of the autocorrelation function ss (τ -d) reaches a maximum value, the phase sequence φ r,i (n) and phi t,i The time delay between (n) is:
in combination with the low precision synchronization signal, the excitation signal is aligned with the response signal on the time axis.
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