阅读说明：本技术 高含硫气田弱磁油管双层管柱缺陷的电涡流检测方法 (Current vortex detection method for defects of double-layer pipe column of weak magnetic oil pipe in high-sulfur-content gas field ) 是由 陈友荣 金合丽 刘半藤 周莹 任条娟 于 2020-07-20 设计创作，主要内容包括：本发明涉及一种高含硫气田弱磁油管双层管柱缺陷的电涡流检测方法,所述方法包括数据预处理、极点对称模态分解改进、希尔伯特变化、特征提取、SVM训练和识别等方法,可输出缺陷类型,从而提高缺陷分类的识别率,本发明能很好的去除弱磁油管偏心引起的数据线性偏移；通过基于分段时间插值方法、改进极点对称模态分解方法,对预处理后数据进行分解,保证插值节点的单调性,本方法可较好地识别2×90挤压、双面挤压、单面挤压、3×90挤压、4×90挤压和弯曲6类缺陷,提高了每一类缺陷识别率和平均缺陷识别率。(The invention relates to an eddy current detection method for a high-sulfur-content gas field weak magnetic oil pipe double-layer pipe column defect, which comprises the methods of data preprocessing, pole symmetric modal decomposition improvement, Hilbert change, feature extraction, SVM training, recognition and the like, and can output the defect type, thereby improving the recognition rate of defect classification; the preprocessed data are decomposed by improving a pole symmetric modal decomposition method based on a segmented time interpolation method, so that monotonicity of an interpolation node is guaranteed, 6 types of defects of 2 x 90 extrusion, double-sided extrusion, single-sided extrusion, 3 x 90 extrusion, 4 x 90 extrusion and bending can be well identified, and identification rate of each type of defects and average defect identification rate are improved.)

1. An eddy current testing method for defects of a double-layer pipe column of a weak magnetic oil pipe in a high-sulfur-content gas field is characterized by comprising the following steps of:

(1) if the current eddy current detection data of the training sample exist, the current state is set as the training state;

(2) judging whether the training sample is in a training state or not, if so, reading eddy current detection data of the training sample, and continuing the step (4), otherwise, continuing the step (3);

(3) detecting position points of a plurality of depths of the double-layer pipe column of the weak magnetic flow oil pipe, reading eddy current detection data, and continuing to the step (4);

(4) smoothing the eddy current detection data by adopting an average method;

(5) analyzing defect data in the eddy current detection data obtained in the step (4), determining the positions of trough points, the starting position and the ending position of the eddy current detection data, dividing the eddy current detection data into a plurality of data segments according to the positions of the trough points, processing each data segment, and removing linear offset of the eddy current detection data;

(6) converting the eddy current detection data obtained in the step (5) into time domain eddy current detection data according to the sampling time of each position point in the eddy current detection data, taking a plurality of position points as interpolation nodes, and interpolating the time point of each interpolation node to obtain a signal to be decomposed;

(7) searching local extreme points of a signal to be decomposed, connecting two adjacent extreme points, searching middle points of the adjacent extreme points, supplementing the middle points of the boundary points of the initial point and the first extreme point, supplementing the middle points of the boundary points of the end point and the last extreme point according to the initial point and the end point of the signal to be decomposed, obtaining all the middle points, constructing interpolation curves among different middle points, and calculating the mean value of the interpolation curves;

(8) obtaining an IMF component according to the signal to be decomposed, and obtaining a residual signal obtained by subtracting the obtained IMF from the signal to be decomposed;

(9) if the residual signal is a monotonous signal or the value of the residual signal is smaller than a preset threshold value, outputting a plurality of IMF components obtained currently, otherwise, taking the residual signal as a new signal to be decomposed, and continuing the step (7);

(10) performing Hilbert transformation on the IMF components, and calculating instantaneous amplitude, instantaneous phase, instantaneous frequency and marginal spectrum to obtain a plurality of characteristic parameters;

(11) if the current state is not the training state, the characteristic parameters obtained in the step (10) are characteristic parameters for detection, the step (12) is continued, if the current state is the training state, the characteristic parameters obtained in the step (10) are used as characteristic parameter samples, an SVM algorithm based on a Gaussian kernel function is adopted to train the selected characteristic parameter samples, model parameters of an SVM algorithm model based on the Gaussian kernel function are obtained, at least one classifier is constructed according to the trained parameters, the current state is made to be the recognition state, and the step (12) is carried out;

(12) and adopting the constructed classifier to detect the defects of the characteristic parameters for detection.

2. The eddy current testing method for the defects of the double-layer pipe column of the weak magnetic oil pipe in the high sulfur-containing gas field according to claim 1, wherein in the step (4), the following formula is adopted for smoothing:

wherein the content of the first and second substances,eddy current test data representing the h-th depth of the ith sample point,

3. The eddy current testing method for the defects of the double-layer pipe column of the weak magnetic oil pipe in the high sulfur-containing gas field according to claim 1, wherein in the step (5), the linear offset of eddy current testing data is removed by adopting the following formula:

wherein the content of the first and second substances,eddy current test data representing the h-th depth of the ith sample point,preprocessed data representing the h-th depth of the ith sample point,represents the jth valley point depth of the ith sample point,

4. The eddy current testing method for the defects of the double-layer pipe column of the weak magnetic oil pipe in the high sulfur-containing gas field according to claim 1, wherein in the step (5), the positions of the wave trough points are determined by adopting the following steps:

(5.1) smoothing the data according to the step (4)It is subjected to derivation to obtain a derivative of

(5.2) if the sampling depths h and h +1 satisfy the condition 1 according to the derivative:

(5.3) if the sampling depths h and h +1 satisfy the condition 2:wherein, V₁If the derivative difference judgment threshold is represented, the position h +1 is the valley point 2 to be selected, the step (5.6) is skipped, and otherwise, the step (5.4) is skipped;

(5.4) if the sampling positions h and h +1 satisfy the condition 3:if the position h +1 is the valley point 3 to be selected, jumping to the step (5.7), otherwise, jumping to the step (5.8);

(5.5) if the front 5 point derivatives or the rear 5 point derivatives of the to-be-selected valley points 1 both meet the formula (3), considering the to-be-selected valley points 1 as valley points, recording the positions of the valley points, and jumping to the step (5.8), otherwise, directly jumping to the step (5.8);

wherein, V₂Indicating a derivative decision threshold value, η indicating a point quantity selection value;

(5.6) if the front 5 point derivatives or the rear 5 point derivatives of the to-be-selected valley points 2 both meet the formula (4), considering the to-be-selected valley points 2 as valley points, recording the positions of the valley points, and jumping to the step (5.8), otherwise, directly jumping to the step (5.8);

(5.7) if the front 5 point derivatives or the rear 5 point derivatives of the to-be-selected valley points 3 both meet the formula (5), considering the to-be-selected valley points 3 as valley points, recording the positions of the valley points, and jumping to the step (5.8), otherwise, directly jumping to the step (5.8);

and (5.8) judging whether all sampling positions are finished, if not, h is h +1, jumping to the step (5.2), otherwise, outputting the positions of all valley points.

5. The eddy current testing method for the defects of the double-layer pipe column of the weak magnetic oil pipe in the high sulfur-containing gas field according to claim 1, wherein in the step (6), the following steps are adopted to take a plurality of position points as interpolation nodes, and each time point is interpolated to obtain a signal to be decomposed:

using a plurality of position points as interpolation nodes, and using each interpolation node as an unknown function

wherein f is_k(t) represents the function value inserted at the current time t,

6. the eddy current testing method for the defects of the double-layer pipe column of the weak magnetic oil pipe in the high sulfur-containing gas field is characterized in that in the step (10), the instantaneous amplitude, the instantaneous phase, the instantaneous frequency and the marginal spectrum are calculated by adopting the following formulas:

(10.1) dividing each IMF component c_i(t) performing a Hilbert transform to obtain a transformed component

(10.2) calculating the instantaneous amplitude a by equation (15)_i(t)：

(10.3) calculating instantaneous phase θ by equation (16)_i(t):

(10.4) calculating instantaneous frequency w by equation (17)_i(t)：

(10.5) calculating a marginal spectrum of the eddy current test data by equation (18):

wherein h (w, t) represents the Hilbert spectrum of the time-frequency distribution.

7. The eddy current testing method for the defects of the double-layer pipe column of the weak magnetic oil pipe in the high sulfur-containing gas field according to claim 1, wherein the step of detecting the defects is carried out by using a single electric currentIn step (11), the Gaussian kernel function K (χ)_J,χ_I) Comprises the following steps:

K(χ_J,χ_I)＝exp(-γ×||χ_J-χ_I||) (20)

wherein, K (χ)_J,χ_I) Representing the Gaussian kernel function,. chi_J,χ_IRepresenting a characteristic parameter sample and gamma representing a gaussian kernel parameter.

Technical Field

The invention relates to the technical field of nondestructive testing, in particular to an eddy current testing method for detecting defects of a double-layer pipe column of a weak magnetic oil pipe in a high-sulfur-content gas field.

Background

In the current society, natural gas is the life line of national industrial development, and the nation pays attention to the exploration and development of natural gas. The plain gas field is the largest integral sea-phase high-sulfur-content gas field in China at present and has high content of hydrogen sulfide. The hydrogen sulfide not only can cause serious corrosion to underground exploration and production equipment such as weak magnetic oil pipes, casings and the like, but also can cause danger to lives in a large range once leakage occurs, so that the development of a plain gas field is a worldwide problem. In the process of gas field development, due to the reasons of manual misoperation or salt-gypsum layer creep and the like, the double-layer pipe column of the underground inner-layer oil pipe and the outer-layer casing pipe can be deformed in various types, and even hydrogen sulfide leakage can be caused seriously. In order to safely develop a plain gas field and require that the outer casing of the double-layer pipe column cannot be seriously deformed, the outer casing of the underground double-layer pipe column of the gas field needs to be monitored, and measures such as shutdown, repair, well sealing and the like are timely carried out when the outer casing is deformed. The well logging methods commonly used at present comprise multi-arm well diameter well logging, well temperature well logging, ultrasonic imaging well logging, eddy current well logging and the like. Due to the complex development environments of the air flow, the temperature and the like of the plain gas field, the hydrogen sulfide exists, and the inner layer weak magnetic oil pipe cannot be directly extracted, the outer casing of the double-layer pipe column cannot be detected by adopting logging methods such as multi-arm well diameter logging, well temperature logging, ultrasonic imaging logging and the like. Considering that the inner oil pipe is made of nickel-based alloy and has weak magnetism and little influence on eddy current, an eddy current logging technology is selected to measure the underground double-layer pipe column, and the response of the outer casing to the eddy current is analyzed through eddy current signals capable of penetrating through the weak magnetic oil pipe, so that the deformation of the outer casing is detected in a nondestructive mode.

At present, the nondestructive testing technology based on the electric eddy current is widely applied in many fields. These applications are primarily through eddy current inspection equipment, acquiring eddy current signals, and analyzing and processing the eddy current signals to identify defects. The feature extraction method based on the specific eddy current signal is one of the core methods of nondestructive testing, so some researchers are focused on researching an isochronous frequency domain analysis method based on fourier transform and wavelet transform to extract features such as frequency spectrum separation points, harmonic components, fundamental components and the like, but these methods need to select a transform basis function in advance, have no adaptivity to signal analysis, and all adopt a fixed transform scale, and the transformed signal cannot fully reflect the original signal, so that certain loss exists. Therefore, some researchers have focused on studying the feature extraction method based on the hilbert-yellow transform. The hilbert-yellow transform method is not limited by signal stationarity and linearity, and can adaptively transform signals according to the characteristic time scale of original signals to obtain Intrinsic Mode Functions (IMFs), instantaneous frequencies and boundary spectrums capable of reflecting the characteristics of the signals. However, these methods mainly use an emd (empirical Mode decomposition) method to decompose data, and have problems of large calculation amount, Mode mixing, overshoot, and the like when decomposing the eddy current detection signal. In order to solve the problem of EMD, Wang J L et al propose an Extreme-point Symmetric Mode Decomposition (ESMD), but ESMD adopts a cubic spline interpolation method, and has the problems of poor local characteristics, incapability of ensuring monotonicity of interpolation nodes, and the like when interpolation is performed on interpolation nodes.

Meanwhile, the defect type of the object can be effectively detected by adopting a proper classification and identification method according to the characteristic signal, so that part of scholars are focused on researching the classification and identification method based on the eddy current data, and part of scholars construct a deep neural network model based on the eddy current data to realize the identification of the defect, but the deep neural network model needs a large number of training samples. And the other part of scholars construct classification models such as BP (back propagation) neural network and radial basis function neural network based on eddy current data to realize defect identification, but the classification models have the problem of overfitting.

In summary, the current methods have the following problems: firstly, the common time-frequency domain analysis method has no adaptivity and the transformed signal has certain signal loss; secondly, aiming at eddy current detection data, the interpolation node monotonicity cannot be guaranteed by an ESMD method; thirdly, the common machine learning method is easy to generate the over-fitting problem, and the deep neural network is suitable for large sample data.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides an eddy current detection method for the defects of a double-layer pipe column of a weak magnetic oil pipe in a high-sulfur-content gas field, and the identification rate of defect classification is improved.

In order to achieve the above object, the present invention has the following configurations:

the invention provides an eddy current detection method for defects of a double-layer pipe column of a weak magnetic oil pipe in a high-sulfur-content gas field, which comprises the following steps of:

(1) if the current eddy current detection data of the training sample exist, the current state is set as the training state;

(2) judging whether the training sample is in a training state or not, if so, reading eddy current detection data of the training sample, and continuing the step (4), otherwise, continuing the step (3);

(3) detecting position points of a plurality of depths of the double-layer pipe column of the weak magnetic flow oil pipe, reading eddy current detection data, and continuing to the step (4);

(4) smoothing the eddy current detection data by adopting an average method;

(5) analyzing defect data in the eddy current detection data obtained in the step (4), determining the positions of trough points, the starting position and the ending position of the eddy current detection data, dividing the eddy current detection data into a plurality of data segments according to the positions of the trough points, processing each data segment, and removing linear offset of the eddy current detection data;

(6) converting the eddy current detection data obtained in the step (5) into time domain eddy current detection data according to the sampling time of each position point in the eddy current detection data, taking a plurality of position points as interpolation nodes, and interpolating time points between each interpolation node to obtain a signal to be decomposed;

(7) searching local extreme points of a signal to be decomposed, connecting two adjacent extreme points, searching middle points of the adjacent extreme points, supplementing the middle points of the boundary points of the initial point and the first extreme point, supplementing the middle points of the boundary points of the end point and the last extreme point according to the initial point and the end point of the signal to be decomposed, obtaining all the middle points, constructing interpolation curves among different middle points, and calculating the mean value of the interpolation curves;

(8) obtaining an IMF component according to the signal to be decomposed, and obtaining a residual signal obtained by subtracting the obtained IMF from the signal to be decomposed;

(9) if the residual signal is a monotonous signal or the value of the residual signal is smaller than a preset threshold value, outputting a plurality of IMF components (signal components) obtained currently, otherwise, taking the residual signal as a new signal to be decomposed, and continuing the step (7);

(10) performing Hilbert transformation on the IMF components, and calculating instantaneous amplitude, instantaneous phase, instantaneous frequency and marginal spectrum to obtain a plurality of characteristic parameters;

(11) if the current state is not the training state, the characteristic parameters obtained in the step (10) are characteristic parameters for detection, the step (12) is continued, if the current state is the training state, the characteristic parameters obtained in the step (10) are used as characteristic parameter samples, an SVM algorithm based on a Gaussian kernel function is adopted to train the selected characteristic parameter samples, model parameters of an SVM algorithm model based on the Gaussian kernel function are obtained, at least one classifier is constructed according to the trained parameters, the current state is made to be the recognition state, and the step (12) is carried out;

(12) and adopting the constructed classifier to detect the defects of the characteristic parameters for detection.

Optionally, in the step (4), smoothing is performed by using the following formula:

wherein the content of the first and second substances,eddy current test data representing the h-th depth of the ith sample point,

indicating smoothed eddy current data, m indicating the maximum depth of the data, and α indicating a sliding window.

Optionally, in the step (5), the linear offset of the eddy current test data is removed by using the following formula:

wherein the content of the first and second substances,preprocessed data representing the h-th depth of the ith sample point,

represents the jth valley point depth of the ith sample point,valley point position representing the ith sample pointElectrical eddy current test data.

Optionally, in the step (5), the valley point position is determined by the following steps:

(5.1) smoothing the data according to the step (4)

It is subjected to derivation to obtain a derivative ofLet h equal to 1;

(5.2) if the sampling depths h and h +1 satisfy the condition 1 according to the derivative:

orIf the position h +1 is the valley point 1 to be selected, jumping to the step (5.5), otherwise, jumping to the step (5.3);

(5.3) if the sampling depths h and h +1 satisfy the condition 2:

wherein, V₁A derivative difference determination threshold value is indicated,if the position h +1 is the valley point 2 to be selected, jumping to the step (5.6), otherwise, jumping to the step (5.4);

(5.4) if the sampling positions h and h +1 satisfy the condition 3:

if the position h +1 is the valley point 3 to be selected, jumping to the step (5.7), otherwise, jumping to the step (5.8);

(5.5) if the front 5 point derivatives or the rear 5 point derivatives of the to-be-selected valley points 1 both meet the formula (3), considering the to-be-selected valley points 1 as valley points, recording the positions of the valley points, and jumping to the step (5.8), otherwise, directly jumping to the step (5.8);

or

Wherein, V₂Indicating a derivative decision threshold value, η indicating a point quantity selection value;

(5.6) if the front 5 point derivatives or the rear 5 point derivatives of the to-be-selected valley points 2 both meet the formula (4), considering the to-be-selected valley points 2 as valley points, recording the positions of the valley points, and jumping to the step (5.8), otherwise, directly jumping to the step (5.8);

and is

(5.7) if the front 5 point derivatives or the rear 5 point derivatives of the to-be-selected valley points 3 both meet the formula (5), considering the to-be-selected valley points 3 as valley points, recording the positions of the valley points, and jumping to the step (5.8), otherwise, directly jumping to the step (5.8);

and is

And (5.8) judging whether all sampling positions are finished, if not, h is h +1, jumping to the step (5.2), otherwise, outputting the positions of all valley points.

Optionally, in the step (6), the following steps are adopted to take a plurality of position points as interpolation nodes, and the time point between each interpolation node is interpolated to obtain a signal to be decomposed:

using a plurality of position points as interpolation nodes, and using each interpolation node as an unknown function

The function value of (a) is determined,the preprocessed data representing the h depth of the ith sample point is obtained, and the derivative value f' (t) of the interpolation node is obtained_k) From two adjacent interpolation nodesAnd its derivative value, [ t ]_k,t_k+1]The interval is divided into a plurality of time points with intervals of 0.001 second, and each time point is interpolated by adopting a following segmented cubic Hermit interpolation formula (6) to obtain a signal to be decomposed;

wherein f is_k(t) represents the function value inserted at the current time t,ψ_k(t) and ψ_k+1(t) can be expressed as:

optionally, in the step (10), the instantaneous amplitude, the instantaneous phase, the instantaneous frequency and the margin spectrum are calculated by using the following formulas:

(10.1) dividing each IMF component c_i(t) performing a Hilbert transform to obtain a transformed component

(10.2) calculating the instantaneous amplitude a by equation (15)_i(t)：

(10.3) calculating instantaneous phase θ by equation (16)_i(t):

(10.4) calculating instantaneous frequency w by equation (17)_i(t)：

(10.5) calculating a marginal spectrum of the eddy current test data by equation (18):

wherein h (w, t) represents the Hilbert spectrum of the time-frequency distribution.

Optionally, in the step (11), the gaussian kernel function K (χ)_J,χ_I) Comprises the following steps:

K(χ_J,χ_I)＝exp(-γ×||χ_J-χ_I||) (20)

wherein, K (χ)_J,χ_I) Representing the Gaussian kernel function,. chi_J,χ_IRepresenting a characteristic parameter sample and gamma representing a gaussian kernel parameter.

In summary, compared with the prior art, the invention provides a preprocessing method for eddy current detection data, which can well remove data linear offset caused by the eccentricity of a weak magnetic oil pipe; decomposing the preprocessed data by improving a pole symmetric modal decomposition method based on a segmented thrice Hermit interpolation method, ensuring monotonicity of interpolation nodes and obtaining a plurality of Intrinsic Mode Functions (IMF); secondly, analyzing the obtained IMF components by using a Hilbert transform method, extracting 6 characteristic parameters such as instantaneous amplitude variance and marginal spectrum variance of the first component IMF1, mean value and instantaneous amplitude variance of the second component IMF2 and instantaneous amplitude variance and instantaneous phase variance of the third component IMF3, well representing 6 defects such as 2 × 90 extrusion, double-sided extrusion, single-sided extrusion, 3 × 90 extrusion, 4 × 90 extrusion and bending, and finally training and identifying the characteristic parameters by using an SVM model based on a Gaussian kernel function to avoid the problem of overfitting; the method can better identify 6 types of defects of 2 multiplied by 90 extrusion, double-sided extrusion, single-sided extrusion, 3 multiplied by 90 extrusion, 4 multiplied by 90 extrusion and bending, and improves the identification rate of each type of defects and the average identification rate of the defects.

Drawings

FIG. 1 is a flow chart of the eddy current testing method for detecting defects of a double-layer pipe column of a weak magnetic oil pipe in a high sulfur-containing gas field according to an embodiment of the invention;

fig. 2 is a block flow diagram of a valley point location determination method of the present invention.

Detailed Description

Example embodiments will now be described more fully with reference to the accompanying drawings. Example embodiments may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of example embodiments to those skilled in the art. The same reference numerals in the drawings denote the same or similar structures, and thus their repetitive description will be omitted.

The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring the invention.

As shown in fig. 1 and 2, in order to solve the technical problems in the prior art, the present invention provides an eddy current testing method for detecting defects of a double-layer pipe column of a weak magnetic oil pipe in a high sulfur-containing gas field, comprising the following steps:

(1) initializing sliding window α, derivative difference determination threshold V₁Derivative judgment threshold V₂And (3) waiting parameters, and if the eddy current detection data of the training sample exist, making the current state be the training state;

(2) if the current state is the training state, reading the eddy current testing data of the training sample, and jumping to the step (4), otherwise, jumping to the step (3);

(3) an electromagnetic flaw detection tester MTD-J produced by GoWell company and composed of 1 longitudinal long probe A, 2 transverse cross probes B and BB and 1 longitudinal short probe C is adopted to detect the double-layer tubular column of the weak magnetic oil pipe. In a sampling period, a longitudinal long probe, a transverse cross probe and a longitudinal short probe of the logging instrument sequentially sample the same depth of the underground double-layer pipe column. And the 11 time points acquired by the longitudinal long probe A are respectively 0.01, 0.06, 0.11, 0.17, 0.26, 0.35, 0.47, 0.57, 0.67, 0.77 and 0.87, and the sampling data A1-A11 are output. And 3 time points acquired by the B transverse cross probe are respectively 0.9, 0.92 and 0.96, and sampling data B1-B3 are output. The 3 time points acquired by the BB transverse cross probe are 1.04, 1.06 and 1.07 respectively, and the sampled data BB1-BB3 are output. The 6 time points acquired by the C longitudinal short probe are 1.09, 1.11, 1.12, 1.13, 1.14 and 1.15 respectively, and sampling data C1-C6 are output. Outputting eddy current detection data consisting of 23 point data of A1-A11, B1-B3, BB1-BB3 and C1-C6 in each depth;

4) respectively preprocessing each point data in the eddy current detection data of 23 points with different depths, namely smoothing the eddy current data and removing jitter by adopting a mean value method according to the following formula (1);

wherein the content of the first and second substances,

eddy current test data representing the h-th depth of the ith sample point,

indicating smoothed eddy current data, m indicating the maximum depth of the data, and α indicating a sliding window.

5) Analyzing the defect data to determine valley point locations

And the starting position of the eddy current dataAnd an end position

Where e represents the number of valley point positions plus 1. According to the position of the valley point, the original current vortex data is obtained

Divided into e +1 segments. Processing each section through a formula (2), removing linear offset of eddy current detection data, and obtaining preprocessed eddy current detection data;

wherein the content of the first and second substances,preprocessed data representing the h-th depth of the ith sample point,

represents the jth valley point depth of the ith sample point,valley point position representing the ith sample pointElectrical eddy current test data. As shown in fig. 2, the method for determining the valley point position of the defect data includes the steps of:

5.1) based on the smoothed dataIt is subjected to derivation to obtain a derivative of

Let h equal to 1;

5.2) depending on the derivative, if the sampling positions h and h +1 satisfy the condition 1:or

The position h +1 is the valley point 1 to be selected, and the step 5.5) is skipped, otherwise, the step 5.3) is skipped.

5.3) if the sampling positions h and h +1 satisfy the condition 2:

wherein, V₁If the difference of the derivatives is judged to be the threshold value, the position h +1 is the valley point 2 to be selected, and the step 5 is skipped6), otherwise jump to step 5.4).

5.4) if the sampling positions h and h +1 satisfy the condition 3:the position h +1 is the valley point 3 to be selected, and the step 5.7) is skipped, otherwise, the step 5.8) is skipped.

5.5) if the front 5 point derivatives or the rear 5 point derivatives of the to-be-selected valley points 1 both meet the formula (3), the to-be-selected valley points 1 are considered to be valley points, the positions of the valley points are recorded, and the step 5.8 is skipped), otherwise, the step 5.8 is directly skipped).

Or

Wherein, V₂Indicating a derivative decision threshold value, η indicating a point quantity selection value.

5.6) if the first 5 point derivatives or the last 5 point derivatives of the to-be-selected valley points 2 both meet the formula (4), the to-be-selected valley points 2 are considered to be valley points, the positions of the valley points are recorded, and the step 5.8 is skipped), otherwise, the step 5.8 is directly skipped).

And is

5.7) if the front 5 point derivatives or the rear 5 point derivatives of the to-be-selected valley points 3 both meet the formula (5), the to-be-selected valley points 3 are considered to be valley points, the positions of the valley points are recorded, and the step 5.8 is skipped), otherwise, the step 5.8 is directly skipped).

And is

5.8) judging whether all sampling positions are finished. If not, h is h +1, jump to step 5.2), otherwise output the positions of all valley points.

6) Let n be 1, and convert the electric eddy current testing data after data preprocessing into time domain electric eddy current testing data according to the sampling time of 23 different types of data

Wherein, t_kThe sampling time of the sampling point k is expressed in 1 msec. The 23 points are used as interpolation nodes, and each interpolation node is used as an unknown function

And the derivative value f' (t) of the interpolation node is obtained_k). According to two adjacent interpolation nodesAnd its derivative value, [ t ]_k,t_k+1]The interval is divided into a plurality of time points with intervals of 0.001 second, and each time point is interpolated by adopting the following segmented cubic Hermit interpolation formula (6) to obtain a signal to be decomposed.

Wherein f is_k(t) represents the function value inserted at the current time t.ψ_k(t) and ψ_k+1(t) can be expressed as:

7) searching local extreme points of the signal to be decomposed, connecting two adjacent extreme points, and searching the middle point F of the adjacent extreme points_p. Supplementing the midpoint F of the boundary point between the initial point and the first extreme point according to the initial point and the end point of the signal to be decomposed₀The middle point F of the boundary point between the end point of the supplement and the last extreme point_eAll midpoints are obtained. Constructing interpolation curves among 3 different midpoints for 3p +1, 3p +2 and 3(p +1) (p is 0,1, 2.,) by using formula (6), wherein p represents a midpoint classification serial number, and calculating a mean value l of the 3 interpolation curves_n。

8) Subtracting l from the signal to be decomposed_nAn IMF was obtained.

c_n(t)＝f_D(t)-l_n(11)

Wherein f is_D(t) represents the signal to be decomposed at the current time t, c_n(t) represents the nth IMF. Subtracting the component c from the original signal_n(t), obtaining a new original signal.

r_n(t)＝f_D(t)-c_n(t) (12)

Wherein r is_n(t) represents the original signal after the nth processing. Let the current signal to be decomposed f_D(t) is r_n(t)。

9) If the residual signal r_n(t) is a monotonous signal or the value is smaller than the preset threshold, the original signal f (t) can be expressed as formula (13), and the currently obtained n IMF components c are output₁(t)，c₂(t),...c_n(t), otherwise, n is n +1, and the step 7) is skipped again to obtain the next IMF component.

10) Performing Hilbert transform on the first 3 IMF components, calculating instantaneous amplitude, instantaneous phase, instantaneous frequency and marginal spectrum, and selecting 6 characteristic parameters such as instantaneous amplitude variance and marginal spectrum bandwidth of a first component IMF1, mean value and instantaneous amplitude variance of a second component IMF2, instantaneous amplitude variance and instantaneous phase variance of a third component IMF 3; the method for calculating the instantaneous amplitude, the instantaneous phase, the instantaneous amplitude and the marginal spectrum comprises the following steps:

10.1) dividing each IMF component c by equation (14)_i(t) performing a Hilbert transform to obtain a transformed component

10.2) calculating the instantaneous amplitude a by equation (15)_i(t)：

10.3) calculating instantaneous phase θ by equation (16)_i(t):

10.4) calculating the instantaneous frequency w by equation (17)_i(t)：

10.5) calculating the marginal spectrum of the eddy current test data by equation (18):

wherein h (w, t) represents the Hilbert spectrum of time-frequency distribution, namely

Where Re represents the real part of the function.

11) If the current state is not the training state, jumping to step 12), otherwise adopting the Gaussian kernel based function K (χ)_J,χ_I) The SVM algorithm trains the selected characteristic parameter samples to obtain a Lagrange multiplier lambda of an SVM algorithm model based on a Gaussian kernel function_qBias, etc. And (3) constructing 6 classifiers according to the trained parameters by a formula (21), respectively judging the defects of 6 different types, setting the current state as the recognition state, and jumping to the step 2). Wherein the Gaussian kernel function K (χ)_J,χ_I) Can be expressed as

K(χ_J,χ_I)＝exp(-γ×||χ_J-χ_I||) (20)

Wherein, K (χ)_J,χ_I) Representing the Gaussian kernel function,. chi_J,χ_IRepresenting a characteristic parameter sample, gamma is a Gaussian kernel parameter, and | | represents a norm.

Wherein P (x) represents a decision function, N represents the number of input characteristic values, b represents a bias, and λ_qRepresents the optimal solution of the lagrange multiplier after training and satisfies the condition that lambda is more than or equal to 0_qC ≦ C, C denotes a penalty factor, Y_qAnd representing the label corresponding to the characteristic sample.

12) And identifying the defects according to the 6 classifiers trained by the SVM and the extracted 6 characteristic parameters, and if the defects exist, recording the defect type and the depth information. And if the detection is not finished, jumping to the step 2), otherwise, outputting the types and depth information of the defects of the currently detected weak magnetic oil pipe double-layer pipe column.

In summary, compared with the prior art, the invention provides a preprocessing method for eddy current detection data, which can well remove data linear offset caused by the eccentricity of a weak magnetic oil pipe; decomposing the preprocessed data by improving a pole symmetric modal decomposition method based on a segmented thrice Hermit interpolation method, ensuring monotonicity of interpolation nodes and obtaining a plurality of Intrinsic Mode Functions (IMF); secondly, analyzing the obtained IMF components by using a Hilbert transform method, extracting 6 characteristic parameters such as instantaneous amplitude variance and marginal spectrum variance of the first component IMF1, mean value and instantaneous amplitude variance of the second component IMF2 and instantaneous amplitude variance and instantaneous phase variance of the third component IMF3, well representing 6 defects such as 2 × 90 extrusion, double-sided extrusion, single-sided extrusion, 3 × 90 extrusion, 4 × 90 extrusion and bending, and finally training and identifying the characteristic parameters by using an SVM model based on a Gaussian kernel function to avoid the problem of overfitting; the method can better identify 6 types of defects of 2 multiplied by 90 extrusion, double-sided extrusion, single-sided extrusion, 3 multiplied by 90 extrusion, 4 multiplied by 90 extrusion and bending, and improves the identification rate of each type of defects and the average identification rate of the defects.

In this specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.