Synthetic seismic record calibration is the basis of structural interpretation and reservoir prediction.The accuracy of synthetic seismic record calibration directly determines the accuracy of subsequent structural interpretation and reservoir prediction,and only when the polarity of the corresponding seismic section is accurately determined,the calibration can be accurate.Therefore,determining the polarity of seismic data is the most basic work.In practical production applications,the conventional method which uses the positive and negative polar waves to produce the synthetic seismic records and then compare the correspondence between the two and the seismic trace often fails to determine the polarity of the seismic profile.In this paper,the polarity determination of the Sulige gas field was taken as an example and,according to the principle of phase scanning,the forward modeling and formula derivation were conducted.The results show that the phase of the seismic profile and phase correction quantities are opposite.In the actual data analysis,the phase correction can be obtained by using the phase scanning to compare the well impedance and the relative impedance of the seismic traces nearby the borehole,and then the phase of the actual seismic profile can be obtained to determine the polarity of the seismic profile.The method is intuitive and has good operation and popularization in practical application.
In seismic data interpretation, the calibration of synthetic seismic records is the basis for structural interpretation and reservoir prediction. The accuracy of the calibration results of synthetic seismic records directly determines the accuracy of subsequent structural interpretation and reservoir prediction. Then there is a thousand miles [ 1 , 2 ] . It is not uncommon to calibrate and use the polarity of the two (wavelet and seismic section) oppositely due to the inability to recognize the polarity of the seismic section [ 3 , 4 ] . To make an accurate synthetic seismic record, the polarity of the seismic section must be clearly defined, because it is related to the polarity of the used wavelet and the accuracy and precision of the synthetic seismic record [ 5 ] , and in well-seismic calibration, the polarity of seismic data The judgment is the most basic work and the precondition of reservoir prediction research [ 6 ] . Only by accurately judging the polarity of the wavelet corresponding to the seismic section, can the top and bottom of the reservoir be accurately matched with the peaks and troughs on the seismic section, and the reservoir can be accurately understood through the study of the seismic section and its polarity. Different understandings will lead to different interpretations and prediction results [ 7,8 ] . Like the seismic section, the seismic wavelet is also polarized. It can be considered that the polarity of the seismic section is the polarity of the seismic wavelet. Therefore, the polarity of the seismic wavelet needs to be determined according to the polarity of the seismic section next to the well, that is, the positive polarity The profile is calibrated by positive polarity wavelet, and the negative polarity profile is calibrated by negative polarity wavelet [ 9 ] .
Since the polarity of seismic data is often changed due to acquisition or processing [ 8 ] , it is very important for seismic data interpreters to find out the polarity of seismic data, and its importance has also been discussed above. Therefore, no matter how the polarity of the seismic data has changed, interpreters must use various means to identify the polarity of the seismic data. It was stipulated in the 42nd SEG Annual Meeting in 1975 [ 10 ] that the unified standard for field records is that the first arrival wave takes off downward and the recorded value is negative, which is called SEG normal record, and the polarity problem inside the seismic record has not been involved here. In 1994, Han Wengong [ 11 ] made a clear explanation of the polarity, and defined the concepts of positive and negative polarity of the seismic section. That is, the SEG reverses the normal polarity.
Around the discrimination of the polarity of the seismic section, some methods have been proposed [ 3 , 8 , 12 - 13 ] , including the acoustic wave synthetic seismogram method, the "single and double track" section discrimination method, the extraction wavelet discrimination method, and the reverse calculation method of the synthetic seismogram. The average velocity method, the model discrimination method, and the method of determining the polarity using VSP logging velocity data. The common feature of different methods is that the residual wavelet on the processing result section is assumed to be zero phase. These methods are all derived from relatively strict assumptions and data requirements. , it is difficult to be widely used in actual production.
The polarity judgment of conventional seismic data generally uses positive polarity wavelet and negative polarity wavelet to make synthetic seismic records respectively, and then compares the corresponding relationship between the two and seismic traces. Corresponding wavelets, it is usually expressed that the seismic section corresponding to positive polarity wavelet is positive polarity seismic data, and the section corresponding to negative polarity wavelet is negative polarity seismic data [ 14 ] . But when faced with actual data, it is difficult to use this method to determine the polarity of seismic data [ 4 , 12 , 15 ] , the reason is that the phase of actual seismic data is often not completely on 0° or 180°, but often in the middle a certain phase. This time, the phase scanning method is used to judge the polarity of the seismic section by comparing the correlation between the well impedance and the relative impedance of the side channel inversion, and the phase of the seismic section. This method is intuitive and has good operation and compatibility in practical applications. Promotional.
1 Determination of polarity of seismic section
The author first judges the polarity of the seismic section on the basis of determining the phase of the seismic section. As for the method of determining the phase, scholars at home and abroad mostly use the phase scanning method to determine the phase of the seismic section, because the amplitude and waveform are compared in the time domain [ 16 , 17 ] . This time, the conversion of the seismic section into an impedance section is directly compared with the uphole wave impedance, which is very intuitive and operable.
1.1 Principle of phase scanning method
The principle of the phase scanning method [ 20 ] is to directly compare the logging curve with the phase scanning side-by-hole inversion trace, and the purpose is to make the inversion trace and the logging curve have good comparability. The specific method is: do a phase conversion filter every 30° for the inversion relative velocity trace beside the well. Arrange the phase scanning traces and display them on a piece of paper, and then use the acoustic logging curve filtered by the seismic frequency band to compare with the bypass traces at each phase angle to find the bypass traces most similar to the logging curves , the phase angle corresponding to it is the phase correction value to be found. As shown in Fig. 1 , the side-hole inversion trace with 60° phase correction has the best comparability with the acoustic logging curve, and this 60° is the phase correction value we want to determine.
1.2 Formula derivation
The corresponding Hilbert transform of the seismic trace with known fixed phase θ is:
The Hilbert transform corresponding to the zero-phase seismic trace is:
To transform it into a zero-phase seismic trace, it is only necessary to rotate Z with a certain angle of phase, that is, phase correction. Let the rotation angle (phase correction amount) be θ a , then:
From formula (1), (2), (3) get:
From formula (4) get:
Equation (5) shows that the phase correction amount is the opposite number of the seismic trace phase. Where: A is the instantaneous amplitude.
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