CN112764090A - Seismic source nonlinear scanning signal design method based on forced vibration - Google Patents
Seismic source nonlinear scanning signal design method based on forced vibration Download PDFInfo
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Abstract
The invention relates to a seismic source nonlinear scanning signal design method based on forced vibration, and belongs to the technical field of geophysical exploration. The method considers the actual surface condition, determines the target frequency-energy value by utilizing the vibration characteristic information of the near-surface forced vibration system when the vibroseis is excited by scanning, further determines the frequency change rate, the instantaneous frequency, the instantaneous phase and the instantaneous output data of the nonlinear scanning signal, and realizes the design of the vibroseis nonlinear scanning signal. The method can effectively adapt to different earth surfaces, enhances the construction adaptability of the controllable seismic source in a complex environment, effectively improves the signal-to-noise ratio of seismic source construction data, and is closer to practical production application.
Description
Technical Field
The invention relates to a seismic source nonlinear scanning signal design method based on forced vibration, and belongs to the technical field of geophysical exploration.
Background
In the vibroseis seismic data acquisition construction, a scanning signal drives a vibration system to excite seismic waves through a vibroseis electric control system and a hydraulic control system, and the scanning signal is one of key factors influencing the quality of the seismic waves. The seismic waves are transmitted to a ground instrument acquisition system through the earth, and are correlated through scanning signals to form single shot records. The obtained seismic record frequency spectrum is narrowed and biased to low frequency under the filtering action of a detector, an instrument, earth absorption attenuation and the like, and the influence cannot be changed artificially.
The vibroseis scanning signals are linear and nonlinear, wherein the linear scanning signals are most widely used Chirp signals, the scanning frequency of the signals is uniform, and the scanning energy is evenly distributed. The linear scanning signals used by the controllable seismic source cannot output energy with enough intensity in a low-frequency part (below 5 Hz) due to the limitation of a mechanical-hydraulic system, and even if a high driving amplitude is designed, the distortion of the part is high, so that the fundamental wave output value with practical production significance is still low, and the low-frequency exploration with real significance cannot be realized. The non-linear scanning signal is originally provided aiming at the non-linear characteristic of seismic wave absorption attenuation, and the Goupil-law repetition point is used for researching the characteristic of an autocorrelation function aiming at the non-linear scanning signal; zhang hongle et al provide a scanning signal for improving the characteristics of related wavelets, mainly emphasizes the energy near the dominant frequency; the Cao takes good care of carrying out attenuation compensation optimization on the exponential or logarithmic scanning signals by utilizing the quantitative analysis of the non-line scanning factors and the spectral characteristics of the main target layer sections; the blue-plus technique proposes a sweep rate energy-dependent sweep technique that improves seismic resolution by increasing sweep time in the high frequency band. The method mainly compensates high frequency, generally, exponential or logarithmic scanning signal design is carried out according to ideal wavelets or earth absorption attenuation curves, Wanghuazhou and the like propose the amplitude spectrum characteristics of the target layer reflection wavelets, and different scanning modes are simultaneously excited, so that target layer reflection waves are superposed in the same direction, and reflection seismic signals with high signal-to-noise ratio and wide spectrum are obtained. Wefff and et al propose a method for designing a vibroseis nonlinear scanning signal based on a target stratum frequency spectrum, and enhance the energy of a target stratum dominant frequency band range by utilizing nonlinear scanning.
The earth surface type is not considered in the current nonlinear scanning signal design, so that the designed nonlinear scanning signal cannot adapt to the actual earth surface type, and the quality of the seismic signal is influenced.
Disclosure of Invention
The invention aims to provide a seismic source nonlinear scanning signal design method based on forced vibration, and aims to solve the problem that the accuracy of signal design is poor due to the fact that the ground surface condition is not considered in the nonlinear scanning signal designed at present.
The invention provides a seismic source nonlinear scanning signal design method based on forced vibration for solving the technical problems, which comprises the following steps:
1) obtaining vibration characteristic information of a near-surface forced vibration system when a vibroseis is excited by scanning, wherein the vibration characteristic information is represented by frequency-energy data;
2) determining resonance frequency and dominant frequency band according to the vibration characteristic information of the near-surface forced vibration system;
3) taking the dominant frequency band as an enhanced scanning energy frequency band, and taking 1.5-2 times of the maximum output value of the controllable seismic source as an energy design value of the frequency band;
calculating the ratio of the resonance frequency amplitude value determined in the step 2) to the energy design value of the reinforced scanning energy band, dividing the frequency-energy data obtained in the step 1) by the ratio to obtain initial target frequency-energy data, uniformly modifying the energy value of the dominant frequency band, namely the reinforced scanning frequency band, into the energy design value, and keeping the energy value after proportional conversion unchanged in the low-frequency part of the non-reinforced scanning frequency band; modifying the energy value of the middle-high frequency part of the non-reinforced scanning frequency band into the designed maximum instantaneous force of the controllable seismic source, and fitting the discrete energy value in the full frequency band, wherein the fitting result is the target frequency-energy value;
4) and calculating the frequency change rate, instantaneous frequency, instantaneous phase and instantaneous output data of the nonlinear scanning signal according to the target frequency-energy value and the performance parameters of the vibroseis vibration system, and completing the design of the vibroseis nonlinear scanning signal based on forced vibration.
The method considers the actual surface condition, determines the target frequency-energy value by utilizing the vibration characteristic information of the near-surface forced vibration system when the vibroseis is excited by scanning, further determines the frequency change rate, the instantaneous frequency, the instantaneous phase and the instantaneous output data of the nonlinear scanning signal, and realizes the design of the vibroseis nonlinear scanning signal. The method can effectively adapt to different ground surfaces, enhances the construction adaptability of the controllable seismic source in a complex environment, effectively improves the signal-to-noise ratio of seismic source construction data, and is closer to practical production application.
Further, in order to quickly and accurately acquire the vibration characteristic information of the near-surface forced vibration system, the vibration characteristic information in the step 1) is obtained by adopting a single-frequency scanning mode: and (3) counting the amplitude attribute of the first arrival of the direct wave of the receiving channel closest to the seismic source or the receiving channel with the minimum offset distance in each single-frequency excitation seismic record, and establishing a fitting curve between the single-frequency scanning frequency and the statistical amplitude attribute, wherein the fitting curve is the frequency-energy data to be obtained.
Further, in order to quickly and accurately acquire the vibration characteristic information of the near-surface forced vibration system, the vibration characteristic information in the step 1) is obtained by adopting a cannon data mode: carrying out linear dynamic correction on the single shot record by using first arrival pickup, and correcting the first arrival waves to the same moment; analyzing an amplitude spectrum of the first arrival information analysis of the direct waves near the seismic source in the linear dynamic correction seismic records of different single shots, wherein the result is frequency-energy data; the method comprises the steps that receiving channels in a certain offset range from a seismic source in the arrangement are received by the near-seismic source, and the direct wave first arrival information is 1 complete period waveform seismic record covering first arrival waves.
Further, the energy in the frequency-energy data is a root mean square amplitude.
Further, the energy value of the non-emphasized scanning energy band range among the discrete energy values in the entire frequency band range is converted into an amplitude value or a theoretical energy value, which is related to the designed driving amplitude.
Further, the fitting method in the step 3) is a smooth spline fitting or a gaussian fitting method.
Further, the calculation formula of the frequency change rate of the nonlinear scanning signal is as follows:
SR(f)＝β·[Fconstraint(f)/Ftarget(f)]2(Fe-Fs)/SL
Fconstraint(f) is the vibroseis frequency-upper output limit; ftarget(f) Is a target frequency-energy value; feIs the scan cut-off frequency; fsIs the scanning starting frequency; SL is scanning duration; beta is a scale factor; SR (f) is the rate of frequency change; f is the instantaneous frequency.
Drawings
FIG. 1 is a flow chart of a method of forced vibration based seismic source nonlinear sweep signal design according to the present invention;
FIG. 2 is a schematic diagram of amplitude A of forced vibration versus driving force frequency f;
FIG. 3 is a simplified physical model diagram of a trapping body under forced vibration conditions;
FIG. 4 is a schematic diagram of the output limiting factor of the vibroseis at different frequencies;
FIG. 5 is a diagram showing the statistical results of single-frequency excitation frequency-energy (root mean square amplitude) on the near-surface of a grassland in an embodiment of the present invention (offset 30m, time 0-200 ms);
FIG. 6 is a schematic diagram of a scanning energy enhancement section for the grassland near-surface resonance frequency in the embodiment of the invention;
FIG. 7 is a graph of target frequency versus energy values obtained by smooth spline fitting for a grassland near-surface type in an embodiment of the present invention;
FIG. 8 is a schematic diagram of the upper limit of instantaneous output of a Nomad65 neo-type vibroseis (1-20Hz, design maximum drive amplitude 75%) according to an embodiment of the present invention;
FIG. 9 is a graph of the rate of change of frequency obtained in an embodiment of the present invention;
FIG. 10 is a time-instantaneous frequency curve of a non-linear sweep signal obtained in an embodiment of the present invention;
FIG. 11 is a time-instantaneous phase curve of a non-linear scanning signal obtained in an embodiment of the present invention;
FIG. 12 is a time-instantaneous output curve of a non-linear scan signal obtained in an embodiment of the present invention;
FIG. 13-a is a time-frequency curve of a linear sweep signal in an exemplary embodiment of the present invention;
FIG. 13-b is a time-output curve of a linear scan signal according to an exemplary embodiment of the present invention;
FIG. 13-c is a graph of linear sweep signal energy versus frequency for a validation example of the present invention;
FIG. 13-d is a time-amplitude curve of wavelets associated with a linear scanning signal in accordance with an illustrative embodiment of the present invention;
FIG. 14-a is a time-frequency curve of a non-linear sweep signal in an exemplary embodiment of the present invention;
FIG. 14-b is a time-output curve of a non-linear scan signal according to an exemplary embodiment of the present invention;
FIG. 14-c is a non-linear sweep signal energy versus frequency curve in an exemplary embodiment of the present invention;
FIG. 14-d is a time-amplitude curve of a wavelet associated with a non-linear scanning signal in an exemplary embodiment of the present invention;
FIG. 15 is a schematic diagram showing the correlation of wavelets obtained from different scanning modes.
Detailed Description
The following further describes embodiments of the present invention with reference to the drawings.
As shown in fig. 2, forced vibration is that the vibration system is under the action of a periodic external force, the generated vibration is called forced vibration, the resonance frequency is an inherent property of the forced vibration system, and forced vibration has the following regular characteristics:
when f isDriving device＝fFixing deviceWhen the system reaches the resonance state A ═ Am；fDriving deviceAnd fFixing deviceThe closer together, the greater the amplitude of the forced vibration; f. ofDriving deviceAnd fFixing deviceThe further the phase difference, the smaller the amplitude of the forced vibration; when resonance occurs, in a period, the energy provided by the outside is equal to the energy consumed by the system overcoming the resistance to work.
The part of the controllable seismic source directly acted by the flat plate in the vibration process is used as a capturing body to synchronously vibrate along with the flat plate, so that the purpose of transmitting the mechanical vibration energy of the controllable seismic source to an underground medium is achieved, and finally, the transmission of elastic waves required by seismic exploration is formed. "Capture" and deep geologic body are formed by an approximate elastic-damping system (K)g，Dg) The connection can be considered to constitute a simple vibration system. It is forced to vibrate under the action of the vibroseis, as shown in figure 3.
According to the characteristics of the forced vibration system, when the driving frequency of the controllable seismic source reaches the resonance frequency of the forced vibration system in the graph 2, the response amplitude of the capture body reaches the maximum, and meanwhile, the output efficiency of the controllable seismic source energy to the underground medium reaches the maximum.
According to the method, the amplitude-frequency characteristic and the time-frequency characteristic of the signal are optimally designed according to a nonlinear scanning signal design theory, the energy output of the seismic source in the range of the resonant frequency and the dominant frequency band is increased by utilizing the self-defined nonlinear scanning signal, and the adaptability and the output efficiency of the controllable seismic source to various complex earth surface environments are improved.
The vibroseis sweep signal is a set of instantaneous amplitude data with respect to time, the instantaneous amplitude S (t) being derived from the instantaneous phase
And the output function A (t) as shown in equation (1).
The instantaneous phase can be calculated from the instantaneous frequency, which is the time domain integral of the rate of change of frequency. The central problem in nonlinear sweep signal design is therefore how to determine the frequency rate of change function that is satisfied while ensuring that the instantaneous output does not exceed the upper performance limit of the vibroseis. The rate of change of frequency is expressed as shown in equation (2):
SR(f)＝β·[Fconstraint(f)/Ftarget(f)]2(Fe-Fs)/SL (2)
Fconstraint(f) is the vibroseis frequency-upper output limit; ftarget(f) Is a target frequency-energy value; feIs the scan cut-off frequency; fsIs the scanning starting frequency; SL is scanning duration; beta is a scale factor; SR (f) is the rate of frequency change; f is the instantaneous frequency.
The vibroseis is influenced by a mechanical structure and a hydraulic system of the vibroseis, and the process of executing scanning signals is limited by a curve in fig. 4, namely the instantaneous output corresponding to any instantaneous frequency of the scanning signals cannot exceed the output limit (the frequency of the vibroseis-the upper output limit) of the vibroseis. The limit includes a displacement limit F caused by the displacement limit of the weight in the low frequency banddisp(f) Hydraulic system pump flow limit Fflow(f) Medium and high frequency range artificially set maximum driveAmplitude Fset(f)。
The target frequency-energy value is a group of energy value data which are designed according to the resonance frequency and the dominant frequency band of the forced vibration system and continuously change along with the increment or the decrement of the instantaneous frequency, the meaning of the energy value in the group of energy value is the accumulated total excitation energy of the controllable seismic source at a certain frequency, and the target frequency-energy value at any frequency can be larger than the instantaneous output upper limit of the controllable seismic source due to the fact that the meaning of the energy value is different from that of the instantaneous output. In the signal design process, the target frequency-energy value directly determines the energy spectrum curve form of the nonlinear scanning signal.
The scaling factor is a fixed constant, and ensures that the final frequency calculated by using the current frequency change rate reaches the designed cut-off frequency.
The rate of change of frequency is the first derivative of the instantaneous frequency with respect to time. For a linear scanning signal, the rate is a fixed constant; for non-linear scanning signals, the rate is a variable that varies continuously over time. According to the formula (2), the method can be obtained by calculating parameters such as a vibroseis parameter, a target frequency-energy value, a starting frequency, a cut-off frequency, a scanning length and the like.
Based on the analysis, the invention provides a design method of a vibroseis nonlinear scanning signal. The method comprises the steps that by means of the resonance frequency characteristic of a near-surface elastic-damping system, firstly, the vibration characteristic information of a near-surface forced vibration system during scanning excitation of a vibroseis is obtained, and the resonance frequency and the dominant frequency band are determined according to the vibration characteristic information of the near-surface forced vibration system; then designing a target frequency-energy value of the near-surface forced vibration system according to the determined resonance frequency and the dominant frequency band; and finally, calculating the frequency change rate of the nonlinear scanning signal according to the target frequency-energy value, the performance parameters of the vibroseis vibration system and the set scanning parameters, and further calculating instantaneous frequency, instantaneous phase and instantaneous output data to realize the design of the vibroseis nonlinear scanning signal. The implementation flow of the method is shown in fig. 1, and the specific implementation process is as follows.
1. And obtaining the vibration characteristic information of the near-surface forced vibration system when the vibroseis is excited by scanning.
According to the forced vibration system in fig. 3, the resonant frequency and the dominant frequency band property can be obtained by analyzing the vibration characteristic information received by the detector when the vibroseis is excited by scanning. The acquisition mode generally adopts a single-frequency scanning recording mode, and actual single shot data can be simplified and adopted as supplement in production.
1) Single frequency scanning mode: aiming at a certain specific near-surface condition, the controllable seismic sources adopt single-frequency one-by-one scanning excitation and conventional arrangement receiving to obtain seismic records excited by the single frequency. The single frequency point generally covers the scanning excitation frequency range during the conventional production of the earth surface, and each frequency is scanned once in a single frequency mode. And after all single-frequency point scanning excitation is finished, acquiring seismic records corresponding to all frequency points of the frequency band. The single-frequency scanning driving amplitude parameter is obtained through tests, and the safety of equipment and the distortion are not over standard.
For example, a series of single frequency scans at 1Hz, 2Hz...96Hz are performed on the vibroseis. The scan length, ramp length and force parameters are shown in table 1. The maximum driving amplitude of the controllable seismic source with the low frequency range of 1-20Hz is changed rapidly, so that the frequency increment is set to be 1Hz for ensuring the accuracy of an analysis result; the maximum driving amplitude of the medium-high frequency range controllable seismic source is stable, and the frequency increment is set to be 3 Hz; the driving amplitude is not more than the system limit as the upper limit in a low frequency range (such as a Nomad65neo seismic source is 1-7 Hz), and the driving amplitude designed in a medium and high frequency range has three types of 50%, 60% and 70% of the maximum output of the controllable seismic source.
TABLE 1
2) And (3) blasting data: aiming at a certain specific near-surface condition, a controllable seismic source adopts production parameter scanning excitation and conventional arrangement receiving to obtain a seismic record of actual scanning excitation; the scanning frequency range is generally not less than 5 octaves, and in order to eliminate errors caused by contingency, a certain amount of single shot data under a certain specific near-surface condition needs to be selected, and the number of the single shot data is generally not less than 5.
For example, the controllable seismic source is excited by linear scanning signals, and the adopted parameters are scanning length 16s, scanning frequency range 3Hz-96Hz, slope length 1s and output parameter 70%.
2. And determining the resonance frequency and the dominant frequency band according to the vibration characteristic information of the near-surface forced vibration system.
By analyzing seismic records and production cannon data obtained by single-frequency scanning, the resonance frequency and the dominant frequency band of the forced vibration system under different near-surface conditions can be determined. In order to eliminate the influence of medium-deep factors such as refracted waves, reflected waves, absorption attenuation effects and the like, the energy effect of seismic waves generated by the near-surface contacting with the flat plate when the controllable seismic source is excited is better analyzed, and the near-surface vibration characteristics are analyzed by selecting the first arrival information of direct waves near the seismic source.
1) Single frequency scanning mode: the single-frequency scanning seismic record under a certain near-surface condition is subjected to statistics on the root-mean-square amplitude of the first arrival information of the direct waves near the seismic source in the single-frequency excitation seismic record one by one, and a series of frequency-energy (root-mean-square amplitude) statistical data of the near-surface condition are obtained. The term "near the seismic source" refers to the reception of the receiving channel closest to the seismic source or with the smallest offset distance in the arrangement, but avoids the strong interference of the seismic point, and the term "direct wave first arrival information" refers to the 1 complete cycle waveform record covering the first arrival. And the analysis time window offset range is fixed during statistics and is not changed along with the change of the single-frequency excitation seismic record. And sequencing a series of statistical data of energy (root-mean-square amplitude) under a certain specific near-surface condition obtained by statistics according to the ascending sequence of the corresponding single-frequency scanning frequency, and calculating a fitting curve with the maximum correlation coefficient with the statistical amplitude. The frequency corresponding to the maximum value of the curve is the resonance frequency of the near-surface forced vibration system, and the frequency range of the main energy is the dominant frequency band.
2) And (3) blasting data: aiming at a plurality of actual shot data under a certain specific near-surface condition, firstly, linear motion correction is carried out on a single shot record by utilizing first arrival pickup, and first arrival waves are corrected to the same moment. And analyzing the amplitude spectrum of the first arrival information analysis of the direct waves near the seismic source in the linear dynamic correction seismic records of different single shots, and storing the result as frequency-energy (root-mean-square amplitude) data. And (3) taking the frequency-energy (root-mean-square amplitude) data envelopes of the single shots with different near-surface conditions as final frequency-energy (root-mean-square amplitude) statistical data under the conditions. The term "near the seismic source" refers to the reception of the reception trace in the arrangement within a certain offset range from the seismic source, and the term "direct wave first arrival information" refers to the 1 complete period waveform seismic record covering the first arrival. The analysis time window offset range is fixed during statistics and is not changed along with the actual single-shot data replacement. And (3) analyzing the obtained apparent main frequency of the first arrival information amplitude spectrum of the direct wave near the seismic source under a certain near-surface condition, namely the resonance frequency of the near-surface forced vibration system, wherein the frequency range of the main energy is the dominant frequency band. The actual width of the dominant band may be determined according to actual conditions, for example, a band range in which 90% of energy is located may be selected.
In this embodiment, a single-frequency scanning mode of the grassland surface is adopted, and the first arrival information of the direct waves near the seismic source, such as the seismic records with the offset of 30m, is selected as the analysis object, and the frequency-energy (root-mean-square amplitude) statistical result and the fitting curve of the single-frequency scanning are shown in fig. 5. It can be seen that in the medium and high frequency band, when the earth surface near the vibroseis flat plate is driven by external forces with different frequencies, the response amplitude near 23Hz reaches the maximum value, the response amplitude of 10-50Hz is obviously higher than other frequency ranges, the 23Hz is considered as the resonance frequency of the elastic-damping system near the surface of the grassland, and the 10-50Hz is considered as the dominant frequency band range.
3. And designing a target frequency-energy value of the near-surface forced vibration system according to the determined resonance frequency and the dominant frequency band.
The frequency-energy (root mean square amplitude) statistics contain the resonance frequency and dominant band information of the near-surface forced vibration system, which can only be used for nonlinear sweep signal design if it is converted to "target frequency-energy value" suitable for different near-surface conditions according to equation (2).
In the first step, the frequency band range of the enhancement scan energy and its energy design value are determined.
The dominant frequency band is used as an enhanced scanning energy frequency band (the invention enhances the energy of the scanning signal in the frequency band by changing the time-frequency characteristic of the scanning signal, namely prolonging the scanning time without changing the magnitude of the output force), and 1.5 to 2 times of the maximum instantaneous output value of the controllable seismic source is used as a design value of the enhanced scanning energy of the frequency band.
And secondly, modifying the frequency-energy (root-mean-square amplitude) data of a certain specific near-surface obtained by statistics into target frequency-energy data.
And calculating the ratio of the resonance frequency amplitude value to the design value of the reinforcement scan energy, and dividing the frequency-energy data in the vibration characteristic information by the ratio to obtain initial target frequency-energy data. The energy values of the dominant band, i.e. the enhancement scan band, are uniformly modified to the energy design values. Keeping the energy value after proportional conversion unchanged in the low-frequency part of the non-reinforced scanning frequency band; and in the middle-high frequency part of the non-reinforced scanning frequency band, the energy value is modified into the driving amplitude of the designed controllable seismic source. The frequency-energy data is discrete energy values for each frequency point over the entire frequency band.
And thirdly, fitting the discrete target frequency-energy value.
Since the discrete energy values of the frequency points are obtained in the last step, and the calculated frequency change rate of the nonlinear scanning signal is directed at the continuously changing frequency data, the discrete frequency-energy data needs to be fitted, and the target frequency-energy value of the continuous change is calculated through the fitting. The fitting can adopt a smooth spline fitting mode or a Gaussian fitting mode and the like.
In the present example, as can be seen from fig. 5, the resonance frequency of the near-surface of the grassland is 23Hz, and the dominant frequency band is 10 to 50Hz, as shown in fig. 6. The dominant band range is defined as the emphasis scan energy band.
The specific energy enhancing method for the enhanced scanning energy band is as follows: firstly, manually modifying the amplitude scatter data counted in the step (2); modifying the amplitude value of 10-50Hz, adjusting the amplitude of 50-96Hz to the driving amplitude of the designed controllable seismic source (for example, the driving amplitude is 200,550N according to the 75% driving amplitude of the Nomad65neo type controllable seismic source), keeping the amplitude of 1-10Hz unchanged, and taking the black scattered points in the graph 7 as the modified frequency-energy data; and then, carrying out curve fitting on the adjusted scattered point data by using Matlab software, wherein the fitting result is shown as a curve in FIG. 7 by using a smooth spline.
4. And calculating the frequency change rate, instantaneous frequency, instantaneous phase and instantaneous output data of the nonlinear scanning signal to realize the design of the nonlinear scanning signal.
According to the design theory of the nonlinear scanning signals, the frequency change rate of the nonlinear scanning signals is calculated by using the target frequency-energy value of a certain near-surface type, the performance parameters of the vibroseis vibration system and the set scanning parameters determined in the third step, and then the instantaneous frequency, the instantaneous phase and the instantaneous output data are determined. The specific implementation flow is as follows.
1) And solving the upper limit of the instantaneous output value of the controllable seismic source according to the seismic source parameters, the designed driving amplitude and the scanning parameters.
2) And (3) calculating the frequency change rate aiming at a specific near-surface condition according to a formula (2), wherein a target frequency-energy value is obtained in the step (3), the frequency change rate continuously changes along with the frequency, the numerical value of the target frequency band is smaller, namely the total energy of the scanning signal in the frequency band is increased through the upper limit of the instantaneous output value of the controllable seismic source and the long scanning time, the numerical value outside the target frequency band is higher, and the output energy of the controllable seismic source in the low-frequency and high-frequency ranges is reduced.
3) Calculating time-instantaneous output data for the nonlinear sweep signal under a particular near-surface condition according to equations (1) and (3): the instantaneous frequency is obtained by frequency change rate integral, then the instantaneous phase is calculated by the instantaneous frequency, the instantaneous output is determined by the ramp factor and the upper limit F of the instantaneous output value of the controllable seismic sourceconstrain(f) And (6) calculating.
A(t)＝W(t)Fconstrain(f) (3)
4) And converting the time-instantaneous output data aiming at a specific near-surface type into a text file with a corresponding format according to the requirements of the vibroseis to be imported.
In this embodiment, the results of fitting the grassland near-surface, Nomad65neo vibroseis and smooth spline are selected, and the corresponding calculation processes of the frequency change rate, instantaneous frequency, instantaneous phase and instantaneous output data of the nonlinear scanning signal are as follows:
in a first step, an upper limit of the instantaneous force output value is determined.
Setting a non-linear scanning signal start frequency Fs3Hz, cut-off frequency Fe96Hz, the sweep length SL 16s, the ramp type cosine ramp, the ramp length TL 1s, the time sampling interval dt 0.005s, and the frequency sampling interval df 0.05 Hz. The maximum static pressure weight F can be known from the Nomad65Neo vibroseis operation manualmaxMass of the weight Mr3500Kg, maximum weight displacement XmaxPump flow rate Qm3/smaxThe plate area Ap is 0.0133m2For protecting the seismic source, the maximum driving amplitude F is designedset＝0.75*FmaxThe distribution of the upper limit of the instantaneous output value of the seismic source of the model in the low frequency band (1-20Hz) is shown in FIG. 8.
In the second step, the frequency change rate SR (f) of the nonlinear scanning signal is generated.
It was determined through testing that β 1.995 can meet the design requirements when the frequency rate of change sr (f) is as shown in fig. 10.
And thirdly, generating time-instantaneous output data.
The time-instantaneous frequency change curve and the time-instantaneous phase change curve (as shown in fig. 10 and 11) of the non-linear scanning signal can be obtained by integrating the frequency change rate sr (f) in fig. 9 with time, and then the time-instantaneous output of the non-linear scanning signal can be obtained by calculating according to the formula (1) and the formula (6), and the result is shown in fig. 12.
And fourthly, converting the text file into a text file.
And converting the text file into a text file according to the determined results of the first three steps, wherein the results are shown in table 2.
TABLE 2
Verification case
To further illustrate the effect of the present invention, the following example is to compare the linear scanning and the non-linear scanning for production with the typical grassland surface of the inner Mongolia flag.
The surface is first tested with the linear sweep signals most commonly used in production. The linear sweep parameters were a start frequency of 3Hz, a cut-off frequency of 96Hz, a sweep length of 16s, and a ramp length of 1s, as shown in FIGS. 13-a, 13-b, 13-c, and 13-d. The excitation energy of the linear scanning signals is evenly distributed to different frequencies, the frequency ranges of the excitation energy consumed in the vibration process of the flat plate drive capture body under different surface types are different by adopting the scanning mode, and meanwhile, the effective energy ratio of the actually input deep underground medium is lower.
The nonlinear scanning parameter results obtained by the nonlinear scanning signal design method are shown in figures 14-a, 14-b, 14-c and 14-d, the scheme is that nonlinear scanning signals are designed by combining grassland surface resonance frequency and dominant frequency band distribution, compared with designed linear scanning signals, the nonlinear scanning signal design method strengthens target frequency band energy, reduces energy loss near the surface, and improves vibroseis excitation energy downloading efficiency and adaptability to different surface types.
The two seismic record related wavelet pairs with different scanning modes are shown in fig. 15, and it can be seen from the graph that the energy of the seismic record wavelet under nonlinear scanning is obviously stronger than that of linear scanning for production, which shows that the two aspects of time-frequency characteristic and amplitude-frequency characteristic of scanning signals are optimized simultaneously based on forced vibration characteristic, and the downloading efficiency of the excitation energy of the controllable seismic source can be effectively increased. Meanwhile, as the target frequency band of the nonlinear scanning enhanced energy covers the range of the effective wave frequency band, the nonlinear scanning can not only increase the energy of effective signals in seismic data to improve the signal-to-noise ratio, but also widen the effective wave frequency band and improve the resolution.
The invention can design and optimize self-defined nonlinear scanning signals aiming at different types of tables, and effectively improves the signal-to-noise ratio of the obtained data by enhancing the vibration energy of the effective frequency band and improving the downloading efficiency of the excitation energy. Compared with the traditional linear scanning signal and low-frequency-broadening nonlinear scanning signal design method, the scanning signal design method can effectively adapt to different earth surfaces, and enhances the construction adaptability of the controllable seismic source in a complex environment, so that the method is more scientific, reasonable, accurate and reliable, and has great guiding significance for improving the seismic exploration data quality and the geological effect.
The method can adapt to different types of tables to design corresponding scanning signals, and has stronger scientificity, accuracy, convenience and adaptability compared with the prior production using the same scanning signal. The technical method can effectively relieve the problem of poor signal-to-noise ratio of the obtained data in seismic source construction, is closer to practical production application, can accelerate the conversion application of scientific and technological achievements, and fully plays a supporting and leading role of scientific and technological innovation on enterprise development. With the continuous progress of geophysical prospecting technology, the requirement on the quality of seismic data is higher, and the method has unique technical advantages and very wide market application prospect in the face of complex and various surface conditions of domestic and foreign seismic exploration work areas.
Claims (7)
1. A method for designing a seismic source nonlinear scanning signal based on forced vibration is characterized by comprising the following steps:
1) obtaining vibration characteristic information of a near-surface forced vibration system when a vibroseis is excited by scanning, wherein the vibration characteristic information is represented by frequency-energy data;
2) determining resonance frequency and dominant frequency band according to the vibration characteristic information of the near-surface forced vibration system;
3) taking the dominant frequency band as an enhanced scanning energy frequency band, and taking 1.5-2 times of the maximum instantaneous output value of the controllable seismic source as an energy design value of the frequency band;
calculating the ratio of the resonance frequency amplitude value determined in the step 2) to the energy design value of the reinforced scanning energy band, dividing the frequency-energy data obtained in the step 1) by the ratio to obtain initial target frequency-energy data, uniformly modifying the energy value of the dominant frequency band, namely the reinforced scanning frequency band, into the energy design value, and keeping the energy value after proportional conversion unchanged in the low-frequency part of the non-reinforced scanning frequency band; modifying the energy value of the middle-high frequency part of the non-reinforced scanning frequency band into the designed maximum value of the instantaneous output of the controllable seismic source; finally, fitting the discrete energy value in the full frequency band, wherein the fitting result is the target frequency-energy value;
4) and calculating the frequency change rate, instantaneous frequency, instantaneous phase and instantaneous output data of the nonlinear scanning signal according to the target frequency-energy value and the performance parameters of the vibroseis vibration system, and completing the design of the vibroseis nonlinear scanning signal based on forced vibration.
2. The method for designing a seismic source nonlinear scanning signal based on forced vibration according to claim 1, wherein the vibration characteristic information in the step 1) is obtained by a single frequency scanning mode: and (3) counting the amplitude attribute of the first arrival of the direct wave of the receiving channel closest to the seismic source or the receiving channel with the minimum offset distance in each single-frequency excitation seismic record, and establishing a fitting curve between the single-frequency scanning frequency and the statistical amplitude attribute, wherein the fitting curve is the frequency-energy data to be obtained.
3. The method for designing a seismic source nonlinear scanning signal based on forced vibration according to claim 1, wherein the vibration characteristic information in the step 1) is obtained by means of shot data: carrying out linear dynamic correction on the single shot record by using first arrival pickup, and correcting the first arrival waves to the same moment; analyzing an amplitude spectrum of the first arrival information analysis of the direct waves near the seismic source in the linear dynamic correction seismic records of different single shots, wherein the result is frequency-energy data; the method comprises the steps that receiving channels in a certain offset range from a seismic source in the arrangement are received by the near-seismic source, and the direct wave first arrival information is 1 complete period waveform seismic record covering first arrival waves.
4. A method for designing a seismic source nonlinear sweep signal based on forced vibrations according to any of claims 1-3, characterized in that the energy in the frequency-energy data is a root mean square amplitude.
5. A method for designing a seismic source nonlinear scanning signal based on forced vibration according to claim 1, characterized in that the energy values in the non-reinforced energy frequency band range among the discrete energy values in the whole frequency band range are converted into amplitude values to obtain energy values or theoretical energy values, and the theoretical energy values are related to the designed driving amplitude.
6. The forced vibration-based source nonlinear sweep signal design method of claim 1, wherein the fitting method in step 3) is a smooth spline fitting or a gaussian fitting method.
7. The method of forced vibration based source nonlinear sweep signal design according to claim 1, wherein the nonlinear sweep signal frequency rate of change is calculated by the formula:
SR(f)＝β·[Fconstraint(f)/Ftarget(f)]2(Fe-Fs)/SL
Fconstraint(f) is the vibroseis frequency-upper output limit; ftarget(f) Is a target frequency-energy value; feIs the scan cut-off frequency; fsIs the scanning starting frequency; SL is scanning duration; beta is a scale factor; SR (f) is the rate of frequency change; f is the instantaneous frequency.
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