Resolution adjusting method for controllable impact seismic source

Abstract

The invention relates to the technical field of geophysical exploration, and discloses a resolution adjusting method for a controllable impact seismic source, which comprises the following steps: acquiring a longitudinal wave velocity of a target layer of a to-be-imaged region and inversely calculating a target characteristic period of a seismic source output wavelet; according to the preset maximum stroke limit of a gun barrel and the elasticity modulus characteristic of an earth surface medium, a one-dimensional elastic wave propagation theory and a double-path optimization strategy are utilized, the matching target characteristic period is taken as a target, the material type and the physical length of the impact bullet are subjected to joint optimization, and optimal impact bullet parameters are obtained; the parameter is used for field excitation, an actual measurement characteristic period is extracted and compared with a target value, and closed-loop correction is carried out by adjusting the physical length of the bullet on the premise that the material is not changed. The problems of fixed dominant frequency and impedance mismatch of a traditional seismic source are solved, the resolution and the waveform quality can be automatically balanced under the limited stroke, quantitative regulation and control of broadband output are achieved, the energy coupling efficiency is improved, and multi-resolution step-by-step inversion imaging is effectively supported.

Description

Technical Field

[0001] This invention relates to the field of geophysical exploration technology, specifically to a method for adjusting the resolution of a controllable shock source. Background Technology

[0002] Before the construction and operation of underground engineering projects, accurately determining the geological structure of the work area is crucial for preventing geological disasters. Seismic exploration methods, due to their high sensitivity to areas of abnormal wave impedance (such as faults and collapse columns), have become the primary means of determining the structure of underground spaces. However, underground engineering projects (such as coal mine tunnels) require a much higher scale and precision in their focus on the surrounding rock space than traditional surface oil and gas exploration, falling under the category of "small-field, high-precision" detection. The resolution of seismic exploration is directly related to the dominant frequency of the output seismic waves; the theoretical maximum resolution mainly depends on the medium wave velocity and the effective dominant frequency. Therefore, the key to achieving high-resolution imaging lies in whether the seismic source possesses flexible and broadband frequency control capabilities.

[0003] Current mainstream seismic source technologies have certain limitations in frequency and resolution control. For explosive sources, the dominant frequency range of the output seismic wave is basically fixed due to the predetermined chemical reaction process. Changing the charge amount usually only changes the amplitude of the excited seismic wave, but cannot change the phase spectrum and dominant frequency band, making it difficult to adjust it according to geological resolution requirements. In addition, the seismic wavelet waveform generated by explosive excitation is complex with many side lobes, making it difficult to meet the waveform consistency requirements of full waveform inversion imaging. For mechanically swept frequency sources, although the output frequency is controlled by a hydraulic servo system, the vibration output power will decay after exceeding a certain frequency (usually 20Hz), resulting in weak actual control effect in the high-frequency band. Moreover, its essence is a long-period, low-energy-density signal, which differs from high-energy pulse excitation. For drop-weighted seismic sources, their mechanical structure is usually fixed, making it impossible to adjust the output seismic wavelet frequency, and the energy is limited, mainly used for shallow exploration, making it difficult to achieve high signal-to-noise ratio detection of deep body waves.

[0004] In summary, existing mainstream seismic source technologies generally lack good broadband frequency controllability, which limits the ability of a single seismic source to adapt to different resolution requirements in field applications. This limitation further hinders the implementation of multi-resolution stepwise inversion imaging technology, because this approach requires the seismic source to first acquire the background wave velocity field with low-frequency high-fidelity parameters, and then gradually adjust to high-frequency parameters for fine imaging. Therefore, there is an urgent need for a controllable impact source adjustment method that can quantitatively adjust the output resolution while taking into account energy transmission efficiency and waveform quality, in order to meet the needs of refined geological exploration. Summary of the Invention

[0005] To address the problems of insufficient spectrum adjustment capability of existing controllable shock sources under complex geological conditions, difficulty in quantitatively matching excitation parameters according to detection resolution requirements, and easy signal distortion and low energy transmission efficiency caused by wave impedance mismatch, this invention provides a resolution adjustment method for controllable shock sources.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for adjusting the resolution of a controllable shock source includes the following steps: S1. Obtain the longitudinal wave velocity of the target layer in the region to be imaged and resolved. S2. Calculate the target characteristic period of the source output wavelet based on the obtained target layer longitudinal wave velocity; S3. Based on the preset maximum stroke limit of the gun barrel structure and the elastic modulus characteristics of the surface medium, the one-dimensional elastic wave propagation theory model is used to select the target characteristic period as the optimization target and jointly optimize the material type and physical length of the impact bullet to obtain the optimal impact bullet parameters. S4. Utilize the optimal impact bullet parameters to perform on-site excitation measurement and physical length closed-loop correction. Compare the deviation between the measured characteristic period of the actual excitation and the target characteristic period. Under the premise of keeping the material unchanged, eliminate the deviation by adjusting the physical length of the impact bullet until the preset error requirement is met.

[0007] Preferably, in step S1, the method for obtaining the P-wave velocity of the target layer includes any of the following: reviewing existing borehole acoustic logging data or test section velocity test reports in the engineering area, and extracting the velocity parameters of the target strata as the P-wave velocity of the target layer; or conducting experimental excitation before formal operation, using a surface geophone to receive the signal, and calculating the velocity structure through refracted wave travel time curve fitting, shallow seismic tomography, or first arrival wave inversion algorithm to obtain the P-wave velocity of the target layer; or selecting empirical velocity values ​​as the P-wave velocity of the target layer based on the surrounding rock type determined by the geological survey report, by consulting a rock physics handbook or industry experience database.

[0008] Preferably, in step S2, the process of back-calculating the target characteristic period of the source output wavelet based on the P-wave velocity of the target layer includes: using the seismic exploration vertical resolution calculation formula and the P-wave velocity of the target layer to calculate the theoretical maximum vertical resolution that meets the resolution requirements; using the effective cutoff frequency back-calculation formula and the theoretical maximum vertical resolution to calculate the corresponding target effective maximum dominant frequency; and using the target characteristic period engineering estimation formula, engineering conversion factor and target effective maximum dominant frequency to calculate the target characteristic period used for configuring the impact bullet.

[0009] Preferably, in step S3, a dual-path optimization strategy is adopted for the joint optimization calculation of the material type and physical length of the impact bullet. Specifically, this includes: calculating the minimum bullet wave velocity using a preset maximum stroke limit and target characteristic period, and obtaining the reference surface P-wave velocity; determining the relationship between the minimum bullet wave velocity and the reference surface P-wave velocity; when the minimum bullet wave velocity does not exceed the reference surface P-wave velocity, determining that the material wave velocity of the impact bullet is increased to match the reference surface P-wave velocity, and setting the physical length of the impact bullet to the length corresponding to the preset maximum stroke limit, thereby forming the optimal impact bullet parameters; when the minimum bullet wave velocity exceeds the reference surface P-wave velocity, forcibly setting the material wave velocity of the impact bullet to the reference surface P-wave velocity, and back-calculating and reducing the physical length of the impact bullet according to the target characteristic period, so that the physical length of the impact bullet is less than the preset maximum stroke limit, thereby forming the optimal impact bullet parameters.

[0010] Preferably, in step S3, the quantitative mapping relationship between the physical parameters of the impact bullet and the period of the output waveform is established using the theoretical calculation formula of the impact pulse width; the process of back-calculating and reducing the physical length of the impact bullet is calculated using the basic control equation for the configuration of the impact bullet length.

[0011] Preferably, in step S3, the minimum bullet wave velocity is calculated based on the upper limit constraint inequality of material wave velocity; and when determining the material wave velocity of the impact bullet, the material wave velocity of the impact bullet is optimized by using impedance matching to make it approach the reference surface longitudinal wave velocity.

[0012] Preferably, in step S4, the method for extracting the measured characteristic period includes: using the zero-crossing method to identify the time difference between the main lobe amplitude of the wavelet crossing the zero axis as the measured characteristic period; or using the autocorrelation method to calculate the time delay between the sidelobe peak and the main lobe peak of the signal autocorrelation function as the measured characteristic period.

[0013] Preferably, in step S4, the process of eliminating deviation by adjusting the physical length of the impact bullet while keeping the material unchanged includes: obtaining the original length of the impact bullet currently used for firing and the measured characteristic period obtained from the field test; and calculating the corrected new target length using the bullet length linear closed-loop correction formula, the target characteristic period, the measured characteristic period, and the original length.

[0014] Preferably, in step S4, the basis for determining whether the measured characteristic period meets the preset error requirement is: calculating the relative error between the measured characteristic period and the target characteristic period using the correction error convergence criterion formula; determining whether the absolute value of the relative error is less than or equal to 5%, thereby determining whether the preset error requirement is met; wherein, the preset error requirement is that the absolute value of the relative error is less than or equal to 5%.

[0015] Preferably, in step S4, the specific operation of adjusting the physical length of the impact bullet is as follows: according to the calculated new target length, the impact bullet is mechanically machined or replaced with a spare bullet that meets the new target length, and the firing and measurement process is repeated.

[0016] This invention provides a method for adjusting the resolution of a controllable shock source. It has the following beneficial effects: 1. This invention achieves quantitative adjustment of the main frequency band of the source output by calculating the target characteristic period of the source output wavelet based on the longitudinal wave velocity of the target layer, and then jointly optimizing the material and physical length of the impact bullet accordingly. Compared with the defects of traditional explosive sources, which are limited by chemical properties and have a fixed main frequency range, and can only change the amplitude but not the phase, as well as the limitation of frequency sweep sources with severe power attenuation in the high frequency band, this invention can flexibly configure the excitation parameters according to the detection requirements in a wide frequency band, thereby effectively solving the technical problem that a single source is difficult to adapt to exploration tasks with different resolutions.

[0017] 2. The dual-path optimization strategy employed in this invention establishes a mechanism for determining the parameters of the impact projectile under the dual constraints of the preset maximum stroke limit of the gun barrel and the elastic modulus characteristics of the surface medium. This mechanism can prioritize matching the surface impedance to improve energy coupling efficiency when the stroke is sufficient, and obtain high-frequency signals by adjusting the material wave velocity and reducing the physical length when the stroke is limited. Thus, while ensuring the resolution index, the wave impedance of the impact projectile and the surface medium is kept matched, reducing wake interference and energy loss caused by impedance mismatch.

[0018] 3. The resolution adjustment method provided by this invention can support the implementation of multi-resolution stepwise inversion imaging strategies. Since this invention can achieve graded control of output resolution by changing the parameters of the impact bullet, in actual exploration, the background wave velocity field can be obtained first using low-resolution, high-fidelity excitation parameters, and then gradually adjusted to high-resolution parameters for fine imaging. This flexible control capability provides the necessary hardware control foundation for advanced imaging technologies such as full-waveform inversion, which is conducive to improving the convergence speed and inversion accuracy of complex geological structure imaging. Attached Figure Description

[0019] Figure 1 This is a schematic diagram illustrating the implementation status of a controllable shock source launching mechanism according to an embodiment of the present invention in different resolution modes; Figure 2 This is a flowchart illustrating the overall process of the controllable shock source resolution adjustment method of the present invention. Figure 3 This is a flowchart of a dual-path optimization strategy for impact bullet parameters according to an embodiment of the present invention. Figure 4 This is a flowchart of the field excitation measurement and physical length closed-loop correction according to an embodiment of the present invention.

[0020] The components include: 1. Low-pressure gas storage chamber; 2. Solenoid valve; 3. Gun barrel; 4. Impact bullet; 5. Launch stabilization base platform. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] See attached document Figure 1 To achieve high-resolution seismic exploration, the hardware foundation of this invention mainly includes a low-pressure gas storage chamber 1, a solenoid valve 2, a gun barrel 3, an impact projectile 4, and a launch stabilization platform 5. The low-pressure gas storage chamber 1 stores compressed gas at a preset pressure, and its output end is airtightly connected to the rear inlet of the gun barrel 3 via the solenoid valve 2. The gun barrel 3 has an axially extending launch chamber to accommodate and guide the movement of the impact projectile 4. The impact projectile 4 is a replaceable core excitation element, and its material properties and geometric length parameters are configured according to the required detection resolution. The launch stabilization platform 5 is fixed below the gun barrel 3 to support the overall launch mechanism and maintain close contact and coupling with the surface medium of the area to be explored. In operation, the solenoid valve 2 opens, and the gas propels the impact projectile 4 to impact the platform or the ground surface, generating seismic waves.

[0023] See attached document Figure 2 This invention provides a resolution adjustment method for a controllable shock source. This method, based on the aforementioned hardware environment, includes the following steps: S1, Obtain the P-wave velocity of the target layer in the area to be imaged and resolved. Before carrying out seismic exploration operations, the physical property parameters of the medium to be detected, i.e., the P-wave velocity of the target layer, are determined by reviewing existing borehole acoustic logging data in the project area, performing small-scale on-site excitation wave velocity tests and inversions, or estimating based on empirical values ​​of the geological lithology of the surrounding rock. S2, Calculate the target characteristic period of the source output wavelet based on the P-wave velocity of the target layer. Based on the analytical relationship between vertical resolution and effective cutoff frequency in seismic exploration, and combined with the P-wave velocity of the target layer obtained in the previous steps, calculate the effective dominant frequency of the target that meets the current detection and imaging resolution requirements, and convert this frequency into a time-domain control parameter, i.e., the target characteristic period. S3. Calculate the optimal impact projectile parameters based on the target characteristic period and system constraints. Based on the preset maximum stroke limit determined by the barrel 3 structure and the elastic modulus characteristics of the surface medium, a one-dimensional elastic wave propagation theory model is used. With matching the target characteristic period determined in the preceding steps as the optimization objective, a joint optimization calculation is performed on the material type and physical length of the impact projectile 4 to obtain the optimal impact projectile parameters that include specific materials and theoretical lengths. S4. Optimal impact bullet parameters are used for on-site excitation measurement and physical length closed-loop correction. The impact bullet determined in the previous steps is loaded into the gun barrel 3 for the first test. The actual excitation of the seismic wavelet is collected and the measured characteristic period is extracted. The deviation between the measured value and the target characteristic period is compared. While keeping the material unchanged, the deviation is eliminated by adjusting the physical length of the bullet until the measured characteristic period meets the preset error requirements.

[0024] The following section will elaborate on the acquisition of target parameters, construction of theoretical calculation models, multi-constraint optimization strategies, and on-site calibration algorithms involved in each step of this invention, based on the overall process described above.

[0025] In implementing the resolution adjustment method for this controllable shock source, relying on the aforementioned low-pressure gas storage chamber 1, solenoid valve 2, gun barrel 3, and impact projectile 4, the first step (S1) is to obtain the P-wave velocity of the target layer. This step aims to determine the basic physical properties of the medium to be detected, providing reference parameters for subsequent resolution and frequency conversion. The P-wave velocity of the target layer (symbol denoted as...) The velocity of seismic longitudinal waves (PLWs) refers to the propagation velocity of seismic longitudinal waves in the medium of the target area to be imaged and resolved. This value can be selected as a single layer velocity value or a weighted average velocity value within the target depth range, depending on the geological conditions.

[0026] Depending on the specific engineering site conditions, the P-wave velocity of the target layer can be obtained through the following methods. : Firstly, existing engineering data can be utilized. In work areas with basic geological exploration data, existing borehole acoustic logging data curves or test section wave velocity test reports for the target area can be directly accessed to extract velocity parameters of the target formation. Secondly, perform rapid inversion in the field. In the absence of direct data, conduct experimental excitation before formal operation, and use detectors placed on the ground to receive refracted or direct wave signals. Through refracted wave travel time curve fitting, shallow seismic tomography, or first-arrival inversion algorithm, calculate the velocity structure of the shallow or target layer. Third, empirical estimation based on geological lithology. When on-site testing is not feasible, based on the surrounding rock type determined in the geological survey report, consult rock physics handbooks or industry experience databases to select the corresponding empirical velocity value as the initial value. If empirical values ​​are used, a closed-loop calibration process in subsequent steps is required to correct frequency control errors caused by deviations in the empirical values. The P-wave velocity of the target layer is determined using the above method. (Unit: m / s) and the depth range of the target to be detected.

[0027] After determining the medium velocity parameters, the step of inversely calculating the target characteristic period of the source output wavelet is performed (S2). This step translates the spatial resolution requirements of the geological task into time-domain parameters for configuring the impact bullet 4.

[0028] According to the resolution theory of seismic exploration, vertical resolution is limited by the wavelength of the seismic wavelet. To quantify this relationship, the formula for calculating vertical resolution in seismic exploration is used: ; In the above formula, the parameters This represents the theoretically highest vertical resolution, expressed in meters (m), which is the minimum thickness capable of distinguishing two adjacent geological interfaces; parameters The target layer P-wave velocity obtained in step S1, in meters per second (m / s); parameters The target effective maximum frequency required to meet this resolution, measured in Hertz (Hz).

[0029] Based on this formula, it is derived that to achieve the preset resolution... Formula for calculating the required effective cutoff frequency: ; This formula clarifies the condition when the medium velocity... When determining the resolution, if it needs to be increased, then decrease the resolution. The value must be increased to improve the target effective highest dominant frequency of the source output signal. .

[0030] Considering that high-frequency signals attenuate rapidly during the actual propagation of seismic waves through the geological strata, if the earthquake source is directly calculated using theoretical values... Using this as the design frequency would result in insufficient energy to reach the target depth. Therefore, an engineering conversion factor is introduced. The final control target is calculated using the target characteristic periodic engineering estimation formula: ; In the above formula, the parameters This parameter represents the target characteristic period, expressed in seconds (s). It is directly used as the control target for the subsequent design of the physical parameters of the impact bullet. Indicates the target frequency of the engineering design, in Hertz (Hz); Parameter This is an engineering conversion factor, ranging from 0.6 to 0.8. By selecting 0.6 to 0.8 times the theoretically highest main frequency as the design target main frequency, the energy signal-to-noise ratio for deep-sea exploration is ensured while meeting resolution requirements. Through this step, the process from resolution requirements to target characteristic periodicity is completed. Parameter mapping.

[0031] Based on the target characteristic period determined in the previous steps, the step of calculating the optimal impact bullet parameters according to the system constraints is performed (S3). This step transforms the control target in the time domain into the specific physical properties of the impact bullet 4 in the controllable impact source hardware system, that is, determining its material type and geometric length.

[0032] Based on the target characteristic period determined in the preceding steps, step S3 is executed to calculate the optimal impact bullet parameters according to system constraints. This step transforms the time-domain control objective into the specific physical properties of the impact bullet 4 in the controllable impact source hardware system, i.e., determining its material type and geometric length. According to the one-dimensional elastic wave propagation theory and Hopkinson's principle, the duration of the compressive stress wave generated by the flat-headed cylindrical impact bullet 4 directly depends on the bullet's geometric length and material wave velocity. To establish a quantitative mapping relationship between the physical parameters of the impact bullet 4 and the output waveform period, the theoretical calculation formula for impact pulse width is used: ; In the above formula, the parameters The parameter represents the pulse width of the stress wave generated by the impact, corresponding to the characteristic period of the seismic wavelet in an ideal one-dimensional collision model, and is expressed in seconds (s). The physical length of impact bullet 4 is represented in meters (m); parameters This represents the longitudinal wave velocity of the material selected for impact bullet 4, in meters per second (m / s); constant. This represents the multiple factor that allows the stress wave to propagate back and forth once inside the bullet. This is to achieve the target characteristic period determined in step S2. The above relationship can be transformed into the basic governing equation for the design of impact bullet length to guide bullet selection: ; From this equation, it can be seen that during the target characteristic period Given the given conditions, the length of impact bullet 4 Its material wave velocity Proportional. In practical engineering applications, the length design of the impact bullet 4 is subject to the geometric constraints of the mechanical structure of the gun barrel 3. Let the effective acceleration distance of the gun barrel 3 or the maximum allowable loading length of the structure be... Calculated length of impact bullet 4 Geometric constraints must be satisfied Substituting the fundamental governing equations into this condition yields the material wave velocity upper limit constraint inequality (i.e., the stroke constraint condition): ; The above inequality defines that, at a specific resolution (i.e.) ) and stroke limit ( Under the given conditions, the theoretical upper limit of the wave velocity of the bullet material is determined. Simultaneously, to avoid long-tailed wave interference caused by impedance mismatch, impedance matching is required, meaning the wave impedance of the bullet material (primarily manifested as wave velocity when densities are similar) must approach that of the Earth's surface medium. The objective function for impedance matching optimization is constructed as follows: ; In the above function, the symbol The absolute value operator is used; the parameter is... The elastic modulus of the candidate material, in gigapascals (GPa); parameter The equivalent elastic modulus of the Earth's surface medium, expressed in gigapascals (GPa); symbol This indicates a minimum value operation. In actual parameter configuration, this translates to reducing the wave speed of impact bullet 4. As close as possible to the reference surface P-wave velocity .

[0033] Considering the above constraints, this embodiment employs a dual-path optimization strategy to determine the optimal bullet wave velocity. and length See attached document. Figure 3 This strategy first utilizes the material wave velocity upper limit constraint inequality to calculate the maximum stroke. The target feature period is satisfied. Minimum required bullet wave velocity (Right now Then compare it with the reference surface P-wave velocity. The comparison is handled in two ways: Case 1 (not exceeding the limit): When the calculated This indicates that the gun barrel stroke is sufficient for the current resolution requirements. At this point, to suppress the wake blast, the bullet wave velocity is... Increased to the speed of surface waves Consistent; simultaneously, in order to maximize the energy release, the length of the impact bullet 4... Keep it at the maximum allowed value Although the actual excitation period at this time It may be less than the target period. (That is, the frequency is higher than the minimum requirement), but this is in line with the original intention of "high resolution" exploration and achieves the dual goals of impedance matching and energy maximization.

[0034] Case 2 (Exceeding the limit): When the calculated Time. Explanation: If you want to utilize... To achieve the target cycle, the required wave speed must be very high, but this would result in a wave speed far exceeding the Earth's surface wave speed. This generates a severe wake. Therefore, impedance matching is prioritized to force the bullet wave velocity to be controlled. Set to surface wave speed .because To ensure that the cycle is no greater than (That is, the frequency should not be lower than the requirement), the bullet length must be reduced based on the basic control equations designed according to the bullet length. ,at this time .

[0035] Table 1 below shows the parameters based on the above logic, when setting the maximum stroke of barrel 3. Under the condition of m, the results of field measurement and parameter optimization for different surface wave velocity environments and resolution requirements.

[0036] Table 1. Parameters for Joint Optimization of Bullet Material and Length

[0037] Note: In Table 1, the symbol "-" indicates that the calculation path is not activated or applicable under the current criterion logic branch; "0" and "1" in the data represent "no" and "yes" in the logical judgment, respectively.

[0038] Mechanism analysis based on data in Table 1: In the first eight sets of data (e.g., the first row) m / s, m), the calculated maximum demand period s. At this point, if the maximum stroke is used... m, only the bullet wave speed A speed of 450 m / s is sufficient to meet the period requirement. (Based on the reference surface wave velocity.) The speed of the bullet wave is much greater than 450 m / s, falling into the category of "not exceeding". Therefore, the system selects the bullet wave speed... Increase to 1000 m / s to match the surface impedance while maintaining length m. With this configuration, the actual pulse width is narrowed and the frequency is higher than the minimum requirement, which ensures high resolution and obtains greater kinetic energy reserve through the long bullet.

[0039] In the ninth group and subsequent data (e.g., the ninth row) m / s, m, corresponding to a higher wave velocity in the target layer, leads to Extremely small, at 0.0020 s). The calculated value at this time... m / s, which exceeds the speed of surface waves. m / s. Forcing the use of a material with a velocity of 1800 m / s, while meeting the stroke and period requirements, would cause severe wave impedance mismatch. Therefore, this invention forcibly reduces the bullet wave velocity to m / s. Limited to 1600 m / s, in order to achieve an ultra-short period of 0.0020 s at such a low wave velocity, the bullet length must be shortened. Calculations show that the optimal length... m, less than the maximum stroke of 1.8m. This configuration prioritizes ensuring the waveform quality (no wake) and resolution of the signal.

[0040] Through this joint calculation and discrimination process, the present invention can automatically balance the relationship between resolution, waveform quality and excitation energy under the geometric constraints of the barrel 3, and output the optimal impact bullet parameters.

[0041] To demonstrate the specific representation of the calculated optimal impact bullet parameters in the hardware system, and the impact of different resolution requirements on the physical state of the launching mechanism, the following section combines the attached diagram. Figure 1 Please provide an explanation.

[0042] See attached document Figure 1 The figure illustrates the configuration of the physical form (length and material) of the impact bullet 4 as detection requirements change, and this configuration is always constrained by the geometric constraints of the barrel 3. .

[0043] When implementing high-resolution (high-frequency) excitation modes, as shown in the attached... Figure 1 As shown in the upper part, based on the calculation results of the previous steps, the system is configured with a high-velocity, short-length impact projectile 4. At this time, the impact projectile 4 is made of steel or iron with high elastic modulus and high longitudinal wave velocity (…). Metallic materials (values ​​typically greater than 5000 m / s). Due to the target characteristic periodicity. The basic governing equations are designed based on the length of the impact bullet, which is relatively small. Required physical length of bullet Relatively short. As shown in the figure, this length is less than the maximum allowable stroke of barrel 3. In this state, the low-pressure gas storage chamber 1 releases gas through the solenoid valve 2, which propels the shorter impact bullet 4 to accelerate in the firing chamber of the gun barrel 3, impacting the firing stabilization base platform 5 and generating a short-duration pulse stress wave, which excites a seismic wavelet with wide bandwidth and high dominant frequency characteristics. This mode is suitable for shallow fine structure division or hard rock strata exploration.

[0044] When implementing the low-resolution (low-frequency) excitation mode, as shown in the attached... Figure 1The lower part shows the system configured with a low-velocity, long-length impact projectile 4. At this point, in order to obtain a larger target characteristic period... If high-wave velocity materials are continued to be used, the calculated length This will exceed the physical limits of the cannon barrel 3. Therefore, based on the aforementioned constraint optimization logic, the impact bullet 4 is replaced with a polymer material such as nylon or polyurethane. This type of material has a lower longitudinal wave velocity (…). The value is typically between 1500 m / s and 2500 m / s. By reducing... So that in satisfying Under the constraint of stroke, by increasing the physical length This increases the pulse width. As shown in the figure, the physical length of the impact bullet 4 increases, occupying most of the firing cavity space within the barrel 3. The seismic wavelet generated under this configuration has a longer duration and a lower dominant frequency, making it suitable for deep structural exploration or imaging of areas with soft overburden.

[0045] By comparing the appendix Figure 1 As can be seen from the two states, by using the same basic launching platform consisting of a low-pressure gas storage chamber 1, a solenoid valve 2, a gun barrel 3, and a launching stabilization base platform 5, and by replacing the impact bullets 4 with different materials and length specifications, the frequency of the output signal can be adjusted within the limited mechanical stroke range.

[0046] Finally, the step of performing in-situ excitation measurement and physical length closed-loop correction using the optimal impact bullet parameters is executed (S4). (See Appendix) Figure 4 Since the calculation model in the preceding steps is based on the ideal one-dimensional elastic wave theory, in actual operation, the uniformity of the material density of the impact projectile 4, the contact coupling stiffness between the launching stabilizing base platform 5 and the ground surface, and the nonlinear response characteristics of the ground surface medium may all cause deviations between the actual excited seismic wavelet period and the design target. Therefore, it is necessary to fine-tune the physical length of the impact projectile 4 through on-site measurements.

[0047] First, the initial impact projectile 4, made according to the material type and calculated length determined in step S3, is loaded into the gun barrel 3. A broadband geophone is buried near the ground at a preset distance (e.g., 1-2 meters) from the launch stabilization platform 5 to receive the direct wave signal excited by the seismic source. The seismic source system is activated, and the gas in the low-pressure gas storage chamber 1 is released through the solenoid valve 2, driving the impact projectile 4 to impact the launch stabilization platform 5. The geophone simultaneously collects the actual excited seismic wavelet signal.

[0048] Subsequently, time-domain analysis was performed on the acquired measured wavelet signal to extract its actual characteristic period. Specifically, the zero-crossing method was used to identify the time difference between the main lobe amplitude crossing the zero axis, or the autocorrelation method was used to calculate the time delay between the sidelobe peak and the main lobe peak of the signal's autocorrelation function, thereby obtaining the actual excitation characteristic period (denoted as ). ).

[0049] After obtaining the measured data, compare it with the target feature period determined in step S2. A comparison was made. Since the material of impact bullet 4 has been determined, its longitudinal wave velocity... Since the value is constant, the period deviation is corrected by adjusting the physical length of the impact bullet 4. Based on the linear relationship between wavelength and length, the corrected length value is calculated using a linear closed-loop correction formula for bullet length: ; In the above formula, the parameters This indicates the new target length of the corrected impact bullet 4, in meters (m); parameters This indicates the original length of the impact bullet 4 currently being used for firing, in meters (m); Parameter The target feature period is set for step S2, in seconds (s); parameters The characteristic period of the actual excitation obtained in this experiment is expressed in seconds (s).

[0050] Finally, based on the calculated The physical length of the impact bullet 4 is adjusted. Adjustment methods include machining excessively long bullets or replacing them with bullets of a different length. Use spare bullets of the same material. After adjustment, reload and measure. Repeat the above process of "excitation-measurement-calculation-adjustment" until the measured value meets the convergence criterion formula for the correction error: ; In the above formula, the symbol This represents the absolute value operator. (When the actual measurement period...) With target feature period When the relative error is less than or equal to the preset threshold (5% here), the correction is considered complete, and the bullet state at this time is the final configuration used for actual exploration operations.

Claims

1. A method for adjusting the resolution of a controllable shock source, characterized in that, Includes the following steps: S1. Obtain the longitudinal wave velocity of the target layer in the region to be imaged and resolved. S2. Calculate the target characteristic period of the source output wavelet based on the obtained longitudinal wave velocity of the target layer; S3. Based on the preset maximum stroke limit of the gun barrel structure and the elastic modulus characteristics of the surface medium, using the one-dimensional elastic wave propagation theory model, the target characteristic period is matched as the optimization target, and the material type and physical length of the impact bullet are jointly optimized to obtain the optimal impact bullet parameters. S4. Using the optimal impact bullet parameters, perform on-site excitation measurement and physical length closed-loop correction. Compare the deviation between the measured characteristic period of the actual excitation and the target characteristic period. Under the premise of keeping the material unchanged, adjust the physical length of the impact bullet to eliminate the deviation until the preset error requirement is met.

2. The resolution adjustment method for a controllable impact source according to claim 1, characterized in that, In step S1, the method for obtaining the P-wave velocity of the target layer includes any of the following: Review existing borehole acoustic logging data or test section wave velocity test reports in the engineering area, and extract the velocity parameters of the target formation as the longitudinal wave velocity of the target layer; Before formal operation, experimental excitation is carried out, and signals are received using surface geophones. The velocity structure is then calculated by fitting the refracted wave travel time curve, shallow seismic tomography, or first arrival wave inversion algorithm to obtain the P-wave velocity of the target layer. Based on the surrounding rock type determined by the geological survey report, consult the rock physics handbook or industry experience database to select the empirical velocity value as the longitudinal wave velocity of the target layer.

3. The resolution adjustment method for a controllable impact source according to claim 1, characterized in that, In step S2, the process of calculating the target characteristic period of the source output wavelet based on the P-wave velocity of the target layer includes: The theoretical maximum vertical resolution that meets the resolution requirements is calculated using the seismic exploration vertical resolution calculation formula and the P-wave velocity of the target layer. The corresponding target effective maximum frequency is calculated using the inverse formula of effective cutoff frequency and the theoretical maximum vertical resolution. The target characteristic period used for configuring impact bullets is calculated using the target characteristic period engineering estimation formula, engineering conversion factor, and the target's effective highest main frequency.

4. The resolution adjustment method for a controllable impact source according to claim 1, characterized in that, In step S3, the joint optimization calculation of the material type and physical length of the impact bullet adopts a dual-path optimization strategy, specifically including: The minimum bullet wave velocity is calculated using the preset maximum stroke limit and the target characteristic period, and the reference surface longitudinal wave velocity is obtained. Determine the relationship between the minimum bullet wave velocity and the longitudinal wave velocity of the reference ground surface; When the minimum bullet wave velocity does not exceed the reference ground surface longitudinal wave velocity, it is determined that the material wave velocity of the impact bullet is increased to be consistent with the reference ground surface longitudinal wave velocity, and the physical length of the impact bullet is set to the length corresponding to the preset maximum stroke limit, thereby constituting the optimal impact bullet parameters. When the minimum bullet wave velocity exceeds the reference ground longitudinal wave velocity, the material wave velocity of the impact bullet is forcibly set to the reference ground longitudinal wave velocity, and the physical length of the impact bullet is calculated and reduced according to the target characteristic period, so that the physical length of the impact bullet is less than the preset maximum stroke limit, thereby forming the optimal impact bullet parameters.

5. The resolution adjustment method for a controllable impact source according to claim 4, characterized in that, In step S3: A quantitative mapping relationship between the physical parameters of the impact bullet and the period of the output waveform is established using the theoretical calculation formula of impact pulse width. The process of calculating and reducing the physical length of the impact bullet is performed using the basic control equation for impact bullet length configuration.

6. The resolution adjustment method for a controllable impact source according to claim 4, characterized in that, In step S3, the minimum bullet wave velocity is calculated based on the upper limit constraint inequality of material wave velocity; and when determining the material wave velocity of the impact bullet, the impedance matching optimization objective function is used to make the material wave velocity of the impact bullet approach the reference surface longitudinal wave velocity.

7. The resolution adjustment method for a controllable impact source according to claim 1, characterized in that, In step S4, the method for extracting the measured feature period includes: The time difference between the main lobe amplitude of the wavelet crossing the zero axis is identified using the zero-crossing method as the measured characteristic period. Alternatively, the time delay between the sidelobe peak and the main lobe peak of the signal autocorrelation function can be calculated using the autocorrelation method as the measured characteristic period.

8. The resolution adjustment method for a controllable impact source according to claim 1, characterized in that, In step S4, the process of eliminating deviation by adjusting the physical length of the impact bullet while keeping the material unchanged includes: Obtain the original length of the impact bullet currently used for firing and the measured characteristic period obtained from the field test; The corrected new length of the target is calculated using the linear closed-loop correction formula for bullet length, the target characteristic period, the measured characteristic period, and the original length.

9. The resolution adjustment method for a controllable impact source according to claim 8, characterized in that, In step S4, the basis for determining whether the measured feature period meets the preset error requirement is: The relative error between the measured feature period and the target feature period is calculated using the correction error convergence criterion formula; Determine whether the absolute value of the relative error is less than or equal to 5%, thereby determining whether the preset error requirement is met; The preset error requirement is that the absolute value of the relative error is less than or equal to 5%.

10. The resolution adjustment method for a controllable impact source according to claim 8, characterized in that, In step S4, the specific operation for adjusting the physical length of the impact bullet is as follows: Based on the calculated new target length, the impact bullet is mechanically machined or replaced with a spare bullet that conforms to the new target length, and the firing and measurement process is repeated.

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