Method for resolution adjustment of a controllable impact source

Abstract

The present application relates to the technical field of geophysical exploration, and discloses a resolution adjustment method of a controllable impact seismic source, comprising: obtaining the longitudinal wave velocity of a target layer of an area to be imaged and inversely calculating the target characteristic period of a seismic source output wavelet; according to the preset maximum stroke limit of a gun barrel and the elastic modulus characteristics of the surface medium, using one-dimensional elastic wave propagation theory and a double-path optimization strategy, and taking matching the target characteristic period as the target, jointly optimizing the material type and physical length of the impact bullet to obtain optimal impact bullet parameters; using the parameters for field excitation, extracting the measured characteristic period and comparing it with the target value, and under the premise of unchanged material, adjusting the physical length of the bullet to perform closed-loop correction. The present application solves the problems of fixed main frequency and impedance mismatch of the traditional seismic source, can automatically balance the resolution and waveform quality under the limited stroke, realizes quantitative regulation and control of wideband output, improves the energy coupling efficiency, and effectively supports multi-resolution step-by-step inversion imaging.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geophysical exploration, in particular to a resolution adjustment method of a controllable impact seismic source. BACKGROUND

[0002] Before the construction and construction of underground engineering, it is of great significance to accurately explore the geological structure of the operation area to prevent geological disasters. Seismic exploration method has high sensitivity to wave impedance anomaly area (such as fault, collapse column, etc.), and becomes the main means to explore the underground space structure. However, the spatial scale and precision requirement of underground engineering (such as coal mine tunnel) to surrounding rock space is much higher than that of traditional ground oil and gas exploration, which belongs to the category of "small field high precision" detection. The resolution of seismic exploration is directly related to the main frequency of the output seismic wave, and the theoretical highest resolution mainly depends on the medium wave velocity and the effective main frequency. Therefore, the key to realize high resolution imaging lies in whether the seismic source has flexible and wideband frequency regulation capability.

[0003] The current mainstream seismic source technology has certain limitations in frequency and resolution adjustment. For explosive source, it is limited by the established chemical reaction process, and the main frequency range of the output seismic wave is basically fixed. Changing the charge amount can usually only change the amplitude of the excited seismic wave, but cannot change the phase spectrum and main frequency band, making it difficult to adjust according to the geological resolution requirement. In addition, the seismic wavelet waveform generated by explosive excitation is complex, with many side lobes, which is difficult to meet the requirement of waveform consistency for full waveform inversion imaging. For mechanical sweep frequency source, although the output frequency is controlled by hydraulic servo system, the vibration output power will be attenuated when the frequency exceeds a certain value (usually 20Hz), resulting in weak actual regulation effect in high frequency band, and its essence is long period low energy density signal form, which is different from high energy pulse excitation. For drop hammer source, its mechanical structure is usually fixed, and the output seismic wavelet frequency cannot be adjusted, and the energy is limited, which is mainly used for shallow exploration, and it is difficult to realize high signal-to-noise ratio detection of deep body wave.

[0004] In summary, the existing mainstream seismic source technology generally lacks good wideband frequency regulation, which restricts the ability of a single seismic source to adapt to different resolution requirements in field application. This limitation further hinders the implementation of multi-resolution step-by-step inversion imaging technology, because this technology requires the seismic source to first acquire the background wave velocity field with low frequency and high fidelity parameters, and then adjust to high frequency parameters for fine imaging. Therefore, there is an urgent need for a controllable impact seismic source adjustment method that can quantitatively adjust the output resolution, and take into account the energy transmission efficiency and waveform quality, to meet the needs of fine geological exploration. SUMMARY

[0005] In view of the problems of the existing controllable impact source, such as insufficient spectrum adjustment capability under complex geological conditions, difficulty in quantitatively matching the excitation parameters according to the detection resolution requirement, and easy signal distortion and low energy transmission efficiency due to wave impedance mismatch, the present application provides a resolution adjustment method of a controllable impact source.

[0006] To achieve the above object, the present application is implemented by the following technical solutions:

[0007] A resolution adjustment method of a controllable impact source, comprising the following steps:

[0008] S1, obtaining the target layer P-wave velocity of the imaging area to be imaged;

[0009] S2, based on the obtained target layer P-wave velocity, inversely calculating the target characteristic period of the source output wavelet;

[0010] S3, according to the preset maximum stroke limit of the gun barrel structure and the elastic modulus characteristics of the surface medium, using a one-dimensional elastic wave propagation theory model, taking the matching target characteristic period as the optimization target, jointly optimizing the material type and physical length of the impact bullet to obtain the optimal impact bullet parameters;

[0011] S4, using the optimal impact bullet parameters for field excitation and physical length closed-loop correction, comparing the deviation of the actual measured characteristic period and the target characteristic period, and eliminating the deviation by adjusting the physical length of the impact bullet under the premise of keeping the material unchanged until the preset error requirement is met.

[0012] Preferably, in step S1, the way of obtaining the target layer P-wave velocity includes any one of the following: referring to the existing borehole sonic logging data or test section velocity test report of the engineering area, extracting the velocity parameter of the target formation as the target layer P-wave velocity; or performing a test excitation before formal operation, using a surface geophone to receive a signal, and inversely calculating the velocity structure by refraction wave travel time curve fitting, shallow seismic tomography or first arrival wave inversion algorithm to obtain the target layer P-wave velocity; or according to the surrounding rock type determined by the geological survey report, referring to the rock physics manual or industry experience database to select the empirical velocity value as the target layer P-wave velocity.

[0013] Preferably, in step S2, the process of inversely calculating the target characteristic period of the source output wavelet based on the target layer P-wave velocity includes: calculating the theoretical highest vertical resolution meeting the resolution requirement using the seismic exploration vertical resolution calculation formula and the target layer P-wave velocity; calculating the corresponding target effective highest main frequency using the effective cutoff frequency inverse calculation formula and the theoretical highest vertical resolution; and calculating the target characteristic period used for configuring the impact bullet using the target characteristic period engineering estimation formula, the engineering conversion coefficient and the target effective highest main frequency.

[0014] Preferably, in step S3, the material type and physical length of the impact bullet are jointly optimized by using a double-path optimization strategy, which specifically includes: calculating a minimum bullet wave speed by using a preset maximum stroke limit and a target characteristic period, and obtaining a reference ground longitudinal wave speed; determining the size relationship between the minimum bullet wave speed and the reference ground longitudinal wave speed; when the minimum bullet wave speed does not exceed the reference ground longitudinal wave speed, determining that the material wave speed of the impact bullet is raised to be consistent with the reference ground longitudinal wave speed, and setting the physical length of the impact bullet to be the length corresponding to the preset maximum stroke limit, thereby forming the optimal impact bullet parameters; when the minimum bullet wave speed exceeds the reference ground longitudinal wave speed, forcibly setting the material wave speed of the impact bullet to be the reference ground longitudinal wave speed, and inversely 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.

[0015] Preferably, in step S3, the quantitative mapping relationship between the physical parameters of the impact bullet and the output waveform period is established by using an impact pulse width theoretical calculation formula; and the process of inversely calculating and reducing the physical length of the impact bullet is calculated by using an impact bullet length configuration basic control equation.

[0016] Preferably, in step S3, the basis for calculating the minimum bullet wave speed is a material wave speed upper limit constraint inequality; and when determining the material wave speed of the impact bullet, an impedance matching optimization objective function is used to make the material wave speed of the impact bullet approach the reference ground longitudinal wave speed.

[0017] Preferably, in step S4, the method for extracting the measured characteristic period includes: using a zero-crossing method to identify the time difference at which the main lobe amplitude of the sub-wave crosses the zero axis as the measured characteristic period; or using an autocorrelation method to calculate the time delay of the sidelobe peak value and the main lobe peak value of the signal autocorrelation function as the measured characteristic period.

[0018] Preferably, in step S4, the process of eliminating the deviation by adjusting the physical length of the impact bullet under the premise of keeping the material unchanged includes: obtaining the original length of the impact bullet used in the current excitation and the measured characteristic period obtained through the field test; and calculating the corrected target new length by using a bullet length linear closed-loop correction formula, the target characteristic period, the measured characteristic period, and the original length.

[0019] Preferably, in step S4, the basis for determining whether the measured characteristic period meets the preset error requirement is: calculating the relative error of the measured characteristic period and the target characteristic period by using a correction error convergence criterion formula; determining whether the absolute value of the relative error is less than or equal to 5%, so as to determine 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%.

[0020] Preferably, in step S4, the specific operation of adjusting the physical length of the impact bullet is: according to the calculated target new length, the impact bullet is mechanically cut or replaced with a spare bullet conforming to the target new length, and the firing and measuring process is repeated.

[0021] The present application provides a resolution adjustment method of a controllable impact seismic source. The following advantages are provided:

[0022] 1. The present application realizes quantitative adjustment of the main frequency band of the seismic source output by inversely calculating the target characteristic period of the seismic source output wavelet based on the target layer P-wave velocity, and jointly optimizing the material and physical length of the impact bullet accordingly. Compared with the traditional explosive seismic source which is limited by chemical properties and can only change the amplitude but not the phase due to the fixed main frequency range, and the sweep frequency seismic source which has a serious attenuation of output power in the high frequency band, the present application can flexibly configure the firing parameters in a wide frequency band range according to the detection requirements, thereby effectively solving the technical problem that a single seismic source cannot adapt to different resolution exploration tasks.

[0023] 2. The double-path optimization strategy adopted by the present application establishes a judgment mechanism for the parameters of the impact bullet under the double constraints of the maximum stroke limit of the gun barrel and the elastic modulus characteristics of the surface medium. This mechanism can preferentially match the surface impedance to improve the energy coupling efficiency when the stroke is sufficient, and can adjust the material velocity and reduce the physical length to obtain high frequency signals when the stroke is limited, so that the wave impedance of the impact bullet and the surface medium is matched, and the tail wave interference and energy loss caused by impedance mismatch are reduced.

[0024] 3. The resolution adjustment method provided by the present application can support the implementation of a multi-resolution step-by-step inversion imaging strategy. Since the present application can realize step-by-step regulation of the output resolution by changing the parameters of the impact bullet, in actual exploration, the background wave velocity field can be obtained by using low resolution and high fidelity firing parameters first, and then gradually adjusted to high resolution parameters for fine imaging. This flexible regulation capability provides the necessary hardware control basis for advanced imaging technologies such as full waveform inversion, and is beneficial to improving the convergence speed and inversion accuracy of complex geological structure imaging. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The figure is a schematic diagram of the implementation state of the controllable impact seismic source launching mechanism of an embodiment of the present application under different resolution modes;

[0026] Figure 2 The figure is a general flowchart of the resolution adjustment method of the controllable impact seismic source of the present application;

[0027] Figure 3 The figure is a flowchart of the double-path optimization strategy of the impact bullet parameters of an embodiment of the present application;

[0028] Figure 4 Field excitation and physical length closed loop correction flow chart for one embodiment of the present application.

[0029] 1, low-pressure gas storage cavity; 2, electromagnetic valve; 3, barrel; 4, impact bullet; 5, launch stable base platform. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the specification of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0031] Referring to the drawings Figure 1 In order to realize high-resolution seismic exploration, the hardware basis relied on by the present application mainly includes a low-pressure gas storage cavity 1, an electromagnetic valve 2, a barrel 3, an impact bullet 4, and a launch stable base platform 5. The low-pressure gas storage cavity 1 is used to store compressed gas with a preset pressure, and the output end thereof is in airtight connection with the rear end inlet of the barrel 3 through the electromagnetic valve 2. The barrel 3 has a launch cavity extending in the axial direction inside, which is used to accommodate and guide the movement of the impact bullet 4. The impact bullet 4 is a replaceable core excitation element, and the material properties and geometric length parameters thereof are configured according to the requirements of the detection resolution. The launch stable base platform 5 is fixedly connected to the lower side of the barrel 3, and is used to support the whole launch mechanism and keep close contact and coupling with the surface medium of the area to be detected. In the working state, the electromagnetic valve 2 is opened, and the gas pushes the impact bullet 4 to hit the base or the surface to generate seismic waves.

[0032] Referring to the drawings Figure 2 The present application provides a resolution adjustment method of a controllable impact seismic source, which is executed based on the above-mentioned hardware environment and includes the following steps.

[0033] S1, obtaining the target layer P-wave velocity of the imaging resolution area to be detected. Before implementing the seismic exploration operation, the physical property parameters of the medium to be detected, i.e., the target layer P-wave velocity, are determined by referring to the existing borehole sonic logging data of the engineering area, performing wave velocity test inversion of small-scale excitation on site, or estimating the geological lithology experience value of the surrounding rock.

[0034] S2, inversely calculating the target characteristic period of the seismic source output wavelet based on the target layer P-wave velocity. According to the analytical relationship between the vertical resolution and the effective cutoff frequency in the field of seismic exploration, the target effective main frequency corresponding to the current detection imaging resolution requirement is calculated in combination with the target layer P-wave velocity obtained in the previous step, and the frequency is converted into a control parameter in the time domain, i.e., the target characteristic period.

[0035] S3, calculating optimal impact bullet parameters according to target characteristic period and system constraints. According to the preset maximum stroke limit determined by the structure of the barrel 3 and the elastic modulus characteristics of the ground medium, the one-dimensional elastic wave propagation theory model is used to jointly optimize the material type and physical length of the impact bullet 4 to obtain the optimal impact bullet parameters containing the specific material and the theoretical length, by matching the target characteristic period determined in the previous step as the optimization target;

[0036] S4, using the optimal impact bullet parameters for field excitation measurement and physical length closed-loop correction. The impact bullet determined according to the previous step is loaded into the barrel 3 for the first launch test, the actual excited seismic wavelet is collected and the measured characteristic period is extracted, the deviation of the measured value and the target characteristic period is compared, and the deviation is eliminated by adjusting the physical length of the bullet under the premise of keeping the material unchanged, until the measured characteristic period meets the preset error requirement.

[0037] The target parameter acquisition, theoretical calculation model construction, multi-constraint optimization strategy and field correction algorithm involved in each step of the application will be described in detail below in combination with the above overall process.

[0038] In the implementation of the resolution adjustment method of the controllable impact seismic source, relying on the aforementioned low-pressure gas storage cavity 1, electromagnetic valve 2, barrel 3 and impact bullet 4 and other hardware basis, first, the step of acquiring the target layer P-wave velocity (S1) is executed. This step aims to determine the basic physical properties of the medium to be detected, providing a reference parameter for the subsequent resolution and frequency conversion. The target layer P-wave velocity (denoted as Vp) refers to the propagation speed of the seismic P-wave 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 according to the geological conditions.

[0039] For different engineering site conditions, the target layer P-wave velocity can be obtained by the following methods :

[0040] First, use existing engineering data. In the work area with geological survey basic data, directly refer to the existing borehole sonic logging data curve or test section velocity test report of the target area to extract the velocity parameter of the target formation;

[0041] Second, perform on-site rapid inversion. In the absence of direct data, perform a test excitation before formal operation, use the geophones arranged on the ground to receive refracted wave or direct wave signals, and use the refracted wave travel time curve fitting, shallow seismic tomography or first arrival wave inversion algorithm to calculate the velocity structure of the shallow surface layer or the target layer;

[0042] ​Third, empirical estimation based on geological lithology. In the absence of field testing conditions, according to the surrounding rock types determined in the geological survey report, consult the rock physics manual or industry experience database, and select the corresponding empirical velocity value as the initial If the empirical value is used, it needs to be combined with the subsequent step of field closed-loop correction to correct the frequency control error caused by the deviation of the empirical value. Through the above-mentioned manner, the longitudinal wave velocity of the target layer (unit: m / s) and the target depth range to be detected are determined.

[0043] After the medium velocity parameter is determined, the step of inversely calculating the target characteristic period of the source output wavelet (S2) is performed. This step converts the spatial resolution requirement in the geological task into a time domain parameter for configuring the impact bullet 4.

[0044] According to the resolution theory of seismic exploration, the vertical resolution is limited by the wavelength of the seismic wavelet. In order to quantify this relationship, the vertical resolution calculation formula of seismic exploration is used for definition:

[0045] ;

[0046] In the above formula, the parameter represents the theoretical highest vertical resolution, with the unit of meters (m), that is, the minimum thickness that can distinguish two adjacent geological interfaces; the parameter is the longitudinal wave velocity of the target layer obtained in step S1, with the unit of meters per second (m / s); and the parameter is the target effective highest main frequency required to meet the resolution, with the unit of hertz (Hz).

[0047] Based on the formula, the effective cutoff frequency inverse calculation formula required to achieve the preset resolution is derived:

[0048] ;

[0049] This formula clearly shows that when the medium velocity is determined, if the resolution needs to be improved, that is, the value of must be reduced, the target effective highest main frequency of the source output signal must be increased.

[0050] Considering that the high-frequency signal attenuates quickly during the propagation of seismic waves in actual strata, if the source directly uses the theoretically calculated as the design main frequency, it will result in too weak energy reaching the target depth. Therefore, an engineering conversion coefficient is introduced, and the final control target is calculated using the target characteristic period engineering estimation formula:

[0051] ;

[0052] In the above formula, the parameter represents the target feature period, in seconds (s), which is directly used as the control target for the subsequent physical parameter design of the impact bullet 4; the parameter represents the target main frequency of engineering design, in hertz (Hz); the parameter is an engineering conversion coefficient, whose value range is 0.6-0.8. By selecting 0.6-0.8 times of the theoretical highest main frequency as the design target main frequency, the resolution requirement is met while the energy signal-to-noise ratio of deep exploration is ensured. Through this step, the parameter mapping from the resolution requirement to the target feature period is completed.

[0053] Based on the target feature period determined in the previous step, the step of calculating the optimal impact bullet parameters according to the system constraint conditions (S3) is performed. This step converts the control target in the time domain into the specific physical properties of the impact bullet 4 in the controllable impact seismic source hardware system, i.e., determines the material type and geometric length of the impact bullet 4.

[0054] Based on the target feature period determined in the previous step, the step of calculating the optimal impact bullet parameters according to the system constraint conditions (S3) is performed. This step converts the control target in the time domain into the specific physical properties of the impact bullet 4 in the controllable impact seismic source hardware system, i.e., determines the material type and geometric length of the impact bullet 4. According to the one-dimensional elastic wave propagation theory and the Hopkinson bar principle, the duration of the compression stress wave generated by the flat-head cylindrical impact bullet 4 directly depends on the geometric length and material wave speed of the bullet. In order to establish a quantitative mapping relationship between the physical parameters of the impact bullet 4 and the output waveform period, the impact pulse width theoretical calculation formula is adopted:

[0055] ;

[0056] In the above formula, the parameter represents the stress wave pulse width generated by the impact, which corresponds to the feature period of the seismic wavelet under the ideal one-dimensional collision model, in seconds (s); the parameter represents the physical length of the impact bullet 4, in meters (m); the parameter represents the longitudinal wave speed of the selected material of the impact bullet 4, in meters per second (m / s); and the constant represents the multiple factor of the stress wave propagating back and forth inside the bullet once. In order to realize the target feature period determined in step S2, the above relationship is transformed into the impact bullet length design basic control equation for guiding the selection of the bullet:

[0057] ;

[0058] As can be seen from the equation, in the target feature 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):

[0059] ;

[0060] 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:

[0061] ;

[0062] 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 .

[0063] 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:

[0064] Case 1 (not exceeding the limit): When the calculated The calculated maximum period is s. This indicates that the barrel stroke is sufficient for the current resolution requirement. At this time, in order to suppress the tail wave, the bullet wave speed is raised to be consistent with the surface wave speed ; at the same time, in order to maximize the excitation energy, the length of the impact bullet 4 is kept as the maximum allowed value . Although the actual excitation period may be smaller than the target period at this time (i.e., the frequency is higher than the minimum requirement), this meets the initial purpose of exploration with "high resolution", and realizes the dual goals of impedance matching and energy maximization.

[0065] Case two (exceeding the limit): when the calculated is greater than the maximum stroke . This indicates that if the target period is to be achieved, the required wave speed must be very high, but this will result in a wave speed far exceeding the surface wave speed , resulting in serious tail waves. Therefore, impedance matching is prioritized, and the bullet wave speed is set to the surface wave speed . Due to , in order to ensure that the period is not greater than (i.e., the frequency is not lower than the requirement), the basic control equation must be designed according to the impact bullet length to reduce the bullet length, i.e. , at this time .

[0066] The following Table 1 shows the results of field measurement and parameter optimization for different surface wave speed environments and resolution requirements based on the above logic under the condition that the maximum stroke of the barrel 3 is set to m.

[0067] Table 1. Bullet material-length joint optimization calculation parameter table

[0068]

[0069] Note: In Table 1, the symbol "-" indicates that the calculation path is not activated or not applicable under the current criterion logic branch; the data "0" and "1" respectively represent "no" and "yes" in the logic judgment.

[0070] Mechanism analysis combined with Table 1 data:

[0071] In the first eight groups of data (e.g., the first row m / s, m), the calculated maximum period requirement is s. At this time, if the maximum stroke m is used, 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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. .

[0076] 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.

[0077] When implementing the low-resolution (low-frequency) excitation mode, as shown in the attached... Figure 1 The 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.

[0078] 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.

[0079] 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.

[0080] First, the initial impact bullet 4 made according to the material type and the calculated length determined in step S3 is loaded into the barrel 3. A wideband geophone is buried in the near-field ground at a preset distance (e.g. 1-2 meters) from the launch stable base platform 5, for receiving the direct wave signal excited by the seismic source. The gas in the low-pressure gas cavity 1 is released through the electromagnetic valve 2 to drive the impact bullet 4 to hit the launch stable base platform 5, and the geophone synchronously collects the actual excited seismic wavelet signal.

[0081] Subsequently, the actual characteristic period of the collected measured wavelet signal is extracted through time domain analysis. In specific implementation, the zero-crossing method is used to identify the time difference of the amplitude of the main lobe of the wavelet crossing the zero axis, or the autocorrelation method is used to calculate the time delay of the side lobe peak value and the main lobe peak value of the signal autocorrelation function, so as to obtain the actual excited characteristic period (denoted as ).

[0082] After obtaining the measured data, it is compared with the target characteristic period determined in step S2. Since the material of the impact bullet 4 has been determined, its P-wave velocity is a constant value, so 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 linear closed-loop correction formula of the bullet length is used to calculate the corrected length value:

[0083] ;

[0084] In the above formula, the parameter represents the target new length of the corrected impact bullet 4, with the unit of meter (m); the parameter represents the original length of the impact bullet 4 used in the current excitation, with the unit of meter (m); the parameter is the target characteristic period set in step S2, with the unit of second (s); and the parameter is the actual excited characteristic period obtained in this measurement, with the unit of second (s).

[0085] Finally, the physical length of the impact bullet 4 is adjusted according to the calculated , and the adjustment methods include mechanical cutting processing of the too long bullet or replacing it with a spare bullet of the same material with a length of . After the adjustment is completed, the excitation and measurement are performed again. The above process of “excitation-measurement-calculation-adjustment” is repeated until the measured value meets the correction error convergence criterion formula:

[0086] ;

[0087] In the above formula, the symbol represents the absolute value operator. When the measured period the relative error of the target feature period When the relative error is less than or equal to a preset threshold (5% in this case), it is determined that the correction is complete, and the state of the bullet at this time is the final configuration used for actual exploration operations.

Claims

1. A method of resolution adjustment of a controllable impact seismic source, characterized in that, The method comprises the following steps: S1, obtaining the target layer P-wave velocity of the imaging resolution area to be formed; S2, inversely calculating the target characteristic period of the seismic source output wavelet based on the obtained target layer P-wave velocity; S3, according to the preset maximum stroke limit of the gun barrel structure and the elastic modulus characteristics of the surface medium, using a one-dimensional elastic wave propagation theory model, matching the target characteristic period as the optimization target, jointly optimizing the material type and physical length of the impact bullet to obtain the optimal impact bullet parameters; S4, using the optimal impact bullet parameters for field excitation and physical length closed-loop correction, comparing the deviation of the actual measured characteristic period and the target characteristic period, and eliminating the deviation by adjusting the physical length of the impact bullet under the premise of keeping the material unchanged until the preset error requirement is met.

2. A method of resolution adjustment of a controllable seismic source according to claim 1, characterized in that, In step S1, the way to obtain the target layer P-wave velocity includes any one of the following: Refer to the existing borehole acoustic logging data or test section velocity test report of the engineering area, extract the velocity parameter of the target formation as the target layer P-wave velocity; Before formal operation, test excitation is carried out, the surface geophone is used to receive signals, the velocity structure is inversely calculated by refraction wave travel time curve fitting, shallow seismic tomography or first arrival wave inversion algorithm to obtain the target layer P-wave velocity; According to the surrounding rock type determined by the geological survey report, consult the rock physics manual or industry experience database to select the empirical velocity value as the target layer P-wave velocity.

3. The method of resolution adjustment of a controllable seismic source of claim 1, wherein, In step S2, the process of inversely calculating the target characteristic period of the seismic source output wavelet based on the target layer P-wave velocity comprises: Using the seismic exploration vertical resolution calculation formula and the target layer P-wave velocity to calculate the theoretical maximum vertical resolution meeting the resolution requirement; Using the effective cutoff frequency inverse calculation formula and the theoretical maximum vertical resolution to calculate the corresponding target effective maximum main frequency; Using the target characteristic period engineering estimation formula, the engineering conversion coefficient and the target effective maximum main frequency to calculate the target characteristic period used for configuring the impact bullet.

4. The method of resolution adjustment of a controllable seismic source of claim 1, wherein, In step S3, the joint optimization calculation of the material type and physical length of the impact bullet adopts a double-path optimization strategy, which specifically includes: Using the preset maximum stroke limit and the target characteristic period to calculate the minimum bullet wave velocity, and obtaining the reference surface P-wave velocity; Judging the size 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, it is determined that the material wave velocity of the impact bullet is improved to be consistent with the reference surface P-wave velocity, and the physical length of the impact bullet is set 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, the material wave velocity of the impact bullet is forcibly set to the reference surface P-wave velocity, and the physical length of the impact bullet is inversely 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. A method of resolution adjustment of a controllable seismic source according to claim 4, characterized in that, In step S3, The quantitative mapping relationship between the physical parameters of the impact bullet and the period of the output waveform is calculated by using the formula of the impact pulse width theory. The process of inversely calculating and reducing the physical length of the impact bullet is calculated by using the basic control equation of the length configuration of the impact bullet.

6. A method of resolution adjustment of a controllable seismic source according to claim 4, characterized in that, In step S3, the basis for calculating the minimum value of the bullet wave speed is the upper limit constraint inequality of the material wave speed; and when determining the material wave speed of the impact bullet, the impedance matching optimization objective function is used to make the material wave speed of the impact bullet approach the reference ground surface longitudinal wave speed.

7. The method of resolution adjustment of a controllable seismic source of claim 1, wherein, In step S4, the method for extracting the measured characteristic period includes: The time difference of the amplitude of the main lobe of the wavelet crossing the zero axis is identified by using the zero-crossing method as the measured characteristic period; Or the time delay of the sidelobe peak value and the main lobe peak value of the signal autocorrelation function is calculated by using the autocorrelation method as the measured characteristic period.

8. The method of resolution adjustment of a controllable seismic source of claim 1, wherein, In step S4, the process of eliminating the deviation by adjusting the physical length of the impact bullet under the premise of keeping the material unchanged includes: The original length of the impact bullet used in the current excitation and the measured characteristic period obtained by the field test are obtained; The corrected target new length is calculated by using the bullet length linear closed-loop correction formula, the target characteristic period, the measured characteristic period and the original length.

9. A method of resolution adjustment of a controllable seismic source according to claim 8, characterized in that, In step S4, the basis for determining whether the measured characteristic period meets the preset error requirement is: The relative error between the measured characteristic period and the target characteristic period is calculated by using the correction error convergence criterion formula; It is determined whether the absolute value of the relative error is less than or equal to 5%, so as to determine 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. A method of resolution adjustment of a controllable seismic source according to claim 8, characterized in that, In step S4, the specific operation of adjusting the physical length of the impact bullet is: According to the calculated target new length, the impact bullet is mechanically cut or replaced with a spare bullet that meets the target new length, and the excitation and measurement process is repeated.

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