Earthquake excitation depth determination method

Abstract

The invention relates to a seismic excitation depth determination method, and belongs to the technical field of seismic exploration. The method comprises the following steps: firstly, determining a minimum safe excitation depth according to a micro-logging interpretation result, an excitation dose and a grain length, solving ghost reflection amplitude response curves of a dominant frequency and a highest frequency of a target layer, determining an excitation depth range according to a maximum value of the ghost reflection amplitude response curves, solving an autocorrelation wavelet and a frequency spectrum according to micro-logging data, and obtaining a micro-logging result; and determining a favorable excitation depth range according to the autocorrelation wavelet and the frequency spectrum, judging a favorable excitation lithology range, and determining the optimal excitation depth by integrating the minimum safe excitation depth, the excitation depth range determined by ghost reflection, and the favorable excitation depth range and the favorable excitation lithology range determined by micro-logging. According to the method, factors influencing the excitation depth are comprehensively considered, ghost reflection analysis is more visual, and the method is high in applicability.

Description

Technical Field

[0001] The invention relates to a method for determining seismic excitation depth, belonging to the technical field of seismic exploration. Background Art

[0002] In onshore seismic exploration using explosive sources, the excitation depth has a great influence on the quality of seismic data. The design of the excitation depth of onshore seismic exploration usually mainly considers the influence of the excitation medium velocity, lithology and virtual reflection, and is ultimately determined through field tests. The high-speed layer below the diving surface (generally also the top interface of the high-speed layer or the bottom interface of the low-velocity reduction zone) has good excitation conditions. Selecting the high-speed layer below the diving surface for excitation is the basis and key to obtaining good seismic data. In order to improve the seismic excitation effect, it is usually selected to excite in the high-speed layer below the low-velocity reduction zone or the diving surface. Virtual reflection is an important factor to be considered when designing the excitation depth. Virtual reflection refers to when the high-speed layer below the low-velocity reduction layer is excited, due to the large difference in velocity between the high-speed layer and the low-velocity reduction layer, the contact surface between the two can be regarded as a strong reflection surface, and the upgoing wave excited by the source to the interface will be reflected back to the underground again, forming a companion wave of the downgoing wave, which is called a virtual reflection. When designing the seismic excitation depth, the rational use of virtual reflections can obtain high-quality (high energy, wide frequency) excitation wavelets. However, there is a time difference between the virtual reflections and the directly downward seismic waves, which causes the virtual reflections to be superimposed on the rear part of a downward wave, causing a low-frequency response in the seismic record, thereby reducing the resolution of the seismic profile.

[0003] In the existing literature, when considering the influence of virtual reflection on the excitation effect, the frequency response curve of the virtual reflection is mainly obtained, and then the excitation depth is selected according to the effect of the frequency response curve on the frequency band of the seismic data. In the above method, the frequency response curve of the virtual reflection is a series of oscillating curves. This method is not intuitive enough. In addition, the research on the excitation well depth using this method only considers the point source, and lacks the analysis of columnar explosives. Therefore, the existing method of designing the well depth using virtual reflection analysis is insufficient in intuitiveness and applicability.

[0004] The invention patent document with publication number CN104375187A discloses a method for designing the depth of a cannon well based on micro-logging and non-seismic data. This method is based on micro-logging data and uses non-seismic data to conduct surface structure surveys to obtain surface structure interpretation profiles, plot the depth of each excitation point in the diagram, and add the adjusted excitation depth to the length of the explosive column to obtain the comprehensive design depth of the cannon well. However, this method lacks consideration of the stability of the excitation wavelet. The invention patent document with the publication number CN111665536A discloses a well depth design method based on the quantitative constraint of micro-logging wavelets, which includes: extracting the common receiving point gather records of different excitation depths according to the micro-logging data, extracting the first arrival wave of each channel and using the autocorrelation algorithm to obtain the related wavelets, analyzing the wavelet spectrum of each channel to obtain the bandwidth and calculating the octave, calculating the main lobe width of the wavelet of each channel to obtain the center frequency, calculating the main lobe and side lobe energy of the wavelet of each channel to obtain the energy ratio of the side lobe to the main lobe, calculating the main lobe and side lobe peak of the wavelet of each channel to obtain the wavelet clarity, and optimizing the excitation well depth according to the above quantitative analysis results. The above method only considers the quantitative parameters of the micro-logging recording waveform or micro-logging data, and selects the excitation depth based on this, but the factors that actually affect the seismic excitation effect are relatively complex, and there are still deficiencies in the application of the above method in actual production. In addition, the existing excitation well depth design is mostly based on the theory of high-speed layer excitation and ghost reflection, and partly based on the dynamic characteristics of micro-logging, lacking a more comprehensive method for determining the excitation well depth. Summary of the invention

[0005] The purpose of the present invention is to provide a method for determining the seismic excitation depth, so as to solve the problems of insufficient applicability and inaccuracy in the process of determining the seismic exploration excitation depth in the prior art.

[0006] To achieve the above object, the solution of the present invention includes:

[0007] The method for determining the seismic excitation depth of the present invention comprises the following steps:

[0008] 1) Obtain the near-surface lithology, excitation charge amount, charge column length, micro-logging data and micro-logging interpretation results of the target work area;

[0009] 2) Determine the minimum safe excitation depth based on micro-logging interpretation results, excitation charge dosage and charge column length;

[0010] 3) Obtaining the ghost reflection amplitude response curves of the main frequency and the highest frequency of the target layer, and determining the excitation depth range according to the maximum value of the ghost reflection amplitude response curve;

[0011] 4) Obtain autocorrelation wavelet and spectrum from micro-logging data, determine favorable excitation depth range based on autocorrelation wavelet and spectrum, and judge favorable excitation lithology range;

[0012] 5) Determine the optimal excitation depth by comprehensively considering the minimum safe excitation depth, the excitation depth range determined by virtual reflection, the favorable excitation depth range determined by micro-logging, and the favorable excitation lithology range.

[0013] Beneficial effects: The seismic excitation depth determination method of the present invention provides a more comprehensive excitation well depth design method by using virtual reflection, micro-logging and lithology information, obtains the minimum safe excitation depth by using the micro-logging interpretation results and the amount of excitation drugs and the length of the drug column, analyzes the amplitude response of the virtual reflection of the main frequency and the highest frequency of the target layer, intuitively reflects the virtual reflection response characteristics of the seismic exploration frequency component, and uses the amplitude response curve of the virtual reflection to determine the excitation depth range, which is convenient for excitation depth optimization, extracts the micro-logging wavelet and its spectrum profile, determines the favorable excitation depth range and the favorable excitation lithology range according to the wavelet and spectrum characteristics and their stability, and determines the optimal excitation depth by comprehensively considering the minimum safe excitation depth, the excitation depth range determined by virtual reflection, the favorable excitation depth range determined by micro-logging and the favorable excitation lithology range. The method comprehensively considers a variety of factors affecting the excitation depth, has strong applicability, and obtains a more accurate excitation depth.

[0014] Furthermore, the optimal excitation depth determined in the step 5) is greater than the minimum safe excitation depth and is within the intersection of the excitation depth range determined by ghost reflections, the favorable excitation depth range determined by micro-logging, and the favorable excitation lithology range. If there is no intersection in the ranges determined by the four factors, then the optimal excitation depth is determined in the order of the minimum safe excitation depth, the excitation depth range determined by ghost reflections, the favorable excitation depth range determined by micro-logging, and the favorable excitation lithology range.

[0015] Beneficial effects: The excitation depth of the present invention needs to be greater than the minimum safe excitation depth to ensure safety and basic excitation effect during actual operation, and the optimal excitation depth is also within the intersection of the excitation depth range determined by virtual reflection, the favorable excitation depth range determined by micro-logging, and the favorable excitation lithology range, ensuring the optimal excitation effect.

[0016] Furthermore, the minimum safe excitation depth is:

[0017] H0=h L +d=h L +(RL / 2)

[0018] Where H0 is the minimum safe excitation depth, h L is the thickness of the low velocity reduction layer, d is the distance between the top surface of the charge and the top surface of the high velocity layer, R is the explosion radius, and L is the length of the charge.

[0019] Furthermore, the calculation formula of the ghost reflection amplitude response curve is:

[0020] A f,d =Max(s f,d(t))

[0021] In the formula, s f，d (t) is the delayed superposition curve of the seismic wavelet with the main frequency of f and the virtual reflection of the top surface of the high-speed layer when the distance between the top surface of the charge column and the top surface of the high-speed layer is d, A f,d For f，d (t) Maximum value in the delay superposition curve time series, s f，d (t) is obtained from the following formula:

[0022] s f，d (t) = w f (t)+rw f (t-τ)

[0023] Among them, τ is the delay time between the virtual reflection of the high-speed layer top interface and the first downlink wave, r is the reflection coefficient of the high-speed layer top interface incident from bottom to top, and w f (t) is the seismic wavelet with main frequency f, t is the time, not less than the sum of the main period of a seismic wavelet and the virtual reflection of the high-speed layer top interface and the delay time τ of a downlink wave.

[0024] Furthermore, the favorable excitation depth range determined by the logging is a depth section in which the autocorrelation sub-waves of the micro-logging data have fewer side lobes, a wider spectrum width and a stable distribution.

[0025] Furthermore, the excitation depth range is obtained by

[0026] B f =Max(A f,d )

[0027] Where B f A is the ghost reflection amplitude response curve f,d The maximum amplitude value among all different distances d between the top surface of the charge column and the top surface of the high-speed layer, and the excitation depth range is f, respectively, the main frequency f of the target layer dom and the maximum frequency f max The maximum amplitude value at and The range of corresponding depth points.

[0028] Furthermore, the explosion radius is:

[0029] R=kQ 1 / 3

[0030] Where Q is the excitation dose in the target work area, and k is a coefficient related to the excitation medium in the target work area.

[0031] Furthermore, the reflection coefficient r of the high-speed layer top interface incident from bottom to top is:

[0032] r=(v1-v2) / (v1+v2)

[0033] Where v2 is the velocity of the high-speed layer, and v1 is the velocity of the medium above the high-speed layer.

[0034] Furthermore, the delay time is:

[0035] τ=2d / v2

[0036] Where τ is the delay time between the virtual reflection of the interface at the top of the high-speed layer and the first downlink wave, d is the distance between the top surface of the charge column and the top surface of the high-speed layer, and v2 is the velocity of the high-speed layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a schematic diagram of a method flow in an embodiment of the method of the present invention;

[0038] Figure 2 1 is a schematic diagram comparing the determination of the minimum safe excitation depth in the embodiment of the method of the present invention with the prior art;

[0039] Figure 3 It is a graph of delayed superposition of ghost reflections of 35 Hz main frequency Ricker wavelet excited at different depths under the high-speed layer in the embodiment of the method of the present invention;

[0040] Figure 4 It is a ghost reflection amplitude curve response diagram of 35Hz and 70Hz main frequency sub-waves obtained in the embodiment of the method of the present invention;

[0041] Figure 5 The autocorrelation sub-wave and spectrum and lithology histogram obtained by using micro-well logging data in the embodiment of the method of the present invention;

[0042] Figure 6 It is a comprehensive diagram of the design of the excitation well depth according to the velocity, ghost reflection, micro-logging and lithology in the embodiment of the method of the present invention;

[0043] Figure 7 This is a 30-60 Hz frequency division scanning diagram of single shot data of different excitation depth tests in a certain work area in an embodiment of the method of the present invention. DETAILED DESCRIPTION

[0044] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0045] Seismic excitation depth determination method embodiment:

[0046] like Figure 1A method for determining seismic excitation depth is shown, firstly, the minimum safe excitation depth is determined according to the depth of the high-speed layer top interface and the explosion radius, the excitation depth range that is beneficial to seismic frequency protection is determined by the amplitude response of the main frequency and the highest frequency ghost reflection, the more favorable excitation depth range and the favorable excitation lithology range are determined by the micro-logging autocorrelation sub-wave and its spectrum, and the optimal excitation depth is determined according to the weight order of the minimum safe excitation depth, the excitation depth range determined by the ghost reflection, the favorable excitation depth range determined by the micro-logging, and the favorable lithology range. The specific implementation is as follows:

[0047] 1) Obtain the near-surface lithology, excitation charge dosage, charge column length, micro-logging data and micro-logging interpretation results of the target work area.

[0048] In this embodiment, the actual work area is taken as an example, and the excitation charge Q of the target work area is obtained to be 8kg, and the length of the charge column L is obtained to be 2.4m. In this embodiment, the micro-logging at the test point interprets the near surface as a three-layer structure, from top to bottom, respectively, a low-speed layer (first layer), a deceleration layer (second layer) and a high-speed layer (third layer), and the low-speed layer and the deceleration layer are collectively referred to as a low-speed deceleration layer. In this embodiment, taking the actual work area as an example, the micro-logging interpretation results are as follows: For example, the thickness h0 of the low-speed layer is 1.47m, and the velocity v0 is 357m / s, the thickness h1 of the deceleration layer is 8.73m, and the velocity v1 is 1053m / s, and the velocity v2 of the high-speed layer is 1654m / s, so the thickness h0 of the low-speed deceleration layer in this work area is 1.47m, and the velocity v0 is 357m / s, the thickness h1 of the deceleration layer is 8.73m, and the velocity v1 is 1053m / s, and the velocity v2 of the high-speed layer is 1654m / s. L =h0+h1=1.47m+8.73m=10.2m.

[0049] 2) Determine the minimum safe excitation depth based on the micro-logging interpretation results, excitation charge dosage and charge column length.

[0050] Specifically, the minimum safe excitation depth H0 is:

[0051] H0=h L +d=h L +(RL / 2)

[0052] Where H0 is the minimum safe excitation depth, h L is the thickness of the low-speed reduction layer, d is the distance between the top of the charge and the top of the high-speed layer, R is the explosion radius, and L is the length of the charge. In order to ensure the excitation effect, d is generally not less than 1m. The calculation formula for the explosion radius is:

[0053] R=kQ 1 / 3

[0054] Wherein, Q is the target work area excitation dose, k is a coefficient related to the work area excitation medium, and in this embodiment, k is 1.5.

[0055] When the excitation charge Q is 8 kg, the length of the charge column L is 2.4 m, and the thickness of the slow-down layer h L In the example of 10.2m, d = 3m-2.4m / 2 = 1.8m, and the minimum safe excitation depth H0 = 10.2m + 1.8m = 12m. Figure 2 As shown, the conventional method for determining the minimum safe excitation depth is to add the top depth of the high-speed layer to the explosion radius, that is, H0 = h L +R=10.2m+3m=13.2m. The minimum safe excitation depth obtained by the method for obtaining the minimum safe excitation depth in this embodiment is smaller than the excitation depth obtained by the conventional method. The reduction of the minimum safe excitation depth is beneficial to the protection of high-frequency signals and the improvement of the resolution of seismic data.

[0056] 3) Obtain the ghost reflection amplitude response curves of the main frequency and the highest frequency of the target layer, and determine the excitation depth range according to the maximum value of the ghost reflection amplitude response curve.

[0057] Specifically, the expression of the ghost reflection amplitude response curve of the seismic wavelet with the main frequency f is:

[0058] A f,d =Max(s f,d (t)) (1)

[0059] In the formula, s f，d (t) is the delayed superposition curve of the seismic wavelet with the main frequency of f and the virtual reflection of the top surface of the high-speed layer when the distance between the top surface of the charge column and the top surface of the high-speed layer is d, A f,d For f，d (t) Maximum value in the delay superposition curve time series, s f，d (t) is obtained from the following formula:

[0060] s f，d (t) = w f (t)+rw f (t-τ) (2)

[0061] Among them, τ is the delay time between the virtual reflection of the high-speed layer top interface and the first downlink wave, r is the reflection coefficient of the high-speed layer top interface incident from bottom to top, and w f (t) is the seismic wavelet with the main frequency f. Among them, the delay time τ is:

[0062] τ=2d / v2

[0063] The reflection coefficient r of the high-speed layer top interface is:

[0064] r=(v1-v2) / (v1+v2)

[0065] Wherein, d is the distance between the top surface of the charge column and the top surface of the high-speed layer, v2 is the velocity of the high-speed layer, and v1 is the velocity of the deceleration layer above the high-speed layer. The seismic wavelet in this embodiment adopts the Ricker wavelet, and the expression of the Ricker wavelet is:

[0066]

[0067] Based on the amplitude response curve of the virtual reflection of the seismic wavelet with the main frequency f, the maximum amplitude value when the top surface of the charge column is at different distances d from the top surface of the high-speed layer is determined by the following formula:

[0068] B f =Max(A f,d ) (3)

[0069] In the formula, B f A f,d The maximum amplitude value when the top surface of the explosive column is at different distances d from the top surface of the high-speed layer, and the d corresponding to this maximum amplitude value is marked as D f , D f It is the excitation position that makes the amplitude response of the seismic wavelet with the main frequency f the maximum, respectively with the main frequency f of the target layer dom and the maximum frequency f max Substitute f into the above formulas (2), (1), and (3) to calculate the excitation depth range In order to consider the influence of ghost reflection, this embodiment calculates the amplitude response of different main frequency seismic wavelets when the distance d between the top surface of the explosive column and the top surface of the high-speed layer is within the set range according to the set interval, and obtains the favorable excitation depth range. For example, in this embodiment, the amplitude response of 35Hz Ricker wavelet and 70Hz Ricker wavelet is analyzed at intervals of 1m when the distance d between the top surface of the explosive column and the top surface of the high-speed layer is 2-30m. Figure 3 The figure shows the delayed superposition curve of 35Hz Ricker wavelet excited at different depths under the high-speed layer (i.e., the distance between the top surface of the charge column and the top surface of the high-speed layer). The ordinate in the figure is time, in ms, and the abscissa is the distance between the top surface of the charge column and the top surface of the high-speed layer, in m. The maximum amplitude of each curve is obtained to obtain the amplitude response corresponding to each depth, and thus the ghost reflection amplitude response curve of the 35Hz main frequency Ricker wavelet is obtained, see Figure 4 Amplitude response curve of 35Hz Ricker wavelet excited at 2 to 30m below the high-speed layer. Figure 4 The figure shows the amplitude response curves of 35Hz Ricker wavelet and 70Hz Ricker wavelet excited 2 to 30m below the high-speed layer. The maximum amplitude response of 35Hz Ricker wavelet corresponds to the position where the distance d between the top surface of the explosive column and the top surface of the high-speed layer is 8m, and the maximum amplitude response of 70Hz Ricker wavelet corresponds to the position where the distance d between the top surface of the explosive column and the top surface of the high-speed layer is 3 to 4m. Therefore, it can be determined that the distance between the top surface of the explosive column and the top surface of the high-speed layer that is beneficial to 35Hz and 70Hz frequency protection is preferably 3 to 8m, that is, the favorable excitation depth range Hg for:

[0070] H g =h L +d=10.2m+3~8m=13.2~18.2m

[0071] Therefore, the favorable excitation depth range H g In the range of 13.2~18.2m.

[0072] 4) Obtain autocorrelation wavelets and frequency spectra from micro-logging data, determine the favorable excitation depth range based on the autocorrelation wavelets and frequency spectra, and judge the favorable excitation lithology range.

[0073] Specifically, Figure 5 As shown, the autocorrelation operation is performed on each micro-logging data to obtain the autocorrelation wavelet of each channel, and the Fourier transform is performed on the autocorrelation wavelet of each channel to obtain the spectrum. From the autocorrelation wavelet obtained from the micro-logging data in this embodiment, the wavelet excited at a depth of 12 to 30 m has a sharper main lobe and a weaker side lobe. From the obtained autocorrelation wavelet spectrum, the excitation depth has a wider and more stable frequency band above 12 m, so it can be determined that the depth of 12 to 30 m is a favorable excitation depth range. At the same time, from the lithology column chart, the lithology at a depth of 12-30 m is mainly clay and sand-containing clay, the excited wavelet frequency band is wider and the interference background is weaker, while the thick sand layer between 30 and 37 m excites the wavelet interference background. The clay layer, sand-containing clay or mud-sand interlayer in the high-speed layer has better excitation quality. In the lithology column chart, the striped line represents clay, the dotted line represents sand, and different combinations of the two represent changes in sediment content.

[0074] 5) The minimum safe excitation depth, the excitation depth range determined by ghost reflection, the favorable excitation depth range determined by micro-logging, and the favorable excitation lithology range are comprehensively considered, and the optimal excitation depth is determined according to the intersection of the ranges determined by the above four factors. If there is no intersection in the ranges determined by the above four factors, the minimum safe excitation depth, the excitation depth range determined by ghost reflection, the excitation depth range determined by micro-logging data, and the favorable excitation lithology range are determined in descending order of priority.

[0075] Specifically, the optimal excitation depth range is determined by the minimum safe excitation depth and the excitation depth range determined by virtual reflection. On the basis of the determined excitation depth range, the favorable excitation depth range and favorable excitation lithology range determined by micro-well logging are used as reference items. The excitation depth is further optimized according to the obtained results, and finally the optimal excitation depth is comprehensively selected. Among them, the optimal excitation depth is determined to be greater than the minimum safe excitation depth and within the intersection of the excitation depth range determined by virtual reflection, the favorable excitation depth range determined by micro-well logging, and the favorable excitation lithology range. Figure 6As shown, in this embodiment, the minimum safe excitation depth H0 is 12m, and the excitation depth range H g The favorable excitation depth determined by micro-logging is 12-30m. The excitation depth between 12-30m is more favorable. Therefore, the minimum intersection of the above four is the excitation depth of 13.2-18.2m. From the amplitude response of the ghost reflection in this depth range, it can be seen that a smaller excitation depth is favorable for 70Hz high-frequency signals, and a larger excitation depth is favorable for 35Hz main frequency signals. Considering the protection of the main frequency and high-frequency signals, the excitation depth is determined to be 15.2m, that is, 5m below the top interface of the high-speed layer. Figure 6 The thick horizontal line in the middle indicates the location.

[0076] Through the above process, the minimum safe excitation depth is determined according to the depth of the high-speed layer top interface and the explosion radius, the excitation depth range that is beneficial to seismic frequency protection is determined by the amplitude response of the main frequency and the highest frequency ghost reflection, the more favorable excitation depth range is determined by the micro-logging autocorrelation sub-wave and its spectrum, and the lithology range with better excitation effect is clarified. The optimal excitation depth is determined by combining the minimum safe excitation depth, the excitation depth range determined by ghost reflection, the favorable excitation depth range determined by micro-logging, and the favorable excitation lithology range. Figure 7 The 30-60 Hz frequency division scanning diagram of the actual single shot records excited at different depths below the top surface of the high-speed layer of the micro-logging point in the work area is shown. The single shot reflection signal excited at a depth of 5m below the top interface of the high-speed layer has stronger energy and richer details, and has better protection for the main frequency and high-frequency signals, which is better than the single shot records obtained at other excitation depths, verifying the correctness and feasibility of this method to determine the excitation depth.

[0077] Specific implementation methods are given above, but the present invention is not limited to the described implementation methods. The basic idea of ​​the present invention lies in the above basic scheme. For ordinary technicians in this field, it does not take creative work to design various deformed models, formulas, and parameters according to the teachings of the present invention. Changes, modifications, substitutions, and variations of the implementation methods without departing from the principles and spirit of the present invention still fall within the scope of protection of the present invention.

Claims

1. A method for determining seismic excitation depth, characterized in that: The following steps are involved: 1) Obtain the near-surface lithology, excitation charge amount, charge column length, micro-logging data and micro-logging interpretation results of the target work area; 2) Determine the minimum safe excitation depth based on micro-logging interpretation results, excitation charge dosage and charge column length; 3) Obtaining the ghost reflection amplitude response curves of the main frequency and the highest frequency of the target layer, and determining the excitation depth range according to the maximum value of the ghost reflection amplitude response curve; 4) Obtain autocorrelation wavelet and spectrum from micro-logging data, determine favorable excitation depth range based on autocorrelation wavelet and spectrum, and judge favorable excitation lithology range; 5) Determine the optimal excitation depth by comprehensively considering the minimum safe excitation depth, the excitation depth range determined by virtual reflections, the favorable excitation depth range determined by micro-logging, and the favorable excitation lithology range.

2. The method for determining the seismic excitation depth according to claim 1, characterized in that: The optimal excitation depth determined in the step 5) is greater than the minimum safe excitation depth and is within the intersection of the excitation depth range determined by ghost reflection, the favorable excitation depth range determined by micro-logging, and the favorable excitation lithology range. If the ranges determined by the four factors do not have an intersection, they are determined in the order of the minimum safe excitation depth, the excitation depth range determined by ghost reflection, the favorable excitation depth range determined by micro-logging, and the favorable excitation lithology range.

3. The method for determining the seismic excitation depth according to claim 2, characterized in that: The minimum safe excitation depth is: H0=h L +d=h L +(RL / 2) Where H0 is the minimum safe excitation depth, h L is the thickness of the low velocity reduction layer, d is the distance between the top surface of the charge and the top surface of the high velocity layer, R is the explosion radius, and L is the length of the charge.

4. The method for determining the seismic excitation depth according to claim 1, characterized in that: The calculation formula of the ghost reflection amplitude response curve is: A f，d =Max(s f,a (t)) In the formula, s f，d (t) is the delayed superposition curve of the seismic wavelet with the main frequency of f and the virtual reflection of the top surface of the high-speed layer when the distance between the top surface of the charge column and the top surface of the high-speed layer is d, A f,d For f，d (t) Maximum value in the delay superposition curve time series, s f，d (t) is obtained from the following formula: s f，d (t)=w f (t)+rw f (t-τ) Where τ is the delay time between the virtual reflection of the high-speed layer top interface and a downlink wave, r is the reflection coefficient of the high-speed layer top interface incident from bottom to top, and w f (t) is the seismic wavelet with the main frequency f, and t is the time.

5. The method for determining the seismic excitation depth according to claim 2, characterized in that: The favorable excitation depth range is a depth segment with fewer sub-wave side lobes, wider spectrum width and more stable distribution.

6. The method for determining the seismic excitation depth according to claim 4, characterized in that: The excitation depth range is obtained by the following formula: B f =Max(A f，d ) Where B f A is the ghost reflection amplitude response curve f，d The maximum amplitude value among all different distances d between the top surface of the charge column and the top surface of the high-speed layer, and the excitation depth range is f, respectively, the main frequency f of the target layer dom and the maximum frequency f max The maximum amplitude value at and The range of corresponding depth points.

7. The method for determining the seismic excitation depth according to claim 3, characterized in that: The explosion radius is: R=kQ 1 / 3 Where Q is the excitation dose in the target work area, and k is a coefficient related to the excitation medium in the target work area.

8. The method for determining seismic excitation depth according to claim 4, characterized in that: The reflection coefficient r of the high-speed layer top interface incident from bottom to top is: r=(v1-v2) / (v1+v2) Where v2 is the velocity of the high-speed layer, and v1 is the velocity of the medium above the high-speed layer.

9. The method for determining the seismic excitation depth according to claim 4, characterized in that: The delay time is: τ=2d / v2 Where τ is the delay time between the virtual reflection of the interface at the top of the high-speed layer and the first downlink wave, d is the distance between the top surface of the charge column and the top surface of the high-speed layer, and v2 is the velocity of the high-speed layer.

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