Method for inverting high-speed layer of extremely-thick low-speed-reduction belt based on cannon first arrival

Abstract

The invention discloses a method for inverting a high-speed layer of an extremely thick low-speed-reduction zone based on gun first arrival, which comprises the following steps of: 1, in an experimental area with a relatively large burial depth of the high-speed layer, exciting by adopting a point above the top surface of the high-speed layer to obtain a single-shot record of the point, and picking up a first arrival wave by a manual assistance method; 2, the curve slope is obtained from the single shot record, v0, v1 and v2 are further obtained, v0 corresponds to the propagation speed of the excitation point in the low speed reduction zone, v1 corresponds to the reflected wave speed when the seismic wave is propagated in the low speed reduction zone, and v2 corresponds to the reflected wave speed when the seismic wave is propagated downwards to a high-speed layer; 3, drawing a time distance curve graph; and 4, uniformly taking points, calculating numerical values point by point, drawing a corresponding high-speed layer velocity diagram and a high-speed layer burial depth diagram, and completing surface structure investigation work. According to the method, the speed and burial depth values of the high-speed layer of the extremely-thick low-speed-reduction belt can be successfully solved, and the blank of surface layer survey results that a conventional surface layer survey method cannot control the high-speed layer area is filled.

Description

Technical Field

[0001] The invention relates to a method for inverting a high-speed layer in a thick low-velocity reduction zone based on a cannon first arrival, and belongs to the technical field of seismic exploration. Background Art

[0002] Small refraction and single-well micro-logging are conventional methods for investigating surface geological structures. Small refraction is often used to study velocity changes in low-velocity reduction zones and to analyze the first arrival times of direct waves propagating in low-velocity layers and refracted waves at near-surface refraction interfaces. However, the accuracy is low and the construction is restricted by terrain. Single-well micro-logging overcomes the disadvantages of small refraction by drilling in low-velocity layers and using the first arrival of transmitted waves to obtain surface structures. However, it is more expensive and difficult to drill into high-velocity layers.

[0003] Since many current seismic exploration research areas are located in the heart of the desert in arid areas, there are extremely thick low-velocity reduction zones. When using the extremely thick low-velocity reduction zone micro-logging method for surface surveys during construction, it is difficult to drill into the high-speed layer in the heart of the desert, which directly leads to a large difference between the velocity results of the surface survey and the actual situation. Therefore, in order to solve the problem that micro-logging is difficult to measure the velocity of thicker high-speed layers, the present invention is based on the micro-logging surface survey, and uses the reflected wave and refracted wave information of the top interface of the high-speed layer, combined with the principle of seismic wave propagation, to calculate the high-speed layer velocity and burial depth through mathematical methods. Summary of the invention

[0004] The purpose of the present invention is to provide a method for inverting high-speed layers in thick low-velocity zones based on the first arrival of a cannon. In order to solve the problem that micro-logging is difficult to measure the velocity of thick high-speed layers, the research is based on the surface investigation of micro-logging, and the reflection and refraction wave information of the top interface of the high-speed layer is combined with the principle of seismic wave propagation to calculate the velocity and burial depth of the high-speed layer through mathematical methods.

[0005] To achieve the above object, the technical solution adopted by the present invention is: a method for inverting a high-speed layer in a thick low-velocity reduction zone based on the first arrival of a cannon, which comprises the following steps:

[0006] Step 1: In the experimental area where the high-velocity layer is buried at a large depth, a point above the top surface of the high-velocity layer is used for excitation to obtain a single shot record of the point, and the first arrival wave is picked up using a manual assistance method;

[0007] Step 2: Obtain the slope of the curve, k0, k1, k2, from the single shot record, and Further obtain v0, v1, and v2, where v0 corresponds to the propagation velocity of the excitation point in the low velocity reduction zone, v1 corresponds to the reflection wave velocity when the seismic wave propagates in the low velocity reduction zone, and v2 corresponds to the reflection wave velocity when the seismic wave propagates downward to the high-speed layer;

[0008] Step 3: Draw a time-distance curve graph;

[0009] Step 4: Evenly select points in the study area and calculate the values ​​point by point, draw the corresponding high-speed layer velocity map and high-speed layer burial depth map, and complete the surface structure investigation.

[0010] As a further optimization of this scheme, in the step one, when reading the reflection time t of the top surface of the high-speed layer, it cannot be stuck at the center of the phase. According to the first arrival wave point, the time extending upward by 0.75 cycles from the center of the blackened phase is the real first arrival time.

[0011] As a further optimization of this scheme, in the step 2, the arrival time of the first wave in the single shot record is analyzed, and the velocity v value is reversed. At the beginning, what is received is the seismic wave propagating in the low velocity reduction zone, and then what is received is the refracted wave generated by the refraction when the seismic wave propagates to the high-speed zone, wherein v2>v0>v1, which is consistent with the actual single shot record.

[0012] As a further optimization of this solution, in step 3, the buried depth of the high-speed layer in the thick low-velocity reduction zone is calculated according to the earthquake propagation theory combined with the numerical method. Assuming that the average velocity of the strata above the top surface of the high-speed layer is v, the time for the seismic wave to propagate to the top interface of the high-speed layer is t, and the propagation time from the excitation point to the ground is Δt, then:

[0013] to+Δt=2H / v, The obtained H is the buried depth of the high-speed layer.

[0014] The beneficial effects of the present invention are as follows: the present invention accurately identifies the reflection and refraction waves of the top interface of the high-speed layer in single-shot records, draws a time-distance curve diagram, and determines a mathematical calculation method based on the seismic wave propagation theory, thereby successfully obtaining the velocity and burial depth values ​​of the high-speed layer in the extremely thick low-velocity reduction zone, filling the gap in surface survey results in the high-speed layer area that cannot be controlled by conventional surface survey methods.

[0015] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a technical flow chart of the present invention;

[0017] Figure 2 This is a first arrival wave picking diagram of a single shot recording of low-speed reduction zone excitation in the present invention;

[0018] Figure 3 It is a velocity variation diagram of the seismic wave propagating in the low velocity reduction zone of the present invention;

[0019] Figure 4 It is a diagram of the seismic wave propagation mode in the low velocity reduction zone;

[0020] Figure 5 It is a plane diagram of the velocity of high-speed layers in surface surveys of conventional micro-well logging;

[0021] Figure 6 It is a velocity plane map of high-speed layers in surface survey based on the cannon first-arrival inversion method;

[0022] Figure 7 It is a plane diagram of the thickness variation of high-speed layers in surface surveys of conventional micro-well logging;

[0023] Figure 8 It is a plane diagram of the thickness variation of high-velocity layers in surface survey based on the cannon first-arrival inversion method;

[0024] Fig. 9 The surface model is constructed based on the cannon first arrival inversion method. DETAILED DESCRIPTION

[0025] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] A method for inverting a high-speed layer in a thick low-velocity zone based on cannon first arrival, comprising the following steps:

[0027] Step 1: In the experimental area where the high-speed layer is buried at a large depth, a point above the top surface of the high-speed layer is used for excitation to obtain a single shot record of the point, and the first arrival wave is picked up by a manual assistance method (when reading the reflection time t of the top surface of the high-speed layer, it cannot be stuck at the center of the phase. According to the first arrival wave point, it should be extended 0.75 cycles upward from the center of the blackened phase to obtain the real first arrival time);

[0028] Step 2: Single shot record Figure 2 As shown, the slope of the curve, k0, k1, k2, is obtained from the record. Therefore, v0, v1, and v2 can be obtained, where v0 corresponds to the propagation velocity of the excitation point in the low velocity reduction zone, v1 corresponds to the reflected wave velocity when the seismic wave propagates in the low velocity reduction zone, and v2 corresponds to the reflected wave velocity when the seismic wave propagates downward to the high-speed layer. By analyzing the arrival time of the first arrival wave recorded by a single shot, we found that the velocity v value is reversed. This is because the seismic wave propagating in the low velocity reduction zone is received at the beginning, and the refracted wave generated by the refraction when the seismic wave propagates to the high-speed zone is received later. The change in the propagation velocity of the seismic wave is as follows: Figure 3 As shown, therefore v2>v0>v1, which is consistent with the actual single-shot record.

[0029] Step 3: Draw a time-distance curve, according to earthquake propagation theory ( Figure 4 ), combined with numerical methods to calculate the buried depth of the high-speed layer in the thick low-velocity reduction zone, assuming that the average velocity of the strata above the top of the high-speed layer is v, the time it takes for the seismic wave to propagate to the top interface of the high-speed layer is t, and the propagation time from the excitation point to the ground is Δt, then:

[0030] t o +Δt=2H / v, The obtained H is the buried depth of the high-speed layer.

[0031] Step 4: Evenly select points in the study area and calculate the values ​​point by point, draw the corresponding high-speed layer velocity map and high-speed layer burial depth map, and complete the surface structure investigation.

[0032] In this embodiment, a plateau desert block with a typical extremely thick low velocity reduction zone is selected as the test area. First, conventional micro-logging surface surveys are conducted on some test points, the micro-logging depth is changed, the near-surface velocity changes are measured, and the results are statistically interpreted.

[0033] Table 1 Surface parameters of test point A

[0034] Speed ​​v(m / s) Depth(m) 318 6.36 629 17 809 33.63 1210

[0035] Table 2 Surface parameters of test point B

[0036] Speed ​​v(m / s) Depth(m) 378 7.76 597 30.9 816 41.08 957

[0037] The surface conditions map of the work area was drawn based on the surface survey results of the test points. It was found that since the micro-well logging could not reach the high-speed layer, the maximum velocity measured was not the high-speed layer velocity. The surface survey results map had defects and could not provide effective assistance for seismic acquisition work.

[0038] Combined with the existing measurement data, the velocity and depth of the high-speed layer are obtained by the cannon first-arrival inversion method, and the plane diagram of the velocity change of the high-speed layer is drawn ( Figure 6 ) and thickness variation plan ( Figure 8 ), and finally, combined with the research results, it was confirmed that the low-velocity thickness profile was in good agreement with the surface model obtained by the first-arrival inversion of the cannon ( Fig. 9 ), this technical method can fill the gap in surface survey results in high-speed layer areas that cannot be controlled by conventional surface survey methods.

[0039] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the protection scope of the present invention in any form, and all technical solutions obtained by equivalent replacement and the like fall within the protection scope of the present invention. The parts not involved in the present invention are the same as the prior art or can be implemented by the prior art.

Claims

1. A method for inverting high-speed layers in thick low-velocity zones based on first-arrival cannon, characterized in that The steps include: Step 1: In the experimental area where the high-velocity layer is buried at a large depth, a point above the top surface of the high-velocity layer is used for excitation to obtain a single shot record of the point, and the first arrival wave is picked up using a manual assistance method; Step 2: Obtain the slope of the curve, k0, k1, k2, from the single shot record, and Further obtain v0, v1, and v2, where v0 corresponds to the propagation velocity of the excitation point in the low velocity reduction zone, v1 corresponds to the reflection wave velocity when the seismic wave propagates in the low velocity reduction zone, and v2 corresponds to the reflection wave velocity when the seismic wave propagates downward to the high-speed layer; Step 3: Draw a time-distance curve graph; Step 4: Evenly select points in the study area and calculate the values ​​point by point, draw the corresponding high-speed layer velocity map and high-speed layer burial depth map, and complete the surface structure investigation.

2. According to claim 1, a method for inverting high-speed layers in thick low-velocity zones based on cannon first arrivals is characterized in that: In the step 1, when reading the reflection time t of the top surface of the high-speed layer, it cannot be stuck at the center of the phase. According to the first arrival wave point, the time extending 0.75 cycles upward from the center of the blackened phase is the real first arrival time.

3. The method for inverting high-speed layers in thick low-velocity zones based on cannon first arrival according to claim 1 is characterized in that: In the step 2, the arrival time of the first wave in the single shot record is analyzed, and the velocity v value is reversed. At the beginning, what is received is the seismic wave propagating in the low velocity reduction zone, and then what is received is the refracted wave generated by the refraction when the seismic wave propagates to the high-speed zone, where v2>v0>v1, which is consistent with the actual single shot record.

4. The method of inverting high-speed layers in thick low-velocity zones based on cannon first arrival according to claim 1 is characterized in that: In step 3, the buried depth of the high-speed layer in the thick low-velocity reduction zone is calculated based on the earthquake propagation theory combined with the numerical method. Assuming that the average velocity of the strata above the top of the high-speed layer is v, the time for the seismic wave to propagate to the top interface of the high-speed layer is t, and the propagation time from the excitation point to the ground is Δt, then: to+Δt=2H / v, The obtained H is the buried depth of the high-speed layer.

Images