Method for determining thickness of slope accumulation zone

Abstract

The invention relates to a method for determining the thickness of a slope area, and belongs to the technical field of seismic exploration. According to the method for determining the thickness of the slope accumulation zone, the depth of an amplitude interface is determined through the amplitude maximum value of seismic wave data excited by an excitation point of each depth in the same long-distance common receiving point gather data, and then the thickness of the slope accumulation zone is determined. According to the method for determining the thickness of the slope accumulation zone, the determined thickness of the slope accumulation zone is well matched with the excitation effect in production, high analysis accuracy is achieved, meanwhile, micro-logging interpretation can be more visual, simpler and more precise, the interpretation process is easier and more convenient to check, manual modification of first arrival time is avoided, the result is more real and reliable, and the method is suitable for popularization and application. The method is of great significance to accurate design of the well depth of the mountain slope accumulation zone and improvement of seismic data quality.

Description

Technical Field

[0001] The invention relates to a method for determining the thickness of a slope accumulation zone, and belongs to the technical field of seismic exploration. Background Art

[0002] The slope accumulation zone is formed when the weathered rocks are washed down by rain or snow, or accumulated on gentle slopes or at the foot of slopes by gravity. The slope accumulation zone is characterized by looseness, high porosity and high compressibility.

[0003] The slope accumulation zone generally exists in the middle, lower and foot of the slope, with vague stratification roughly parallel to the slope surface. The slope accumulation zone is generally not large in scope, thicker in the middle and thinner at the edge. The elevation changes greatly along the slope, the thickness of the slope accumulation zone changes greatly, and the range of mutation is relatively small; the elevation changes vertically in the slope direction are small, and the thickness of the slope accumulation zone changes slowly. Below the slope accumulation zone is bedrock, which is a rock layer and can be described using the properties of the rock layer. The velocity of the slope accumulation zone is generally 500-1000m / s in the shallow layer, and the velocity is generally 1000-2000m / s in the deep layer due to the increase in water content. The velocity of the lower bedrock is 3000-4000m / s.

[0004] At present, the commonly used near-surface structure survey methods are mainly shallow refraction method and micro-logging method. Relatively speaking, micro-logging method can divide the near-surface velocity layer in detail, and the near-surface structure survey accuracy is relatively high. Micro-logging is to conduct surface structure survey and research work by using the excitation well to generate seismic waves. It directly drills holes in the low-velocity drop zone, and realizes well excitation and ground reception or ground excitation and well reception through drilling. The near-surface structure is studied using the recorded reflection first arrival wave. It provides accurate near-surface velocity and thickness models for point-by-point design of the excitation well depth, improving the stability of the excitation sub-wave, and accurately obtaining the static correction amount.

[0005] When using micro-logging technology to investigate the thickness of the slope accumulation zone, wellhead excitation and in-well reception are generally used. When detailed investigation is required, in-well excitation and ground reception are used. In-well excitation micro-logging uses excitation at different depths in the well, and detection points are arranged near the wellhead on the surface. 3 to 12 detectors are used for reception. The well detection distance (also known as offset distance, that is, the distance from the wellhead to the detector) is generally 1 to 6 meters. According to the surface undulation, the detector layout diagram is usually in the shape of a straight line, a right angle, a fan or a cross. When this traditional method is used to investigate the slope accumulation zone, the seismic wave scattering in the slope accumulation zone is serious, and the interpretation of the thickness of the slope accumulation zone using the first arrival of seismic waves is inaccurate.

[0006] Therefore, a method for determining the thickness of the slope accumulation zone is urgently needed to improve the accuracy of the interpretation results of the slope accumulation zone thickness. Summary of the invention

[0007] The purpose of the present invention is to provide a method for determining the thickness of a slope accumulation zone, which can solve the problem of low accuracy of the results when the micro-logging technology is currently used to investigate the thickness of the slope accumulation zone.

[0008] In order to achieve the above object, the technical solution adopted by the method for determining the thickness of the slope accumulation zone of the present invention is:

[0009] A method for determining the thickness of a slope accumulation zone comprises the following steps:

[0010] (1) Determine the target point on the surface of the slope accumulation zone, where the slope accumulation zone thickness at the target point is the maximum value of the estimated slope accumulation zone thickness; then design a micro-well at the target point, where the depth of the micro-well is 10 to 15 m greater than the maximum value of the estimated slope accumulation zone thickness;

[0011] (2) using a geophone to receive seismic wave data excited by excitation points at different depths in the micro-logging well, to obtain seismic micro-logging data; the geophone is located on the bedrock in an area 100 to 400 m away from the wellhead of the micro-logging well;

[0012] (3) Obtain the maximum amplitude value of the seismic wave data excited by the excitation point at each depth in the same long-channel common receiving point gather data, and then calculate the change rate of the amplitude maximum values ​​corresponding to adjacent excitation points. If the change rate of the amplitude maximum values ​​corresponding to adjacent excitation points is greater than 5, the depth of the amplitude interface is located between the depths of the adjacent excitation points;

[0013] (4) According to the thickness of the slope accumulation zone at the location of the target point, the thickness of the slope accumulation zone at other locations on the surface of the slope accumulation zone is determined; the thickness of the slope accumulation zone at the location of the target point is the depth of the amplitude interface.

[0014] The method for determining the thickness of the slope accumulation zone of the present invention determines the depth of the amplitude interface through the amplitude maximum value of the seismic wave data excited by the excitation point at each depth in the same long-path common receiving point data set, and then determines the thickness of the slope accumulation zone. The slope accumulation zone thickness determined by the method for determining the thickness of the slope accumulation zone of the present invention is consistent with the excitation effect in production, has high analysis accuracy, and can make the micro-logging interpretation more intuitive, simple, and precise, and the interpretation process is easier to check, avoiding artificial modification of the first arrival time, and the results are more real and reliable. It is of great significance for the accurate design of the well depth of the mountain slope accumulation zone and improving the quality of seismic data.

[0015] An obvious feature of the slope accumulation zone is the exposure of bedrock at the edge, or a very thin low-velocity layer. A micro-logging remote detector can be set up to receive direct waves excited from the bedrock and sliding refraction waves excited from the slope accumulation zone.

[0016] The seismic waves excited inside the bedrock have high energy and decay slowly during propagation in the bedrock. The seismic waves excited inside the slope accumulation zone have low excitation energy and decay quickly during propagation in the slope accumulation zone. Then, they pass through the interface between the slope accumulation zone and the bedrock and produce another conversion wave attenuation. When they reach the far receiving point in the bedrock, the energy is small. By placing a detector in the bedrock far away from the excitation point and comparing the amplitude of the seismic wave data excited by the same detector at different depths, a clear amplitude interface can be obtained.

[0017] Spherical diffusion will occur during the propagation of seismic waves. In order to reduce the impact, the receiving point is 100m away. At the same time, in order to ensure that the received seismic wave signal has a certain signal-to-noise ratio, the distance should not exceed 400m. The following formula can be used to analyze the propagation path error: Without considering the influence of elevation (generally the surface of the slope accumulation zone is relatively flat), when the depth of the excitation point is 0m, the propagation path length is 100m, when the depth of the excitation point is 20m, the propagation path length L = (100×100+20×20) 1 / 2 =102m, the error is 2%, and the impact can be ignored.

[0018] It can be understood that when the detector is set on the bedrock in the area 100 to 400 meters away from the micro-logging wellhead, exposed bedrock can be found in the area 100 to 400 meters away from the micro-logging wellhead, or a pit can be dug at the thin edge of the slope accumulation zone to find the bedrock below the surface.

[0019] Preferably, the depth difference between two adjacent excitation points is 0.5 to 1 m. If the depth difference between two adjacent excitation points is too large, it will cause a large error in the depth of the determined amplitude interface. If the depth difference between two adjacent excitation points is too small, it will cause excessive workload and high cost.

[0020] Preferably, the depth of the excitation point is not greater than 3m, the depth difference between two adjacent excitation points is equal, and the depth difference between two adjacent excitation points is 0.5-0.6m; the depth of the excitation point is greater than 3m, the depth difference between two adjacent excitation points is equal, and the depth difference between two adjacent excitation points is 0.8-1m.

[0021] Preferably, the rate of change of the amplitude maximum is determined by a method comprising the following steps: assuming that the excitation point A and the excitation point B are adjacent, the amplitude maximum corresponding to the excitation point A is C, and the amplitude maximum corresponding to the excitation point B is D, then the rate of change of the amplitude maximum corresponding to the excitation point A and the excitation point B is equal to (DC) / C, wherein the depth of the excitation point A is less than the depth of the excitation point B.

[0022] Preferably, the long-channel seismic data refers to seismic data with an offset distance greater than or equal to 100 m. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of near-track seismic data with track number 1 in an embodiment of the present invention;

[0024] Figure 2 Schematic diagram of long-channel seismic data with channel number 5 in an embodiment of the present invention;

[0025] Figure 3 It is a schematic diagram of the result of arranging the seismic waves excited by the excitation points at different depths received by the detector numbered 5 in an embodiment of the present invention from small to large in a two-dimensional coordinate system including a time coordinate according to the depth of the excitation point;

[0026] Figure 4 This is a schematic diagram of the results of thickness analysis by drawing a time-depth curve using a conventional method in an experimental example of the present invention. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below in conjunction with specific embodiments.

[0028] Example

[0029] Taking a certain slope accumulation belt as an example, the method for determining the thickness of the slope accumulation belt in this embodiment specifically includes the following steps:

[0030] (1) Field investigation of the distribution characteristics of the slope accumulation belt, determination of the high-speed layer velocity as 3000 m / s, determination of the target point on the surface of the slope accumulation belt based on the surrounding terrain characteristics, the slope accumulation belt thickness at the target point is the maximum value of the estimated slope accumulation belt thickness, and the slope accumulation belt thickness can be determined by estimating the dip and inclination of the bedrock strata;

[0031] Then, a micro-well logging is designed at the target point, and the target point coordinates are converted into the work area grid coordinates. The position corresponding to the target point is the point corresponding to the line number 7326 and the pile number 2020. In order to ensure that the slope accumulation zone can be penetrated, the micro-well logging depth should be 10 to 15 meters greater than the maximum value of the slope accumulation zone thickness estimation value (the value is 15 meters in this embodiment), that is, the micro-well logging should be penetrated to 10 to 15 meters below the bedrock (the value is 15 meters in this embodiment);

[0032] After drilling the micro-logging well, a detection point is arranged near the wellhead of the micro-logging well. In order to avoid failure to successfully receive seismic signals due to failure of the detector, three detectors are used for reception. The well offset is 1m and they are numbered 1, 2, and 3, corresponding to the corresponding seismic channel numbers.

[0033] Three remote geophones are placed at the outcrop position (on the exposed bedrock) 100 to 400 m (the value in this embodiment is 170 m) away from the micro-logging wellhead, numbered 4, 5, and 6, corresponding to the corresponding seismic track numbers;

[0034] Then, 28 excitation points are set in the micro-logging depth interval of 0.5m to 25m (the depths of the excitation points are shown in Table 1), detonators are used as the source of the excitation seismic waves, and the seismic wave data excited by the excitation points at different depths are received by the geophone, so as to obtain the seismic micro-logging data of all seismic traces;

[0035] (2) Extracting gathers from the seismic micro-logging data to obtain common receiving point gather data, that is, seismic wave data excited by excitation points at different depths received by the same receiving point at different times. For the convenience of display and observation, the seismic wave data are arranged from small to large according to the depth of the excitation point. Since there are a total of 6 detectors, there are a total of 6 groups of common receiving point gather data, and the channel numbers of the seismic data are 1, 2, 3, 4, 5, and 6 respectively; among them, the seismic data with channel numbers 1, 2, and 3 are near-channel seismic data with an offset distance of 1 m, and the seismic data with channel numbers 4, 5, and 6 are far-channel seismic data with an offset distance of 170 m. In this embodiment, the near-channel seismic data refers to the seismic data with an offset distance of less than 10 m, and the far-channel seismic data refers to the seismic data with an offset distance greater than or equal to 100 m.

[0036] The near-track seismic data with track number 1 is as follows Figure 1 As shown, Figure 1 The horizontal axis represents time in ms, and the vertical axis represents the depth of the excitation point (i.e., the distance between the excitation point and the wellhead) in m.

[0037] The distant seismic data of channel number 5 is as follows Figure 2 As shown, Figure 2 The horizontal axis represents time in ms, and the vertical axis represents the depth of the excitation point (i.e., the distance between the excitation point and the wellhead) in m.

[0038] (3) Normalizing the amplitude in the same distant channel common receiving point gather data. Taking the seismic data with channel number 5 as an example, the normalization method includes the following steps:

[0039] ① First, obtain the maximum amplitude value of the seismic wave data excited by the excitation point at each depth in the same common receiving point gather data, as shown in Table 1;

[0040] ② Compare the maximum amplitude values ​​of seismic wave data excited by the excitation points at different depths in the same common receiving point gather data, and obtain the maximum amplitude value, which is 1842.3162;

[0041] ③ Divide all amplitudes in the same common receiving point gather data by the maximum amplitude to obtain the normalized amplitude data (normalized amplitude value);

[0042] Table 1 Maximum amplitude and normalized amplitude of seismic wave data excited by excitation points at different depths in seismic data with channel number 5

[0043]

[0044]

[0045] Note: The normalized amplitude values ​​in Table 1 represent the normalized amplitude values ​​corresponding to the maximum amplitude of the seismic wave data excited by the excitation point at a certain depth.

[0046] (4) Determine the amplitude boundary surface according to the curve of the amplitude maximum value of the seismic wave data excited by the excitation points at each depth versus the depth. The specific steps are as follows: According to the depth of each excitation point and the amplitude maximum value of the seismic wave data excited by each excitation point in Table 1, when the depth of the excitation point increases from 15 m to 16 m, the amplitude maximum value of the seismic wave data changes sharply, and the change rate of the amplitude maximum value reaches 7.58, that is, the depth of the amplitude boundary surface is between 15 and 16 m;

[0047] In this step, in order to determine the depth of the amplitude interface, the full waveform of the same common receiving point gather (the distant channel seismic data with channel number 5) can also be reduced in size according to a certain ratio. The maximum amplitude (1842.3162) is reduced and displayed as 20 in the display graph. The normalized amplitude value obtained in step (3) is multiplied by 20, which is the display value of the corresponding amplitude value in the display graph.

[0048] The seismic waves excited by the excitation points at different depths received by the detector numbered 5 are reduced and arranged from small to large in a two-dimensional coordinate system including the time coordinate according to the depth of the excitation point. Figure 3 As shown, Figure 3 The horizontal axis represents time in ms, the time grid is 20ms, and the vertical axis represents the depth of the excitation point (i.e. the distance between the excitation point and the wellhead) in m. Figure 3 It can be seen that the depth of the amplitude interface is between 15 and 16 m;

[0049] The depth of the amplitude interface is the thickness of the slope accumulation zone at the target point, that is, the thickness of the slope accumulation zone at the target point is 15 to 16 meters, and then the thickness of the slope accumulation zone at other locations on the surface of the slope accumulation zone is determined.

[0050] After obtaining the thickness of the slope accumulation zone at the target point, the thickness of the slope accumulation zone at other locations on the surface of the slope accumulation zone can be obtained by jointly calculating the inclination and dip of the bedrock at the bottom of the slope accumulation zone and the thickness of the slope accumulation zone at the target point with the coordinate information of the target point.

[0051] Experimental example

[0052] In order to compare with the conventional method, the method of drawing time-depth curve is used to determine the stratified velocity and stratified thickness of the slope accumulation zone at the target point. Then the thickness of each low-velocity layer is accumulated to obtain the thickness of the slope accumulation zone at the target point.

[0053] Among them, the method of drawing the time-depth curve is as follows:

[0054] (1) The horizontal direction is the time axis, with the unit of ms, and the vertical direction is the depth axis, with the unit of m. Each near-track first-arrival seismic data (the first-arrival seismic data with track number 1) corresponds to a depth-time pair, which is plotted to the corresponding position of the coordinate system to obtain each data point;

[0055] (2) The data points that can be fitted into a straight line are connected end to end. The resulting line segment represents a layer. The slope of the line segment indicates the velocity magnitude of the layer. The intersection of the two line segments of two adjacent layers is the layer interface between the two adjacent layers. The thickness of each layer is the difference in depth between the top interface and the bottom interface of the layer, that is, the difference in depth between the two end points (connection points) of the line segment representing the layer.

[0056] The time-depth curve is plotted as follows: Figure 4 As shown, Figure 4 The horizontal axis is time, in ms, and the vertical axis is depth, in m; Figure 4 It can be seen that the thickness of the slope accumulation zone at the target point determined by the traditional interpretation method is 12.67m.

[0057] Therefore, the thickness of the slope accumulation zone at the location of the target point determined by the present invention differs by about 3 m from the thickness of the slope accumulation zone at the location of the target point determined by the conventional interpretation method.

[0058] In order to verify the accuracy of the thickness of the slope accumulation zone determined by the conventional method and the method of the present invention, a cannon excitation test was used for verification. The results showed that the method for determining the thickness of the slope accumulation zone of the embodiment was more accurate and reliable, while the traditional method had a large error.

Claims

1. A method for determining the thickness of a slope accumulation zone, characterized in that: The following steps are involved: (1) Determine the target point on the surface of the slope accumulation zone, where the slope accumulation zone thickness at the target point is the maximum value of the estimated slope accumulation zone thickness; then design a micro-well at the target point, where the depth of the micro-well is 10 to 15 m greater than the maximum value of the estimated slope accumulation zone thickness; (2) using a geophone to receive seismic wave data excited by excitation points at different depths in the micro-logging well, to obtain seismic micro-logging data; the geophone is located on the bedrock in an area 100 to 400 m away from the wellhead of the micro-logging well; (3) Obtain the maximum amplitude value of the seismic wave data excited by the excitation point at each depth in the same long-channel common receiving point gather data, and then calculate the change rate of the amplitude maximum values ​​corresponding to adjacent excitation points. If the change rate of the amplitude maximum values ​​corresponding to adjacent excitation points is greater than 5, the depth of the amplitude interface is located between the depths of the adjacent excitation points; (4) According to the thickness of the slope accumulation zone at the location of the target point, the thickness of the slope accumulation zone at other locations on the surface of the slope accumulation zone is determined; the thickness of the slope accumulation zone at the location of the target point is the depth of the amplitude interface.

2. The method for determining the thickness of the slope accumulation zone according to claim 1, characterized in that: The depth difference between two adjacent excitation points is 0.5 to 1 m.

3. The method for determining the thickness of the slope accumulation zone according to claim 1, characterized in that: The depth of the excitation point is not more than 3m, the depth difference between two adjacent excitation points is equal, and the depth difference between two adjacent excitation points is 0.5-0.6m; The depth of the excitation point is greater than 3m, the depth difference between two adjacent excitation points is equal, and the depth difference between two adjacent excitation points is 0.8-1m.

4. The method for determining the thickness of the slope accumulation zone according to any one of claims 1 to 3, characterized in that: The rate of change of the amplitude maximum is determined by a method comprising the following steps: assuming that the excitation point A and the excitation point B are adjacent, the amplitude maximum corresponding to the excitation point A is C, and the amplitude maximum corresponding to the excitation point B is D, then the rate of change of the amplitude maximum corresponding to the excitation point A and the excitation point B is equal to (DC) / C, wherein the depth of the excitation point A is less than the depth of the excitation point B.

5. The method for determining the thickness of the slope accumulation zone according to any one of claims 1 to 3, characterized in that: Long-channel seismic data refers to seismic data with an offset distance greater than or equal to 100m.

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