A method of detecting compressional waves for near-surface absorption attenuation calculations

Abstract

The application discloses a method for detecting longitudinal waves used for near-surface absorption attenuation calculation, and establishes a deep well logging according to the minimum distance interval between an excitation point and a receiving point, the deep well logging depth is greater than or equal to the sum of the minimum distance between the excitation point and the receiving point and the thickness of a low velocity layer, the seismic wave is generated within the deep well logging within the range of 0.5 meters from the ground, the longitudinal wave data of the deep formation is obtained by receiving at the position below the depth greater than the minimum distance between the excitation point and the receiving point, the relative position between the excitation point and the receiving point is designed according to the minimum distance between the excitation point and the receiving point, meanwhile, the development degree of the surface wave can be weakened by exciting and receiving in the same well, the longitudinal wave information of the near-surface propagation without the interference of the surface wave can be obtained, and the high-precision near-surface absorption attenuation parameters can be ensured, the longitudinal wave data without the interference of the surface wave is obtained by using the velocity difference between the surface wave and the longitudinal wave, and the precision of the near-surface formation absorption attenuation investigation result is improved.

Description

Technical Field

[0001] This invention belongs to the field of geophysical exploration, specifically relating to a method for detecting longitudinal waves used in near-surface absorption attenuation calculations. Background Technology

[0002] When seismic waves propagate through strata, the geological medium absorbs and attenuates them, reducing their frequency and weakening their energy. Because near-surface strata are relatively loose and exhibit significant longitudinal and lateral variations, the absorption and attenuation of seismic waves are particularly severe, greatly impacting seismic data. In seismic exploration, to obtain high-resolution and high signal-to-noise ratio seismic data, it is often necessary to investigate the absorption and attenuation of seismic waves by near-surface strata, i.e., to investigate and obtain the absorption and attenuation coefficient of near-surface strata.

[0003] There are two main categories of existing methods for investigating the absorption attenuation coefficient of near-surface strata. The first category involves directly measuring the absorption attenuation coefficient of stratum samples using instruments. The second category involves observing the changes in amplitude, frequency, and phase of seismic waves during their propagation through the strata and then calculating the absorption attenuation coefficient using formulas. In seismic exploration, the second method is commonly used. This method utilizes dual-well micrologging to obtain seismic waves (two wells are drilled near the surface, one for excitation and the other for reception, with a 5-meter interval between the wells), and then calculates the coefficient using mathematical methods.

[0004] The main problem with the two existing methods is that after the seismic source triggers the seismic waves, a complex wave field is generated, including various seismic waves such as P-waves, S-waves, and surface waves. Research shows that calculating the formation absorption attenuation coefficient primarily utilizes P-wave data unaffected by surface wave interference, resulting in the most accurate calculation. However, in actual data acquisition, improper placement of the trigger and receiver points can lead to the acquisition of a mixture of various waves (mainly surface waves and P-waves), severely impacting the accuracy of the obtained formation absorption attenuation parameters and making it impossible to accurately acquire surface absorption attenuation data, thus causing some misguidance in earthquake research. Summary of the Invention

[0005] The purpose of this invention is to provide a method for detecting longitudinal waves used in near-surface absorption attenuation calculations, thereby overcoming the shortcomings of the prior art.

[0006] A method for detecting P-waves used in near-surface absorption attenuation calculations includes the following steps:

[0007] S1, establish deep well logging based on the minimum distance interval between the excitation point and the receiver point, and the deep well logging depth is greater than or equal to the sum of the minimum distance between the excitation point and the receiver point and the thickness of the low-rate-drop layer;

[0008] S2 generates seismic waves within 0.5 meters above the ground in deep well logging. The deep P-wave data of the formation is obtained by receiving the waves at a depth below the minimum distance between the excitation point and the receiver point in deep well logging.

[0009] Preferably, utilizing the velocity difference between surface waves and longitudinal waves, the time length of one longitudinal wave period is calculated as 1 / f seconds.

[0010] The minimum distance between the excitation point and the receiver point can be obtained:

[0011]

[0012] X min V represents the minimum distance between the excitation and reception points, expressed in meters. 纵 V represents the propagation velocity of P-waves in near-surface strata, measured in meters per second. 面 f represents the surface wave propagation velocity in the near-surface strata, in meters per second; f represents the dominant frequency of the P-wave in the near-surface strata, in Hertz.

[0013] Preferably, the longitudinal wave propagation velocity V in the near-surface strata 纵 The dominant frequency f of P-waves and the propagation velocity V of surface waves in near-surface strata. 面 Obtained through existing seismic data or surface surveys.

[0014] Preferably, the drilling depth for deep well logging is greater than or equal to X. min +H, where H is the thickness of the low-deceleration layer.

[0015] Preferably, H is obtained using single-well micro-logging, low-refractive-index, ground-penetrating radar, and first arrival tomography methods.

[0016] Preferably, the absorption attenuation Q value is obtained by analyzing the changes in the shape of the P-wave during its propagation through the strata, based on the acquired P-wave data and the absorption attenuation Q value of the P-wave.

[0017] Preferably, a shallow well is set at intervals on one side of the deep well logging, a seismic wave exciter is set at the bottom of the deep well logging, and the seismic wave receiver set in the shallow well logging is used to receive and obtain the P-wave data of the shallow formation.

[0018] Preferably, the difference between the drilling depth of shallow well logging and the drilling depth of deep well logging is X. min .

[0019] Preferably, the distance from the ground in the well is X min Multiple seismic wave receivers can be installed at intervals below the depth.

[0020] Preferably, the well spacing between deep well logging and shallow well logging is 1-3 meters.

[0021] Compared with the prior art, the present invention has the following beneficial technical effects:

[0022] This invention discloses a method for detecting P-waves used in near-surface absorption attenuation calculations. A deep well logging system is established based on the minimum distance interval between the excitation and reception points. The depth of the deep well logging is greater than or equal to the sum of the minimum distance between the excitation and reception points and the thickness of the low-velocity layer. Seismic waves are generated within 0.5 meters of the wellhead. Deep P-wave data is obtained by receiving the waves at a depth below the surface greater than the minimum distance between the excitation and reception points within the deep well logging system. The minimum distance between the excitation and reception points guides the design of their relative positions. Simultaneously, simultaneous excitation and reception within the same well can reduce the development of surface waves, allowing for the acquisition of near-surface propagating P-wave information unaffected by surface wave interference, ensuring high-precision near-surface absorption attenuation parameters. Furthermore, compared to dual-well micrologging, this method is more cost-effective.

[0023] Preferably, the present invention utilizes the velocity difference between surface waves and P-waves to calculate the time length of a P-wave period, thereby obtaining P-wave data that is not affected by surface waves, thus improving the accuracy of near-surface strata absorption attenuation survey results. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the P-wave data detection structure in the deep formation of an embodiment of the present invention.

[0025] Figure 2 This is a schematic diagram of the P-wave data detection structure in the shallow strata of an embodiment of the present invention.

[0026] Figure 3 This is a diagram showing the transmission data of wave and longitudinal wave signals at different distances between the excitation point and the receiving point in an embodiment of the present invention.

[0027] Figure 4 This is a diagram of surface wave and longitudinal wave data obtained at a distance of 1 meter between the receiving point and the excitation point in an embodiment of the present invention.

[0028] Figure 5 This is a diagram of surface wave and longitudinal wave data obtained at a distance of 14 meters between the receiving point and the excitation point in an embodiment of the present invention.

[0029] Figure 6 This is a near-surface absorption attenuation map calculated from P-wave data obtained using the method of this invention in an embodiment of the invention.

[0030] Figure 7 This is a near-surface absorption attenuation map calculated from P-wave data obtained using existing methods in an embodiment of the present invention.

[0031] In the diagram, 1 is the seismic wave generator; 2 is the seismic wave receiver. Detailed Implementation

[0032] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0033] A method for detecting P-waves used in near-surface absorption attenuation calculations includes the following steps:

[0034] S1. Based on the minimum distance interval between the excitation point and the receiver point, establish a deep well logging and a shallow well logging, wherein the depth of the deep well logging is greater than or equal to the sum of the minimum distance between the excitation point and the receiver point and the thickness of the low-velocity layer; the depth of the shallow well logging is the difference between the depth of the deep well logging and the minimum distance between the excitation point and the receiver point.

[0035] This invention utilizes the velocity difference between surface waves and longitudinal waves, where the longitudinal wave propagates faster than the surface wave. For the same propagation distance, a time difference exists between the two waves, and this time difference increases with the distance traveled. In calculating absorption attenuation, a longitudinal wave period is required, with a duration of 1 / f seconds. This means the propagation time difference between the longitudinal wave and the surface wave must be greater than one longitudinal wave period.

[0036] Right now:

[0037]

[0038] The minimum distance between the excitation point and the receiving point can be obtained from the above formula:

[0039]

[0040] In the formula, X min V represents the minimum distance between the excitation and reception points, expressed in meters. 纵 V represents the propagation velocity of P-waves in near-surface strata, measured in meters per second. 面 f represents the surface wave propagation velocity in the near-surface strata, in meters per second; f represents the dominant frequency of the P-wave in the near-surface strata, in Hertz.

[0041] P-wave propagation velocity V in near-surface strata 纵 The dominant frequency f of P-waves and the propagation velocity V of surface waves in near-surface strata. 面 Obtained through existing seismic data or surface surveys.

[0042] The distance between two logging wells of different depths is 1-3 meters, that is, the distance between deep well logging and shallow well logging is 1-3 meters.

[0043] Specifically, deep well logging refers to drilling depths greater than or equal to X. min +H, The difference between the drilling depth in shallow well logging and the drilling depth in deep well logging is X. min .

[0044] Wherein, H is the thickness of the low-velocity layer, obtained using single-well micrologging, low-refractive-index, ground-penetrating radar, and first arrival tomography methods.

[0045] S2 generates seismic waves within a 0.5-meter range from the surface in a deep well logging operation. A seismic wave receiver, positioned at a depth greater than the minimum distance between the excitation and reception points within the deep well logging operation, receives and acquires deep P-wave data. Using this acquired P-wave data, the absorption attenuation Q-value is obtained by analyzing the morphological changes of the P-wave during propagation through the formation. The near-surface absorption attenuation Q-value is calculated using the rise time method, spectral ratio method, centroid frequency shift method, or spectral simulation method.

[0046] Specifically, a seismic wave generator is used as the source to generate seismic waves.

[0047] S3: A seismic wave exciter is installed at the bottom of the deep well logging hole. The seismic wave receiver installed in the shallow well logging hole receives and obtains the P-wave data of the shallow formation. Based on the P-wave data of the deep formation and the P-wave data of the shallow formation obtained in S2 and S3, the absorption attenuation data of the deep formation and the absorption attenuation data of the shallow formation are calculated respectively.

[0048] In S2, to accurately calculate the absorption attenuation data of deep formations and improve its detection accuracy, deep well logging is employed. In deep well logging at a depth greater than or equal to the sum of the minimum distance between the excitation point and the receiver point and the thickness of the low-velocity layer, a seismic receiver is installed below the minimum distance between the excitation point and the receiver point. The seismic receiver is then used to obtain the distance X from the surface in the well. min Seismic wave data at depths below a certain point.

[0049] Based on the formation depth, at a distance X from the surface in the well. min At depths below the target depth, multiple seismic wave receivers can be placed at intervals to improve the accuracy of the detection data.

[0050] Multiple seismic wave receivers are set up at equal intervals, allowing for comparison and verification of data from each receiver to further validate the accuracy of the data and provide strong support for deep stratum absorption and attenuation data.

[0051] Depending on the specific formation thickness, if a single excitation cannot collect data from the well at a distance X from the surface... min The absorption attenuation data for the entire formation section down to the bottom of the well is collected in segments. Seismic wave receivers are set up in each segment, and steps S2 and S3 are repeated until data that meets the requirements is collected.

[0052] Step S3 is to obtain the distance X from the ground to the well. min Based on the absorption and attenuation data of the formation at a certain depth, a seismic wave generator was placed at the bottom of the well as a source to generate seismic waves. Then, freely movable seismic wave receivers were evenly distributed from the surface to a depth X in the well. min Within the formation, seismic waves are generated at the bottom of the well, while a receiver simultaneously receives them. If a single generation fails to capture the distance X from the surface to the well depth, the seismic activity will continue. min For the absorption attenuation data of a section of the strata, the process is carried out in segments, with the receiver moved and the above steps repeated until data that meets the requirements is collected.

[0053] like Figure 1 As shown, for the P-wave data setup in the deep formation described above, a seismic wave exciter 1 is installed at a depth of L = 0.5 meters above the ground in the deep well logging. The depth of the excitation point from the ground in the deep well logging is greater than the minimum distance X between the excitation point and the receiver point. min Multiple seismic wave receivers 2 are set up below the depth; seismic wave exciter 1 is used as the source to release seismic wave signals, and seismic wave receivers 2 at different depths are used to obtain deep longitudinal wave data of the strata.

[0054] like Figure 2 As shown, for the setting of P-wave data of shallow formations in the above method, a shallow well logging with a distance Y of 1-3 meters is set on one side of the deep well logging. Multiple seismic wave receivers 2 are set at intervals in the shallow well logging. A seismic wave exciter 1 is set at the bottom of the deep well logging. The seismic wave exciter 1 is used as the source to release seismic wave signals. The P-wave data of shallow formations are obtained by using the seismic wave receivers 2 of different depth layers set at intervals in the shallow well logging.

[0055] Table 1 below shows the P-wave velocity, surface wave velocity, and dominant frequency of seismic waves in different regions.

[0056] Table 1

[0057]

[0058]

[0059] Example 1

[0060] Experimental testing was conducted in the Kuqa area:

[0061] A deep well logging and a shallow well logging are established based on the minimum distance interval between the excitation point and the receiver point. The depth of the deep well logging is greater than or equal to the sum of the minimum distance between the excitation point and the receiver point and the thickness of the low-velocity layer. The depth of the shallow well logging is the difference between the depth of the deep well logging and the minimum distance between the excitation point and the receiver point.

[0062] According to the seismic wave period calculation formula: T=1 / f, it can be calculated that, in the Kuqa area, through experiments, the period of the first complete waveform from the initial wave to the final wave is 20 milliseconds, that is, the propagation time difference between the P-wave and the surface wave is at least 20ms.

[0063] Then, experiments were conducted using receivers and excitations at different distances, such as... Figure 4 and Figure 5 The figures shown are surface wave and longitudinal wave data obtained at a distance of 1 meter between the receiving point and the excitation point, and surface wave and longitudinal wave data obtained at a distance of 14 meters between the receiving point and the excitation point, respectively. Figure 3 As shown, surface wave and longitudinal wave signal transmission data are transmitted at different distances between the excitation point and the receiving point. The corresponding experimental data are shown in Table 2 below. The distance between the excitation point and the receiving point needs to be 16 meters to avoid interference from the surface wave on the first wavelet of the reflected wave. Therefore, based on the minimum distance of 16 meters between the excitation point and the receiving point, deep wells and shallow wells are set up.

[0064] Table 2. Experiments with receiver and excitation points at different distances.

[0065]

[0066]

[0067] Based on the single-well micrologging method, the thickness of the low-rate-drop layer is 10m, and the drilling depth of the deep well is set at 35m, while the drilling depth of the shallow well is 19m.

[0068] Seismic waves were generated at a depth of 0.5 meters from the wellhead in a deep well logging operation. Ten seismic wave receivers were installed at intervals up to 16 meters below the wellhead to acquire deep P-wave data. Using this acquired P-wave data, the absorption attenuation Q-value was obtained by analyzing the changes in the P-wave's morphology during propagation through the formation. The near-surface absorption attenuation Q-value was calculated using the rise time method, spectral ratio method, centroid frequency shift method, or spectral simulation method. The results are as follows: Figure 6 As shown, the near-surface absorption attenuation map calculated from the P-wave data obtained by the method of this invention exhibits a clear and regular trend in the variation of P-wave properties, which is consistent with the previous method. Figure 7 As shown, the near-surface absorption attenuation map obtained using existing methods exhibits drastic fluctuations and lacks regularity in the changes of P-wave properties.

[0069] A seismic wave exciter is installed at the bottom of a deep well logging hole, and eight seismic wave receivers are installed in a shallow well logging hole to receive and acquire P-wave data of the shallow formation. Based on the P-wave data of the deep formation and the P-wave data of the shallow formation acquired by S2 and S3, the absorption attenuation data of the deep formation and the absorption attenuation data of the shallow formation are calculated respectively.

Claims

1. A method for detecting longitudinal waves used in near-surface absorption attenuation calculations, characterized in that, Includes the following steps: S1, establish deep well logging based on the minimum distance interval between the excitation point and the receiver point, and the deep well logging depth is greater than or equal to the sum of the minimum distance between the excitation point and the receiver point and the thickness of the low-rate-drop layer; S2 generates seismic waves within 0.5 meters above the ground in deep well logging, and receives and acquires deep longitudinal wave data of the formation at a depth below the minimum distance between the excitation point and the receiver point in deep well logging. Utilizing the velocity difference between surface waves and longitudinal waves, the duration of one longitudinal wave period is calculated as: 1 / ƒ seconds. The minimum distance between the excitation point and the receiver point can be obtained: X min The minimum distance between the excitation point and the receiver point, in meters; V 纵 V represents the propagation velocity of P-waves in near-surface strata, measured in meters per second. 面 ƒ represents the surface wave propagation velocity in near-surface strata, in meters per second; ƒ represents the dominant frequency of P-waves in near-surface strata, in Hertz. P-wave propagation velocity V in near-surface strata 纵 The dominant frequency of longitudinal waves ƒ and the propagation velocity of surface waves V in the near-surface strata 面 Obtained through existing seismic data or surface surveys.

2. The method for detecting longitudinal waves for near-surface absorption attenuation calculation according to claim 1, characterized in that, Deep well logging drilling depth greater than or equal to X min +H, where H is the thickness of the low-deceleration layer.

3. The method for detecting longitudinal waves for near-surface absorption attenuation calculation according to claim 2, characterized in that, H was obtained using single-well micrologging, low refraction, ground-penetrating radar, and first arrival tomography methods.

4. The method for detecting longitudinal waves for near-surface absorption attenuation calculation according to claim 1, characterized in that, Using the acquired P-wave data, the absorption attenuation Q-value is obtained by analyzing the changes in the P-wave morphology during its propagation through the strata.

5. The method for detecting longitudinal waves for near-surface absorption attenuation calculation according to claim 1, characterized in that, A shallow well is set up at intervals on one side of the deep well logging. A seismic wave exciter is set at the bottom of the deep well logging, and the seismic wave receiver set in the shallow well logging is used to receive and obtain the P-wave data of the shallow formation.

6. The method for detecting longitudinal waves for near-surface absorption attenuation calculation according to claim 5, characterized in that, The difference between the drilling depth in shallow well logging and the drilling depth in deep well logging is X. min .

7. The method for detecting longitudinal waves for near-surface absorption attenuation calculation according to claim 1, characterized in that, X distance from the ground in the well min Multiple seismic wave receivers can be installed at intervals below the depth.

8. A method for detecting longitudinal waves for near-surface absorption attenuation calculation according to claim 5, characterized in that, The well spacing between deep well logging and shallow well logging is 1-3 meters.

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