Modeling method of near-surface velocity of piedmont zone gravel layer

Abstract

The application discloses a modeling method for a mountain front gravel layer near-surface velocity, which comprises the following steps: calculating the average velocity of a low-velocity layer, the top velocity of a low-velocity gravel layer, the delay time of a low-velocity drop layer, the relative change rate of the gravel layer velocity with the depth, the total thickness of the low-velocity drop layer, the thickness of the low-velocity layer and the thickness of the gravel layer, and the gravel layer velocity, and finally establishing a mountain front gravel layer near-surface velocity model, which meets the demand for a high-precision mountain front gravel layer near-surface velocity model and improves the oil and gas exploration effect.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of seismic exploration data processing of oil, and relates to a modeling method of near-surface velocity, in particular to a modeling method of near-surface velocity of mountain front gravel layer. BACKGROUND

[0002] Seismic exploration is a main method for seeking and exploring oil and natural gas. Main work includes three steps of seismic data acquisition, processing and interpretation, wherein seismic data processing includes static correction, denoising, deconvolution, dynamic and static correction, velocity analysis, stacking and migration, etc., to provide data results for seismic data interpretation.

[0003] In the process of seismic data processing, a basic seismic theory assumes that an excitation point and a receiving point are on the same horizontal plane, and stratum seismic wave velocity is uniform, but actual surface elevation is undulating, and a set of weathered layer or unconsolidated sediment called low-velocity zone exists in near-surface. Due to different weathering degrees and consolidation degrees at different positions, the thickness and velocity of the low-velocity zone change laterally relative to underlying strata, and the lateral change of the low-velocity zone distorts hyperbolic shape of seismic exploration reflected wave travel time, and affects seismic imaging precision, so the distortion should be eliminated in seismic data processing.

[0004] At present, the elimination method of the distortion mainly includes static correction and near-surface velocity modeling, including model method, refraction method and tomography method, etc. Although these methods have respective advantages in aspects of stable refraction area data processing and inversion of near-surface velocity structure change trend, etc., they are difficult to meet the demand of surface modeling of depth domain migration imaging, especially the demand of near-surface velocity modeling of mountain front alluvial fan area.

[0005] The mountain front alluvial fan is a fan-shaped accumulation body formed by a river out of a mountain pass, and is generally a set of unconsolidated rock with continuous medium characteristics, and the maximum thickness of the mountain front gravel layer exceeds hundreds of meters. The underlying stratum of the mountain front gravel layer is a consolidated rock or an old stratum, and the overlying stratum is loose sediment. The characteristics of great thickness and severe lateral change of the mountain front alluvial fan seriously affect the seismic imaging precision, and make the near-surface velocity model of the mountain front gravel layer have low accuracy. SUMMARY

[0006] The present application aims to provide a modeling method of near-surface velocity of mountain front gravel layer, and based on micro-logging data and combined with data in ultra-deep micro-logging, low-velocity layer, gravel layer velocity, thickness or relative change rate of velocity with depth are calculated, and finally a near-surface velocity model of mountain front gravel layer with high accuracy is established.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows:

[0008] The application discloses a modeling method for a near-surface velocity of a piedmont gravel layer, and comprises the following steps in sequence:

[0009] S1. calculating an average velocity of a low-velocity layer, a top velocity of a low-velocity gravel layer and a delay time of a low-decreasing-velocity layer

[0010] The average velocity of the low-velocity layer at the micro-logging survey point, the thickness of the micro-logging survey point and the top velocity of the low-velocity gravel layer at the micro-logging survey point are obtained by interpreting the micro-logging data and layering.

[0011] The top velocity of the low-velocity gravel layer is the top interface velocity of the underlying gravel layer of the low-velocity layer.

[0012] The average velocity of the low-velocity layer at the shot-receiver point is obtained by using the average velocity of the low-velocity layer at the micro-logging survey point and the geodetic coordinates of the micro-logging survey point and the shot-receiver point; and the top velocity of the low-velocity gravel layer at the shot-receiver point is obtained by using the top velocity of the low-velocity gravel layer at the micro-logging survey point.

[0013] The first arrival time of the refracted wave is selected, and the delay time of the low-decreasing-velocity layer at the shot-receiver point and the refracted layer velocity at the shot-receiver point are calculated according to the generalized reciprocal method.

[0014] S2. calculating a relative variation rate of the gravel layer velocity with depth

[0015] The time-depth data of the gravel layer in the super-deep micro-logging are extracted, fitted and discretized to obtain the maximum value of the fitted vertical travel time.

[0016] The average velocity of the low-velocity gravel layer is calculated for each micro-logging based on the deepest micro-logging data, the position of each micro-logging corresponding to the reference micro-logging is iteratively searched, the data are corrected and combined, and the relative variation rate of the gravel layer velocity with depth is obtained through data fitting.

[0017] S3. calculating the total thickness of the low-decreasing-velocity layer

[0018] The delay time and the thickness of the gravel layer are calculated by using the average velocity of the low-velocity layer at the micro-logging survey point, the thickness of the micro-logging survey point, the top velocity of the low-velocity gravel layer at the micro-logging survey point, the delay time of the low-decreasing-velocity layer at the shot-receiver point, the refracted layer velocity at the shot-receiver point and the relative variation rate of the gravel layer velocity with depth, the thickness value range of the gravel layer is corrected, and the bottom interface elevation of the low-decreasing-velocity layer at the micro-logging survey point is calculated through iterative calculation.

[0019] The bottom interface elevation of the low-decreasing-velocity layer at the shot-receiver point is obtained by using the bottom interface elevation of the low-decreasing-velocity layer at the micro-logging survey point and the geodetic coordinates of the micro-logging survey point and the shot-receiver point through the Kriging interpolation method, and the total thickness of the low-decreasing-velocity layer is calculated.

[0020] S4. calculating the thickness of the low-velocity layer and the thickness of the gravel layer

[0021] The low velocity layer thickness is corrected by the initial thickness of the low velocity layer, the theoretical delay time of the low velocity layer and the theoretical delay time of the gravel layer, and the low velocity layer thickness of the shot point and the gravel layer thickness of the shot point are calculated.

[0022] S5. Calculate the gravel layer velocity

[0023] The average velocity of the gravel layer and the gravel layer velocity at any depth are calculated according to the following formula by using the top velocity of the low velocity gravel layer of the micro-logging survey point, the gravel layer thickness of the shot point and the relative change rate of the gravel layer velocity with depth,

[0024]

[0025]

[0026] wherein m is the shot point serial number, v m,1 is the average velocity of the gravel layer, v m,1 (d) is the gravel layer velocity at any depth, β is the relative change rate of the gravel layer velocity with depth (calculated by S2), v m,1,T is the top velocity of the low velocity gravel layer of the shot point, d is the depth from the surface, h m,1 is the gravel layer thickness of the shot point (calculated by S4), h m,0 is the initial thickness of the low velocity layer, H m is the total thickness of the low velocity layer.

[0027] S6. Establish the near-surface velocity model of the gravel layer in the piedmont zone

[0028] The velocity model at the following depth is established by using the low velocity layer thickness and velocity of the shot point, the gravel layer thickness of the shot point and the gravel layer velocity of the shot point:

[0029]

[0030] or

[0031]

[0032] wherein the value of m is 1, 2, …, Np, wherein Np is the total number of shot points.

[0033] As a limitation of the present application, in step S1, the representation method of the micro-logging data is (t i，j , d i，j ), which represents the vertical travel time at the observation point at the jth depth of the ith micro-logging, wherein t is the vertical travel time, and i and j are both sample serial numbers.

[0034] As another limitation of the present application, in step S2, the formula used for fitting is:

[0035] t = (1 - (1 + βh) α-1 / (v0β(1-α)(1+βh) α-1 )

[0036] Wherein, v0 is the velocity of the top interface of the gravel layer, h is the observed depth of the gravel layer, and a is a parameter for adjusting the velocity variation rate.

[0037] As a third limitation of the present application, the delay time of the gravel layer is the low-velocity layer delay time and the gravel layer delay time.

[0038] As a fourth limitation of the present application, in step S4, the theoretical delay time of the gravel layer is calculated by the following formula:

[0039] kv = (H m -h m，0 ) / (kt1-kt0);

[0040]

[0041] Wherein, kv is the average velocity of the gravel layer section, kt o is the time at the top of the gravel layer, kt1 is the time at the bottom of the gravel layer, Γ m，1 is the theoretical delay time of the gravel layer, and v m，2 is the refractor velocity at the shot point. Wherein, kv is the average velocity of the gravel layer section, which is divided by the thickness of the gravel layer section.

[0042] Compared with the prior art, the technical progress achieved by the present application is that:

[0043] The present application proposes a method for comprehensively utilizing micrologging (micrologging with an investigation depth of less than 80 meters) and super-deep micrologging (micrologging with an investigation depth of more than 300 meters and gravel layer thickness and velocity parameters) data to jointly establish a near-surface velocity model of the gravel layer in the mountain front zone.

[0044] The method of the present application obtains data such as the average velocity of the low-velocity layer, the top velocity of the low-velocity gravel layer, the delay time of the low-velocity layer, the relative change rate of the gravel layer velocity with depth, the total thickness of the low-velocity layer, the thickness of the low-velocity layer, and the thickness and velocity of the gravel layer by calculation, and the near-surface velocity model of the gravel layer in the mountain front zone established by the method has high accuracy and significantly distorts the hyperbolic shape of the lateral variation of the low-velocity zone on the travel time of the reflected wave in seismic exploration.

[0045] The present application can be applied to the field of oil seismic exploration data processing technology, further meets the demand for high-precision near-surface velocity models of the gravel layer in the mountain front zone, and improves the effectiveness of oil and gas exploration.

[0046] The present application will be described in further detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0047] Figure 1 This is a diagram showing the ultra-deep micro-logging data results of the gravel layer obtained in step S2 of Embodiment 1 of the present invention;

[0048] Figure 2 This is a result diagram of the data obtained in step S2 of Embodiment 1 of the present invention, showing the relative depths of different micro-logging wells and placing them at the corresponding depths.

[0049] Figure 3 This is a result diagram of the relative time differences of different micro-logging data obtained in step S2 of Embodiment 1 of the present invention, and the data placed at the corresponding time according to the corresponding time.

[0050] Figure 4 This is a diagram showing the total thickness of the low-rate-deceleration layer obtained in step S3 of Embodiment 1 of the present invention.

[0051] Figure 5 This is a diagram showing the low-velocity layer thickness at the shot receiver obtained in step S4 of Embodiment 1 of the present invention.

[0052] Figure 6 This is a diagram showing the gravel layer thickness at the gun inspection point obtained in step S4 of Embodiment 1 of the present invention.

[0053] Figure 7 This is a graph showing the velocity results of gravel layers at arbitrary depths obtained in step S5 of Embodiment 1 of the present invention. Detailed Implementation

[0054] Example 1: A method for modeling near-surface velocities in gravel layers of a mountain piedmont.

[0055] The modeling method of near-surface velocity of the gravel layer in the foreland zone of this invention is used to model all the micrologging data, ultra-deep micrologging data, geodetic coordinates of shot receivers, elevation data, and first arrival data of seismic data collected within a range of more than 100 km on the southern margin of the Gar Basin.

[0056] In this embodiment, the subscripts have the following meanings: 0 represents the low-velocity layer, 1 represents the gravel layer, 2 represents the refractive layer; i represents the micrologging sequence number, j represents the observation point sequence number, k represents the ultra-deep micrologging sequence number, and m represents the shot detection point sequence number.

[0057] This embodiment includes the following steps performed sequentially:

[0058] S1. Calculate the average velocity of the low-velocity layer, the top velocity of the low-velocity gravel layer, and the delay of the low-decline layer.

[0059] (11) Extract the average velocity of the low-velocity layer and the top velocity of the low-velocity gravel layer from the micro-logging survey points.

[0060] The micrologging data from the low-velocity layer survey is denoted as (t).i，j , d i，j , d is any depth from the surface, the microlog data represents the vertical travel time t at the observation point of the jth depth d in the ith microlog.

[0061] The microlog interpretation method is applied to interpret the stratification of the ith microlog, and the average velocity of the low-velocity layer extracted through the interpretation stratification is denoted as v i，0 , the thickness of the microlog survey point is h i，0 , the top velocity of the low-velocity gravel layer of the microlog survey point is v i，1，T . Denote the number of microlog wells as N w , respectively i = 1, 2, …, N w .

[0062] (12) Calculate the average velocity of the low-velocity layer and the top velocity of the low-velocity gravel layer of all shot points in the work area

[0063] Denote the total number of shot points in the work area as Np, and m is the shot point number, m = 1, 2, …, Np, using the Kriging interpolation method, according to the geodetic coordinates of the microlog survey points and the shot points, use N w The average velocity of the low-velocity layer v i，0 of N m，0 The top velocity of the low-velocity gravel layer v w of N i，1，T The top velocity of the low-velocity gravel layer v m，1，T of Np shot points is interpolated.

[0064] (13) Calculate the low-velocity layer delay time of all shot points in the work area

[0065] Select the first arrival time of the refracted wave, calculate the low-velocity layer delay time and the refracted layer velocity of the shot point according to the generalized reciprocal method, denote the low-velocity layer delay time of each shot point as τ m , and the refracted layer velocity as v m，2 .

[0066] S2. Calculate the relative change rate of the gravel layer velocity with depth

[0067] (21) Extract the time-depth observation data of the gravel layer in the super-deep microlog

[0068] Denote the super-deep microlog data of the gravel layer as (t i，j , d i，j ), which represents the depth d of the jth observation point in the ith super-deep microlog. The observed vertical travel time is t, denote the number of low-velocity layer observation points extracted through the interpretation stratification as N iρ , and the number of gravel layer observation points as N i，1 , then the time-depth observation data of the gravel layer at this observation point is:

[0069]

[0070] j = N i，0 +1, N i，0 +2,..., N i，0 +N i，1 , k = j-N i，0 =1, 2,..., N i，1

[0071] The time-depth observation data of the gravel layer is arranged according to the super-deep micro logging data of the gravel layer, and the result is shown in FIG. 2; Figure 1

[0072] The thickness of the low-velocity gravel layer observed by each super-deep micro logging is different, and the maximum depth is taken, and the maximum depth of the gravel layer observed in all micro logging data is recorded as dmax=max(d 1，i，k ).

[0073] (22) Fitting and discretizing the gravel layer of each super-deep micro logging

[0074] Assuming that the relative change rate of velocity with depth is β, and the velocity of the top interface of the low-velocity gravel layer (i.e. h=0) is v0, then the velocity of the gravel layer at any depth is:

[0075] v(h)=v0(1+βh) a

[0076] ① The observation data of all micro logging gravel layers are fitted according to the following formula:

[0077] t=(1-(1+βh) α-1 / (v0β(1-α)(1+βh) α-1 )

[0078] In the formula, t is the observation time of the gravel layer, h is the observation depth of the gravel layer, the depth and time data of each micro logging gravel layer are calculated according to the fitting formula, and are recorded as: (Rt i，k , Rd i，k ), Rd and Rt in the data are respectively the vertical travel time calculated according to the fitting formula on the depth Rd, and the subscripts i and k respectively represent the observation point of the kth depth of the gravel layer of the ith super-deep micro logging.

[0079] ② Calculate the maximum value of the fitting vertical time in all super-deep micro logging

[0080] According to the formula

[0081] t=(1-(1+βh) a-1 / (v0β(1-α)(1+βh) α-1 )​

[0082] By fitting the data, we obtain (mRt) i，k mRd i，k ), representing the observation point at the k-th depth of the gravel layer in the i-th ultra-deep micro-logging well, with the vertical travel time calculated by the fitting formula at depth mRd as mRd, where k represents 1 to dmax.

[0083] Take all micro-logging mRt i，k The maximum value is denoted as maxmRt. k .

[0084] (23) Calculate the relative relationships of gravel sections (i.e., gravel layers in ultra-deep micro-logging) in each well.

[0085] With (maxmRt) k mRd i，k Using the baseline as a reference, calculate the relative depth and time difference between each well and this baseline. The minimum and maximum depths of the baseline data are denoted as mh, respectively. 2，0 mh 2，1 .

[0086] For (maxmRt) k mRd i，k According to the formula

[0087] t=(1-(1+βh) α-1 / (v0β(1-α)(1+βh) α-1 )

[0088] By fitting the data, we obtain mv0, mα, and mβ.

[0089] The calculation process is described using data from a single well as an example. Specifically, the following steps ①-④ are used with the halving method to determine which segment of the gravel layer in the current ultra-deep micro-logging system has the same average velocity as the deepest segment of the gravel layer. ① Calculate the average velocity of the entire gravel segment using the following formula:

[0090] Va=(mRd i，Ni1 -,mRd i，1 ) / (mRt i，Ni1 -mRt i，1 ),

[0091] The corresponding minimum and maximum depths are denoted as h, respectively. 1，0 with h 1，1 .

[0092] h 2，0 =h 1，0 +(mh 2，1 -h 1，1 )

[0093] h 2，1= h 1，1 + (mh 2，1 -h 1，1 )

[0094] 2. Put the ultra-deep microlog in the center of the reference well, i.e. align the center of the well gravel layer with the center of the reference well gravel layer.

[0095] h0 = (h 1，0 +h 2，0 ) / 2

[0096] h1(h 1，1 +h 2， 1) / 2

[0097] h0 and h1 are the positions of the top of the ultra-deep microlog gravel layer and the bottom of the ultra-deep microlog gravel layer, respectively, corresponding to the reference ultra-deep microlog.

[0098] 3. Use the following formula:

[0099] t = (1 - (1 + mβh mα-1 / (mv0mβ(1 - mα)(1 + mβh mα-1 )

[0100] Calculate h0, h1, and the corresponding times ct o , ct1.

[0101] 4. Delt = (h1 - h1) / (ct1 - ct0) - Va

[0102] If Delt meets the required error (generally 0.0001 km / s), h0 is the vertical relative difference of the microlog.

[0103] Otherwise,

[0104]

[0105] Repeat steps 2 to 4 until the calculated Delt meets the requirements.

[0106] Use the formula t = (1 - (1 + mβh mα-1 / (mv0mβ(1 - mα)(1 + mβh mα-1 ) to calculate the time dt corresponding to h0.

[0107] Record h0 as dh i and dt as dt i for each well.

[0108] Calculate the relative depths of different micrologs according to the method of step (23) above and place the data at the corresponding depths according to the depths, and the results are shown in Table 1. Figure 2 Table 1 Figure 2The average velocity of the low velocity gravel layer of the leftmost well is the same as the average velocity of the reference well at the same position, i.e. Figure 2 The leftmost curve is placed at about 70-230m, indicating that the average velocity of the low velocity gravel layer is the same as the reference curve at the 70-230m section.

[0109] (24) Correct all data:

[0110] ft i，k = t1 i，k + dt i

[0111] fh i，k = d1 i，k + dh i

[0112] The corrected data of all microlog gravel layers are fitted according to the following formula:

[0113] t = (1 - (1 + βh) α-1 / (v0β(1-α)(1+βh) α-1 )

[0114] The fitting obtains v0, the parameter α of the adjustment velocity variation rate, and the relative variation rate β of the gravel layer velocity with depth. During the fitting, t corresponds to ft i，k , and h corresponds to fh i，k .

[0115] The relative time difference and position difference of different micrologs are calculated according to the method of the above step (23); and Figure 2 The correction is performed according to the method of the above step (24), and the result is shown in Figure 3 It can be seen that the consistency is good, indicating that the method of the present application can significantly improve the accuracy of the result.

[0116] S3. Calculate the total thickness of the low velocity layer

[0117] Record the surface elevation of the shot point measured in the field as G m，S， The low velocity layer bottom boundary elevation is G m，1，T , the low velocity layer thickness to be calculated is H m , the average velocity v i，0 of the low velocity layer of the microlog survey point obtained by the step S1, the thickness h i，0 , the low velocity gravel layer top velocity v i，1，T , the shot point low velocity layer delay time τ m , the refraction layer velocity v m，2 , and the relative variation rate β of the gravel layer velocity with depth obtained by the step S2 of the technical disclosure, are used to calculate the total thickness of the low velocity layer according to the following steps.

[0118] (31) Calculate the elevation of the bottom boundary of the low-velocity layer at the micro-logging survey point.

[0119] Let h ∈ (h_i) be the range of gravel layer thickness at the i-th micro-logging point. min h max Without loss of generality, h is taken. min =0,h max =1000, calculate the elevation of the bottom boundary of the low-velocity layer at this survey point according to the following steps:

[0120] ① Extracting the retardation time of the low-deceleration layer and the velocity of the refractive layer

[0121] Let the geodetic coordinates of the i-th micrologging survey point be located at the m-th shot receiver point. Then the time delay τ of the low-velocity layer at that survey point is... i and the velocity v of the refractive layer i，2 for

[0122]

[0123] ② Calculating the delay of the gravel layer

[0124] The formula for calculating the delay in the low-speed layer is:

[0125]

[0126] Where, τ i，0 When the low-velocity layer is delayed, h is the observation depth of the gravel layer.

[0127] Gravel layer delay τ i，1 The calculation formula is:

[0128] τ i，1 =τ i -τ i，0 .

[0129] ③ Calculate the initial thickness h of the gravel layer i，1 The formula is:

[0130] h i，1 =(h min +h max ) / 2.

[0131] ④ Calculate the thickness of the gravel layer as h. i，1 Theoretical delay time

[0132] Use the formula:

[0133]

[0134] h=(Ln(v / v0)-α) / αβ

[0135] Calculate v i，1 The corresponding fh0.

[0136] Using the formula:

[0137] t = (1 - (1 + βh α-1 ) / (v0β(1 - α)(1 + βh α-1 )),

[0138] Substitute v0, α, β calculated in step S2, calculate fh0and h i The corresponding time ft0and ft1,

[0139] fv= (h i，1 -fh0) / (ft1-ft0)

[0140]

[0141] ⑤Correct the gravel layer thickness value range

[0142]

[0143] Given the relative error ε of the theoretical value and the observed value at a given delay, if |Γ i，1 -τ i，1 | / τ i，1 < ε, then h i，1 is the gravel layer thickness to be calculated, otherwise repeat steps ③ to ⑤ until h i，1 that meets the requirements of ε is calculated.

[0144] ⑥Calculate the low velocity layer bottom boundary elevation G i，1，B , the formula is:

[0145] G i，1，B = G i，S -(h i，0 +h i，1 )

[0146] (32) Calculate the low velocity layer bottom boundary elevation of all micro logging survey points

[0147] Take i = 1, 2, …, N W , respectively, according to the method provided in (31) of step S3 of the invention, calculate the low velocity layer bottom boundary elevation of N W micro logging survey points G i，1，B .

[0148] (33) Calculate the total thickness of the low velocity layer of all shot points in the work area

[0149] Use the Kriging interpolation method, according to the geodetic coordinates of the micro logging survey points and the shot points, use N W low velocity layer bottom boundary elevations G i，1，B of NP The low-velocity layer bottom elevation G of the mth shot point m，1，B Then the total thickness H of the low-velocity layer is calculated according to the following formula m :

[0150] H m = G m，S - G m，1，B ;

[0151] The total thickness of the low-velocity layer calculated by the method of the present application is shown in the following table, where the horizontal and vertical coordinates are grid coordinates with a unit of meters. Figure 4

[0152] S4. Calculate the thickness of the low-velocity layer and the thickness of the gravel layer

[0153] (41) Calculate the thickness of the low-velocity layer and the thickness of the gravel layer of the mth shot point

[0154] Let m be the shot point number, and the thickness range of the low-velocity layer of the mth shot point is h∈(h min , h max ), without loss of generality, take h min = 0, h max = 1000, and calculate the thickness of the low-velocity layer and the thickness of the gravel layer of the shot point according to the following steps.

[0155] ① Calculate the initial thickness of the low-velocity layer

[0156] h m，0 = (h min +h max ) / 2

[0157] ② Calculate the theoretical delay time of the low-velocity layer

[0158]

[0159] ③ Calculate the theoretical delay time of the gravel layer

[0160] Use the formula:

[0161] h = (Ln(v / v0) - α) / αβ

[0162] Calculate v m，1 corresponding to kh0.

[0163] Use the formula:

[0164] t = (1 - (1 + βh) α-1 / (v0β(1 - α)(1 + βh) α-1 ),

[0165] Substitute v0, α, and β calculated in step S2 to calculate kh0and H m -h m，0 ​Corresponding time kt0 and kt1,

[0166] kv = (H m -h m，0 ) / (kt1-kt0)

[0167]

[0168] ④ Correct the low-speed layer thickness value range

[0169]

[0170] Given the relative error ε of the theoretical value and the observed value at a given delay, if |Γ m，0 +Γ m，1 -τ m | / τ m < ε, then h m，0 is the low-speed layer thickness, otherwise repeat steps ① to ④ until h m，0 that meets the ε requirement.

[0171] ⑤ Calculate the gravel layer thickness

[0172] h m，1 = H m -h m，0

[0173] (42) Calculate the low-speed layer thickness and gravel layer thickness of all shot points

[0174] Take m = 1, 2, …, N P , N P is the total number of shot points in the work area, and the low-speed layer thickness and gravel layer thickness of N P shot points are calculated by the method provided in (41) of step S4 of the invention. The results are shown in Figure 5 and Figure 6 , where the horizontal and vertical coordinates are grid coordinates in meters.

[0175] As can be seen from Figure 5 and Figure 6 , the gravel layer thickness is much greater than the low-speed layer thickness, so the cost of micrologging through the gravel layer is very high, and therefore the number of samples is extremely small.

[0176] S5. Calculate the gravel layer velocity

[0177] Take m = 1, 2, …, N P , N P is the total number of shot points in the work area, and the average velocity of the gravel layer and the gravel layer velocity at any depth of N P shot points in the work area are calculated according to the following formula.

[0178] ① Calculate the average velocity of gravel layer

[0179]

[0180] ② Calculate the velocity of gravel layer at any depth

[0181] Let d be any depth from the ground surface, calculate N according to the following formula P The velocity of gravel layer at any depth of the shot point of the work area:

[0182] v m，1 (d) = v m，1，T × [1 + (d-h m，0 )β], h m，0 ≤ d ≤ H m ;

[0183] The results are shown in Figure 7 The horizontal and vertical coordinates in the figure are grid coordinates, with units of meters.

[0184] S6. Establishing a near-surface velocity model of the gravel layer in the piedmont zone

[0185] The velocity model at the following depth is established using the low-velocity layer thickness and velocity of the shot point, the gravel layer thickness of the shot point, and the gravel layer velocity of the shot point:

[0186]

[0187] Or

[0188]

[0189] Wherein, the value of m is 1, 2, …, Np, wherein Np is the total number of shot points.

[0190] Compared with the pre-stack depth migration profile of the surface model obtained by the conventional tomographic inversion method based on the first arrival, the same phase axis continuity is obviously improved, especially the phase axis in the shallow layer is obviously improved, indicating that the near-surface velocity model of the gravel layer in the piedmont zone established by the method is more accurate.

[0191] It should be noted that the above only describes the preferred embodiments of the present application and is not intended to limit the present application. Although the present application has been described in detail with reference to the above embodiments, those skilled in the art can still modify the technical solutions described in the above embodiments or make equivalent replacements to some technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the claims of the present application.

Claims

1. A method for modeling near-surface velocities in a gravel layer in a mountain foreland zone, characterized in that, It comprises the following steps in sequence: S1. Calculate the low-speed layer average velocity, low-speed gravel layer top velocity and low-decreasing velocity layer delay time The micro-logging data is interpreted and stratified by using the micro-logging interpretation method, and the low-speed layer average velocity of the micro-logging survey point, the thickness of the micro-logging survey point and the low-speed gravel layer top velocity of the micro-logging survey point are obtained; The low-speed layer average velocity of the shot-receiver point is interpolated by using the low-speed layer average velocity of the micro-logging survey point according to the geodetic coordinates of the micro-logging survey point and the shot-receiver point by using the Kriging interpolation method; The low-speed gravel layer top velocity of the shot-receiver point is interpolated by using the low-speed gravel layer top velocity of the micro-logging survey point; The low-decreasing velocity layer delay time of the shot-receiver point and the refracted layer velocity of the shot-receiver point are calculated according to the generalized reciprocal method by selecting the first arrival time of the refracted wave; S2. Calculate the relative change rate of the gravel layer velocity with depth The time-depth data of the gravel layer in the ultra-deep micro-logging is extracted, fitted and discretized to calculate the maximum value of the fitted vertical travel time; the deepest micro-logging data is taken as the reference to calculate the low-speed gravel layer average velocity of each micro-logging, and the position of each micro-logging corresponding to the reference micro-logging is iteratively searched; after correction and data merging, the relative change rate of the gravel layer velocity with depth is obtained through data fitting; The formula used for fitting is: Where t is the vertical travel time, h is the observed depth of the gravel layer, and the velocity of the top interface of the gravel layer is obtained by fitting , the parameter of adjusting the velocity change rate , the relative change rate of the velocity of the gravel layer with depth ; S3. Calculate the total thickness of the low-decreasing velocity layer The delay time and thickness of the gravel layer are calculated by using the low-speed layer average velocity of the micro-logging survey point, the thickness of the micro-logging survey point, the low-speed gravel layer top velocity of the micro-logging survey point, the low-decreasing velocity layer delay time of the shot-receiver point, the refracted layer velocity of the shot-receiver point and the relative change rate of the gravel layer velocity with depth; the thickness value range of the gravel layer is corrected, and the low-decreasing velocity layer bottom boundary elevation of the micro-logging survey point is calculated through iterative calculation; The low-decreasing velocity layer bottom boundary elevation of the shot-receiver point is interpolated by using the low-decreasing velocity layer bottom boundary elevation of the micro-logging survey point according to the geodetic coordinates of the micro-logging survey point and the shot-receiver point by using the Kriging interpolation method, and the total thickness of the low-decreasing velocity layer is calculated; S4. Calculate the low-speed layer thickness and the gravel layer thickness The total thickness of the low-decreasing velocity layer is used to correct the low-speed layer thickness value range by using the initial thickness of the low-speed layer, the theoretical delay time of the low-speed layer and the theoretical delay time of the gravel layer, and the low-speed layer thickness of the shot-receiver point and the gravel layer thickness of the shot-receiver point are calculated; The theoretical delay time of the gravel layer is calculated by the following formula: ； ； Wherein, kv is the average velocity of the gravel layer section, kt0 is the time of the top of the gravel layer, kt1 is the time of the bottom of the gravel layer, is the theoretical delay time of the gravel layer, v m,2 is the refracted layer velocity of the shot point, kv 2 is the square of the average velocity of the gravel layer section; S5. Calculate the gravel layer velocity The gravel layer average velocity and the gravel layer velocity at any depth are calculated according to the following formula by using the low-speed gravel layer top velocity of the micro-logging survey point, the gravel layer thickness of the shot-receiver point and the relative change rate of the gravel layer velocity with depth, ； ； where m is the shotpoint number, v m,1 is the average velocity of the gravel layer, v m ,1 (d) is the velocity of the gravel layer at any depth, v is the relative change of the velocity of the gravel layer with depth, v m ,1,T is the top velocity of the low velocity gravel layer at the shotpoint, d is any depth from the surface, h m ,1 is the thickness of the gravel layer at the shotpoint, h m,0 is the initial thickness of the low velocity layer, H m is the total thickness of the low velocity layer; S6. Establish the near-surface velocity model of the gravel layer in the mountain front zone The velocity model at the following depth is established by using the low-speed layer thickness and velocity of the shot-receiver point, the gravel layer thickness of the shot-receiver point and the gravel layer velocity of the shot-receiver point: Or Wherein, the value of m is 1, 2, …, Np, wherein, Np is the total number of shot points, v m,0 is the low-velocity layer average velocity of Np shot points, and N W is the low-velocity layer average velocity of Nm micro-logging survey points i,0 is interpolated v m,0 , the micro-logging interpretation method is applied to interpret and stratify the i-th micro-logging, and the low-velocity layer average velocity extracted through the interpretation and stratification is v i,0 .

2. The method of claim 1, wherein, In step S1, the representation method of the microlog data is , which represents the vertical travel time at the observation point of the jth depth of the ith microlog, where i and j are both sample serial numbers.

3. The method of claim 1, wherein: In step S3, the delay time of the gravel layer is the low-speed layer delay time and the gravel layer delay time.

Images