CN109765617B - Method for suppressing multiple reflection refraction wave by vertical double seismic sources based on kinematics - Google Patents

Abstract

The invention discloses a method for suppressing multiple reflection refraction waves by vertical double seismic sources based on kinematics, which is used for deducing a time distance curve of the multiple reflection refraction waves and target layer reflection waves when an excitation well depth exists on the basis of a multiple reflection refraction wave propagation path, and providing a vertical double seismic source combined excitation method on the basis of the curve. The combined double seismic sources are arranged at different depths of the same position on the ground, the vertical relative distance of the two seismic sources is controlled, the two seismic waves generated by the two seismic sources are simultaneously excited, and time difference interference cancellation exists in the propagation process, so that the purposes of suppressing multiple reflection refracted waves and reducing the influence on the reflected waves of a target layer are achieved. From multiple reflection refraction wave generation source, the problem that the resolution ratio and the signal-to-noise ratio of the reflected wave of a target layer in seismic records are low due to low-speed zone development in seismic exploration work areas such as loess tablelands and sand lands is solved, the overall working efficiency of seismic exploration is improved, and fine interpretation of coal and coal bed gas reservoirs is facilitated.

Description

Method for suppressing multiple reflection refraction wave by vertical double seismic sources based on kinematics

Technical Field

The invention discloses a method for suppressing multiple-reflection refracted waves by vertical double seismic sources based on kinematics, and belongs to the technical field of oil-gas seismic exploration.

Background

China is a country with the widest distribution and the largest thickness of loess in the world, and abundant energy resources are stored under the coverage of the loess, and mainly coal, coal bed gas and petroleum and natural gas. The fossil energy sources are mainly explored by means of artificial earthquakes. Due to the unique physical properties of the loess plateau, the development of pores, looseness, dryness, low density and low wave velocity, the problems of serious seismic wave energy attenuation, serious high-frequency attenuation caused by low signal-to-noise ratio, development of strong interference such as surface wave, ringing, multiple waves and the like are caused. With the change of the energy policy from extensive type to intensive type, in seismic exploration, refined exploration is increasingly emphasized, so that how to eliminate interference waves in artificial seismic exploration, target layer reflected waves are highlighted, and the realization of shallow reservoir refined seismic interpretation becomes vital.

Among the interference waves, the multiple reflection refracted wave is a common interference wave which is difficult to suppress by conventional collection processing. For seismic exploration with a relatively shallow target layer burial depth, such as coal field seismic exploration, multiple reflection refracted waves generally appear at a few hundred milliseconds, and generally show the same or similar wave field characteristics as target layer reflected waves, so that the wave field characteristics are difficult to eliminate, and target layer effective waves are covered and difficult to identify. If the multiple reflected refracted waves cannot be correctly identified, the multiple reflected refracted waves are not eliminated as interference waves, and the judgment of the position of the target layer and the subsequent explanation are influenced. Generally, the objective of suppressing multiple-reflection refracted waves is achieved through a series of mathematical operations, such as filtering, predictive subtraction and sparse inversion, in the seismic processing stage. The characteristics of the time distance curve are nearly consistent with those of the primary emission wave, so that the elimination of the apparent velocity difference in seismic processing is difficult to realize.

Due to the special geological structure of the low-speed zone, the seismic data acquired in the field have low resolution and low signal-to-noise ratio. Therefore, the expected effect of suppressing multiple reflection refracted waves is difficult to achieve in the processing work, reflected waves of a target layer cannot be clearly separated from the refracted waves, the position of the target layer is difficult to judge, and shallow reservoir prediction and development parameter evaluation cannot be effectively guided.

The first prior art is as follows:

the processing and pressing means for pressing multiple reflection refraction waves commonly used at the present stage is as follows: the method comprises the steps of adopting a multiple wave combination adaptive attenuation method, based on a multiple wave elimination and extended wiener filtering combination adaptive attenuation method of a same-phase axis optimization tracking technology, firstly, based on predicted multiple wave information, calculating by using the same-phase axis optimization tracking technology and a short-time window FK apparent velocity filtering method, and separating original records into quasi-primary wave records and quasi-multiple wave records; and then, eliminating residual multiples in the quasi-primary record by using an extended wiener filtering method, and simultaneously recovering damaged primary information in the original quasi-primary record.

(1) Obtaining a multiple record m (x, t) by utilizing a free interface multiple prediction method (SRMP), and carrying out dynamic correction and superposition processing on the multiple record m (x, t) to obtain a multiple superposition velocity spectrum, wherein a corresponding calculation formula is

Wherein N (N is more than or equal to 1 and less than or equal to N) is a track number; x is the number of_nIs the offset of the nth track, pair E_m(V, τ) after smoothing, the velocity value V is superimposed on m (x, t)₀Zero offset time τ₀On the hyperbolic in-phase axis of (A), will be in spectrum E_m(v, τ) is formed with (v)₀，τ₀) Energy of the nodular structure at the central extremum;

(2) finding the spectrum E by using contour tracing method_m(V, τ) above by (V)₀，τ₀) The distribution range of the energy of the cluster structure which is the central extreme value is searched out, the position of the extreme value point is searched out, and then the coordinate (V) of the point is used for searching the energy of the cluster structure₀，τ₀) To fit the corresponding homophasic axes in the spatio-temporal domain, i.e. the travel times t of the respective roads traveled thereby_nCan be represented as

Then with the originally tracked extreme position (V)₀，τ₀) A local velocity spectrum of an original record d (x, t) is created for the center, and accurate parameters v '0 and tau' 0 of the same-phase axis can be obtained by scanning within a certain speed and time range, so that the optimized tracking process of the multiple same-phase axis is realized;

(3) obtaining travel time t of seismic channel where each multiple homophase axis passes_mThen, the short-time window FK apparent velocity filtering method is used to eliminate the original record d (x, t) and the multiple record m (x, t), and the removed multiple in-phase is obtainedQuasi-primary wave recording behind axis d_p(x, t) and the remaining multiples record m_l(x, t), and the removed multiple in-phase axes are synthesized into a quasi-multiple record d_m(x, t) and hyperbolic multiples log m_h(x,t)；

Recording d for quasi-multiples using extended wiener filtering_m(x, t) and quasi-primary recording d_p(x, t) performing a second adaptive attenuation to filter out the quasi-multiple recordings d_m(x, t) and further eliminates residual multiples from quasi-primary record d p (x, t), the corresponding process is represented as:

wherein ELSF represents an extended least squares filter operator; p is a radical of₁Contained in (x, t) is the primary information filtered out from record d m (x, t); p is a radical of₂(x, t) is an erasure record d_pThe result of the residual multiples in (x, t), the final attenuated multiples, is recorded as follows:

p(x,t)＝p₁(x,t)+p₂(x,t) (d)

p (x, t) is the final result of the combined adaptive attenuation.

Disadvantages of the first prior art

The method for suppressing multiple waves in seismic processing has the characteristic of 'reliability degree' depending on field acquisition of seismic data, and in some exploration work areas, high-quality seismic acquisition data are often difficult to obtain due to large surface relief, development of low-speed zones and complex shallow structures. In some work areas with shallow target layer burial depth (400m-600m) and near far-middle offset distances, the visual velocity of multiple reflection refracted waves and target layer reflection waves is very close to each other in seismic record, and intersection points exist, so that the first scheme has limitations in principle by adopting a pressing method. In the pressing process, the reflected waves of the target layer can be synchronously pressed while the refracted waves are pressed for multiple times, so that the reflected waves of the target layer are lost in a section of offset after being processed, the explanation of a subsequent reservoir is greatly influenced, and the reservoir development is difficult to guide.

Prior art 2

In the construction method of field earthquake collection, combined excitation is carried out along the direction of a horizontal lateral line, and the aim of suppressing interference waves is achieved by improving downward propagation energy and then improving the signal to noise ratio.

The utility model provides a loess highland coal mine district big combination base distance combination arouses seismic prospecting technique, adopts big combination base distance, little dose combination well to make up and arouse, its characterized in that, the step is as follows:

q1: low speed belt investigation was performed: determining the thickness of the low and deceleration belts, and the implementation method comprises the following steps: adopting unequal track spacing, namely, the track spacing at two ends of the receiving track is small, and the track spacing in the middle is large; adopting an encounter time-distance curve observation system to collect and blast at a zero offset distance end point, wherein the depth of an excited pit is 0.5-1m, and the dosage is 0.5-1 kg;

wherein, the meeting time-distance curve observation system is an observation system used for carrying out low-speed zone investigation: respectively exciting two ends with unequal channel spacing, and receiving by using the same receiving array, namely a detector, to obtain two seismic records with opposite directions for explaining the thickness of a low-speed zone at the point; respectively picking the first arrival time of the two records to obtain a relation graph of the first arrival time and the offset, and calculating to obtain the speed and the thickness of each layer;

q2: seismic data acquisition: selecting 2-3m in a deceleration zone as the well depth of combined excitation, and keeping the horizontal elevation of each well consistent when the combined well is excited; the single well dosage is 1-3 Kg; the excitation source is promoted to excite the surface wave, and the impact on the soil medium is increased.

Q3: analyzing the effective wave energy and frequency, and determining the number of wells excited by the combination, wherein the range value is 3-7 excited well combinations; the combined base distance of the combined well is set to be 10 m;

q4: receiving seismic signals by using a combined detector, wherein the detector is embedded by selecting 3-5 piles, or is linearly combined along a measuring line, and the combined base distance is 0-5 m;

q5: the observation system adopts a conventional observation system. As a preferable mode of the above technical solution, the specific parameters during the excitation in the Q1 step are: the depth of an excitation pit is 1m, the dosage is 0.5kg, the sampling rate is 0.5ms, the recording length is 0.5s, and the arrangement length is 90m respectively; the obtained record is first-arrival picked up, and the low and reduced speed belt speed and thickness are explained. Preferably, in the step Q2-Q3, the combination mode of the combination wells is linear combination along the direction of the seismic survey line or area combination with the central excitation point as the center.

The second prior art has the defects

The horizontal direction earthquake combination excitation solves the main functions and aims to suppress random interference through signal combination, increases the energy of earthquake waves which are transmitted downwards through large base distance and small dosage, suppresses interference waves, improves the signal to noise ratio, does not purposefully eliminate multiple reflection refraction waves, and has the effect on fine interpretation of earthquake sections after stacking because the multiple reflection refraction wave information still exists in an earthquake data body.

The existing seismic processing and seismic acquisition methods do not suppress and eliminate the multiple-reflection refracted wave from a multiple-reflection refracted wave generation source head, but take remedial measures after the multiple-reflection refracted wave is generated.

Disclosure of Invention

In order to solve the defects of the prior art, the invention discloses a method for seismic exploration by vertical double-seismic-source excitation based on kinematic characteristics, which solves the problems of low resolution and low signal-to-noise ratio of target layer reflected waves in seismic records caused by low-speed zone development in seismic exploration work areas such as loess tablelands, sand lands and the like from a multiple reflection refracted wave generating source, improves the overall working efficiency of seismic exploration, and is favorable for the fine interpretation of coal and coal bed gas reservoirs.

The invention is realized by the following technical scheme:

the method for suppressing multiple reflection refracted waves by using the vertical double seismic sources based on kinematics comprises the following steps:

step 1) deriving a time-distance curve equation of multiple reflection refraction waves excited by well depth below the earth surface and target layer reflection waves based on multiple reflection refraction wave propagation paths;

step 2) carrying out low-speed zone survey on the seismic exploration work area, mainly surveying the speed and thickness information of the low-speed zone, and judging the coal bed burial depth;

step 3) determining the seismic wave main frequency received by the geophone;

step 4) carrying out single shot test, observing unprocessed seismic records, and finding out the area where the multiple reflected refracted waves at the offset distance interfere the most seriously with the reflected waves of the target layer;

step 5) selecting a proper combined excitation well distance delta d according to the stratum parameters and the equipment parameters, wherein the combined excitation well distance delta d has a suppression effect on multiple reflection refraction waves, and simultaneously does not have a suppression effect on target layer reflection waves;

and 6) in the area selected in the step 4), setting combined double seismic sources at different depths of the same position on the ground, controlling the vertical relative distance of the two seismic sources to be a combined excitation parameter well spacing delta d, and simultaneously exciting the two seismic sources to perform seismic exploration.

Preferably, the derivation process of the time distance curve of the reflected refracted wave in the presence of the excitation well depth is as follows:

firstly, a plurality of sections of refracted waves with parallel rays and interfaces do not exist in an ideal isotropic layered medium, the amplitude of a multi-reflection-refracted wave is reflected to be larger than that of a primary refracted wave through seismic recording, and a multi-refraction wave path is determined through spectral element forward modeling verification based on the two points;

and step two, deducing a time distance curve of multiple reflection refracted waves when the excitation well depth exists:

on the multiple refracted wave path, measuring the time required for the seismic wave to propagate from the seismic source to the ground detector along the multiple refracted wave path;

the propagation of the seismic wavelet includes two layers: an upper low-speed layer and a lower high-speed layer; the thickness of the overlying low-speed layer is known to be h₁Seismic wave velocity of overburden low velocity zone is V₁The depth of the excitation point from the ground is d, and the seismic wave velocity of the lower high-speed layer is V₂Setting the offset as x,

according to the seismic exploration principle, a reflected wave time distance curve is obtained and written as a formula (1):

in the formula, t_{Inverse direction}For the propagation time of the reflected wave, X_{Inverse direction}Is the transverse distance, V, of the excitation point and the detection point₁Seismic wave velocity of overburden low velocity layer, h₁For overlying low-velocity layer thickness, t₀The self-excitation and self-recovery time is the self-excitation and self-recovery time of the ground excitation of the upper low-speed layer vertically propagating to the interface between the two layers and then returning to the ground excitation point;

obtaining a refracted wave time distance curve formula according to the seismic exploration principle:

f(θ)＝arcsin(V₁/V₂)

in the formula, t_{Folding device}For refracted wave propagation time, X_{Folding device}Is the transverse distance, V, of the excitation point and the detection point₁For seismic wave velocity, V, of overlying low-velocity layer₂Seismic wave velocity of underlying high-velocity layer, h₁To cover the thickness of the low-velocity layer, h₂The thickness of the lower covering high-speed layer is shown, and theta is a refraction wave critical angle;

and performing composite operation on the time distance curve of the reflected wave and the refracted wave to obtain a time distance curve formula (3) when the multiple reflection-refracted wave is excited below the ground surface with the well depth:

simplifying to obtain:

f(θ)＝arcsin(V₁/V₂)

in the formula (I), the compound is shown in the specification,t_{reversely folded}For multiple reflection-refraction wave propagation time, X_{Reversely folded}Is the transverse distance, V, of the excitation point and the detection point₁For seismic wave velocity, V, of overlying low-velocity layer₂Seismic wave velocity of underlying high-velocity layer, h₁To cover the thickness of the low-velocity layer, h₂The thickness of the lower cladding high-speed layer, d is the distance (well depth) from the excitation point to the earth surface, and theta is the critical angle of the refracted wave.

Preferably, the derivation process of the time distance curve of the target layer reflected wave in the presence of the excitation well depth is as follows:

according to the propagation path of the reflected wave of the target layer, the thickness of the low-speed layer is h₁Velocity of V₁The high-speed layer has a thickness of h₂Velocity of V₂Total thickness is H, offset is x, excitation well depth is d, alpha is incident angle, H ═ H₁+h₂；

The speed is replaced by a root mean square speed approximation, the time-distance curve is an explicit function, and the following root mean square speed formula is used:

in the formula, V_rRoot mean square velocity, i is the surface from top to bottom, i is the ith stratum, t_iIs the propagation time of seismic waves in the ith formation, V_iThe seismic wave velocity in the ith stratum;

it can be seen that the time interval curve of the target layer reflected wave in the presence of the excitation well depth is represented as:

in the formula, t_rFor the purpose of the target layer reflection wave propagation time, V_rRoot mean square velocity, X_rThe transverse distance between an excitation point and a demodulator probe, the alpha reflection angle, the distance (well depth) from the excitation point to the earth surface, and H is the total thickness of the stratum;

simplifying to obtain:

preferably, the combined excitation well spacing Δ d is obtained by:

let two vertical excitation points and one depth be d₁And the other is d₂The low-speed layer speed is V₁High layer velocity of V₂The time difference is delta t, and theta is a critical angle;

for two different well depths d₁And d₂Obtaining a time-distance curve t of two multiple reflection-refraction waves_{Reverse folding 1}And t_{Reverse folding 2}Time distance curve t of two multiple reflection-refraction waves_{Reverse folding 1}And t_{Reverse folding 2}The subtraction yields:

Δt＝|t_{reverse folding 1}-t_{Reverse folding 2}|，Δd＝|d₁-d₂|

The method is simplified as follows:

the time difference Δ t is selected to be 2n +1 times 1/2 cycles, n is 0, 1, 2, 3 …, and the range of the combined excitation well spacing Δ d is:

f(θ)＝arcsin(V₁/V₂)

let V₁/V₂Z is the wave speed ratio of the low-speed layer to the high-speed layer, and is obtained according to the Pythagorean theorem

The period T is the reciprocal of the seismic dominant frequency f

The velocity V of the overlying low-velocity layer is selected according to the seismic dominant frequency f of the known seismic exploration₁Lower velocity of high velocity zone V₂And further determining the range of the combined well spacing delta d playing a role in suppressing the multiple reflection refraction wave as follows:

z∈(0,1)

the influence on the reflected wave of the target layer is reduced while the vertical combined seismic source is excited and pressed to reflect the refracted wave for multiple times;

according to the target layer reflection wave time distance curve when the well depth is excited according to the formula (7):

since the depth of the target layer is far greater than the well depth of the excitation point in the actual exploration situation, the formula (13) is simplified to obtain:

the relative position relation of the combined excitation points influences the reflected wave of the target layer, and the depth of the excitation points at two vertical positions is set as d₁And the other is d₂Corresponding propagation times are respectively t_r1And t_r2Then, then

Subtracting the corresponding parts of the left and right sides with equal signs of the upper and lower formulas in the formula (15) respectively to obtain

Then:

selecting nT and nT +1/2t according to the time difference delta t_rAnd avoiding the interference cancellation of reflected waves, namely:

the size of the well spacing delta d is preferably combined according to the relation between the offset and the burial depth, the multiple reflection refraction wave is pressed on the premise of not interfering the target layer reflection wave,

small offset X_rWhen H/2, determining the range of the combined well spacing delta d playing a role in suppressing the multiple reflection refraction wave as follows:

middle offset distance X_rWhen H, the range of the combined well spacing Δ d for suppressing the multiple-reflection refracted wave is determined as follows:

large offset distance X_rWhen the average value of the well distance Δ d is 2H, the range of the combined well distance Δ d for suppressing the multiple-reflection refracted wave is determined as follows:

compared with the prior art, the invention has the following beneficial effects:

the invention provides a vertical double-seismic-source combined excitation method based on the time distance curve of multiple reflection refraction waves and target layer reflection waves at different excitation depths deduced by seismic wave propagation rules and based on multiple reflection refraction wave propagation characteristics. Setting a combined double seismic sources at the same position on the ground at different depths, controlling the vertical relative distance of the two seismic sources, simultaneously exciting, and generating different t by the two seismic sources₀The seismic waves of time have time difference interference cancellation in the propagation process, and the purposes of suppressing multiple reflection refracted waves and reducing the influence on the reflected waves of a target layer are achieved. From multiple reflection refraction wave generation source, the problem that in seismic exploration work areas such as loess tablelands and sand lands, due to the fact that low-speed zones develop, target layer reflection wave resolution and signal-to-noise ratio are low in seismic records is solved, a large number of complex mathematical operations in the seismic data processing process are avoided, the overall working efficiency of seismic exploration is improved, and fine interpretation of coal and coal bed gas reservoirs is facilitated. The method provided by the invention has good practical application value.

Drawings

The invention is further described below with reference to the accompanying drawings.

FIG. 1 is a vertical dual source combined excitation method;

FIG. 2 is a schematic diagram of a multiple reflection refracted wave propagation path;

FIG. 3 is a schematic diagram of multiple reflection refracted wave propagation paths in the presence of an excitation well depth;

FIG. 4 shows the target layer reflected wave propagation path in the presence of an excitation well depth.

FIG. 5 is a comparison graph I of the time interval curve and the forward modeling;

FIG. 6 is a comparison graph II of the time interval curve and the forward modeling;

FIG. 7 is a single shot seismic record chart for a test work area;

FIG. 8 is a diagram of a vertical dual-source combined excitation seismic record of a test work area.

Detailed Description

The present invention will be described in further detail with reference to specific examples, but the scope of the present invention is not limited to these examples, and all changes or equivalent substitutions that do not depart from the spirit of the present invention are intended to be included within the scope of the present invention.

A seismic wavelet is a piece of signal with a defined start time, limited energy and a certain duration, which is the basic unit in a seismic recording. A wavelet may be defined by its amplitude spectrum and phase spectrum, the type of phase spectrum may be zero phase, constant phase, minimum phase, mixed phase, etc.; for zero-phase and constant-phase wavelets, which can be simply regarded as a set of sine waves of different amplitudes and frequencies, all sine waves are in zero phase or in constant phase, two columns of seismic wavelets generate interference phenomena in the process of propagating in a medium, the interference is cancelled at some positions, and the interference is strengthened at some positions. The method is used for eliminating multiple reflected-refracted waves.

The method for suppressing multiple reflection refracted waves by using the vertical double seismic sources based on kinematics comprises the following steps:

1.1, step 1) deducing a time distance curve between the multiple reflection refracted wave and a target layer reflection wave when the excitation well depth exists on the basis of the multiple reflection refracted wave propagation path;

1.1.1 the derivation process of the time distance curve of the reflected refracted wave in the presence of the excitation well depth is as follows:

firstly, a plurality of sections of refracted waves with parallel rays and interfaces do not exist in an ideal isotropic layered medium, the amplitude of a multi-reflection-refracted wave is reflected to be larger than that of a primary refracted wave through seismic recording, and the path of the multi-reflection wave is determined through spectral element forward modeling verification based on the two points, as shown in fig. 2;

and step two, deducing a time distance curve of multiple reflection refracted waves when the excitation well depth exists:

measuring the time required for seismic waves to propagate from a seismic source (point O) to a ground detector (point E) along the multiple refracted wave path on the multiple refracted wave path;

the earthquakeThe propagation of the sub-wave includes two layers: an upper low-speed layer and a lower high-speed layer; the thickness of the overlying low-speed layer is known to be h₁Seismic wave velocity of overburden low velocity zone is V₁The depth of the excitation point from the ground is d, and the seismic wave velocity of the lower high-speed layer is V₂Setting the offset as x, and according to the seismic exploration principle, writing a reflected wave time-distance curve into a formula (1):

in the formula, t_{Inverse direction}For the propagation time of the reflected wave, X_{Inverse direction}Is the transverse distance, V, of the excitation point and the detection point₁Seismic wave velocity of overburden low velocity layer, h₁For overlying low-velocity layer thickness, t₀The self-excitation and self-recovery time is the self-excitation and self-recovery time of the ground excitation of the overlying low-speed layer vertically propagating to the interface between the two layers and then returning to the ground excitation point.

According to the seismic exploration principle, the time distance curve of the obtained refracted wave is expressed as a formula (2):

f(θ)＝arcsin(V₁/V₂)

in the formula, t_{Folding device}For refracted wave propagation time, X_{Folding device}Is the transverse distance, V, of the excitation point and the detection point₁For seismic wave velocity, V, of overlying low-velocity layer₂Seismic wave velocity of underlying high-velocity layer, h₁To cover the thickness of the low-velocity layer, h₂The thickness of the lower covering high-speed layer is shown, and theta is a refraction wave critical angle;

and performing composite operation on the time distance curve of the reflected wave and the refracted wave to obtain a time distance curve formula (3) when the multiple reflection-refracted wave is excited below the ground surface with the well depth:

simplifying to obtain:

f(θ)＝arcsin(V₁/V₂)

in the formula, t_{Reversely folded}For multiple reflection-refraction wave propagation time, X_{Reversely folded}Is the transverse distance, V, of the excitation point and the detection point₁For seismic wave velocity, V, of overlying low-velocity layer₂Seismic wave velocity of underlying high-velocity layer, h₁To cover the thickness of the low-velocity layer, h₂The thickness of the lower cladding high-speed layer is d, the distance (well depth) theta from an excitation point to the ground surface is a refraction wave critical angle;

the propagation time-distance curve of the multiple reflection refraction wave is obtained through the formula (4), and the propagation time of the multiple reflection refraction wave and the transverse distance X between the excitation point and the detection point can be known_{Reversely folded}Distance d from excitation point to earth surface, velocity V of seismic wave of overlying low-velocity layer₁Velocity V of seismic waves in the underlying high-velocity layer₂Thickness h of overlying low-speed layer₁It is related.

1.1.2 the derivation process of the time distance curve of the target layer reflected wave in the presence of the excitation well depth is as follows:

according to the propagation path of the reflected wave of the target layer, the thickness of the low-speed layer is h₁Velocity of V₁The high-speed layer has a thickness of h₂Velocity of V₂Total thickness is H, offset is X, excitation well depth is d, alpha is incident angle, H ═ H₁+h₂；

The speed is replaced by a root mean square speed approximation, the time-distance curve is an explicit function, and the following root mean square speed formula is used:

in the formula, V_rRoot mean square velocity, i is the earth's surface from aboveTo the lower i-th formation, t_iIs the propagation time of seismic waves in the ith formation, V_iThe seismic wave velocity in the ith stratum;

it is understood that the time interval curve of the target layer reflected wave in the presence of the excitation well depth is expressed by equation (6):

in the formula, t_rFor the purpose of the target layer reflection wave propagation time, V_rRoot mean square velocity, X_rThe transverse distance between an excitation point and a demodulator probe, the alpha reflection angle, the distance (well depth) from the excitation point to the earth surface, and H is the total thickness of the stratum;

simplifying to obtain:

1.1.3 comparison of time-distance curves with forward results

1.1.3.1 setting low speed layer thickness h₁80m, high speed layer thickness h₂400m, low-speed layer longitudinal wave speed V₁800m/s, high-speed layer longitudinal wave velocity V₂2500m/s and the well depth d is 0m, and according to the deduced time distance curve of the multiple reflection refracted wave and the target layer reflection wave when the well depth exists, as shown in fig. 5, a comparison graph I of the obtained time distance curve and the forward simulation is obtained;

1.1.3.2 setting low speed layer thickness h₁80m, high speed layer thickness h₂500m, low-speed layer longitudinal wave velocity V₁800m/s, high-speed layer longitudinal wave velocity V₂2500m/s, well depth d 0m, which is derived from the existence of well depthA time distance curve of the secondary reflected refracted wave and the target layer reflected wave, as shown in fig. 6, is a comparison graph two of the obtained time distance curve and the forward modeling;

fig. 5 and 6 verify that multiple reflection of the refracted wave can cause an interference phenomenon with the target layer reflected wave.

1.2, step 2) carrying out low-speed zone survey on the seismic exploration work area, mainly surveying the speed and thickness information of the low-speed zone, and judging the coal bed burial depth;

1.3, determining the main frequency of seismic waves received by the geophone in step 3);

1.4, performing single shot test in step 4), observing unprocessed seismic records, and finding out an area where the multiple reflected refracted waves at the offset distance interfere with the reflected waves of a target layer most seriously;

1.5, selecting an appropriate combined excitation well spacing delta d according to the formation parameters and the equipment parameters, wherein the combined excitation well spacing delta d has a suppression effect on multiple-reflection refracted waves, and does not have a suppression effect on target layer reflected waves;

the combined excitation well spacing deltad is obtained by the following steps:

let d be the depth of two vertical excitation points in the stratum₁And the other is d₂The low-speed layer speed is V₁High layer velocity of V₂The time difference is delta t, and theta is a critical angle;

for two different well depths d₁And d₂Obtaining a time-distance curve t of two multiple reflection-refraction waves_{Reverse folding 1}And t_{Reverse folding 2}，

f(θ)＝arcsin(V₁/V₂)

Subtracting two equal-sign ends of the two time distance curves respectively to obtain:

Δt＝|t_{reverse folding 1}-t_{Reverse folding 2}|

Δd＝|d₁-d₂|

The method is simplified as follows:

the time difference Δ t is selected to be 2n +1 times 1/2 cycles (n is 0, 1, 2, 3 …), when the combined excitation interval Δ d ranges from:

f(θ)＝arcsin(V₁/V₂)

let V₁/V₂Z is the wave speed ratio of the low-speed layer to the high-speed layer, and is obtained according to the Pythagorean theorem

The period T is the reciprocal of the seismic dominant frequency f

The seismic dominant frequency f, the overburden low velocity V, which can be selected according to known seismic exploration₁Lower velocity of high velocity zone V₂And further determining the range of the combined well spacing delta d playing a role in suppressing the multiple reflection refraction wave as follows:

z∈(0,1)

the influence on the reflected wave of the target layer is reduced while the vertical combined seismic source is excited and pressed to reflect the refracted wave for multiple times;

according to the target layer reflection wave time distance curve when the well depth is excited according to the formula (7):

since the depth of the target layer is far greater than the well depth of the excitation point in the actual exploration situation, the formula (13) is simplified to obtain:

the relative position relation of the combined excitation points influences the reflected wave of the target layer, and the depth of the excitation points at two vertical positions is set as d₁And the other is d₂Corresponding propagation times are respectively t_r1And t_r2Then, then

Subtracting the corresponding parts of the left and right sides with equal signs of the upper and lower formulas in the formula (15) respectively to obtain

Then:

selecting nT and nT +1/2t according to the time difference delta t_rAnd avoiding the interference cancellation of reflected waves, namely:

the size of the well distance delta d is preferably combined according to the relation between the offset distance and the burial depth, the multiple reflection refraction wave is pressed on the premise of not interfering the target layer reflection wave,

small offset X_rWhen H/2, determining the range of the combined well spacing delta d playing a role in suppressing the multiple reflection refraction wave as follows:

middle offset distance X_rWhen H, the range of the combined well spacing Δ d for suppressing the multiple-reflection refracted wave is determined as follows:

large offset distance X_rWhen the average value of the well distance Δ d is 2H, the range of the combined well distance Δ d for suppressing the multiple-reflection refracted wave is determined as follows:

1.6 step 6) in the area selected in the step 4), setting combined double seismic sources at different depths of the same position on the ground, controlling the vertical relative distance of the two seismic sources as a combined excitation parameter well spacing delta d, and simultaneously exciting the two seismic sources to perform seismic exploration.

2.1 tests were carried out using the current method with the present invention,

a certain work area is located in the Liulin city of Shanxi province, the landform of a typical loess highland is covered by a huge-thick loess area, the thickness is 0-300m, the soil mainly comprises sub-sandy soil and sub-clay, the loess layer has a loose structure and low seismic wave speed (200-800m/s), and the absorption and attenuation effects on seismic waves are strong. The main coal mining layer is No. 4 coal mining layer of Shanxi group. The buried depth is 400m-600m, the thickness is about 4m, and the thickness of the coal seam is stable.

And selecting a flat area with flat terrain and surface relief smaller than meter in the work area for testing. According to stratum information obtained by small refraction and micro logging, the burial depth of a target layer is about 580m, the thickness of a low-speed layer is 80m, the longitudinal wave speed is 600m/s, the thickness of a high-speed layer is 500m, and the longitudinal wave speed is 2000 m/s.

According to the calculation of the formula (16), the purpose of suppressing multiple reflection refraction waves can be achieved by selecting 10m for determining the combined well spacing delta d. Conventional single shot excitation followed by vertical dual source combined excitation as shown in figure 7 and figure 8 are both unprocessed seismic records. According to the coal seam depth information obtained by drilling, coal seam reflected waves should appear in a dotted line frame, and people can see through observation that only multiple reflection refracted waves can be seen in the dotted line frame of fig. 7, and the coal seam reflected waves can be clearly seen in the dotted line frame of fig. 8. The invention has obvious pressing effect on multiple shallow refracted waves and highlights coal seam reflected waves.

According to the invention, the well distance delta d is excited by selecting a proper combination according to stratum parameters and equipment parameters, two rows of seismic waves (multiple reflection refraction waves) meet and interfere destructively in the process of low-speed zone motion propagation, the reflected waves of a target layer are protected, and the purpose of suppressing the seismic interference waves of the multiple reflection refraction waves is achieved. From multiple reflection refraction wave generation source, the problem that in seismic exploration work areas such as loess tablelands and sand lands, due to the fact that low-speed zones develop, target layer reflection wave resolution and signal-to-noise ratio are low in seismic records is solved, a large number of complex mathematical operations in the seismic data processing process are avoided, the overall working efficiency of seismic exploration is improved, and fine interpretation of coal and coal bed gas reservoirs is facilitated.

The present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (1)

1. The method for suppressing the multiple-reflection refracted wave by the vertical double seismic sources based on the kinematics is characterized by comprising the following steps of:

step 1) deriving a time-distance curve equation of multiple reflection refraction waves excited by well depth below the earth surface and target layer reflection waves based on multiple reflection refraction wave propagation paths;

step 2) carrying out low-speed zone survey on the seismic exploration work area, mainly surveying the speed and thickness information of the low-speed zone, and judging the coal bed burial depth;

step 3) determining the seismic wave main frequency received by the geophone;

step 4) carrying out single shot test, observing unprocessed seismic records, and finding out the area where the multiple reflected refracted waves at the offset distance interfere the most seriously with the reflected waves of the target layer;

step 5) selecting a proper combined excitation well distance delta d according to the stratum parameters and the equipment parameters, wherein the combined excitation well distance delta d has a suppression effect on multiple reflection refraction waves, and simultaneously does not have a suppression effect on target layer reflection waves;

step 6) in the area selected in the step 4), setting combined double seismic sources at different depths of the same position on the ground, controlling the vertical relative distance of the two seismic sources to be a combined excitation parameter well spacing delta d, and simultaneously exciting the two seismic sources to perform seismic exploration;

the step 1) specifically comprises the following steps:

the derivation process of the time distance curve of the reflected refraction wave when the excitation well depth exists is as follows:

firstly, a plurality of sections of refracted waves with parallel rays and interfaces do not exist in an ideal isotropic layered medium, the amplitude of a multi-reflection-refracted wave is reflected to be larger than that of a primary refracted wave through seismic recording, and a multi-refraction wave path is determined through spectral element forward modeling verification based on the two points;

and step two, deducing a time distance curve of multiple reflection refracted waves when the excitation well depth exists:

on the multiple refracted wave path, measuring the time required for the seismic wave to propagate from the seismic source to the ground detector along the multiple refracted wave path;

the propagation of seismic sub-waves includes two layers: an upper low-speed layer and a lower high-speed layer; the thickness of the overlying low-speed layer is known to be h₁Seismic wave velocity of overburden low velocity zone is V₁The depth of the excitation point from the ground is d, and the seismic wave velocity of the lower high-speed layer is V₂Setting the offset as x,

according to the seismic exploration principle, a reflected wave time distance curve is obtained and written as a formula (1):

in the formula, t_{Inverse direction}Is a reflected wavePropagation time, X_{Inverse direction}Is the transverse distance, V, of the excitation point and the detection point₁Seismic wave velocity of overburden low velocity layer, h₁For overlying low-velocity layer thickness, t₀The self-excitation and self-recovery time is the self-excitation and self-recovery time of the ground excitation of the upper low-speed layer vertically propagating to the interface between the two layers and then returning to the ground excitation point;

obtaining a refracted wave time distance curve formula according to the seismic exploration principle:

in the formula, t_{Folding device}For refracted wave propagation time, X_{Folding device}Is the transverse distance, V, of the excitation point and the detection point₁For seismic wave velocity, V, of overlying low-velocity layer₂Seismic wave velocity of underlying high-velocity layer, h₁To cover the thickness of the low-velocity layer, h₂The thickness of the lower covering high-speed layer is shown, and theta is a refraction wave critical angle;

and performing composite operation on the time distance curve of the reflected wave and the refracted wave to obtain a time distance curve formula (3) when the multiple reflection-refracted wave is excited below the ground surface with the well depth:

simplifying to obtain:

in the formula, t_{Reversely folded}For multiple reflection-refraction wave propagation time, X_{Reversely folded}Is the transverse distance, V, of the excitation point and the detection point₁For seismic wave velocity, V, of overlying low-velocity layer₂Seismic wave velocity of underlying high-velocity layer, h₁To cover the thickness of the low-velocity layer, h₂The thickness of the lower cladding high-speed layer, d is the distance from an excitation point to the ground surface, and theta is the critical angle of a refracted wave;

the derivation process of the time distance curve of the target layer reflected wave in the presence of the excitation well depth is as follows:

according to the propagation path of the reflected wave of the target layer, the thickness of the low-speed layer is h₁Velocity of V₁The high-speed layer has a thickness of h₂Velocity of V₂Total thickness is H, offset is x, excitation well depth is d, alpha is incident angle, H ═ H₁+h₂；

The speed is replaced by a root mean square speed approximation, the time-distance curve is an explicit function, and the following root mean square speed formula is used:

in the formula, V_rRoot mean square velocity, i is the surface from top to bottom, i is the ith stratum, t_iIs the propagation time of seismic waves in the ith formation, V_iThe seismic wave velocity in the ith stratum;

it can be seen that the time interval curve of the target layer reflected wave in the presence of the excitation well depth is represented as:

in the formula, t_rFor the purpose of the target layer reflection wave propagation time, V_rRoot mean square velocity, X_rThe transverse distance between an excitation point and a demodulator probe, the alpha reflection angle and the distance H between the excitation point and the ground surface are respectively the total thickness of the stratum;

simplifying to obtain:

the combined excitation well spacing delta d in the step 5) is obtained through the following steps:

let two vertical excitation points and one depth be d₁And the other is d₂The low-speed layer speed is V₁High layer velocity of V₂The time difference is delta t, and theta is a critical angle;

for two different well depths d₁And d₂Obtaining a time-distance curve t of two multiple reflection-refraction waves_{Reverse folding 1}And t_{Reverse folding 2}Time distance curve t of two multiple reflection-refraction waves_{Reverse folding 1}And t_{Reverse folding 2}The subtraction yields:

Δt＝|t_{reverse folding 1}-t_{Reverse folding 2}|，Δd＝|d₁-d₂|

The method is simplified as follows:

the time difference Δ t is selected to be 2n +1 times 1/2 cycles, n is 0, 1, 2, 3 …, and the range of the combined excitation well spacing Δ d is:

let V₁/V₂Z is the wave speed ratio of the low-speed layer to the high-speed layer, and is obtained according to the Pythagorean theorem

The period T is the reciprocal of the seismic dominant frequency f

The velocity V of the overlying low-velocity layer is selected according to the seismic dominant frequency f of the known seismic exploration₁Lower velocity of high velocity zone V₂And further determining the range of the combined well spacing delta d playing a role in suppressing the multiple reflection refraction wave as follows:

the influence on the reflected wave of the target layer is reduced while the vertical combined seismic source is excited and pressed to reflect the refracted wave for multiple times;

according to the target layer reflection wave time distance curve when the well depth is excited according to the formula (7):

since the depth of the target layer is far greater than the well depth of the excitation point in the actual exploration situation, the formula (13) is simplified to obtain:

the relative position relation of the combined excitation points influences the reflected wave of the target layer, and the depth of the excitation points at two vertical positions is set as d₁And the other is d₂Corresponding propagation times are respectively t_r1And t_r2Then, then

In the formula (15), corresponding parts of the upper and lower equal-sign left and right sides are respectively subtracted to obtain delta t_r＝|t_r1-t_r2|,Δd＝|d₁-d₂If yes, then:

selecting nT and nT +1/2t according to the time difference delta t_rAnd avoiding the interference cancellation of reflected waves, namely:

the size of the well spacing delta d is preferably combined according to the relation between the offset and the burial depth, the multiple reflection refraction wave is pressed on the premise of not interfering the target layer reflection wave,

small offset X_rWhen H/2, determining the range of the combined well spacing delta d playing a role in suppressing the multiple reflection refraction wave as follows:

middle offset distance X_rWhen H, the range of the combined well spacing Δ d for suppressing the multiple-reflection refracted wave is determined as follows:

large offset distance X_rWhen the average value of the well distance Δ d is 2H, the range of the combined well distance Δ d for suppressing the multiple-reflection refracted wave is determined as follows:

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