A design method for earthquake observation system based on energy loss minimization

Abstract

The present invention relates to the field of oil and gas exploration, and specifically to a seismic observation system design method based on the goal of minimizing energy loss. In view of the situation where high-loss geological bodies absorb and attenuate seismic wave energy violently, causing the energy flow or energy density of the underlying rock mass to decrease sharply, a low-energy loss observation system design technology with the goal of "minimizing energy loss" is proposed. This observation scheme enhances the illumination and energy density focused on the in-situ system in the foreland belt by adding shot points in the effective recharge area. Compared with the seismic wave excitation and reception on the nappe, this scheme greatly reduces the propagation path length of seismic rays in the nappe with high propagation loss, and reduces the energy loss in the propagation of seismic waves. Compared with the conventional enhanced observation scheme of adding encrypted shot points on the nappe, it not only greatly reduces the drilling construction cost, but also reduces the safety risks brought by construction operations in the cliffs of the nappe belt.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas exploration, and in particular to a seismic observation system design method based on the goal of minimizing energy loss. Background Art

[0002] The foreland belt refers to the area where the orogenic movement causes the front belt of the mountain range to be thrust, the foot of the mountain, the edge of the basin, etc. It is widely developed in the basin-mountain combination zone on the edge of the Neogene basin. It consists of three parts: the reverse fault, the overlying rock system (i.e. the nappe) and the underlying rock system (also known as the original system). Various buried structures are developed in the underlying rock system of the foreland belt, which is a long-term stable and favorable oil and gas preservation unit. The area has good oil and gas accumulation conditions. However, due to the extremely low signal-to-noise ratio of seismic data in the foreland belt, the oil and gas exploration of the buried structures under the nappe in the foreland belt is seriously restricted.

[0003] The fundamental reason for the low signal-to-noise ratio of seismic data in the piedmont belt is that it is difficult to solve the problem of large loss of seismic wave energy during the excitation and propagation of seismic waves using conventional seismic acquisition methods. The greatest loss of seismic wave energy comes from the thrust nappe, which is often composed of high-speed dense rock masses such as Proterozoic metamorphic complexes, Paleozoic carbonate rocks and igneous rocks, and has huge loss in seismic wave excitation. The broken zone produced by the high-density fracture cutting of the thrust nappe causes disordered scattering of seismic waves, and the energy flux density of seismic waves propagating to the underlying original rock mass is sharply reduced. The high energy dissipation of the thrust nappe has an energy shielding effect on the underlying original system, and also causes a large amount of random noise and serious complication of the wave field. The thrust nappe has become the primary factor restricting the quality of seismic data in the piedmont belt.

[0004] In the past, in the conventional seismic acquisition and production in the piedmont belt, the following measures were taken to address the low signal-to-noise ratio problem in the piedmont belt: (1) Strengthen the test of excitation and receiving factors in the nappe belt, and select the best excitation well depth, which is generally 2-4 meters deeper than the clastic rock area. (2) In the nappe belt, the surface structure survey was intensified by using methods such as intensified micro-logging points to guide the design of well location and depth and improve the quality of static correction; (3) In the nappe belt, an "enhanced" observation scheme of intensified shot points (lines) was adopted. Practice has shown that the above three schemes have no significant effect on improving the quality of seismic data in the piedmont belt. Summary of the invention

[0005] The purpose of the present invention is to overcome the low signal-to-noise ratio problem existing in the prior art caused by the high loss of seismic wave excitation and propagation energy in the overthrow body, and to provide a low-loss seismic acquisition and observation system design scheme with the goal of "minimizing the energy loss" of seismic waves. The method is mainly to increase the density of shot points in the low-loss clastic rock areas of the Mesozoic and Cenozoic eras where the tectonic action in the foreland is weak, the rock strata are relatively complete, and the seismic excitation and propagation losses are small, and the characteristics of low loss of seismic wave excitation and propagation energy in this area are utilized to inject high-quality seismic wave energy into the underlying in-situ system of the overthrow body, thereby achieving the purpose of improving the signal-to-noise ratio of the foreland belt and the imaging quality of the buried structure.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] A method for designing a seismic observation system based on an energy loss minimization objective comprises the following steps:

[0008] Step 1: Conduct near-surface geological surveys in the target piedmont area to confirm the distribution range of high-loss areas or high-loss geological bodies, and divide the boundary line AB between the high-loss area and the low-loss area;

[0009] Step 2: According to the seismic data quality or signal-to-noise ratio, energy loss intensity, and target layer burial depth of the high-loss geological body, the boundary line CD of the low-energy-loss area is determined, and the area between the boundary line AB and the boundary line CD is the effective recharge area;

[0010] Step 3: Determine the replenishment density according to the expected seismic data quality target; in the effective replenishment area, encrypt the seismic wave excitation points and receiving points to replenish the seismic wave energy of the target layer underlying the high-loss geological body or the thrust body; thus, form a regional seismic observation system in the piedmont zone.

[0011] The present invention found in the seismic exploration of the piedmont zone that the quality distribution of seismic data in the piedmont zone has obvious zoning characteristics. The nappe has typical high-energy loss rock physical characteristics, which causes a huge difference in data quality between the nappe and the Mesozoic and Cenozoic foreland areas. At the same time, the high-loss nappe has an "energy shielding" effect on the underlying in situ system. Although the underlying system rock formation is relatively complete, it is difficult to image due to weak energy. The structural-sedimentary evolution process of the basin-mountain combination zone is difficult to restore, and the hidden favorable structure is difficult to implement.

[0012] The rapid loss of seismic wave energy by the nappe makes it difficult to effectively illuminate the stable in-situ rock formations in the underlying reverse structural area, resulting in weak energy, low signal-to-noise ratio, and difficulty in imaging. Based on this problem, the present invention designs an observation scheme with the goal of minimizing seismic wave energy loss. This method is contrary to the actual need of the conventional enhanced observation system to increase the number of excitation points in the part with the worst seismic data (on the high-loss nappe belt), but selects a certain range of areas in the Mesozoic-Cenozoic clastic rock outcrop area at the front edge of the nappe as an effective replenishment area with low energy loss as a shot point density area to form a low-loss seismic observation system. In the technical solution of the present invention, the dividing line AB serves as the starting boundary of the effective replenishment area, and the enclosure CD serves as the terminal boundary of the effective replenishment area. By determining the area from the starting boundary to the terminal boundary, the regional range of the effective replenishment area is reasonably planned.

[0013] On the one hand, this observation scheme enhances the illumination and energy density focused on the in-situ system of the foreland belt by adding more shot points in the effective recharge area. On the other hand, compared with the seismic wave excitation reception on the nappe, this scheme greatly reduces the propagation path length of seismic rays in the nappe with high propagation loss, and reduces the amount of energy loss in the propagation of seismic waves, thereby significantly improving the energy flux density and illumination of the underlying in-situ system, improving the imaging quality of the in-situ system of the overthrust structural belt, and the economic feasibility of the designed scheme. Compared with the conventional enhanced observation scheme of adding encrypted shot points on the nappe, it not only greatly reduces the drilling construction cost, but also reduces the safety risks brought by construction operations in the cliffs of the nappe belt.

[0014] As a preferred technical solution of the present invention, in step 1, the near-surface geological survey specifically includes using geological maps combined with surface field surveys to determine the location and range of high-loss rock layers; in the piedmont zone, the nappe distribution area characterized by surface exposure of carbonate rocks, metamorphic rocks, and igneous rocks corresponds to the high-loss area of ​​seismic wave excitation and propagation; the clastic rock area of ​​the foreland basin at the front edge of the thrust belt is a low-loss area for seismic wave excitation, propagation, and reception. When there is a stable weathering layer on the surface of the nappe belt, the nappe belt area can be used as a medium-loss area for receiving energy to increase the energy supply of the nappe belt.

[0015] In the process of geological exploration, high-loss areas or high-loss geological bodies include the following three types of rock / rock formation outcrops or geological bodies: 1. There are high-loss formations exposed near the surface, which makes it impossible to effectively convert the source excitation energy into seismic wave downward energy. In the high-loss excitation area, the surface generally exposes limestone, dolomite, igneous rock and other chemical rocks, crystalline rocks, dense high-speed rock masses, or is composed of undercompacted, loose strong viscosity layer and plastic layer, and the excitation coupling between the source and the rock mass is poor. The overburden in the orogenic belt is often composed of dolomite, limestone, and igneous rock, which are typical rock formations with high loss of seismic wave excitation energy. The exposed areas of these formations are high-loss areas. In addition, the undercompacted Quaternary cover area and loess plateau area are also high-loss areas for seismic excitation. In the high-loss excitation area, all receiving channels on the seismic single shot have weak energy.

[0016] Second, the underground geological bodies with high loss of seismic wave propagation energy produce strong reflection, scattering and absorption losses on the downward energy of seismic waves, causing a sharp decrease in the propagation energy or energy flux density of seismic waves. The orogenic nappes cause disordered scattering of seismic waves in different directions on the interface due to the drastic changes in the stratum attitude, or due to the strong scattering of broken rocks in the fault zone, or due to the existence of geological bodies such as karst caves where seismic waves cannot be effectively propagated, or due to the shielding effect of strong reflection interfaces, causing a sharp decrease in propagation energy density, and elastic waves cannot be effectively propagated. This is a typical high-loss propagation area. The high-loss propagation area is manifested in a single seismic shot as a sharp weakening of the receiving channel energy in the high-loss geological body area, while the receiving channel energy is significantly stronger in the area where the seismic wave propagation path does not pass through the high-loss body.

[0017] 3. There are high-loss areas of seismic wave receiving energy near the surface, which are generally caused by the reflection of the uploaded energy by the shallow strong reflection interface, or the strong absorption of the uploaded seismic wave energy by the shallow strong viscous layer and plastic layer, or the poor coupling between the surface soil and the geophone, which cannot effectively resonate. The high-loss energy area often has a superposition of high losses in seismic wave excitation, propagation or reception.

[0018] Based on the above three typical high-loss areas or high-loss geological body characteristics, the boundary line AB between the seismic wave energy high-loss area and the seismic wave energy low-loss area can be determined.

[0019] As the preferred technical solution of the present invention, energy loss minimization and quality maximization have a dual relationship. The smaller the loss factor, the larger the quality factor and the higher the data quality. Ls, Lr and Lp represent the energy loss factors of seismic waves in excitation, reception and propagation, respectively, and Qs, Qr and Qp represent the quality factors of seismic waves in excitation, reception and propagation. The seismic wave energy loss factor is correlated with the elastic mechanical characteristics and parameters of rock, lithology, rock combination and later transformation intensity.

[0020] In seismic exploration, the loss factor L, quality factor Q, and absorption coefficient α have the following relationship:

[0021]

[0022]

[0023] Among them, δ is the logarithmic absorption rate; E represents the seismic wave energy, f represents the seismic wave frequency, and V represents the seismic wave velocity.

[0024] The quality factor Q has an equivalent effect to the signal-to-noise ratio and can be replaced by the signal-to-noise ratio of seismic data. The signal-to-noise ratio of seismic data can be obtained through a variety of calculation methods such as the energy method, frequency method, power spectrum method, and cross-correlation method.

[0025] The loss factor L is used to describe the attenuation of seismic wave energy absorption by the medium within a wavelength λ (or a period T) from the reference point; the closer the medium is to perfect elasticity, the smaller the energy loss factor. In seismic exploration, the strength of seismic wave energy loss can be directly determined by the signal-to-noise ratio of seismic data, and the two are negatively correlated.

[0026] As a preferred technical solution of the present invention, in step 2, the specific method for confirming the confinement line CD is as follows:

[0027] Based on the geological interpretation results of the previous seismic survey data, the burial depth of the target layer of the reverse structure in the footwall of the thrust belt or the footwall rock system is determined, including the predicted maximum burial depth and minimum burial depth; the burial depth of the target layer is used as the basis for determining the width between the enclosing line CD and the AB boundary.

[0028] As a preferred technical solution of the present invention, in step 2, the specific confirmation method of the enclosure CD is according to the following steps: selected according to the forward simulation of the geological model and the conventional formula calculation. In the case of an accurate geological model, the scope and boundary of the effective recharge area can be determined based on technical methods such as model forward modeling, ray tracing and lighting analysis.

[0029] In the piedmont zone, the scope and density of the effective recharge area are determined by a comprehensive analysis of the following factors: the burial depth of the target layer of the underlying strata of the nappe belt, the magnitude of the energy loss factor of the nappe belt, the quality of seismic data in the high-loss zone (signal-to-noise ratio), and the expected data quality target.

[0030] The scope of the effective recharge area, that is, the distance from the CD boundary to the AB boundary, can be selected by the above two methods: one is to select according to the buried depth of the underlying rock system target layer: the range is determined by taking the 1 / 2 depth close to the main target layer as the maximum width between the CD and AB boundaries. The second is to select according to the forward simulation of the geological model and the calculation of the conventional formula. In the case of an accurate geological model, the scope and boundary of the "effective recharge area" can be determined based on technical methods such as model forward modeling, ray tracing and lighting analysis; the criterion for the rationality of the selection of the recharge area range is: the area has the characteristics of low loss of seismic wave excitation energy and low loss of received energy, and at the same time, it can effectively illuminate and focus the underlying rock system target layer shielded by the nappe, and because the incident angle is small, the distance that the seismic wave ray passes through the nappe is the shortest, so it has the characteristics of low propagation energy loss.

[0031] The range of the effective replenishment area, that is, the position of the CD line, or the length W from the CD line to the AB line, is selected based on two main considerations: First, the maximum offset X allowed by velocity analysis and dynamic correction stretching must be met. max The empirical method is to select the depth close to the main target layer as the maximum offset, that is,

[0032]

[0033] The second is to be located in the blind area of ​​the refracted wave where total reflection occurs, that is

[0034] D max =h0 tanθ c (4)

[0035] Among them, h0 is the depth of the target layer, θ c is the critical angle for total reflection at the rock layer interface, which is related to the velocity difference of the rock layer in the area:

[0036]

[0037] V1 and V2 are the velocities of the upper and lower layers of the interface, respectively.

[0038] Through the above relationship, the division position or (interval) of the CD line can be preferably determined. Based on this method, the most preferred effective recharge area range can be determined.

[0039] The energy loss of seismic waves can be divided into excitation loss, propagation loss and reception loss. The total energy loss of seismic waves is the superposition of excitation loss, propagation loss and reception loss. Through research, it is found that the energy loss of earthquakes in the piedmont zone has the following characteristics:

[0040] The basin-margin clastic rock area at the front edge of the nappe has weak tectonic action and relatively complete rock formations, which is a low-loss area for earthquake excitation, reception and transmission. The original rock mass beneath the nappe is mainly affected by the gravity of the nappe, and the lateral pushing force of the nappe is weak. The structural deformation is mainly folding, and the rock formations are relatively complete, which is a low-loss area for seismic wave transmission. The greatest loss of seismic wave energy in the piedmont belt comes from the thrust nappe, which is composed of Proterozoic metamorphic complex and Paleozoic carbonate high-speed rock blocks dominated by dolomite, and the conditions for seismic wave excitation are extremely harsh.

[0041] In addition, when the surface of the crystalline rock of the nappe has a stable weathering crust, it can form a good coupling with the detector, and the receiving energy loss of the surface of the nappe is small. Therefore, the deterioration of receiving conditions is not the decisive factor restricting the data quality in the carbonate rock outcrop area. Therefore, in the design of the low-loss observation system, the area with a stable weathering layer on the surface of the nappe belt can also be used as a receiving energy supplement area.

[0042] As a preferred technical solution of the present invention, in the piedmont regional earthquake observation system, the energy replenishment methods include the following three methods:

[0043] 1. Only low-loss excitation energy replenishment is carried out, and the shot lines / shot points are encrypted in the "effective replenishment area"; for 2D seismic line exploration, according to the expected data quality target, on the basis of normal shot spacing, at least 1 times of shot points are selected to be encrypted in the "effective replenishment area" with low excitation energy loss; for 3D seismic exploration, additional shot lines or shot points are selected in the "effective replenishment area".

[0044] Second, only low-loss receiving energy replenishment is carried out. Specifically, the receiving lines or receiving points are encrypted only in the "effective replenishment area"; or the detection points / lines are encrypted in the low-loss receiving favorable area of ​​foreland clastic rock and the relatively favorable area of ​​overthrow belt for receiving energy replenishment; the "effective replenishment area" with low energy loss is also the low-loss receiving energy area, and the method of encrypting the receiving points is similar to the first solution.

[0045] 3. Low-loss excitation and dual energy replenishment can be carried out. It is possible to choose to increase the density of guns and detectors only in the low-loss area (effective replenishment area) for foreland excitation and reception, or to increase the density of detectors in the favorable area for foreland debris reception and the relatively favorable area for overthrow belt reception while performing excitation energy replenishment in the (effective replenishment area). The number or density of encrypted excitation points and receiving points is still based on the energy loss intensity of the overthrow belt. This scheme increases the workload and cost compared with the previous two schemes, but the energy density of reflected waves in the overthrow body and the underlying rock system is greater. According to data analysis, in the high-loss area of ​​seismic wave excitation energy such as limestone, metamorphic rock, igneous rock in the overthrow belt, when there is a stable weathering crust on the surface, the detector can form a good coupling with the ground, and the receiving energy loss is moderate. At this time, the overthrow belt can be used as a relatively favorable receiving area.

[0046] As a preferred technical solution of the present invention, it also includes step 4: implementing seismic data acquisition through the seismic observation system;

[0047] Step 5: Process the collected seismic data to obtain a seismic profile.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] The technical solution of the present invention greatly reduces the propagation path length of seismic rays in the high-loss nappe, reduces the energy loss in the propagation of seismic waves, and thus achieves: 1) significantly improves the energy flux density and illumination of the underlying in-situ system, and improves the imaging quality of the in-situ system of the overthrust structural belt; 2) the designed solution is economical and feasible. Compared with the conventional enhanced observation solution of adding encrypted shot points on the nappe, it not only greatly reduces the drilling construction cost, but also reduces the safety risks brought by construction operations in the cliffs of the nappe belt.

[0050] After adopting the low-loss observation system, the reflected wave energy in the overthrust body and the underlying in-situ system, which appeared as a "blank zone" in the normal observation system acquisition, has been greatly improved, and the imaging quality is expected to reach the threshold of the basic needs of structural interpretation. Compared with the excitation in the metamorphic rock area of ​​the overthrust body, the seismic excitation in the low-loss excitation energy replenishment area reduces 70% of the energy excitation loss and 60% of the return energy propagation loss. Using the low-loss observation system, the depth of the target layer is selected as the range width of the low-loss excitation energy replenishment area during construction, and the encrypted shot points are doubled. Compared with the conventional uniform shot check point observation system, the imaging quality of the buried structure in the footwall of the thrust nappe belt is expected to increase by 155%. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 This is a typical seismic profile of the piedmont zone's geological structure characteristics;

[0052] Figure 2 This is a schematic diagram of the energy recharge area for the low-loss observation system;

[0053] Figure 3 Schematic diagram of earthquake propagation loss;

[0054] Figure 4 (a) is the NW04-326 line profile collected in a mountain front zone in 2004;

[0055] Figure 4 (b) is a high-density re-collection profile of the same location in a certain piedmont zone collected 20 years ago; DETAILED DESCRIPTION

[0056] The present invention is further described in detail below in conjunction with test examples and specific implementation methods. However, this should not be understood as the scope of the above subject matter of the present invention being limited to the following embodiments, and all technologies realized based on the content of the present invention belong to the scope of the present invention.

[0057] Example 1

[0058] Based on the analysis and research of seismic acquisition in a certain piedmont belt, this paper proposes an observation system design scheme with the goal of "minimizing energy loss" in view of the inherent rock physics characteristics of the high-loss nappe in the piedmont belt:

[0059] Based on a detailed investigation of the near-surface structure of a certain piedmont belt and a comparative analysis of the lateral variation characteristics of the seismic data quality, the present invention proposes a design scheme for a low-loss seismic acquisition and observation system with the goal of "minimizing the energy loss" of seismic waves. The method is to increase the density of shot points in the Mesozoic and Cenozoic low-loss clastic rock areas where the piedmont tectonic action is relatively weak, the rock strata are relatively complete, and the seismic excitation and propagation losses are small, and utilize the characteristics of low loss of seismic wave excitation and propagation energy in this area to inject high-quality seismic wave energy into the underlying in-situ system of the thrust body, thereby achieving the purpose of improving the signal-to-noise ratio of the piedmont belt and the imaging quality of the buried structure.

[0060] Specifically, the seismic observation system design method based on the energy loss minimization objective proposed in the present invention comprises the following steps:

[0061] Step 1: Conduct near-surface geological surveys in the target piedmont area to confirm the distribution range of high-loss areas or high-loss geological bodies, and divide the boundary line AB between the high-loss area and the low-loss area; Figure 1The figure shows the geological structure characteristics of a typical seismic profile in the piedmont belt. Through research on this area, it is shown that the basin-margin clastic rock area at the front edge of the nappe is weakly tectonic, and the rock strata are relatively complete, which is a low-loss area for seismic excitation, reception and transmission. The original rock mass under the nappe is mainly affected by the gravity of the nappe, and the lateral pushing force of the nappe is weak. The structural deformation is mainly folding, and the rock strata are relatively complete, which is a low-loss area for seismic wave transmission. The maximum loss of seismic wave energy in the piedmont belt comes from the thrust nappe, which is composed of Proterozoic metamorphic complex and Paleozoic carbonate high-speed rock blocks dominated by dolomite, and the seismic wave excitation conditions are extremely poor. In addition, it is found that when the surface of the crystalline rock of the nappe has a stable weathering crust, it can form a good coupling with the detector, and the energy loss of the surface of the nappe is small. Therefore, the deterioration of the receiving conditions is not the decisive factor that restricts the data quality in the carbonate rock outcrop area. The fundamental way to improve the quality of seismic data is to try to reduce the energy loss in the excitation and propagation of seismic waves in the nappe.

[0062] Step 2: According to the seismic data quality or signal-to-noise ratio, energy loss intensity, and target layer burial depth of the high-loss geological body, the boundary line CD of the low-energy-loss area is determined, and the area between the boundary line AB and the boundary line CD is the effective recharge area;

[0063] As previously studied, the excitation and propagation losses of seismic waves in the piedmont nappe are much greater than those in the foreland clastic rock area, but the difference in receiving losses between the two is small when there is a weathering layer on the surface. Based on this feature, the contribution of intensified shot points in the metamorphic rock system of the nappe belt to improving the imaging quality of the nappe and the underlying rock system is extremely limited. At the same time, the propagation loss caused by the earthquake passing through the nappe twice is huge. If the intensified shot points on the nappe are moved to the "effective supply area" where the Mesozoic and Cenozoic clastic rocks are exposed in the foreland, not only will the excitation loss be greatly reduced, but the length of the ray passing through the high-loss nappe will be greatly shortened. The energy "effective supply area" can be used to reduce the excitation and propagation losses of seismic waves, and the characteristics of fully illuminating the underlying rock system can be realized, and a low-loss observation system can be designed. Energy injection into the target layer of the reverse cover structure area of ​​the underlying rock system can be achieved, the energy flux density and illumination can be increased, and the imaging quality of the concealed structure can be improved. This seismic acquisition and observation method, which aims to minimize the losses in seismic wave excitation, reception and propagation, avoids geological bodies with high loss factors, and adds shot check points in media with high quality factors to inject replenishment of seismic wave energy into the buried area, is defined as a low-loss observation system.

[0064] like Figure 2-3 Shown is a schematic diagram of the energy replenishment area of ​​the low-loss observation system;

[0065] In the normal 3D or 2D seismic acquisition area, according to the burial depth of the underlying rock system target layer, a certain range is selected in the clastic rock outcrop area of ​​the foreland basin at the front edge of the thrust belt as the "effective supply area" of low-loss excitation energy, such as Figure 2In the range enclosed by the ABCD points shown, the seismic excitation quality is good, the explosive energy can be converted into the energy of the downward transmitted seismic wave in a large proportion, the main body of the seismic ray propagates in the stable rock mass, the distance through the nappe is short, and the propagation loss is small, which is a low-loss shot gathering energy replenishment area. The range of the low-loss replenishment area, that is, the distance from the CD boundary to the AB boundary, can be selected according to the following two methods: one is to select according to the buried depth of the target layer of the underlying rock system. The buried depth of the target layer can be used as the width between the CD and AB boundaries to determine its range. The second is to select according to the forward simulation of the geological model. In the case of an accurate geological model, the range and boundary of the "effective replenishment area" can be determined based on technical methods such as model forward modeling, ray tracing and lighting analysis. The "effective replenishment area" is generally located at the edge of the Mesozoic and Cenozoic foreland basin adjacent to the nappe, with high seismic wave excitation and reception quality factors, and can be used as a low-loss excitation and reception energy replenishment area.

[0066] More specifically, the range of the effective replenishment area, that is, the position of the CD line, or the length W from the CD line to the AB line, is mainly selected from two aspects: first, it must meet the maximum offset X allowed by velocity analysis and dynamic correction stretching. max The empirical method is to select the depth close to the main target layer as the maximum offset.

[0067]

[0068] Second, it must be located in the blind area of ​​the refracted wave where total reflection occurs.

[0069] D max =h0 tanθ c

[0070] Among them, h0 is the depth of the target layer, θ c is the critical angle for total reflection at the rock interface, which is related to the velocity difference of the rock surface in the area.

[0071]

[0072] V1 and V2 are the velocities of the upper and lower layers of the interface, respectively.

[0073] Once the depth of the target layer and the quality dividing point D (any point on the AB line) are determined, the distance between the CD line and the AB line can be calculated. This distance is the maximum distance along the dip of the formation.

[0074] Step 3: Determine the replenishment density according to the expected seismic data quality target; in the effective replenishment area, encrypt the seismic wave excitation points and receiving points to replenish the seismic wave energy of the target layer underlying the high-loss geological body or the thrust body; thus, form a regional seismic observation system in the piedmont zone.

[0075] Specifically, the replenishment methods of the regional seismic observation system in the piedmont belt include the following three methods: the nappe exposes high-speed rock masses such as carbonate rocks and igneous rocks, but when there is a stable weathering crust on the surface of the nappe, it can form a good coupling with the detector, and the receiving energy loss in the nappe is small, which is a more favorable area for seismic wave reception, and the foreland clastic rock area is a favorable receiving area.

[0076] The low-loss observation system design includes the following three schemes:

[0077] The first option (I): Low-loss excitation energy replenishment is performed only in the "effective replenishment area".

[0078] The second option (II): Low-loss receiving energy replenishment is only carried out in the "effective replenishment area". It is possible to choose to increase the number of detection points in the low-loss receiving area of ​​the foreland (ordinary type, II1), or to increase the number of detection points in the favorable receiving area of ​​the foreland clastic rock and the relatively favorable receiving area of ​​the overthrust belt (expanded type, II2) for receiving energy replenishment.

[0079] The third option (III): low-loss excitation and dual energy replenishment. You can choose to increase the number of shots and receivers (III1) only in the low-loss area ("effective replenishment area") for foreland excitation and reception, or increase the number of receivers (III2) in the favorable foreland reception area and the relatively favorable area for overthrow belt reception while performing excitation energy replenishment in the "effective replenishment area". This option increases the cost compared to the first two options, but the energy density of reflected waves in the overthrow body and the underlying rock system is higher.

[0080] like Figure 4 (a) shows the NW04-326 line profile collected in 2004. Figure 4 (b) is the high-density re-collected profile at the same position of the line, which is shortened to the south and extended to the north. Figure 4 As shown in the figure, by encrypting the high-density acquisition profiles of the shot checkpoints, not only the structural characteristics of the underlying rock system are significantly improved, but also the double reflections in the thrust body and the embedded structural characteristics of the frontal zone are clearer, reflecting the effectiveness of foreland clastic rock excitation and receiving low-loss areas for effective energy supplement in improving imaging quality.

[0081] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for designing a seismic observation system based on the goal of minimizing energy loss, characterized in that: The steps include: Step 1: Conduct near-surface geological surveys in the target piedmont area to confirm the distribution range of high-loss geological bodies and demarcate the boundary line AB between the high-loss seismic wave energy area and the low-loss seismic wave energy area; Step 2: According to the signal-to-noise ratio, energy loss intensity, and buried depth of the target layer of the high-loss geological body, the boundary line CD of the low-energy-loss area is determined, and the area between the boundary line AB and the boundary line CD is the effective recharge area; The specific confirmation method of the CD is as follows: Based on the geological interpretation results of the previous seismic survey data, determine the burial depth of the target layer of the thrust structure or the footwall rock system of the thrust belt, including the predicted maximum burial depth and minimum burial depth; use the burial depth of the target layer as the basis for determining the width between the CD and AB boundaries; or, The specific confirmation method of the CD of the enclosure is based on the following steps: selection based on the forward simulation of the geological model and the calculation of the conventional formula; in the case of an accurate geological model, the scope and boundary of the "effective recharge area" are determined based on the forward modeling, ray tracing and lighting analysis techniques; Step 3: Determine the replenishment density according to the expected seismic data quality target; in the effective replenishment area, encrypt the seismic wave excitation points and receiving points to replenish the seismic wave energy of the target layer underlying the high-loss geological body or the thrust body; thus, form a regional seismic observation system in the piedmont zone.

2. The method for designing a seismic observation system based on the energy loss minimization objective according to claim 1, characterized in that: In step 1, the near-surface geological survey specifically includes the use of geological maps combined with surface field surveys to determine the location and range of high-loss rock strata; in the foreland zone, the distribution area of ​​thrust bodies characterized by surface exposed carbonate rocks, metamorphic rocks, and igneous rocks corresponds to the high-loss area of ​​seismic wave excitation and propagation; the clastic rock area of ​​the foreland basin at the front edge of the thrust belt is a low-loss area for seismic wave excitation, propagation and reception.

3. The method for designing a seismic observation system based on the energy loss minimization objective according to claim 2, characterized in that: Minimizing energy loss and maximizing quality have a dual relationship. The loss factor L, quality factor Q, and absorption coefficient α have the following relationship: （1） （2）； in, δ is the logarithmic absorption rate; E represents the seismic wave energy, f represents the seismic wave frequency, and V represents the seismic wave velocity.

4. The method for designing a seismic observation system based on the energy loss minimization objective according to claim 1, characterized in that: In the piedmont regional earthquake observation system, the energy replenishment methods include the following three methods:

1. Only low-loss excitation energy replenishment is performed; 2. Only low-loss receiving energy replenishment is performed; 3. Carry out low-loss excitation and receive dual energy supply.

5. The method for designing a seismic observation system based on the energy loss minimization objective according to claim 4, characterized in that: In the first method, at least one times of the number of artillery lines / artillery points is selected to be encrypted in the effective replenishment area.

6. The method for designing a seismic observation system based on the energy loss minimization objective according to claim 4, characterized in that: In the second method, the low-loss receiving energy replenishment method includes: encrypting the receiving lines or receiving points in the effective replenishment area; encrypting the detection points / lines in the foreland clastic rock receiving favorable area and the thrust belt receiving more favorable area for receiving energy replenishment.

7. The method for designing a seismic observation system based on the energy loss minimization objective according to claim 1, characterized in that: The method also includes step 4: collecting seismic data through the seismic observation system; and step 5: processing the collected seismic data to obtain a seismic profile.

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