Analysis of wave field characteristics of 1. unit structure
The seismic wave field of unit structure refers to the seismic response of local structural units such as dimples, bumps and faults on the horizontal superimposed profile under the condition of uniform medium (single reflection interface).
1) gyrating wave
There is a small depression on the geological section, or a concave interface is formed near the fault due to traction. When the radius of curvature is less than the buried depth, as described in Chapter 2, a rotating wave field with inverted positions of reflection point and receiving point will be formed on the horizontal stacked profile. Fig. 5-2-2(a) is the record of the rotating wave field of two small depressions, and fig. 5-2-2(b) is the cross section after offset homing. The rotating wave has been homing, and the original shape of the two small depressions has been restored.
The rotating wave field has the following characteristics:
A. The gyration wave is bow-shaped, and its gyration range is related to the buried depth and bending degree of the interface. The deeper the interface, the greater the bending degree and the larger the raceway, and vice versa. When the curvature center of the concave interface is just on the ground, the self-excited and self-collected light will focus on one point.
B the concave interface, like concave mirror, has the function of energy focusing. Especially at the tangent point (also called the turning point) of the reflected wave and the turning wave at the plane interface, the two waves are tangent and have strong amplitude.
The wave field of C rotating wave has the shape of anticline, and the vertex of anticline should be the bottom point of small depression. It is precisely because of the "anticline" shape of the in-phase axis that the rotating wave is easily mistaken for the reflection of the underground anticline structure and should be paid attention to. In the early 1970s, a western oil company mistakenly interpreted the spin wave as anticline structure, which led to drilling errors. In order to remember this lesson, they used the rotating waveform as the cover of the textbook as a warning.
Fig. 5-2-2 Rotating Waves on Horizontal Superposition Section (a) and Migration Section (b)
Figure 5-2-3 Anticline Interface and its t0 Time Profile of Self-excitation and Self-collection
2) Divergent wave
The lower part of Figure 5-2-3 is an anticlinal interface. On the horizontal superimposed profile, the reflected wave of the anticline interface is still anticline-like, but its upward uplift amplitude and amplitude are increased compared with the actual anticline, as shown in the upper part of Figure 5-2-3.
The anticline interface is like a convex mirror, which has the function of diffusing energy, so it is called divergent wave.
3) Diffraction wave
Diffraction waves will be generated at lithologic abrupt points, such as breakpoints, pinch-out points and corners on erosion surfaces.
Figure 5-2-4 Breakpoint of Diffraction Wave
Figure 5-2-4 shows the diffraction wave generated by Gudian fault in Songliao Basin, China. The survey line is perpendicular to the fault strike, and the downward bending in-phase axis can be clearly seen on the profile, which is the diffraction wave generated by the breakpoint.
Figure 5-2-5 shows the diffraction wave generated on the erosion surface.
Figure 5-2-5 Diffraction Wave on Erosion Surface
Diffraction wave has the following characteristics:
A. In the case of homogeneous medium, the geometric shape of diffracted wave on the horizontal superimposed section is hyperbola, which has been proved theoretically. The diffraction wave is figuratively likened to "anticline", and the top of the "anticline" is the position of the diffraction point. If the diffraction wave is generated by a breakpoint, then the diffraction point is a breakpoint.
B diffracted waves have the strongest energy at the diffraction point, and then weaken to both sides. The amplitude also depends on the lithologic differences on both sides of the diffraction point. The difference is large, the amplitude is strong, and vice versa. In addition, it also depends on the relative position of the receiving point and the diffraction point. If the receiving point is directly above the diffraction point, the energy is strong, and if the receiving point is far from the diffraction point, the energy is weak.
The diffraction wave generated by the breakpoint is tangent to the reflected wave of the plane interface at the diffraction point, and the diffraction wave is divided into two halves from the tangent point, and the phase difference between the two halves is 180. On the profile, the outer half branch is obvious, and the inner half branch is often submerged by strong reflection but not obvious. In this way, the phenomenon of "layered (fault) waves are constantly reflected and diffracted" will appear on the horizontal superimposed profile.
4) Profile wave
When the fault distance is large, the rock wave impedance on both sides of the fault plane is obviously different, and the profile is smooth. The fault plane itself is a reflection interface, and the reflected wave generated on this interface is called profile wave. Fig. 5-2-6 is the section wave on the self-excited and self-collected section.
Fig. 5-2-7 is a simple schematic diagram of t0 time profile of normal fault self-excitation and self-collection.
The profile wave has the following characteristics:
Figure 5-2-6 Cross-sectional reflection wave
A. The cross-section wave often intersects with the reflected wave of the falling plate obliquely, and there is a diffraction wave at the edge breaking point, which constitutes a wave image of reflection diffraction, cross-section wave diffraction and cross-section wave diffraction (Figure 5-2-7).
Fig. 5-2-7 Schematic diagram of t0 time profile of normal fault self-excitation and self-collection.
B-section waves are sometimes strong, sometimes weak, sometimes absent, and appear intermittently, which is related to the change of lithology on both sides of the section and the fluctuation of reflection coefficient.
In addition to the above four kinds of waves related to special geological structures, the following two kinds of special seismic waves are also common in horizontal stacked profiles.
5) Multiple wave
In the acquisition and processing of seismic reflection data, although many methods are used to suppress multiples, in areas where multiples are well developed (especially at sea, even if long array and high coverage are adopted, it is tried to increase the residual time difference of multiples to weaken multiples), but this effort has certain limitations (because the array length is generally required to be equal to the depth of exploration target layer, so it cannot be designed too long, and the coverage times are also restricted by surface conditions and production efficiency).
Fig. 5-2-8 is a cross-sectional view of multiple waves at sea.
Figure 5-2-8 Offshore Multi-section Map
The multiples on the horizontal stacked profile have the following characteristics (which can also be used as identification marks):
A. dip angle and t 0 time stamp. This symbol is more obvious for full-scale multiples, which are approximately equal to integer multiples of multiples.
B. speed sign. Multiple waves show the characteristics of low speed in velocity spectrum.
C. emerging signs. If multiple waves are generated in the shallow layer with relatively gentle occurrence, secondary waves and tertiary waves will appear in the middle and deep layers of the profile, which will interfere with the true reflection of the middle and deep layers with a certain dip angle, and the multiple waves will interfere obliquely with the primary reflection of the middle and deep layers, resulting in difficulties in comparison.
The generation of multiple waves often tells us that there are special lithologic bodies (such as igneous rocks) with strong wave impedance surface underground. In this respect, multiple is a useful information.
6) Side wave
When the survey line is parallel to the strike of strata, waves from outside the vertical plane of the survey line often appear on the horizontal superimposed profile, which are called side waves.
Fig. 5-2-9 is a schematic diagram illustrating the formation mechanism of shear waves. Fig. 5-2-9a is a simple normal fault model, in which the main survey line and the tie survey line (X is the main survey line and Y is the tie survey line) are arranged on the surface, and the normals of the sag and the fault can be made at the intersection point S of the survey lines. Figure 5-2-9b shows that there can be two ray planes on the tie line, and Figure 5-2-9 c makes a theoretical t0 time (self-excitation and self-reception) profile, where t0B is the theoretical t0 time of the descending plate and t0A is the theoretical t0 time of the cross section, that is, the arrival time of the side wave received on the tie line through the point S on the surface.
Figure 5-2-9 Formation Mechanism of Side Wave
A, b and c are explained in the text.
Fig. 5-2- 10 shows the lateral reflection of Gudian fault in Songliao basin. On the right side of the diagram is the structure diagram of the workspace. On the seismic interpretation profile of line 1480, there is a group of strong and continuous abnormal reflections, which are inconsistent with the occurrence of upper and lower reflection layers near1s. Where did it come from? Combined with the geological structure characteristics of the work area and the geological interpretation of the section, the abnormal wave can be reasonably explained even after the structure is mapped. This also shows that the correlation of profiles is a process of repeated understanding and comprehensive interpretation.
Figure 5-2- 10 side wave
2. Analysis of seismic wave field characteristics of complex structures.
1) single interface complex structure wave field
If the interface of a stratum under study fluctuates greatly, and the structures such as anticline, syncline and fault are developed, then the complex combination of the above special waves will appear on the horizontal superimposed time profile, and various phenomena such as tangency, skew and interference will appear between them, forming a complex wave image.
2) Wave field of multi-layer interface complex structure
If there are several structural layers on the geological section, the development of each layer structure may or may not be inherited. According to the principle of self-excited and self-received imaging of horizontal superimposed profile, the wave propagating upward along the normal ray from the deepest reflection interface will deflect the propagation direction on all interfaces of the overlying medium, which will make the formed image inconsistent with the real geological structure and produce complex phenomena such as "false structure" and "false breakpoint".
In order to simplify the discussion, a mathematical simulation method only considering the kinematic characteristics of seismic waves is adopted.
Fig. 5-2- 1 1 t 0 time profile of three-layer interface ray tracing theory
A. the second interface; B. Level 3 interface; C. the fourth oblique interface; D. general theoretical t0 diagram of three-layer interface
Figure 5-2- 1 1 is the theoretical t0 time profile of three-layer interface layered media calculated by ray tracing forward. The second interface of layered media fluctuates greatly and consists of two small depressions and small protrusions. The t0 time profile of this layer is shown in Figure 5-2-11a. Tangent connection and oblique interference occur among reflected wave, diffracted wave, rotating wave and divergent wave, and the geometry is like two nested "bows". In spatial distribution, it seems that there are four upward uplift reflection in-phase axes. This complex wave field image can't directly reflect the true shape of geological structure, which often leads to illusion or even error in interpretation.
The third interface of the layered medium is horizontal, and the corresponding theoretical t0 time profile is shown in Figure 5-2-11b. As the wave propagates upward from the interface along the normal, the ray "focuses" toward the center through the concave part of the first interface and "diverges" to both sides at the convex part, which causes the theoretical t0 time profile of the horizontal interface to fluctuate synchronously with the overlying interface. The influence of this overlying complex structure on the wave field of the underlying simple structure is called velocity trap in the interpretation of conventional seismic data. Due to the uneven velocity in the lateral direction, the rays propagated by the wave deflect, resulting in unequal t0 time, resulting in the so-called false structure. The greater the lateral change of velocity (the greater the wave velocity difference between the upper and lower interfaces), the greater the influence.
Similarly, the wave field of the fourth oblique interface in Figure 5-2- 1 1 c can be analyzed. And figure 5-2- 1 1 d is the total complex wave field of the three-layer interface.
Figure 5-2- 12 is the actual horizontal superimposed profile of the continental slope in the South China Sea. As can be seen from the figure, the seabed topography fluctuates greatly, with submarine grooves, gentle platforms and narrow and steep seamounts. Due to the velocity trap caused by the dramatic change of topography, the fluctuation of each reflection layer below the seabed on the horizontal superimposed profile is almost exactly the same as the topographic fluctuation (synchronous fluctuation). The anticline and syncline shown on the profile are the illusion caused by the pull-up or pull-down of reflection time caused by the shallowness and depth of the low-speed layer of seawater, and are not the real form of the structure. When interpreting this profile, special attention should be paid to the influence of seabed topography.
Figure 5-2- 12 Submarine Topographic Seismic Profile of South China Sea Continental Slope
T2- lower interface reflection of Paleogene Yuehai Formation: T4- lower interface reflection of Paleogene Hanjiang Formation; T5 —— Internal reflection of Paleogene Zhujiang Formation; T7- Lower Tertiary Zhuhai Formation bottom interface reflection; T8-the reflection of the new boundary.
The influence of overlying depression and uplift structure on the wave field of underlying simple structure is analyzed above, and there is also the influence of overlying fault structure on the wave field of underlying structure in fact. Fig. 5-2- 13 is a model in which the overlying interface is a normal fault and the underlying interface is a horizontal interface. Assuming V2 > V 1, the wave field of the normal fault is the same as that in Figure 5-2-7 (diffraction wave is not considered here), and the wave field of the underlying horizontal interface becomes three mutually staggered in-phase axes, resulting in false breakpoints.
From the above analysis of the wave field, we can see that the horizontal superimposed profile is not a simple image of the geological profile, and there are inherent connections (similarities) and differences (differences) between them. Generally speaking, when the structure is simple, the in-phase axis of the reflected wave can directly reflect the geometric shape of the structure; When the structure is complex, three kinds of artifacts often appear on the horizontal stack profile: one is the migration effect caused by self-excited and self-received imaging of the horizontal stack profile; Second, the illusion related to velocity, or the influence of complex structures such as overlying depression, uplift and fault on the seismic wave field of the underlying interface; The third illusion is the lateral wave on the seismic profile, but a reflection interface has two reflected waves on the seismic profile. In order to overcome this, 3D seismic work should be carried out.
Fig. 5-2- 13 fault's influence on the underlying wave field
3. Wave field characteristics of special geological bodies such as buried hill, diapir structure and reef in earthquake.
Wave field characteristics of 1) buried hill
Buried hill refers to the paleotopography below the unconformity surface, which is often composed of carbonate strata and can form traps under certain conditions. Huabei Oilfield in China is an oil and gas reservoir with buried hill as the main body.
Fig. 5-2- 14 is a buried hill seismic profile with complex wave field. The top surface of buried hill is unconformity surface, which has the characteristics of unconformity surface reflection wave, showing low frequency, strong phase and multiphase waveform, accompanied by diffraction wave, profile wave, rotation wave and lateral wave.
Fig. 5-2- 14 horizontal superimposed profile of buried hill
We should be especially careful when comparing such seismic profiles. It is necessary to understand the ins and outs of various waves and their relationship, and refer to the migration profile to help explain.
2) Wave field characteristics of diapir structure
Salt dome or mud dome diaphragm is an important type of oil storage structure, which can form stratigraphic trap oil and gas reservoirs with surrounding rocks.
Fig. 5-2- 15 is the migration profile of the salt dome anticline in Qianjiang sag, Hubei province, China. From the profile, it can be seen that the reflected wave occurrence between the top and bottom of the salt source layer is not harmonious, which is characterized by the thick top and thin wing of the salt source layer and the weak convex on the bottom plate. Because the salt dome itself has no good layered structure, there are only sporadic reflection events.
Fig. 5-2- 15 migration profile of salt dome anticline
3) Wave field characteristics of reefs
Reef in marine carbonate rocks is an important phenomenon in petroleum exploration, which can form reef oil fields. Fig. 5-2- 16 is the seismic profile of the marginal reef in the Pearl River Mouth Basin of China. On the profile, the reef shows the characteristics of strong reflection at the top of the reef, no reflection in the reef, flooding on both sides, bending under the reef, side bottom diffraction, abnormal velocity and mound reflection (see Figure 5-2- 12 for the geological age of each reflection layer on the profile).
Fig. 5-2- 16 Seismic Profile of Platform Edge Reef
In seismic data interpretation, it is very important to identify and compare various seismic waves on seismic profiles and analyze seismic wave fields. At present, there is another earthquake simulation method, that is, the essence is to establish an initial geological model according to the preliminary interpretation results, calculate the theoretical seismic wave field, and compare it with the actual wave field to make the interpretation scheme more reasonable.