(A) Difficulties and countermeasures analysis
1. Analysis of processing difficulties
The seismic profile involved in this book is very large, with many types of geological units and complex structures. In addition, the surface seismic geological conditions are changeable, the construction years, construction methods and acquisition equipment of seismic acquisition are different, and the data quality is quite different, which brings many problems and difficulties to data processing. After careful analysis of the original data, it is considered that the main difficulties in this data processing are:
Processing method of (1) low SNR data
There are two main reasons for the low data signal-to-noise ratio. First, the reflection characteristics of underground seismic geological horizons are clear, the impedance value of the interface between upper and lower strata is large, and the reflection energy is strong. However, the surface acquisition conditions are complex, the excitation and reception conditions are poor, and the interference noise from underground and outside the surface is serious, resulting in low signal-to-noise ratio of data, such as Pazhou Bay area in Jianghan. Second, the reflection characteristics of underground seismic geological horizons are unclear, the impedance value of the upper and lower strata interface is small, and the reflection energy is weak. Although the surface excitation and reception conditions are good, effective reflection signals cannot be received, resulting in low signal-to-noise ratio of data, such as Paleozoic exposed areas at both ends of SA and LH survey lines in Jianghan Basin, Datong Lake area and southern mountainous areas of Xinyang Basin. For the first case, as long as the selected processing method is correct, good contour effect can still be obtained, but the second case is more difficult.
(2) Consistency processing of amplitude, frequency and phase of seismic data.
Due to the large span of large profile survey line, diverse geological structures and complex and changeable surface excitation and reception conditions, under the unified acquisition factors, the energy and frequency recorded by the original single shot in each section of the survey line or between shots are very inconsistent, which directly affects the profile effect of processing, such as different seismic sources and different lithology of excitation and reception.
(3) field static correction processing
The survey lines involved in field static correction are mainly Jianghan SA and LH survey lines. Due to the complex underground geological structure, these static correction problems are characterized by unclear line segments, poor data quality and great difficulty in processing.
(4) Pre-stack migration imaging of complex structures.
Because seismic profiles are used in regional geological exploration, there are many types of geological structural units involved, many large faults, and the buried depth of strata is also very different. For example, on the LH survey line, the Silurian system is exposed at the shallowest place, and the deepest place is about 6000m m, so it is very difficult to establish the migration geological model and calculate the migration speed.
Step 2: Countermeasures
In view of the above difficulties, the following countermeasures are taken to deal with large earthquake profiles:
A. Based on the principle of improving the signal-to-noise ratio of the profile and ensuring the reliability of the data;
B. Do a good job in noise analysis, adopt targeted noise suppression and removal methods to suppress noise interference to the maximum extent and improve the signal-to-noise ratio of data;
C, using surface consistency amplitude processing, surface consistency deconvolution and other surface consistency processing methods to eliminate the differences in energy and frequency of seismic data and solve the problem of surface consistency;
D, using elevation correction, refraction wave static correction, chromatographic inversion static correction and other methods to improve the superposition effect of static correction;
E) Do a good job in velocity analysis, improve the picking accuracy of stacking velocity by combining constant-speed scanning with fine velocity spectrum, properly encrypt the longitudinal and transverse velocity analysis points in complex structural areas, and improve the stacking imaging effect through fine velocity analysis;
F. Do a good job in the combination of processing and interpretation personnel, establish relatively accurate geological model and velocity model, and strive to improve the accuracy of migration imaging.
(2) Processing key technology-prestack depth migration technology.
Pre-stack depth migration methods mainly include K-Shikhov integral pre-stack depth migration and wave equation pre-stack depth migration. Kerk-Shikhov prestack depth migration is fast and accurate, but the imaging effect is greatly affected because of its different approximation degree and some limitations in methods (such as multi-travel path, false frequency and false amplitude processing). Compared with K-Shikhov prestack depth migration, wave equation migration is realized by wave field continuation, regardless of travel time and amplitude, which can correctly and automatically deal with all kinds of complex wave propagation, shielding zone and phase shift, but it is relatively simple to realize. At the same time, the imaging results are no longer sensitive to the high frequency error of velocity, and the velocity trend is more important than the velocity details, thus making accurate depth imaging easier. Wave equation prestack depth migration is the most expensive and time-consuming algorithm in seismic data processing at present. In recent years, with the appearance of PC cluster parallel system, the hardware price has dropped sharply, which also makes it possible to prestack depth migration of wave equation. It has become the hotspot, frontier and development direction of global seismic imaging research, and represents the latest generation of seismic imaging technology. Because of the advantages of prestack depth migration method, it is much more accurate than other migration algorithms for salt dome imaging, buried hill imaging, overthrust nappe structure and steep dip structure imaging, and the lateral position under complex structures, which is of great significance for clarifying the position of structures and further understanding the shape of structures.
This book uses Marvel Comics software, and its pre-stack imaging methods include:
1) gram Shikhov prestack time migration
-straight ray prestack time migration
-Bending ray prestack time migration
Two-speed prestack time migration of bending ray based on floating datum.
Bending ray two-speed prestack time migration based on real surface
2) Kirchhoff prestack depth migration
-Pre-stack depth migration based on floating datum
-prestack depth migration based on true surface
3) prestack depth migration of wide-angle finite difference wave equation
4) Pre-stack depth migration of bidirectional wave equation
In this processing, high-precision Kriging-Shikhov prestack depth migration imaging method is adopted.
(3) Analysis of treatment effect
1. South and North China NHB-07 section
It can be seen from the processed profile (Figure 3-42) of South China Survey Line that the Paleozoic Cambrian-Ordovician and Carboniferous-Permian reflection wave groups have strong energy, clear characteristics, good lateral continuity and stable energy; Niqiuji sag and Lu Yi sag were contiguous in Paleozoic, with clear structural contact, clear faults and structural shapes, which can basically be tracked and compared in the whole region.
The overall reflection characteristics of Niqiuji sag are obvious and the interlayer relationship is clear. The reflection of TN at the bottom of Neogene has good continuity and can be tracked continuously. On the whole, the reflection of TE at the bottom of Paleogene in Niqiuji sag is continuous, but there are also intermittent traces. The TMz reflection at the bottom of Mesozoic has general continuity and can be traced, while the TC reflection at the bottom of Carboniferous has good continuity and can be traced continuously. The reflection continuity of TPZ at the bottom of Paleozoic is general and can be traced.
It can be seen that the Paleogene in Lu Yi sag is unconformity contact, and the Tpz reflection event at the bottom of Neogene has good continuity, strong energy and high signal-to-noise ratio, so it can be tracked continuously. TE reflection at the bottom of Paleogene is generally continuous and can be traced in Lu Yi sag. The TC at the bottom of Carboniferous and TPZ at the bottom of Paleozoic have good continuity and can be traced continuously.
The target layers of Fuyang sag, Linquan sag, Taihe uplift and Dancheng uplift are shallow, and the reflection time is about 0.4s The in-phase axis in the target layer has good continuity and high signal-to-noise ratio, which can continuously track the reflection at the bottom of Paleozoic.
Changshan uplift-Xinyang basin is located at the southern end of NHB 07 survey line. According to the old data in the past, there is no good seismic reflection below Paleogene in Changshan Uplift, and the Cambrian and Ordovician are not completely preserved, but some depressions are preserved, and the erosion phenomenon in the uplift area is serious. It is a uplift area dominated by old strata. There is no Upper Cambrian in this area, but only Middle and Lower Cambrian.
From the overall profile processing effect, compared with the old data, the relationship between Changshan Uplift and Xinyang Basin is clearer through fault contact, with reliable structural form and clear wave group characteristics. The whole Changshan uplift structure is gentle, the Cambrian and Ordovician systems are not completely preserved, the uplift area is seriously eroded, and the data quality at its southern end is poor.
2. Jianghan Plain 2006-LH, SA Profile
Fig. 3-42 Comparison of the effects of this prestack depth migration processing and the original processing profile 3-42 NHB-07 (overall)
On the whole, the signal-to-noise ratio of Linxiang-Huangpi (2006-LH) line is low, among which the seismic reflection wave groups of Keli and Weizhou structures are strong, with clear features and complete horizons. Jiangnan orogenic belt and Qinling-Dabie orogenic belt at both ends of the survey line are affected by complex surface and underground geological conditions, with low signal-to-noise ratio and poor data quality. Comparing the local section of Weizhou structure with the old section, it can be seen that the wave group characteristics, imaging effect of reflection layer and signal-to-noise ratio of data in the new section are all improved compared with the old section (Figure 3-43).
The overall signal-to-noise ratio of Songzi-Anlu (2006-SA) survey line is low, and the data quality of Jingzhou-Daye thrust interference zone is good, and the seismic reflection level is complete, which is easy to compare and interpret. The fold belt in western Hunan and Hubei and the Qinling-Dabie orogenic belt at both ends of the survey line are affected by complex surface and underground geological conditions, with low signal-to-noise ratio and poor quality of data. Compared with the old section (a part of Zhongxiang arc fold belt), it can be seen that the wave group characteristics, fracture structure and deep imaging effect of the new section are improved compared with the old section (Figure 3-44).
(4) Suggestions on treatment measures
Through prestack gathers processing and prestack depth migration processing of two-dimensional profile, the following suggestions are put forward for data processing:
A. Do a good job in the preliminary analysis of seismic data processing, identify the key processing points and geological problems that need to be solved, determine the processing flow adopted on this basis, and carry out various processing methods and parameter tests in a targeted manner. Therefore, the combination of processing and geological interpretation is very important for data processing.
B. It is very important to do a good job in surface static correction for long-span 2D data processing. It is suggested to do a good job in unified calculation of surface correction, which can effectively improve the signal-to-noise ratio in complex mountainous areas by combining micro-logging acquisition with post-surface correction based on seismic data.
C. The surface consistency processing of wavelets such as amplitude, phase and frequency in prestack gathers processing, especially the data splicing processing of different years, different observation systems and different excitation and reception factors, must first do well the wavelet shaping and surface consistency processing between blocks, so as to ensure the true and reliable final imaging.
D. In view of the characteristics of some structural depressions, such as large profile, large lateral span, deep buried stratum and weak energy, as well as the characteristics of large dip angle of fault steps and large lateral velocity change, pre-stack depth domain imaging processing plays a key role in further improving the imaging of complex structures and deep structures. Advanced prestack depth migration imaging technology is the best imaging method choice for 2D and 3D processing. Using K-Shikhov integration method or wave equation prestack depth migration with more fidelity amplitude-preserving algorithm can realize the correct imaging of complex structures to the maximum extent. Pre-stack depth migration can effectively solve the imaging problem of complex structures when the velocity changes laterally. After prestack depth migration, seismic data not only regress accurately in space, but also provide reliable velocity field information. Compared with time migration profile, depth migration profile has the characteristics of real, intuitive and easy interpretation of underground structure.