The Research And Application Of The Deep Seismic Reflection Profiling In Orogens In West China

In west China, there lies the world famous No.3 pole - Qinghai-Tibetan plateau, the widest desert basin -Tarim Basin in Asia, and the typical orogenic belt - Tianshan orogen in central Asia, et al. These units not only make of majesty and grand tectonic physiognomy scenery for west China, also offer us a natural laboratory to research on the leading science of continental dynamics, such as the formation of Qinghai-Tibetan plateau and intra-continental deformed orogeny and basin-forming. Especially these geology process is still on-going, for that we can use deep probe method to observe the being behavior, to understand the deep dynamic background for the purpose of forecasts and alleviates the geology calamity, to supply basic gist for the developing of continental dynamics theory.Deep seismic reflection profiling is recognized globally as an avant-courier technology to probe the deep earth. The deep seismic reflection profiling has been applied in many range and basin area to detect the deep structure and has obtain many important discovery since the 1970's. It greatly promote the development of the earth science. Our scholars have deployed the deep seismic reflection profiling experimentation in orgoenic in west China since 1992.It is both hydrocarbon enrichment areas and earthquakes disaster zones in western China orogens. Qiangtang basin situated in the hinterland of the thickened Qinghai-Tibetan orogenic belt where is located in the east of Tethys domain. It is a region of the most hydrocarbon potential and the most intact marine strata within the Qinghai-Tibetan plateau. There are new discovery of Kela-2 and Dina-2 oil & gas fields and also Jiashi strong earthquake swarm along the junction of Tianshan orogen and Tarim basin. Recently some deep seismic reflection profiles take its high resolution advantage have been deployed in these orogenic belts are gradually open out the inside information of resource and disaster hides in deep earth.However, it is also very hard to probe the Qinghai-Tibetan plateau and its surrounding orgoenic belts. As is known to us, there are two main issues that restrict the deep seismic prospecting in the orogenic zones. One is the complicated earth's surface condition, typically as undulating landform, alternating new information and old outcrop, lateral variation of LVZ. The other is the complicated subsurface structures, i.e., concealed faults or over-thrusts are developed in upper crustal, and channel flow are obvious in lower crustal.Due to these complicated surface and subsurface conductions, the absorption of high frequency of seismic wave field is serious, and shallow low-frequency interference wave disturb the deep reflection events. So it is hard to obtain the deep structure information and some time takes a risk of no gains with high pains in these orognenic areas. And the lateral variation of LVZ brings the asymmetry travel-time delay to the seismic wave lead to complex statics correction problem. As is well known, the statics problem is the threshold to the seismic data processing in mountain area. Only if the best methods for the mountain area statics problem have been resolved, the profile can be stack naturally.As we know that seismic wave has definite amplitude and frequency characteristic when travelling throw the stratum. We may reduce the risk by the analysis of the characteristic of seismic wave, finding the main issues that hinder the seismic acquisition process, and then promote corresponding methods to improve the profile quality. It is also an efficient approach to calculate the statics based on the accurate velocity building for complex near surface to solve the statics correction issues in the complicated area.Through field practice and room theory analysis, the author has studied the key problem when applied the deep seismic reflection profile in orgonenic belts in west China. The paper has tested the technology and method to improve the acquisition quality under the complicated geology condition, and about twelve seismic survey lines obtained in the last decade and four new seismic acquisition lines recently deployed in Qiantang basin and one deep seismic reflection profile in the junction belt of Tianshan and Tarim (TT2007) are analyzed in this paper for the application of statics correction which is the key process during the seismic data process in complex areas.The paper divides into two main parts, accordingly.Part one is based on the experience of field acquisition and back to the room study of the interference factors analysis and corresponding countermeasures for data acquisition and process to improve the S/N ratio of the oil seismic data and the new finished deep seismic reflection profiling. Through the analysis of attenuation by earth absorption, surface-consistence statistical autocorrelation of seismic data, the author summed up the general characters of the data and the factors of interference the wave spreading, furthermore, carried out the test of coherent noise attenuation. Practices show that the complicated near-surface velocity structure is the key factor that influence the seismic field acquisition and data process. For the more knowledge of the near-surface structures the better for the job of seismic field acquisition, noise attenuation, and statics correction. So the establishment of near-surface velocity structures is the main research content of this paper. Two reflection seismic profiles that linking up and crossing the central uplift of the Qiangtang basin were tested for the first arrival tomographic method and the results reveal the central uplift the undulate character of the high velocity structure both vertical and horizontal. Based on the near-surface structure created by these inversion methods the author did forward simulation to analyze the actually problem during this area exploration, such as the blocking effect of the permafrost and the relationship between the well depth and seismic wave frequency. As for the optimize study of shooting the author researched the inconsistent of the survey line and adjacent shots assembles ,and in recording aspect, discussed the relationships between the receiver arrange and coherent noise attenuation, group interval and the spatial aliasing. The research results indicate that after acquisition technology improvement and data process parameters optimization, the quality of seismic profile are much more improved than before.The second part of this paper is based on the experience of field acquisition and focus on the statics correction problems of the junction belts between the southwest Tianshan and Tarim basin, the author carried out the application of near-surface velocity. Where there the complicated near-surface made the seismic rays are no-perpendicularity to the surface especially when the undulating landform and lateral variation of the LVL are all sharp, it will lead to long, medium and short wavelength statics problems. The author first analyzed the major factors to the statics problems that come from objectivity and subjectivity, and then discussed the applicability of the common statics correction methods, such as, elevation statics, model statics, refraction statics, and tomography statics during seismic data processing in the basin and range junction with complex structure character. Based on the subsection of the basin, range and its junction seismic data using tomographic statics correction method to process, compare and analyze the results, to deal with the big fall of the altitude and sharp variety of the LVL especially under the junction part, the author bought forward the statics correction technology work flow, i.e., combination of the tomographic method to create the velocity model and calculate the floating statics values for the shot and receiver station with elevation method to calculate the final datum statics values. In order to make the statics value accord with the huge low velocity of Tarim basin, and the shallow high velocity of Tianshan range, and the sharp changed velocity of the junction zone, the author used multi-line tying process, after the matching process the statics were applied during the velocity analysis and NMO, at last add the elevation statics to CMP ensembles.Upon the research of the two parts contents in acquisition technology and data process application in the two areas, this paper arrives at some new understandings of the deep seismic reflection profiling in these areas as follows:(1)The research results of the near-surface velocity model using deep seismic reflection profiling data1) The tomographic travel-rays assemble where the structure is complex underground. The tomographic image reveals the undulate character of the high velocity structure both vertical and horizontal. And its thickness has negative correlation with the age of the outcrop in the central uplift of Qiangtang.2) There is a distinct velocity variety in the junction of the Tianshan and Tarim basin. The method of adopting subsection tomograhpic and use multi-line tying process to combine with elevation method can solve the statics problem in TT2007 deep seismic reflection profile and improve the quality of the profile.3) The research course of creating the forward model based on the inversion method using the interpretation results of shallow refraction survey and the first arrivals data, to simulate the field shots and to do the theoretic research in approximately the real-surface condition, can be the guideline to supervise the field work. This will be an efficient way to improve the seismic acquisition quality.(2) Research results and countermeasures for the deep seismic acquisition in Qiangtang area.1) Theory analysis and results of seismic records simulation indicates that explode in or under the frozen layer can have better energy than above the high-velocity frozen layer and can reduce the effect of the wave-front spreading in this layer. The 'ice' may be melt with the soil to form mud and may stuff the aiguille when drilling in the permafrost. In case of that, it will be better to use special drilling equipment to get rid of the mud and make the aiguille clear.2) Model forward simulation shows that the deeper the drill hole the broader the record frequency and the less the interference wave. The real field shots also validate the fact that the shallower the record the serious the air blast and surface wave. Concluded from the experiment and comparison with the field shots in this area, it is recommended that the single well depth be more than 18m for the exploration of upper crustal.3) The frequency of coherent noise are low, about around 10Hz. When these wave in the record with the time dip angle great than 50ms, it will produce spatial aliasing in the F-K domain. So it is difficult to get rid of this kind of noise and it may influence the interpretation of the deep refection event. Even the ever test of 20mgroup interval record has spatial aliasing. So it is recommended to reduce the group interval considering the costallowable during the field work to decrease the possibility of spatial aliasing.