| Cross-hole seismic prospecting is a new seismic survey method which is arised in 1980s. In this method, seismic sources are placed in a well and geophones are located in one or several wells nearby to acquire seismic wave motivated by the sources, then using tomography or reflection wave imaging technique by analyzing data from two adjacent wells to obtain the geological texture between two wells. Compared to surface seismic prospecting, acquisition processing of cross-hole seismic prospecting has characteristics of short propagation distance for wave energy, close to detecting targets, keeping away from low velocity layer, et al. Hence, it can acquire seismic data containing high frequency and high signal-to-noise ratio property, which improve the exploratory accuracy dramatically. Cross-hole seismic prospecting which has the property of high accuracy and high resolution has been applied in structural fine imaging and detailed reservoir description for petroleum and gas prospecting, engineering geological and hydrogeologic exploration, civil engineering safety detecting and other geophysical operations.Since cross-hole seismic technique was presented in last century, it has continuously been a hot topic to researchers. Up until now, the main research subject of cross-hole seismic prospecting is two-dimensional (2D) cross-hole imaging, i.e., learn the underground structure between two wells by acquiring data between two adjacent wells and using travel time inversion technique to the data. However, a number of shortages are existed in this 2D prospecting method: 1) The acquired seismic wave fields across wells are the responds of wave fields in three-dimensional space. Therefore, the entire information of 3D spatial wave field cannot be comprehensively reflected by studying the simulated wave records in two-dimensional condition; 2) 2D travel time imaging only uses data from two adjacent wells to image, and information from multi-well distributed in three-dimensional (3D) space is not taken advantage effectively, which leads to a result that the use factor of spatial well site is low; 3) Using 2D cross-hole travel time imaging can only obtain underground plain structure with two wells belong to the plain and it cannot acquire the information belong to the lateral of profile, then imaging results has a bad property of transversal continuity, this affects the interpretation and evaluation of 3D geological texture; 4) 2D cross-hole travel time imaging is also facing problems that this method cannot image the object where there is a large dip angle interface between two wells (Dip angel of interface is greater than 45°) and accuracy of imaging is under the influence of lateral structure and so on. In addition, the rapidly developing reverse time migration method of cross-hole seismic research is restricted to 2D space, and it still cannot solve the problems like utilize of multi-well and imaging in 3D space.On account of reviewing cross-hole seismic technique and methods of reverse time migration imaging (RTM) at present, aiming at the existing issues in current studies, a research of cross-hole 3D elastic wave field numerical modeling and 3D acoustic wave RTM based on multi-well data is developed in this paper. Cross-hole wave field characteristic and propagation laws are learnt and analyzed through cross-hole 3D elastic wave field numerical modeling, then true cross-hole 3D geological texture are acquired by RTM, which can remedy the shortage in 2D cross-hole imaging and offer more actual and accurate 3D geological information for cross-hole seismic survey. Main research contents and achievement is shown summarily as follow:(1) In finite difference numerical modeling, boundary condition is one of the crucial research contents. In this paper, starting from elastic wave theory and according to basic idea of wave equation staggered grid finite difference, discretization formulas of convolution perfectly matched layer (CPML) in 3D staggered grid finite difference schemes are deduced based on analyzing the theory of non-fission CPML.3D elastic wave equation numerical modeling contained CPML is realized by using FORTRAN Language. Compared with conventional PML, the diversity of absorbing effects on occasions of normal incidence and incidence with a big offset (grazing angle), required memory space for numerical simulation, computational efficiency and other aspects for two boundary conditions is learnt in this paper. The research shows that CPML proposed in this paper has the capability of better absorbing effect for wave field incident to boundary with various offsets. At meanwhile, it has the property of occupying less memory space, and has higher computational efficiency et al. Applying CPML in 3D elastic wave field numerical modeling can obtain a better result.(2) The calculated quantity should be considered in 3D numerical modeling. In order to fulfill the requirement of large-scale computing for 3D elastic wave field numerical modeling, OpenMP technique is implemented in parallel computing for 3D elastic wave equation. The calculated efficiency of OpenMP parallel computing is analyzed, which lays a basis for follow-up large-scale computing of 3D wave field numerical modeling and RTM. Based on the research contents mentioned above, investigation of cross-hole 3D elastic wave field numerical modeling and signature analysis is carried out in this paper. The wave field of typical 3D cross-hole geologic body models like bedded geologic models, local inhomogeneous body models, large dip angle interface models and lateral high-speed media models et al are also analyzed by numerical modeling. The impact of local inhomogeneous bodies and lateral high-speed media on wave form in the regions nearby has been mainly discussed. By numerical modeling to typical 3D cross-hole geologic models, wave field is acquired in modeling containing lateral reflected waves, refracted waves and diffracted waves, etc. Which enriches the type of cross-hole wave field. By analyzing the impact on the travel time field nearby from these two types of lateral high-speed media, it gives a deep understanding on characteristics of cross-hole wave propagation. These lay basis and theoretical foundation for identification, analysis and processing of cross-hole seismic data.(3) Due to the fact that 3D cross-hole wave field is complex compared to a certain extent, starting out with theories of polarization, in this paper a method of pressure wave (P wave) and shear wave (S wave) separating for cross-hole 3D three-component (3C) seismic data is proposed in order to obtain independent P wave field based on the difference of cross-well seismic waves in the apparent velocity and polarization. In this method, azimuth correction of position placed geophones for acquired 3C data can obtain wave field recordings of radial component, vertical radial component and tangential radial component. In addition, τ-p transform with high resolution is adopted to separate up-going waves and down-going waves. Ultimately, independent P wave and S wave recordings are gained from cross-hole seismic data. The methods proposed in this article take full advantage of the cross-hole wave field properties of apparent velocity and polarization, and avoid the issues of estimating incident angle in traditional wave field separation methods. The experimental research about P wave and S wave separating for theoretical models’ 3C data, verifies efficiency and feasibility of these methods presented in this paper. At meantime, it also lays a basis for 3D acoustic wave equation migration imaging.(4) 3D RTM based on acoustic equation is studied in this paper. On account of analyzing theories of RTM, fundamental principles of cross-correlation, excitation time and amplitude ratio imaging condition technique in RTM are investigated, and implementing process and advantages as well as disadvantages of these techniques are also discussed in this research. Considering the imaging effect and calculated quantity as basis overall, this article adopts excitation time imaging condition as the research technique. Meanwhile, in order to satisfy the requirement of travel time accuracy for RTM, Fast Marching Method (FMM) which has the property of high precision and stability is proposed for 3D cross-hole travel time computing. Firstly, this paper illustrates the basic principles of FMM in detail, and then provides the implementing process of FMM. Subsequently, computing parameter fulfilled imaging accuracy is acknowledged by theoretical model testing, in the meantime the stability and efficiency of FMM in 3D travel time computing are demonstrated within complex models. Ultimately, analyzes production mechanism of noise in RTM processing and various types of denoising methods, a method combined Poynting vector denoising with Laplace denoising is proposed for 3D RTM denoising, and it improve the Signal to Noise Ratio in 3D RTM. RTM for theoretical bedded media models demonstrates that denoising method presented in this article has an effective result for noise eliminating.(5) 3D cross-hole RTM method based on multi-well data is developed in this research. On account of research on 3D elastic wave field numerical modeling, separation of P and S waves fields and RTM,3D multi-well cross-hole RTM method based on acoustic wave equation is accomplished in this article. This paper illustrates the imaging results of typical geologic models like bedded geologic models, local inhomogeneous body models, large dip angle interface models and fault models.3D structure distributions in the underground space among wells are studied by multi-well imaging, which solves the issues that transverse structural changing cannot be acquired in 2D imaging as well as CT imaging technique cannot get a correct result if a large dip angle interface exists in underground media. This gives a basis for 3D cross-hole geologic structural interpretation, and provides a new method and idea for cross-hole seismic prospecting.(6) Based on 3D multi-well cross-hole RTM, this paper presents research effects for 3D multi-well cross-hole RTM with various types of recording geometries on basis of two layers media models. The research shows that in two kinds of prospecting regions illustrated in article, reliable imaging results can be obtained when using 12 and 6 well sites, respectively. Aiming at cross-hole elastic wave recordings, research indicates:Imaging process for 3D multi-well cross-hole RTM flows as Azimuth correction for 3C data-Separation of up-going and down-going waves-Separation of P and S waves fields-Integrate P wave fields-RTM for multi-well data. In the case of bedded models, after separating P and S wave fields from acquired 3C elastic wave fields recording, adopting the separated P wave fields on 3D multi-well acoustic wave RTM can obtain imaging results which are similar to imaging results of acoustic wave equation. This demonstrates the validity and feasibility of processing flow proposed in this paper. Ultimately, multi-well imaging by using the data combined with noise shows that this method has a potential capability of noise immunity and feasibility, which gives a basis for 3D cross-hole seismic prospecting.Reviewing the contents mentioned above, this article presents a research on numerical modeling and characteristic analysis for 3D cross-hole elastic wave field. Propagation laws of 3D elastic wave field have been obtained and this paper gives a further understanding on wave propagating properties across wells, and it lays a foundation and theoretical supporting for distinguishing, analyzing and processing of cross-hole seismic data. On account of the recording geometries for multi-well,3D cross-hole RTM based on multi-well is proposed and true cross-hole 3D geological texture are acquired, which improves the lateral resolution. Studying impact of survey layouts on imaging offers wells minimum number for fulfilling imaging and processing flow for multi-well cross-hole RTM, which provides a new method and idea for cross-hole seismic prospecting. |
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