High Precision Imaging And Applied Research Of Data Processing For Mineral Seismic Exploration

Using seismic method for deep mineral exploration, can make up for the shortcoming of probing depth of the traditional exploration method, such as gravity, magnetic and electrical prospecting, it provides the necessary techniques for the second prospecting space. However, metallic mines geological conditions are often complex and undulate; ore bodies underground present complex structure, steep dip, small scale; the impedance has little difference between the ore body and the surrounding rock; and in the ore body often present the heterogeneous nature. The subsequent seismic data processing faces many problems. In this paper, with these characteristics of metallic mines, we study a series of seismic processing techniques to improve the resolution and signal-to-noise ratio of the data, some of these technologies are first proposed, some of these technologies are applied research. With comprehensive application, it provides the technical support for deep exploration and development of mineral resources.The resolutions of seismic exploration are the vertical resolution and horizontal resolution. It is generally recognized that the vertical resolution is the tuning thickness "quarter wavelength", the horizontal resolution is the first Fresnel zone radius. Pre-stack depth migration focuses the Fresnel zone on a point, and then it greatly improves the resolution of seismic data. The resolutions of pre-stack depth migration only have relation to the spatial sampling rate in theory. We compare three pre-stack migration methods in two-dimensional complex Marmousi model, which are Kirchhoff integral migration, one-way wave-equation migration and reverse-time migration. By comparison, the lines in the image of Kirchhoff integral migration are thicker, target reservoir is not clear, there are some blind areas and caustic areas, and the imaging is not very well; the image of one-way wave equation migration has higher resolution, but the imaging of high dip angle faults and small-scale fault blocks are indistinct; the reverse time migration has the best imaging result and the resolution is high, many features of details are retained, the imaging of three faults, fault blocks, anticline, high-velocity anomaly and target reservoir are clearer, and the locations are also very accurate. If we neglect the imaging accuracy, the locations of target reservoir obtained by three methods are same correspondent with the speed model. We made a further study on imaging technique of reverse time migration and summed up the present problem and difficulty of reverse time migration we stated:1. Due to the multiple solutions of time consistency imaging conditions, cross-correlation imaging condition cause low-frequency false by mistakenly correlating the refraction wave, diving wave, inverse scattering wave with received wavefield,which deteriorates the imaging quality of shallow;2. The cost of computation and memory is high;3; All problems which numerical calculation faces. Also I summarized to solve these problems and difficulties of the program:1. I comprehensively apply a variety of imaging conditions and the various filtering methods of migrated section, in order to suppress low-frequency false;2.Plane wave migration, multi-step migration strategy and the use of high-performance scientific computing, including multi-node parallel computing, GPU general computing technologies are applied in order to cope with the high computation and high memory overhead;3. I use higher order finite difference solving wave equation, absorbing boundary and other technology to improve the calculation precision. Aiming at the characters of metal mine seismic exploration, certain technologies mentioned above are selected to eliminate the pseudomorph and improve calculation efficiency, mainly including high order wavefield continuation,normalized imaging conditions,the filter of imaging results,the methods such as high-performance parallel computing, and reverse-time migration trial calculation of metal mine model was carried out. Taking the concealed mineral deposit model accreted of copper mine and delafossite in Hubei province for example, reverse-time migration imaging formation was done. Slender symbiotic copper ore body concealed in the mineral deposit and the interface of delafossite and copper mine could be clearly figured out by the results of imaging, and many detail characters could be depicted.Pre-stack depth migration respect to post-stack and time migration is more serious dependence on the velocity. The error of migration caused by velocity error has been more than caused by migration algorithm. The study on high efficiency and high precision velocity mode-building method is important. We studied the three pre-stack depth migration methods on velocity sensitive issues, and given analytic equation between the velocity error and the depth error. We analysis the "Smile" and "frown" phenomenon in the depth migrated section and common image gathers (CIG). Through the analysis of horizontal layered model, high-velocity model and Marmousi model, we give below conclusion:the migration depths of three methods with the same velocity perturbation are the same, in other words, they have the same depth error, namely the similar velocity error sensitivity. Consider the imaging effect and computational efficiency, using the advantages of above two imaging methods, we proposed the combined velocity model-building method which based on the Kirchhoff integral migration and reverse time migration, it combines the efficient computing of Kirchhoff integral migration and the high accuracy imaging of reverse time migration. The following eight steps describe the process of combined velocity model-building method:First, Establish the "initial velocity model" by general velocity analysis; Second, Use Kirchhoff integral migration to image section with "initial velocity model"; Third, Form the CIG or ADCIG of the initial velocity model, use the relationship between the imaging depth error and formation parameters error to update the velocity model (RCA strategy layer by layer); Fourth, Repeat steps 2 to 3, the process of updating velocity must be iterative calculation 2-3 times, it will be form "middle velocity model" with major structures, we call it is "part MVA"; Fifth, Use reverse time migration to image section with "middle velocity model"; Sixth, Same to step 3; Seventh, Repeat steps 5 to 6, the process of updating velocity must be iterative calculation 1-2 times, it will be form "final velocity model" with the complex structures of steep dip angle and small scale; Eighth, Use reverse time migration to image section with "final velocity model", as final imaging results. Taking a mushroom-shaped high-speed metal mining model as an example, the velocity analysis using RCA at each layer, we should choose the appropriate amount of control point; the residual points are available through interpolation and smoothing. We use the smooth function between adjacent layers, increase the control point near the complex structure and the acute velocity regions to improve the accuracy of the velocity analysis. The combined velocity modeling can effectively get high quality imaging section and velocity model of metal mine.Another characteristic of mineral mining seismic data is low signal-to-noise rate, the noise is strong, and effective signal is relatively weak. Low signal-to-noise ratio of seismic data will directly reduce the resolution of seismic section, we give the relationship between the resolution and the signal-to-noise ratio, analysis of seismic data obtained in the metallic mine to maintain signal-to-noise ratio greater than 2 is necessary. According path can be integral to Radon transform respectively linear, parabolic, polynomial, hyperbolic, elliptic transform. Phase shift hyperbolic Dix hyperbolic equation is far offset error for the larger substitute. Compared with conventional transformation, far offset hyperbolic reflection energy is better focused. Linear, parabolic, polynomial Radon transform operator has invariance when, in the frequency domain can be achieved. Time-the spatial domain into the frequency of the problem-to solve the spatial domain, the benefits of doing so is the speed of Fourier transform is very fast and can be carried out in each frequency component Radon transform, and avoid solving all the time--offset the speed of the large sample matrix problems, and enhance the computing speed. Toeplitz operator matrix structure of (uniform sampling parameters), can be used Levinsion fast recursive algorithm to solve. High resolution Radon transform is a frequency domain space sparse bound algorithm, in the inversion iteration process, in accordance with the results of the previous iteration, through Bayesian principles will be weighted matrix and the results of the previous iteration link be new weighted matrix; weighted matrix and then solve the equation, the frequency domain by the sparse solution. As operator matrix weighted matrix does not have the presence of Toeplitz structure, but still Hermite matrix, we can use the Cholesky decomposition method, LU decomposition method, the faster algorithm is conjugate gradient algorithm. In foreign, some scholars have begun studying time-space domain to direct calculation precision hyperbolic Radon transform algorithm, the algorithm used in least squares method to the most rapid decline in law. Conjugate gradient method fast convergence, the process for stability and the high accuracy solution, which has greatly improved its calculation speed. Subsurface media is often anisotropic, in order to consistent with the real subsurface media, anisotropy parameters should be considered in the integral path of Radon transform. For actual data processing, de-noising can receive better results, based on non-hyperbolic travel time offset function, anisotropic Radon transform formula has been defined, Compared to general Radon transform formula, anisotropy parametersη(anisotropy parameters) has been added, this parameters can be indicated by anisotropy parameters epsilon and delta, re-writtenτin the form of t, we obtain anisotropic inverse transform formula. We use simulation seismic records which added Gaussian random noise, and compared conventional Radon transform, high-resolution Radon transform and 2D mask filtering de-noising, we concluded that 2D mask filtering de-noising has a better effect of random noise suppression. By a variety of methods of a comparative study, which contains FK filtering,high-resolution linear Radon transform, high-resolution parabolic Radon transform and 2D mask filtering, on a middle shooting seismic record, we concluded that Radon transform combined with mask filtering has a best result of filtering and de-noising. De-noising a actual metal mining seismic data,and comparing de-noising results and difference profile, we concluded that Radon transform and mask filtering can suppress strong surface waves and strong linear interference, and then substantially raise signal to noise ratio of seismic data.We did characteristic processing and research on seismic test data of Jinchang Nickel-Copper Mining Area. According to geological mapping data of this area, we made nickel-copper geological model, where the layered nickel-copper orebody with dip was distributed in surrounding rock at 50～60 degrees. Then we did numerical simulating and wave-field characteristic analysis with high-order difference wave-equation in this model. Because layer spacing is tiny, the inclined events caused by inclined layers lead to mutual interferences, which are confounded and coupled with diffraction energy from pinch outs and catastrophe points, that's why the wave-field is very complicated. We did conventional seismic processing and pre-stack depth migration processing on the test data, respectively. Mineral vein from the post-stack section of the former is stretched seriously, slim lined orebody entirely moves upward, diffracted wave and scattered wave seriously affect the quality of the post-stack section, this will cause error of interpretation; the migration image from the latter has higher resolution and restores the locations of mineral vein and orebody, the mineral vein boundary can be distinguished clearly, it's easier to process geologic interpretation combined with geologic data. Based on the observation system and seismic data of Jinchang Nickel-Copper Mining Area, we processed the real data using processing software system with self-owned intellectual property rights and several self-developped characteristic processing modules, including Velocity Analysis Technique of High-resolution to improve accuracy of velocity spectrum picking; Surface-consistent Deconvolution Technique of Gabor Transformation to remove uncoupled effect between seismometer and bedrock caused by near surface weathering and to enhance the frequency band of seismic data; Combined Transformation Method of Suppression of Random Noise in Curvelet field to enhance seismic signal/noise ratio; Radon Transformation and Adaptive Mute Mask Filtering and Denoising Technique to suppress strong linear interference and extract weak reflection signals. Compared with post-stack section of conventional processing, synthetically using these characteristic processing techniques can greatly improve resolution and signal/noise ratio of seismic data of metallic orebody,obtain seismic stacked section of high quality and provide technical support for extension and distribution morphology of deep metallic orebody.