Cross-hole Seismic First Arrival Wave Travel-time Tomography And Inversion Method Research

Cross-hole tomography is an important part of cross-hole seismic techniques, with thewide application of cross-hole seismic technology in oil field exploration, therefore, the studyof cross-hole seismic technology and perfection is particularly important. At present, based onthe ray theory of first-break travel-time tomography technology has become one of the mainmethods of cross-hole seismic tomography. Because first-break wave is easy to identify, highstability and carrying a large amount of information, so the first-break wave travel-timetomography can provide seismic exploration with more efficient, more accurate velocityanalysis.Thesis mainly studied the primary wave travel-time based on the theory of the raytomography, its main divided into two parts, the forward ray tracing and inversion imaging.For forward ray tracing by using linear interpolation of travel-time ray tracing algorithm.Tomography inversion of the article adopts damping least-square LSQR algorithm of the highprecision and fast convergence rate, deducing the iterative formula of the algorithm andintroducing the algorithm for selecting the damping factor in standard. On this basis, the useof Fortran programming language based on ray theory is the cross-hole seismic first-breakwaves travel-time tomography imaging and inversion technique, and the large sparse matrixcompression storage are introduced in programming. The method has been tested for thevertical level layered model, slope cross-hole of the layered mode and bending interface layermodel, three main factors of tomography are discussed in detail: ray coverage, grid size andinversion algorithm.The results showed that:1) travel-time linear interpolation ray tracing algorithm is easyto programming and the inversion results of high precision.2) Compared with LSQRalgorithm, the damping LSQR algorithm is of high precision, the iteration speed, strongpracticability and can obtain ideal inversion result.3) through the theoretical model of the testget satisfactory results, further illustrates the forward (LTI) algorithm and the inversiondamping LSQR algorithm is the correctness and reliability, provides correct and efficienttheoretical support for the practical work of the seismic exploration.4)On the theory, the closer for ray and the smaller the grid size, the higher the inversion precision, but consideringthe actual cost, to arrange the number of shot points and geophone, to ensure that the cost andimprove the inversion accuracy.