The Research On The Forward Calculation And Constrained Inversion Of The 3D Salt Dome Model's Gravity Anomaly

Gravity surveys have been used in solving regional geological problems, plotting structure units and faults, studying sediment base depth and lithological distribution, which can provide gravity information for oil and gas exploration. It is well known that fault trap, salt dome, pillows etc. have good conditions in reservoiring oil and gas, so they have become major targets in oil and gas exploration. However, the seismic method often has difficulties recovering the geometry of the base salt with steep flanks and diapiric root. This problem may lead to incorrect subsalt imaging and inaccurate interpretation of subsalt geological structures. Gravity methods can help to construct the distribution of the geological units. The thesis carries out the constraint inversion of the data of 3D salt dome model by by simulating geological unit, thus providing good applications for the oil and gas exploration of the gravity method.Firstly, on the basis of researching on the forward calculation of the regular geological bodies, I study rapid computation of gravitational attraction of three-dimensional bodies of arbitrary shape by Talwani, design the proper 3D salt dome model and carry out the forward calculation of this model by program. Secondly, the anomalous source is divided into a large number of rectangular prisms with fixed size and unknown physical properties. Base on the geometry relation of the prisms, I study the fast forward calculation of the density property model and realize it by program, which paves the way for the sensitivity matrix fast construction.Thirdly, in the inversion I construct the objective functionΦ( m ) =φd +μφm. Solutions are obtained by minimizing a global objective function composed of the model objective function and data misfit function. Smoothness constraints can be incorporated into the objective function, which can improve the numerical stability of the inversion. The tendency to put structure at the surface can be overcome by introducing a depth weighing to counteract this natural decay. Proper depth weighing function can approximately compensates for the decay with depth. The objective function can be realized using a conjugate gradient approach. If we know some prior information of geological units, we can give the value to the certain parameter(s), or impose the certain bound for the inverse parameter(s), thus decreasing the ambiguity. In the thesis I use the absolute constraint directly, imposing the upper and lower bound for the parameters and during the iterative process, chopping off all the values above the upper bound and below the lower bound. The method can be done easily and generally has good results.Finally, I design different 2D and 3D models especially the salt dome model and carry out forward calculations to simulate the real data, then realize the constrained inversion using these data. The numerical tests show the validity and feasibility of the methods above.