Gravity Data Inversion Using Simulated Annealing Method (Case Study: Aman Abad Region-Arak City)

Optimization methods such as Simulated Annealing (SA), Genetic Algorithm (GA), Ant Colony Optimization (ACO) and Partial Swarm Optimization (PSO) are very popular for solving inverse problems. Gravity inversion is one of the fields of geophysical exploration that mainly used for mining and oil exploration. In this research optimization method is considered for gravity inversion of a sedimentary basin to find out its geometry. A new method for 3D inversion of gravity data is developed. In such optimization methods before anything else having a forward model is necessary. This model is the relationship between Bouguer gravity anomalies and a combination of prisms. The gravity anomalies of the density interface are generated by equating the material below the interface to a series of juxtaposing rectangular blocks. The stochastic optimization method that is used for solving inverse problem is Simulated Annealing. With a try and error method at first the cost function of a lot of choices is measured. Then the minimum value between them is used as the primary set of variables for introducing to model that is the start configuration of model variables. After that the repeating algorithm starts until the cost function reaches to as small as possible value by considering the geophysical constraints of the place that is being studied. Finally the geometry of prisms that is the depth of each prism is achieved. This geometry shows the situation of anomaly source. For better inversion regional anomaly is used with introducing some unknown constants to the model. The values of unknown parameters of model are extracted in a continuous range that is obtained from priori information of the region. At first the algorithm was used in solving a synthetic problem that is defined by developing random data in an arbitrary region, so with using forward model again and again finally the cost function reaches to its minimum value. For evaluating the success or failure of the algorithm, the contour map of depths that was used in forward model is compared with map that is produced after inversion and discrepancies was considered. The good results of synthetic problem lead to implementing the algorithm for real gravity data from Aman Abad region in Arak city. For this purpose at first the gravity data must be changed to Bouguer gravity anomalies by some reduction. After that the repeated algorithm is implemented and finally the results are compared with some priori information which is obtained by madding boreholes around the region. This priori information claims that the minimum and maximum depth of prisms is between 70 meters to 120 meters. Results of the algorithm are compatible with this information and show the power of algorithm in solving this kind of inverse problem. It must be mentioned that without using this priori information the results of gravity inversion are not unique and many different interpretations can be concluded even interpretations that cannot be accepted in geophysical view. As a norm to find the ability of algorithm in solving this problem the cost function can be helpful which shows the amount of error. The maximum value of it was 0.5 mgal in some regions in this research.