Abstarct: Seismic wave velocity in an aniotropic medium varies in different directions. The velocity anisotropy represents a distinct structure in the scale of seimic wavelengths that is attributed to physical variations. The anisotropy phenomenon in a formation influences different factors such as deviation in the drilling, the production rate and other reservoir quantities. Thus, the determination of anisotropy parameters in order to recognize the above-mentioned factors is necessary. Beside this, in many applications of elastic theory in the reservoir geophysics, the elastic medium is considered to be isotroic. However, the most of rocks in the earth crust are weakly anisotropic. The governing equations in weakly anisotropic media are much easier than the governing equations in strongly anisotropic media. In these equations, the important parameter δ, which is one of three anisotropy parameters introduced by Thomson, controls the most important anisotropy factors in exploration geophysics. The Thomson anisotropy parameters that include ε, δ and γ, are determined from the walkaway vertical seismic profiling (VSP) data. In this thesis, the Thomson anisotropy parameters in an oil field, located west of Iran, was determined from the VSP data and DSI well log using phase slowness method (for the VSP data) and Alford’s rotation technique (for the DSI well log). The determination of the anisotropy parameters from the VSP data in two synthetic and real models was made. The synthetic model was constructed using NORSAR2D software. The results of the phase slowness method in the synthetic model were confirmed by the observations in the oil field. Using the synthetic model with a dipping overburden of maximum 6 degrees, an accuracy of 0.05 for the anisotropy parameter of ε was obtained. For synthetic models with more dipping overburden, the phase slowness method does not lead to acceptable accuracies in the determination of the anisotropy parameter ε. Considering the results of the synthetic horizontal and semi-horizontal models with overburdens of maximum dip of 3 degrees, the determination of the anisotropy parameter δ with an acceptable accuracy was possible. However, the synthetic models with a 6-degree dipping overburden do not yield acceptable results for the anisotropy parameter δ. Therefore, considering the assumption of horizontal overburdens in the determination of the anisotropy parameters using the the phase slowness method is essential. After determining the Thomson anisotropy parameters, the velocity in the anisotropic medium can be calculated using the mathematical relations, the calculated velociy can be compared with the measured velocity in the field. During the transmission of the shear waves from the anisotropic formation, the shear waves are travelled faster in the direction of more compact materials of the formation, and slower in the direction prependicular to the more compact direction. Therefore, the shear waves are decomposed (or polarized) into two fast and slow components. The basis of determination of the anisotropy using the DSI well log is to recognize the shear wave components and their directions. For this, the DSI well log data should be rotated in order to be aligned with the anisotropy principal components. The rotation of the components is carried out using the Alford’s rotation technique. In this method, if the two directions x and y are aligned with the formation anisotropy axes (x and y), the two vertical receivers in the same directions have the same energy level. After applying the rotation, the energy contents of the two fast and slow wave components are compared, and the difference are demonstrated as the anisotropy map and anisotropy log in the investigated borehole. The anisotropy map representing the maximum stress is used for the determination of anisotropy axis or direction, and the anisotropy log indicates the anisotropy values in different depths of the investigated borehole. Considering the obtained results, the anisotropy axis, which is in the direction of the the maximum stress, was determined about 50 degrees and the anisotropy value of the arrival wave time was determined as 23.2 percent. Considering the above-mentioned points, the time difference between the results of the VSP data and anisotropy log obtained from the DSI well log can be compared. However, in this thesis, this comparison was not possible due to the difference in the depths of the two sets of the data (i.e. VSP and well log data).