1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China; 2. Department of Earth Science and Engineering, Hohai University, Nanjing 210098, China; 3. Research Institute of Geotechnical Engineering, Hohai University, Nanjing 210098, China; 4. College of Geo-Exploration Science and Technology, Jilin University ,Changchun 130026,China；5. Institute of Geophysical and Geochemical Exploration，CAGS，Langfang 065000，China
This paper introduces filter factors respectively in downward continuation and derivative calculation for calculating the threedimensional normalized full gradient field of gravity anomaly. The authors deduced the computing formula and the process for achieving its numerical value, and then gave the specific calculation procedure. Characteristics of 3D normalized full gradient of gravity anomaly in the homogeneous oil sphere coronal model were analyzed. It is concluded that, for oil sphere coronal, the 3D normalized full gradient field has the following features: no matter whether it is in a figure of central section or in that of a slice, only when the suitable cutoff frequency is obtained can the "two folders and one low" of typical characteristic signs of oilgas reservoir be shown; otherwise, it is similar to the model of homogeneous sphere coronal which shows characteristics of a single peak anomaly. In addition, the position of "two folders and one low" in the contour map of normalized full gradient field is usually the position and depth of the oil sphere coronal.
