Application Of 3D Gravity Normalized Total Gradient Method In Dachang Gravity Data

The gravity-normalized total gradient method was first proposed by the former Soviet Union scholar Berezkin.It was originally mainly used to search for oil and gas reservoirs,and it can be directly used for oil and ore prospecting under suitable conditions.It uses the analytic function to make the function lose its analytic property at the field source point,and obtains the position of the singularity(field source)through downward continuation.This method can accurately delineate the spatial location of the underground anomaly without prior constraints.For determining the depth of the center of the geological body,the center depth of the anomaly can be determined by selecting the appropriate harmonic number.The normalized total gradient of gravity method can qualitatively or semi-quantitatively determine the spatial location of anomalous bodies.In the early rapid gravity survey,it has unique advantages in delineating the distribution of concealed rock bodies and mineralization prospects.Based on the two-dimensional normalized total gradient method of gravity proposed by Berezkin,this paper is extended to the three-dimensional normalized total gradient method of gravity,and the calculation steps are specifically introduced and deduced.Through the analysis and research of the sphere model,the rectangular parallelepiped model,the complex density distribution model,the contact surface and other models,it is shown that the normalized total gradient extreme value of gravity(G_Hmax)and extreme value positions obtained by using different harmonic coefficients are not the same.When G_Hmax At the maximum,the corresponding position is consistent with the center position of the model,and the harmonic coefficient is the best.The length of the survey line and the distance between the survey points also have an impact on G_Hmax.The longer the survey line length,the greater the extreme value,and the smal er the step size,the higher the calculation accuracy.It does not appear in the slice graph of the three-dimensional gravity normalized total gradient method.The two-dimensional gravity normalization features"two highs and one low",but as the harmonic number increases,it changes from chimney shape to bowl shape.In addition,the best harmonic number can accurately identify the rectangular parallelepiped model.Different depth boundaries are more accurate than traditional boundary recognition methods.In the analysis of the gravity data of Dachang,it is found that the boundary range of the concealed granite rock mass of different depths is an irregular el ipsoid at the top,and the section is an inverted cone,and the overall shape is similar to a mushroom.The Dachang Longboxgai concealed granite body is in the north-south direction as a whole.As the depth deepens,the Longboxgai concealed granite body is slightly convex to the west,with a central depth of H=-1190m;a second-highest value trap appears between Dafulou and Roman Vil age,The depth is about H=-1100m,there is a low-value trap between Roman Vil age and Wangbeng,the depth is about H=-900m.