Characteristics and Applications of Variogram for Gravity and Magnetic Fields

The variogram is used to investigate the regional variation characteristics of gravity and magnetic fields in this study. The range of variogram reflects the coherent area; the nugget effect represents the random disturbance; and the sill means the variation degree. The theoretical simulations involving gravity and magnetic fields show that the coherent range of gravity field is larger than that of magnetic field. The range of potential gravity and magnetic field is mainly determined by the depth of field sources. The variogram for shallow sources is approximate to be a spherical or exponential model, while with the increase of the depth, it nearly becomes a more continuously Gaussian model. The depth of magnetic source is approximately equal to the half of the range, whereas the depth of gravity source is approximately equal to one quarter of the range. The real example of the vertical component magnetic anomaly of the Daye iron-ore deposit in Hubei has geometric anisotropy striking northwest-southeast direction. The variogram reveals that the average depth of magnetite orebodies is about 250m. The decomposed details and approximations of magnetic anomaly using wavelet transform have the coordinate geometric anisotropy. And the estimated depths using variogram ranges are in agreement with those gained by power spectrum method.