Depth estimation of magnetic and gravity sources using normalized source strength calculated from gradient tensor

Abstract

Summary We introduce a new method for interpretation of magnetic and gravity gradient tensor data using the normalized source strength (NSS). The NSS can be derived from eigenvalues of magnetic and gravity gradient tensors. The NSS is proportional to a constant shape factor normalized by the n-th power of the distance between measurement and source points. For magnetic case, it is independent of magnetization direction but proportional to magnitude of the source strength. For both magnetic and gravity fields, the maximum of the NSS occurs exactly above the source point. We also show that the NSS is a homogenous function and its degree of homogeneity is equal to n, the power of the distance between observation and integration points. In our algorithm, we use maxima of the NSS for estimating the horizontal location of the causative body. Then we estimate depth to the source and structural index at that point using the ratio between the NSS and its vertical derivative calculated at two levels; the measurement level and a height h above the measurement level. To discriminate more reliable solutions from spurious ones we reject solutions with unreasonable estimated structural indices. Application of the method is demonstrated on very recent collected aeromagnetic and gravity gradient tensor data sets from McFaulds Lake region, Ontario, Canada.