A comparison of the joint and independent inversions for magnetic and gravity data over kimberlites in Botswana

Complementary independent and joint focusing inversion investigations are applied to the magnetic and gravity datasets for two kimberlite pipes, BK54 and BK55, in Botswana. The magnetic data are high resolution and, clearly, indicate two anomalies in the survey area. Independent inversion of this magnetic data provides a focused image of the subsurface with sharp boundaries for both pipes. The extensions of the pipes are close to those estimated by the available boreholes in the area. The spatial resolution of the gravity data is, on the other hand, relatively poor. The BK54 pipe does not appear on the residual gravity anomaly, while the BK55 is characterized by a positive anomaly. Independent inversion of the gravity data provides the geometry of the BK55 but does not identify the BK54 pipe. While there are some lower density materials in the place of the BK54 pipe, no specific target is revealed. Furthermore and relevant is that the reconstructed model for BK55 is not focused. A joint focusing inversion based on the structural cross-gradient linkage is applied for the combined gravity and magnetic datasets. This algorithm produces structurally similar magnetic susceptibility and density models. A lower density distribution of material, similar to that of the magnetic susceptibility distribution, is revealed for the BK54 pipe. Moreover, in contrast to the result obtained by an independent inversion, the structure for BK54 is connected at depth, and the reconstructed models of BK54 and BK55 share the sparsity. The models are not as sparse as the magnetic susceptibility model that is obtained independently, but they are more focused than the independently obtained density model. A comprehensive comparison of the reconstructed models, and also computational CPU time, for independent and joint inversions is presented.