Joint inversion of surface and borehole magnetic data: A level-set approach

The need to improve the depth resolution of the magnetic susceptibility model recovered from surface magnetic data is a well-known challenge, and it becomes increasingly important as exploration moves to regions under cover and at great depths. Incorporating borehole magnetic data can be an effective means to achieve increased model resolution at depth. The recently developed level-set method for magnetic inversion provides a novel means for constructing the geometric shape of causative bodies and opens a new avenue for the joint inversion of surface and borehole magnetic data for the purpose of achieving improved depth resolution. We have developed an extension of the algorithm to the joint inversion and find that the level-set algorithm can resolve the configuration and spatial separation of complex magnetic sources using the information in the magnetic data from sparse boreholes. We further examine the use of borehole intersection information in estimating the crucially important susceptibility values within the context of level-set inversion and find that the susceptibility value can also be used as a probing parameter to assess the uncertainty in the spatial extent of the causative bodies. We determine that the modified level-set inversion leads to an effective means to image and delineate magnetic causative bodies with complex structure by combining the information from surface magnetic data, borehole magnetic data, and sparse drillhole intersection data.