Joint inversion of surface and three-component borehole magnetic data

The inversion of magnetic data is inherently nonunique with respect to the distance between the source and observation locations. This manifests itsel f as an ambiguity in the source depth when surface data are inverted and as an ambiguity in the distance between the source and boreholes if borehole data are inverted. Joint inversion of surface and borehole data can help to reduce this nonuniqueness. To achieve this, we develop an algorithm for in verting data sets that have arbitrary observation locations in boreholes an d above the surface. The algorithm depends upon weighting functions that co unteract the geometric decay of magnetic kernels with distance from the obs erver. We apply these weighting functions to the inversion of three-compone nt magnetic data collected in boreholes and then to the joint inversion of surface and borehole data. Both synthetic and field data sets are used to i llustrate the new inversion algorithm. When borehole data are inverted dire ctly, three-component data are far more useful in constructing good suscept ibility models than are single-component data. However, either can be used effectively in a joint inversion with surface data to produce models that a re superior to those obtained by inversion of surface data alone.