Controlled Source Magnetics: A Method for Imaging High-resolution Near-surface Magnetic Heterogeneity

Abstract:

Magnetic property in subsurface has been a target of the magnetic method for a wide variety of geophysical applications in mineral, hydrocarbon, groundwater, and environmental arenas. Anomalous magnetic property also affects controlled source electromagnetic (EM) data due to source-driven induced magnetization. At very low frequencies, a few to tens of hertz, the EM response predominantly consists of static-like magnetic field due to induced magnetization. Taking advantage of this property we have developed a numerical procedure to image subsurface magnetic heterogeneity with much improved resolutions. Incidentally, sensitivities of commercially available sensors and geomagnetic noise spectra at these frequencies are reasonably manageable compared to the anticipated magnetic field strength generated by numerical modeling. We, in this study, show that the resolution of three-dimensional inversion result(s) of controlled source magnetic data (fig. 1(a)) is better than that of geomagnetic data (fig. 1(b)). This is because use of EM excitations from different directions reduces non-uniqueness of inverse problem. These results show that a controlled source magnetic method can be a useful exploration tool when higher resolution of magnetic property is needed or strong remnant magnetization hinders the interpretation of magnetic method.