3D Magnetic Characterization of Hot Springs in a Hydrothermal System in the Alvord Basin, Oregon

Abstract

Water is not magnetically susceptible. We utilize this knowledge to map the spring systems in the Borax Lake Hydrothermal System (BLHS). BLHS is an active group of hot springs located in the Alvord Basin. These springs are aligned linearly along a north-south trend. High resolution magnetic data were acquired over this area to map the near-surface magnetic variations. The total field magnetic data were inverted using a 3D inversion algorithm to produce 3D distribution of magnetic susceptibility. Although magnetic data has no depth resolution but imposing constraints during the inversion procedure gives us the flexibility to obtain geologically meaningful results. In particular, we apply depth weighting function in the 3D magnetic inversion (Li and Oldenburg, 1996) to counteract the decay of the magnetic response with depth. The regional magnetic fields that are associated with deeper structures are removed using a spectral basis function approach via linear inversion formalism. The residual data after regional removal are inverted using the 3D code. The 3D nature of the inverse problem is computationally challenging. The inverse problem is solved in the wavelet domain to reduce the computational costs. Interpretation of the 3D subsurface susceptibility distribution from the Alvord basin reveals conduit-like structures of low magnetic susceptibility that are possible channels of preferential fluid flow.