Location of Magnetic Dipoles with Arbitrary Directions of Magnetization in Chongcho Lake, Republic of Korea

The SOAPFI-MD (shape-of-anomaly potential field inversion for magnetic dipoles) program determines locations, strengths, and orientations of buried magnetic dipoles from surface measurements of the magnetic field. It minimizes an objective function that measures the difference between the shape of observed fields and modeled dipole fields using an L2-norm. An application is presented for fields measured by a towed magnetometer in Chongcho Lake, Sokcho, Korea. Previously, many dipoles in Chongcho Lake were modeled using SOAPFI-MD with magnetic dipoles magnetized along the direction of the earth's field. For this study, an iterative procedure varied both location and dipole orientation. The effects of remanent magnetization and self-demagnetization of elongated bodies were approximated for depths exceeding the greatest dimensions of causative bodies such as the archeological artifacts and military debris on the lake bottom. The SOAPFI program allows automatic inversion of gravity and magnetic fields of extended bodies by growth from a small "seed" (initial model) that may produce a field dissimilar to the observed field; the model grows by automatically filling prisms on the periphery of the growing model. The analogous procedure in SOAPFI-MD is to increase the number of dipoles. For given locations and orientations of multiple dipoles with overlapping fields, a system of linear equations determines the relative strengths of the dipoles to minimize the objective function. "Profile-adaptive" filters to pass high-wavenumber signals were applied to the observed signal in intersecting and overlapping profiles and to the modeled dipole fields before evaluation of the objective function.