Location of Magnetic Dipoles in Chongcho Lake, Republic of Korea

Magnetic dipoles were located in Chongcho Lake, Sokcho, Korea, using an adaptation of the SOAPFI (Shape-of-Anomaly Potential Field Inversion) program. Approximately 90 km of magnetic-field profiles were recorded in this 1-km2 lake. The lake bottom contains archeological artifacts and unexploded ordnance. Many of these objects were modeled as dipoles with induced magnetization along the direction of the geomagnetic field. Dipole strengths and locations were automatically derived from anomalies of the total magnetic field. These anomalies were filtered along profiles to pass high-wavenumber signals. "Profile-adaptive" filters of the modeled dipole's field matched filters of the observed field within intersecting and overlapping profiles. This procedure allowed simultaneous inversion of data with and without subtraction of a base station's field. For each data window, one or two dipoles were modeled. The shape-of-anomaly criterion minimizes differences between shapes of modeled and observed gravity or magnetic fields using an L1- or L2- (least-squares) norm. In a general application of this criterion, the SOAPFI program automatically shifts the position of a user-supplied initial model, or "seed". The seed is generally much smaller than the actual causative body. The shape-of-anomaly criterion then allows the inverse model to grow from that seed until the modeled and observed fields are precisely matched. For applications of the SOAPFI program to magnetic dipole modeling, fewer values of the observed field were used to compute the trial values of dipole moments than to compute the objective functions that were minimized to obtain dipole locations.