Use of Commercial-Off-The-Shelf Vehicles for Towed Array Magnetometry: Geophysical Procedures for Vehicle Signature Measurement

摘要

The 1/R3 sensitivity of total field magnetometers makes them highly sensitive to nearby ferrous metal, including that of a towing vehicle. The magnetic signature of a vehicle varies primarily with the orientation of the vehicle in the Earth's magnetic field. It also varies with the orientation of the sensor array relative to the tow vehicle (eg. in a tight turn). In addition, if the vehicle signature is very high and / or the sensor array is very close, small changes in vehicle pitch and roll and sensor array pitch and roll will engender changing amounts of vehicle signature in the data that for practical purposes cannot be filtered out. For the sake of this discussion, we are assuming that the vehicle is far enough away that the primary effect in the data is from changes in vehicle orientation relative to the Earth's magnetic field. When surveys are conducted in a bidirectional up-and-back fashion, the 'bulk signature' of the vehicle is readily seen as differences in magnetic background between outgoing and incoming traverses. These differences (offsets) create stripes or streaks in the data along the direction of travel. Because a GPS antenna is frequently employed in towed array magnetometry, the GPS antenna signature is sometimes seen as an additional streak over the center magnetometer. Simple background leveling techniques used to remove the magnetic signature are not trying to model the vehicle's signature, and thus they usually remove both the vehicle signature as well as the offset caused by the GPS antenna. Depending on the signature removal technique, it may also remove diurnal drift of the Earth's magnetic field and long-wavelength geology. The vehicle signature removal techniques we have employed fall into four categories: 1. Static offset removal 2. Table-based offset removal (the Corps' octant test) 3. Directionally-dependent offset removal 4. Time-domain filtering (dynamic background leveling).