Aeromagnetic and Marine Measurements

Abstract

Modern magnetic measurements have been acquired since the 1940s over land and the 1950s over oceans. Such measurements are collected using magnetometer sensors rigidly fixed to the airframe or towed in a bird for airborne or in a fish in marine surveys using a cable long enough to avoid the ship/airplane magnetic effect. Positioning problems have been considerably reduced by the Global Positioning System (GPS). Considerable progress has been made in geomagnetic instrumentation increasing the accuracy from similar to 10 nT or better in the 1960s to similar to 0.1 nT or more nowadays. Scalar magnetometers, less sensitive to orientation problems than the fluxgate vector instruments, are the most commnonly used for total-field intensity measurement. Optical pumping alkali vapor magnetometers with high sampling rate and high sensitivity are generally used aboard airframes whereas proton precession magnetometers (including Overhauser) are favored at sea. Scalar magnetic anomalies are calculated by subtraction of global core field models like the International Geomagnetic Reference Field (IGRF) after subtraction of an external magnetic field estimate using magnetic observatories or temporary magnetic stations. The external field correction using an auxiliary station is often not possible in marine measurements. However comprehensive models such as CM4 can be used to provide adequate core and external magnetic fields, particularly for almost all early magnetic measurements which were not corrected for the external field. In the case of airborne measurements such global models help to define a reference level for global mapping of the anomaly field. The current marine dataset adequately covers most of the Northern Hemisphere oceanic areas while major gaps are observed in the southern Indian and Pacific oceans. Airborne measurements cover all the world, except oceanic areas and large part of Antarctica. Data are however often not available when owned by private companies. The data released are mainly owned by governmental agencies. The derived airborne/marine magnetic anomaly maps combined with long-wavelength satellite maps help scientists to better understand the structure and the evolution of the lithosphere at local, regional and global scales. Marine magnetic observations are also made at depth, near the seafloor, in order to access shorter wavelengths of the magnetic field for high resolution studies. Airborne High Resolution Anomaly Maps (HRAM) are also nowadays the new trends pushing towards the generalisation of the Unmanned Aerial Vehicles (UAV) or Autonomous Underwater Vehicles (AUV) or Remotely Operated Vehicles (ROV) magnetic surveys.