69 2. Surface geophysical methods Magnetic exploration is primarily used in the first phases of geophysical work in the area. Magnetic surveying, performed with magnetometers (fluxgate magnetometer, proton-precession magnetometer...), is carried out on land, at sea, and in the air. For extensive areas, reconnaissance over both land and sea is conveniently done with an airborne magnetometer. As in the case of gravity, the magnetic field anomaly is simply the observed minus the predicted value at the observation site. If Tobs is the measured total field (corrected for temporal variation (diurnal correction)) and TR the reference field, given by the IGRF tables (International Geomagnetic Reference Field) at the site, the geomagnetic anomaly in the total field ΔT, is given by ΔT = Tobs – TR. The advent of satellites dedicated to measuring the total field T or its vector components has remarkably augmented the global coverage and improved the data for analysis of the earth’s field (the International Geomagnetic Reference Field is revised every five years). Observations of the ΔT anomaly field (or its vertical or horizontal component) over the area of the survey reflect subsurface variations in the magnetization of rock formations. Specific procedures such as derivatives and filtering procedures are useful in separating anomalies. Upward and downward continuation are also used for the determination of regional and residual. A special procedure is to reduce the field to the pole. It consists in the transformation of the anomaly observed at the survey latitude where the field is inclined in an anomaly that would be observed at the magnetic north. For most applications of magnetic surveying, the magnetic effect of the sedimentary rocks may be considered as approximately the same as if the sediments were not present and the magnetic disturbances recorded have their origin at or below the base of the sediments. This is the basis for use of magnetic measurements to map the basement surface. The magnetic method is particularly suitable for mapping basement features such as lineament, faults, shear zones, lithologic contact, etc., which may be hidden from direct view because overlying sedimentary cover. Figure 2.10b shows the magnetic anomaly after pole reduction (Girard, 2017) observed in Martique in the area investigated by a gravimetric survey (Figure 2.7). The volcanic material of various ages highlights generally various responses in link with the varying geomagnetic field. For more information about the magnetic method, we recommend reading the book written by Fairhead (2015). 2.2.3 Electrical and EM methods On the historical side, Electrical methods dedicated to Geosciences began with the Schlumberger Brothers in the late 20’s. In a century, technology started from a simple DC resistivity method and went up to complex Electromagnetism methods implying natural or controlled sources, from 1D to 4D models, from acquisition in boreholes to land, air and sea. EM is now used in, tectonic studies,
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