How Basement Lithology Changes Affect Magnetic Interpretation
Patrick S. Millegan, D. E. Bird, 1998. "How Basement Lithology Changes Affect Magnetic Interpretation", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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Many people are intimidated by the magnetic method, but they are quite willing to attack gravity interpretation. This is usually a result of the inclination/declination issues with magnetics, and the fact that most people view a gravity map as a structural map. Magnetic maps are not structural maps; they are a contouredrepresentation of magnetization changes in the geology. This is harder for people to “visualize” in terms of real rocks and structure. Gravity interpretation seems easier because a gravity high usually correlates with a structural high (except in the case of salt), but simple criteria such as that can cause problems if the interpreter is not careful
On the other hand, some aspects of the magnetic method are quite straightforward. One is the direct mathematical relationship between anomaly wavelength and source depth. Another important relationship is that basement lithology changes typically have amplitudes of hundreds of nanotesla (nT), but structural changes usually cause anomalies of only tens of nT (see Gibson's magnetic susceptibility versus structure illustration later in this volume).
It is also important to note that, depending on magnetic inclination, a fault and a lithology change can have distinctly different magnetic signatures. One might cause a symmetrical anomaly while the other causes an asymmetrical anomaly. This underscores the importance of preliminary models prior to undertaking any gravity or magnetic interpretation.