Amplitude of the magnetic anomaly vector in the interpretation of total-field anomaly at low magnetic latitudes

Abstract

We propose the use of the amplitude of the magnetic anomaly vector (amplitude data) for qualitative interpretation of large areas at low magnetic latitudes. The amplitude data are weakly dependent on the magnetization direction. Hence, the amplitude data require no prior knowledge of the source magnetization direction. The amplitude data produce maxima over the causative sources, allowing the definition of the horizontal projections of the sources. This characteristic is attractive for interpretation at low magnetic latitudes because at these regions the interpretation of the total-field anomaly is not straightforward. We compute the amplitude data using the equivalent-layer technique to transform the total-field anomaly into the three orthogonal components of the magnetic anomaly vector. We analyze the results of tests in synthetic data simulating a main geomagnetic field at high, mid and low latitudes, with sources ranging from symmetric to elongated forms, including a dipping source. These sources, that give rise to the simulated anomalies, have both induced and strong remanent magnetizations. By comparing the amplitude data with the total gradient, we show that the amplitude data locate the horizontal projections of the sources. Thus, the amplitude data can be used to delineate the boundaries of the sources. We apply both the amplitude data and the total gradient to a real total-field anomaly over a large area of the Amazonian Craton, northern Brazil, located at low magnetic latitudes. The amplitude data show a better performance in delineating geologic bodies that are in agreement with the outcropping intrusions in the geologic map. Moreover, the amplitude data revealed new geologic bodies that were not present in the geologic map. The clear alignment of these new bodies with the outcropping intrusions suggested the continuity of these intrusions in depth. This result is a step forward in understanding this area for which the geology is poorly known. Hence, the amplitude data can provide an apparent-geologic map especially in areas at low latitudes with remanently magnetized bodies.