3D magnetic inversion by planting anomalous densities

Leonardo Uieda, Valéria C. F. Barbosa

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This talk presents an adaptation of the gravity-gradient inversion method I developed for my Masters degree dissertation "Robust 3D gravity gradient inversion by planting anomalous densities" to invert magnetic data.

As you may have noticed, there is an error in the title. We do not, in fact, invert magnetic data using density anomalies. This illustrates the perils of copy-pasting combined with a looming deadline.

I have added an open-source implementation of the gravity-gradient inversion method to the Python library Fatiando a Terra. In version 0.1 to 0.4, the code is in module fatiando.gravmag.harvester.

Abstract

AGU abstract ID: GP22A-01

We present a new 3D magnetic inversion algorithm based on the computationally efficient method of planting anomalous densities. The algorithm consists of an iterative growth of the anomalous bodies around prismatic elements called "seeds". These seeds are user-specified and have known magnetizations. Thus, the seeds provide a way for the interpreter to specify the desired skeleton of the anomalous bodies. The inversion algorithm is computationally efficient due to various optimizations made possible by the iterative nature of the growth process. The control provided by the use of seeds allows one to test different hypothesis about the geometry and magnetization of targeted anomalous bodies. To demonstrate this capability, we applied our inversion method to the Morro do Engenho (ME) and A2 magnetic anomalies, central Brazil (Figure 1a). ME is an outcropping alkaline intrusion formed by dunites, peridotites and pyroxenites with known magnetization. A2 is a magnetic anomaly to the Northeast of ME and is thought to be a similar intrusion that is not outcropping. Therefore, a plausible hypothesis is that A2 has the same magnetization as ME. We tested this hypothesis by performing an inversion using a single seed for each body. Both seeds had the same magnetization. Figure 1b shows that the inversion produced residuals up to 2000 nT over A2 (i.e., a poor fit) and less than 400 nT over ME (i.e., an acceptable fit). Figure 1c shows that ME is a compact outcropping body with bottom at approximately 5 km, which is in agreement with previous interpretations. However, the estimate produced by the inversion for A2 is outcropping and is not compact. In summary, the estimate for A2 provides a poor fit to the observations and is not in accordance with the geologic information. This leads to the conclusion that A2 does not have the same magnetization as ME. These results indicate the usefulness and capabilities of the inversion method here proposed.

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