Annihilator transform for magnetic lineaments removal in dike swarms

Abstract

Dike swarms are igneous structures of continental expression accounting for major episodes of magmatism in igneous provinces, mantle plume heads, and continental breakup. In regional magnetic maps, dike swarms are recognized by high-amplitude lineaments indicative of lengthy and juxtaposed magnetized bodies. High-anomaly amplitudes from such tabular (2D) bodies tend to obscure lower amplitude contributions from localized 3D sources, representative of magmatic structures that once served as magma plumbing and storage. The recognition of such subtle signals with conventional filtering techniques is prevented due to spectral overlapping of individual contributions. We have developed a processing scheme to remove contributions from elongated, homogeneous sources to make clear contributions from 3D sources located below, in the middle of, or above a framework of elongated homogeneous sources. The canceling of 2D fields is accomplished by evaluating the horizontal component of the magnetic anomaly along the lineament strike, which for true elongated and homogeneous sources gives a null response. The gradient intensity of the transformed field is then evaluated to enhance residual fields over 3D sources. Lineaments thus removed identify tabular bodies with homogeneous magnetization, interpreted as being indicative of the uniform distribution (mineral type, concentration, and grain-size distribution) of magnetic carrier content in the rock. We evaluated our technique with synthetic data from multiple 2D-3D interfering sources and then applied it to interpret airborne data from the Ponta Grossa Dike Swarm of the Paraná-Etendeka Magmatic Province in Southeastern Brazil.

Well productivity in the Ponta Grossa dike Swarm, Brazil: An integrated study with magnetic data inversion and clustering analysis of model solutions

“…The inference of dikes from remote geophysical data, however, is associated with uncertainty and misrepresentations. Deeper, smaller and closely adjacent dike units, for example are not well recognized due to associated lower anomaly amplitude and superposition of the resulting fields (Mendonça et al, 2019;Castro et al, 2019).…”

Thin Sheet Automatic Interpretation of Dike Swarm Magnetic Transects Using Regularized Second Derivatives

“…We introduce a discontinuous, interval-defined function obtained from rationing the AMA by its second 𝑥-derivative to automatically determine intervals within which a dike unit is expected. The choice for AMA is advised since it is less distorted by shallow sources (geological noise) and more effective in capturing fields from deeper sources (Li et al, 2010;Liu et al, 2015;Mendonça et al, 2019). The use of AMA second 𝑥-derivative aids the identification of intervals where the AMA is concave down to discriminate side-by-side sources with subtle variations in the observed anomalies.…”

Técnicas especializadas para o estudo de lineamentos magnéticos associados a enxame de diques e de anomalias lineares em coberturas basálticas

“…Mesmo em situações que os lineamentos são claramente observados em mapa, esse problema ainda carece de procedimento objetivo para se determinar com precisão qual seria a direção média definida por tais lineamentos. Tal como desenvolvida para diques aflorantes ou subaflorantes, a identificação da direção dos lineamentos pode ser feita aplicando o procedimento 2DFAT -2D Field Annihilator Transform - (Mendonça et al, 2019), baseado no princípio de que um campo magnético nulo é produzido quando avaliado o componente horizontal da anomalia magnética ao longo da direção α coincidente com o strike de um corpo magnético 2D homogêneo. O procedimento 2DFAT basicamente consiste, então, em aplicar uma mudança de componente da anomalia medida para uma direção horizontal coincidente com a direção do lineamento magnético.…”

“…Por sua propriedade de aniquilação, essa transformação anula a contribuição das estruturas 2D homogêneas, preservando, entretanto, as anomalias geradas por fontes 3D ou lineamentos com direções distintas. Conforme aplicada por Mendonça et al (2019), a transformada aniquiladora foi utilizada para verificar a homogeneidade de diques em um enxame e para realçar a distribuição de fontes 3D ao longo do enxame.…”