The tilt derivative applied to AEM conductivity data

Various procedures are used in the processing of potential field data to provide enhanced detection and definition of structural information. The majority of such procedures use the spatial derivatives of the data either individually or in combination. The ability of the tilt angle (or derivative) to provide enhanced mapping of electromagnetic (EM)/conductivity structure is considered here. Although this study considers airborne EM survey data, the concept can equally be applied to ground-based conductivity/resistivity data sets. Following a description of the principles of the tilt derivative (TDR) method, forward modelling studies of concealed EM/conductivity structure are presented. Case studies of the practical application of the procedures to survey data are then performed. The tilt function embodies Automatic Gain Control that normalises the detection and definition of both weak and strong conductivity gradients across an appropriate subsurface depth range. Noise amplification is an inevitable outcome of the procedure. Filtering methods to attenuate undesired artefacts are available and are demonstrated

The application of tilt derivatives to EM conductivity data

In the processing of geophysical potential fields, a wide range of spatial derivatives are available to enhance the information contained in the basic data. Here the ability of the tilt and tilt derivatives to provide enhanced mapping of conductivity data is considered. Tilt and its associated functions are formed by taking combinations of vertical and horizontal derivatives of the data set. A theoretical forward modelling study is carried out to assess the performance of tilt derivatives in relation to the detection and definition of concealed conductivity structure. Case studies of the practical application of the procedures to survey data are performed. The case studies derive from large scale airborne EM data sets but the methods have a general applicability to a wide range of geophysical conductivity and resistivity data. The tilt functions embody Automatic Gain Control that normalise the detection and definition of both weak and strong conductivity gradients across an appropriate subsurface depth range. The use of high order spatial derivatives inevitably results in a degree of noise amplification that is survey and technique specific. Filtering methods for the reduction of undesired, usually high wavenumber, artefacts are available and are shown to be effective

Analytic signal, depth and multispectral interpretation of areas within the Continental Terminal, North-western Nigeria

This study utilized the ability of remote sensing and aeromagnetic data of the Continental Terminal, Northwestern Nigeria, to accentuate structural and lithological geologic features as well as depth estimation and mineral potential for exploitation purposes. The analytic signal and source parameter imaging filtering techniques on the magnetic data helped to identify causative magnetic bodies and space-depth relationship, two important indices in regional and ore body modelling. Depth interpretation result ranged from 120m to 2000m in the entire regolith cover where the basement rocks are overlain by clastic materials. Areas of deeper depths coincided well with regions of higher magnetic anomalies while areas of shallow depth coincided well with regions of lower magnetic anomalies with magnetic values ranging from 33121nT to 32980nT suggestive of a likelihood of having the magnetic unit of the basement at the top of the basement surface. The study identified the structural trend from the aeromagnetic map, showing a perfect agreement with the geologic map and Landsat-8 imagery. The Landsat-8 data was used for structural and lithological mapping and in general geological interpretation. Single band combination of 764 representing Red, Green and Blue were processed from the false colour composite image and pan-sharpened using the pan chromatic band to enhance structural features on the map. The study area showed no promise of hydrocarbon accumulation from the structural interpretation for economic benefits. However, economic-grade minerals were inferred due to magnetic anomalies that suggested accumulation of magnetic materials located within the proximity of lineaments

New insights on structure and tectonics over the Laxmi Ridge using EIGEN6C4 modelled gravity data

1999-2005A comparative analysis has been attempted for the study of structural and tectonic trends over and around (140–190 N latitudes and 640–700 E longitudes) the Laxmi ridge. Initially, three different edge enhancement techniques comprising total horizontal derivative (THD), analytical signal (AS) and Theta map have been tested over synthetic prismatic models with varying depths. It is observed that Theta map technique is relatively suitable for delineation of the edges for the sources at different depths. The free-air gravity (FAG) of the Laxmi ridge and surroundings have been generated using EIGEN6C4 high resolution combined earth gravity modelled data. Upward continued gravity anomaly at 20 Km, 40 km, 60 km, 90 km and 150 km heights have been estimated and these are further enhanced using Theta map technique. The enhanced upward continued maps at different height reveal that the sources of the Laxmi ridge low anomaly is constituted by crust, low density upper mantle materials and recent sediments. Present study reveals that the EIGEN6C4 modelled data could be used effectively to identify various structural features of different wavelength. The study reveals that the major lineaments trends are found along N-S, NE-SW and NW-SE directions followed by E-W, ENE-WSW and NNW-SSE directions including different regional and shallow lineaments trends. The delineated lineaments and their orientations are the results of multiple phases of rifting and breakup of India, Madagascar and Seychelles, since its initial stage

Inconsistent Structure and Motion of the Eastern Median Tectonic Line, Southwest Japan, during the Quaternary

The Median Tectonic Line (MTL) is the largest tectonic line in southwest Japan, and its eastern portion has moved as a right‐lateral fault with a reverse fault component during the Quaternary. Although a high dip of the MTL has been suggested from geomorphological studies, reflection surveys have indicated a low dip of 30–55°. Thus, the MTL shows contradiction between its fault dip and fault motion. In this study, we attempted to re‐estimate the dip of the MTL by gravity anomaly, gravity gradient tensor, and numerical simulation, restoring topographies caused by lateral faulting. The numerical simulations suggested that a fault dip of 70–75° is a reasonable dip of the MTL. These high‐dip faults are able to move as a lateral fault and have the possibility of reverse fault motion. Deformation patterns caused by faults with these dips are in harmony with the accumulated geological and geomorphological evidence for motions of the MTL. On the other hand, gravity and gravity gradient analyses showed only a material boundary with low dip. This suggested that the MTL does not have conspicuous density contrast at the boundary

Inland extent of the Weddell Sea Rift imaged by new aerogeophysical data

Peer reviewedPostprin

Inherited crustal deformation along the East Gondwana margin revealed by seismic anisotropy tomography

The mechanisms of continental growth are a crucial part of plate tectonic theory, yet a clear understanding of the processes involved remains elusive. Here we determine seismic Rayleigh wave phase anisotropy variations in the crust beneath the southern Tasmanides of Australia, a Paleozoic accretionary margin. Our results reveal a complex, thick-skinned pervasive deformation that was driven by the tectonic interaction between the proto-Pacific Ocean and the ancient eastern margin of Gondwana. Stress-induced effects triggered by the collision and entrainment of a microcontinent into the active subduction zone are evident in the anisotropy signature. The paleofracturing trend of failed rifting between Australia and Antarctica is also recorded in the anisotropy pattern as well as a tightly curved feature in central Tasmania. The observed patterns of anisotropy correlate well with recent geodynamic and kinematic models of the Tasmanides and provide a platform from which the spatial extent of deformational domains can be refined

Interpretação geofísico-geológica de uma porção setentrional do cráton amazônico

Orientador: Prof. Dr. Francisco José Fonseca FerreiraCoorientadores: Roberto Gusmão de Oliveira, Nelson Joaquim ReisDissertação (mestrado) - Universidade Federal do Paraná, Setor de Ciências da Terra, Programa de Pós-Graduação em Geologia. Defesa : Curitiba, 28/05/2018Inclui referências: p.107-115Área de concentração: Geologia exploratóriaResumo: Nas últimas décadas o Serviço Geológico do Brasil - CPRM desenvolveu programas de levantamentos aerogeofísicos no território nacional, os quais recobrem 86% do estado de Roraima. A área de estudo incluiu quatro folhas na escala de 1:250.000 (aproximadamente 72.600 km2), de um trato setentrional do Cráton Amazônico, situada na porção central do estado de Roraima (RR), no extremo norte do Brasil. Abrange parte das províncias Rio Negro (1,82-1,52 Ga) e Tapajós-Parima (2,03-1,88 Ga) e respectivos domínios Serra Imeri (DSI), Parima (DPA), Surumu (DSU), Uatumã-Anauá (DUA) e Guiana Central (DGC), este, com maior recobrimento na área de estudo. A integração geofísico-geológica foi realizada através da sobreposição das informações aeromagnetométricas, aerogamaespectrométricas e geológicas disponíveis na literatura. Essas informações foram utilizadas para indicar áreas ao mapeamento e assim apoiar a cartografia e fomentar o conhecimento geológico do estado, contribuindo para o desenvolvimento da região. A interpretação do mapa litogeofísico permitiu indicar assinaturas gamaespectrométricas para unidades ígneas plutônicas, metamórficas e sedimentares e respectivos produtos intempéricos. Em subsuperfície foram interpretados seis domínios magnetométricos. Quando comparados àqueles tectonoestratigráficos, observa-se que apenas um domínio comparece integralmente no DGC. Com a finalidade de estimar a profundidade das fontes magnéticas foi empregada à técnica do espectro de potência, a qual indicou profundidades rasas (até 1 km), intermediárias (entre 1 e 12 km) e profundas (entre 12 e 40 km), observando-se os limites dos domínios magnéticos nos três intervalos estimados. Foi realizada uma modelagem de áreas potenciais para mineralização Fe-Ti-V a partir de dados geofísicos e geoquímicos disponíveis. Destacam-se cinco áreas favoráveis cuja classificação de alvos está em concordância com o grau de favorabilidade obtido tanto pela modelagem espacial quanto pela compatibilidade de unidades geológicas. Palavras-chave: Roraima; Magnetometria; Gamaespectrometria.Abstract: In the last decades, the Geological Service of Brazil has been conducting geophysical survey in the national territory, which already covers 86% of the state of Roraima. The study area included four leaves in the 1: 250,000 (approximately 72,600 km2) scale of a northern tract of the Amazonian Craton, located in the central portion of the state of Roraima (RR), in the extreme north of Brazil. It covers part of the Rio Negro (1.82-1.52 Ga) and Tapajós -Parima (2.03-1.88 Ga) and respective Sierra Imeri (DSI), Parima (DPA), Surumu (DSU), Uatumã - Anauá (DUA) and Central Guyana (DGC), this one, with greater overlap in the study area. Geophysicalgeological integration was performed through the overlapping of magnetometric, gamma ray spectrometry and geological information available in the literature. This information was used to indicate areas for mapping and thus to support cartography and to foster geological knowledge of the state, contributing to the development of the region. The interpretation of the litogeophysical map allowed indicating radiometric signatures for plutonic, metamorphic and sedimentary igneous units and respective weathering products. Six subscriber domains were interpreted in subsurface. When compared to those tectonostratigraphy, it is observed that only one domain appears integrally in the DGC. In order to estimate the depth of the magnetic sources, it was used the power spectrum technique, which indicated shallow (up to 1 km), intermediate (1 to 12 km) and deep (12 to 40 km) depths, the limits of the magnetic domains in the three estimated intervals. A modeling of potential areas for Fe-Ti-V mineralization was performed from available geophysical and geochemical data. We highlight five favorable areas whose classification of targets is in agreement with the degree of favorability obtained by both the spatial modeling and the compatibility of geological units. Keywords: Roraima; Magnetometry; Gamma-ray spectrometry