Aeromagnetic Surveys: Principles, Practice and Interpretation (original) (raw)
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Compilation of the Namibian airborne magnetic surveys: procedures, problems and results
Journal of African Earth Sciences, 1996
Forty one airborne magnetic surveys flown between 1%2 and 1992 over different areas of Namibia, with a total of about 700,000 km, have been compiled as part of a German-Namibian cooperation project to produce a refined compatible countrywide magnetic data set. It was required to compile the data as if the whole country was flown by a single survey at 100 m terrain clearance at the same magnetic epoch. Emphasis was therefore given to the selection of the magnetic reference field model. The creation of a digital terrain model (DTM) was compulsory. LeveIIing of the survey blocks was impeded since adjacent blocks were only rarely overlapping. The magnetic data set is available as both coloured contour maps (scales 1250 000 and 13 000 000) and digital grids with varying cell sires (2000 m, 500 m and 200 m). Shield areas, fold belts and anorogenic intrusions dominate the magnetic anomaly pattern of Namibia. The fold belts are characterized by parallel running, narrow (400 km) magnetic and vi&tally non-magnetic zones. shield areas, covered by a thick sequence of mostly non-magnetic sediments, are distinguished by broad, mid-amphtude anomalies with low gradients. The analysis of magnetic Iinear features reveals the existence of two directional sets of magnetic lineations running across both fold belts and shield areas. Resume-Afin de produire une couverture de donrkes magn&iques compatible et d&ail& couvrant l'ensemble de la Namibie, quarante et tm levb magnetiques aeroportes effect&s entre 1962 et 1992 sur diffkentes n?gIons du pays et total&ant environ 700,000 km ont et6 compiles dans le cadre d'un projet de cooperation german o-namibien. 11 a et6 n6cessaire d'organiser les dorm&s comme si l'ensemble du pays avait et6 couvert par un leve unique B une altitude de survoI de 100 m a une m&ne epoque magnetique. Pour cela, l'accent a et6 mis sur la selection dun modele magnetique de terrain de reference, n6cessairement num&ique. Le niveIlement des divers blocs leves a et6 g&t6 du fait que darts la plupart des cas des blocs limitrophes ne se recouvraient pas. I_es don&es magn&iques sont disponibles, soit sous forme de cartes en couleur des iso-anomalies (khelles 1250 000 et 1:lOOO 000), soit sous forme de grilles numkiques a ceIlule de dimension variable (2000 m, 500 m, 200 m). La repartition des anomalies magrktiques de la Namibie est domi6e par des regions de bouclier, des chames plissees et des intrusions anorogeniques. Les chaInes plissks montrent une altemance de zones paraIl&s et etroites (400 km), magnetiques et virtuellement non-magnetiques. Les bouchers, recouverts d'une epaisse sequence de sediments le plus souvent non-magnetiques,. s'identifient par de larges anomalies d'amplitude moyenne et de faible gradient. L'analyse des hwhmenk magncitiques rkvele l'existence de deux directions pr6fQentielles recoupant tant les chatnes phsskes que les regions de bouclier.
Interpretation of aeromagnetic data over some parts of Mambilla Plateau, Taraba State
International Journal of Physical Sciences, 2018
The data covering Mayo Daga and Gashaka areas of Taraba State has been interpreted by applying source parameter imaging (SPI) and forward and inverse modeling methods. From the quantitative method of interpretation, it was found out that the magnetic intensity within the study area ranges from-129.9 to 186.6 nT in which the area is noticeably marked by both low and high magnetic signatures which may be as a result of several factors such as; susceptibility, degree of strike, difference in magnetic variation in depth and difference in lithology. From the quantitative interpretation, depth estimates obtained when SPI is employed shown minimum to maximum depth to anomalous source that ranges from 400.7 to 2119.2 m. Forward and inverse modeling estimated depths for profiles P1, P2, P3, and P4 were 2372, 2537, 1621 and 1586 m, respectively, with susceptibility values of 0.0754, 0.0251, 0.0028, and 0.001 respectively, suggesting that the bodies causing the anomaly are typical of igneous rocks; basalt and olivine, intermediate igneous rock; granites, and rocks mineral (quartz).
Note on Aeromagnetic Survey with Special Reference to Volcanic Regions(Part 1)
Journal of the Faculty of Science Hokkaido University Series 7 Geophysics, 1966
Merits of aeromagnetic surveys are speediness in their practice and possibility of eliminating topographical effects. The latter depends on flight heights over topographies and their magnetization. Topographical effects are usually disadvantageous for the aim of surveys but, in some cases, those due to volcanic structures are the purpose of surveys. In this paper, the discussion on aeromagnetic survey's are confined to ones of total force. Profiles of magnetic anomaly in total force at various heights over a dipole are examined. Some examples of aeromagnetic surveys affected by the topographies are mentioned. A method of analysis of total force anomaly' is presented: anomaly is attributed to magnetic mass concentrated on a subterranean plane and with some approximations, probable mass distributions are obtained directly from total force anomalies. An example of the analysis is shown for the result of the survey on Lake Towada though the survey is shipbornc.
Total field aeromagnetic anomalies over Kogi-Auchi area have been evaluated in order to map the magnetic lineaments and estimate the depth to basement (sedimentary thicknesses). Aeromagnetic and radiometric data of the area (sheet 246, 247, 266 and 267) were acquired, digitized and analyzed. The slope methods were applied in the analysis of the residual contour map. The result obtained using the slope methods reveals two depth sources in the study area; on the average the deeper magnetic sources range from 2.3 to 4.9 km, while the shallower magnetic sources range from 1.1 to 1.6 km. These shallower sources are probably due to the presence of igneous intrusives and/or magnetized bodies within the sedimentary cover. Visual study of the residual anomaly map shows the presence of igneous intrusive in the entire North and Southwestern part of the study area. The One Vertical Derivative (1VD) lineament map shows the Fault trending in Northwest-Southwest (NW-SW) and Northeast-Southwest (NE-SW) direction and this conforms to the Niger-Benue basin itself. Hydrocarbon exploration is not recommended since the area has low thickness of sediment on the average. All this deduction were reached after due consideration of qualitative and quantitative interpretations supported by geological information of the area.
Evaluation of Aeromagnetic Anomalies Over Okigwe Area, Southeastern Nigeria
Research journal of environmental and earth sciences, 2011
Total field aeromagnetic anomalies over Okigwe area have been evaluated in order to map lineaments and estimate the depths to basement (sedimentary thicknesses). Aeromagnetic contour map of Okigwe (Sheet 312) was acquired, digitized and analyzed. Regional-residual separation and Slope methods were applied. The magnetic lineament map shows major geologic lineaments trending in NE-SW direction with minor ones trending in NW-SE direction. The major trend is in conformity with the trend of the Benue Tough. Visual study of the map shows presence of igneous intrusives in the northeastern part of the area. The results obtained using Slope methods indicate two depth sources in the area. On the average the deeper magnetic sources range from 2.0 to 4.99 km, while the shallower magnetic sources range from 0.4 to 1.99 km. Deeper magnetic sources probably depict depths to Precambrian Basement, whereas shallower sources probably depict depths to igneous intrusives and/or magnetized bodies within the sedimentary covers. Hydrocarbon exploration is not recommended since the area has low thickness of sediments on the average All these deductions were reached at after due consideration to both qualitative and quantitative interpretations supported by geological information of the area.
International Journal of Geosciences, 2017
In this study, we performed some filters to highlight geological structures and/or features which are found in the igneous rocks, between the latitudes 9˚45'N to 10˚45'N and longitudes 13˚15'E to 14˚30'E. The application of the first vertical derivative (FVD) and the horizontal gradient magnitude (HGM) on a total magnetic data over the study area has led to put in evidence: 1) geological features as geological boundaries, faults, dykes, folds on the FVD map; 2) abundant aeromagnetic lineaments probably fractures, dykes and contacts, exhibit a conjugate relationship suggesting a near NE and NW tectonic trends; 3) existence of a possible prominent near E-W compression, characterized by a possible dextral displacement of geological formations by the shear movements; 4) and the magnetic signature of the country rocks.
Interpretation of high resolution aeromagnetic data over southern Benue Trough, southeastern Nigeria
Journal of Earth System Science, 2016
High resolution airborne magnetic data of parts of the southern Benue Trough were digitally processed and analyzed in order to estimate the depth of magnetic sources and to map the distribution and orientation of subsurface structural features. Enhancement techniques applied include, reduction to pole/equator (RTP/RTE), first and second vertical derivatives, horizontal gradients and analytic signal. Results from these procedures show that at least 40% of the sedimentary basin contain shallow (<200 m) magmatic bodies, which in most cases are intermediate to mafic intrusive and hyperbysal rocks, and may occur as sills, dikes or batholiths. Magnetic lineaments with a predominant NE–SW trend appear to be more densely distributed around the basement rocks of the Oban Hills and metamorphosed rocks around the Workum Hills. 3D standard Euler deconvolution and Source Parameter Imaging (SPI TM) techniques were employed for depth estimation. Results from the two methods show similar depth e...
Magnetotelluric Study in Northeastern Botswana
11th SAGA Biennial Technical Meeting and Exhibition, 2009
The proposed boundaries of geological terranes in northeastern Botswana are mainly based on regional magnetic and gravity data, because there are not many outcrops available due to the thick Phanerozoic cover rocks. The extent of the Zimbabwe craton into Botswana as well as the location of the boundaries to its neighbouring mobile belts (Limpopo Belt, Magondi Mobile Belt and Ghanzi-Chobe Belt) are not very well known. Magnetotelluric (MT) profiles of the Southern African MagnetoTelluric EXperiments (SAMTEX) are present in this area and provide information about lithospheric strike directions and the resistivity distributions as well as possible locations of terrane boundaries, which verify some of the proposed terrane boundaries and suggest modification for others (e.g., Ghanzi-Chobe Belt to Magondi Mobile Belt terrane boundary and the western boundary of the Zimbabwe craton).
Global Journal of Geological Sciences, 2019
Interpretation of high-resolution aeromagnetic data over Nasarawa area was carried out to observe near surface source magnetic features that are associated with geological structures in the study area, Prominent lineaments in this study may be related to fractures that were not revealed by previous studies. Some of these lineaments coincide with concealed faults and fractures at Akiri warm spring and vicinity. Because this area has known warm springs, faults or fault intersections at depth can lead to upward movement of warm water that enhances permeability. Thus, mapping concealed fractures provides a focus for follow-up geothermal studies. Fault interpretation was accomplished by synthesizing interpretative maps derived from first vertical derivative and analytical signal along with preliminary depth estimates. Faults were interpreted along linear aeromagnetic anomalies and breaks in anomaly patterns. Many linear features correspond to topographic features, such as drainages. A few of these are inferred to be fault-related. The interpreted faults show criss-crossing pattern of fault zones, some of which appear to step over where they cross, and show zones of west-northwest, north-, and northwest-striking faults that cross west-northwest around Akiri warm spring. North easterly striking faults extend east from this juncture. The associated aeromagnetic anomalies are likely caused by magnetic contrasts associated with rifting of crust beneath the Benue Trough.