Modeling Of Aeromagnetic Data Research Articles

Interpretation and modeling of aeromagnetic data from helicopter surveys of the Morobe goldfield, Papua New Guinea

AbstractWe interpreted and modelled aeromagnetic data from helicopter surveys of the Morobe goldfield, Papua New Guinea, by comparing gridded magnetic data with mapped surface geology. In some cases, geologic units may be recognized and distinguished based on contrasts in magnetic intensity and texture, including the Morobe Granodiorite, Kaindi Metamorphics, Bulolo Volcanics, and Langimar Beds. Magnetic anomalies occur due to intrusions of Edie Porphyry and related igneous rocks that are associated with gold mineralization. Numerous faults control the size and shape of the intrusions. Magnetic anomalies of interest were identified in gridded data from high‐resolution surveys and modelled using flight line data. Two‐dimensional magnetic models included dipping slabs with thicknesses of 100–400 m. Three‐dimensional models were prepared, including a dipping tabular body and a dipping cylinder. A 2008 survey of the Papuan peninsula sponsored by the European Union (EU) provided continuous coverage of the Morobe goldfield area and helped resolve regional features that were only partially covered by previous smaller surveys. In particular, the EU survey covers the full length of the Sunshine Fault and greatly extends coverage of outcrops of the Morobe Granodiorite and Langimar Beds. The EU survey contains isolated anomalies south‐west of Wau that may correspond to intrusions of Edie Porphyry, Morobe Granodiorite.

Structural features derived from a multi-method approach and 2.75D modelling of aeromagnetic data: a case study of the Pitoa–Figuil area (Northern Cameroon)

Structural features derived from a multi-method approach and 2.75D modelling of aeromagnetic data: a case study of the Pitoa–Figuil area (Northern Cameroon)

Multi-scale analysis and modelling of aeromagnetic data over the Bétaré-Oya area in eastern Cameroon, for structural evidence investigations

Abstract. This study was carried out in the Lom series in Cameroon, at the border with Central African Republic, located between the latitudes 5∘30′–6∘ N and the longitudes 13∘30′–14∘45′ E. A multi-scale analysis of aeromagnetic data combining tilt derivative, Euler deconvolution, upward continuation, and 2.75D modelling was used. The following conclusions were drawn. (1) Several major families of faults were mapped. Their orientations are ENE–WSW, E–W, NW–SE, and N–S with a NE–SW prevalence. The latter are predominantly sub-vertical with NW and SW dips and appear to be prospective for future mining investigations. (2) The evidence of compression, folding, and shearing axis was concluded from superposition of null contours of the tilt derivative and Euler deconvolution. The principal evidence of the local tectonics was due to several deformation episodes (D1, D2, and D4) associated with NE–SW, E–W, and NW–SE events, respectively. (3) Depths of interpreted faults range from 1000 to 3400 m. (4) Several linear structures correlating with known mylonitic veins were identified. These are associated with the Lom faults and represent the contacts between the Lom series and the granito-gneissic rocks; we concluded the intense folding was caused by senestral and dextral NE–SW and NW–SE stumps. (5) We propose a structural model of the top of the crust (schists, gneisses, granites) that delineates principal intrusions (porphyroid granite, garnet gneiss, syenites, micaschists, graphite, and garnet gneiss) responsible for the observed anomalies. The 2.75D modelling revealed many faults with a depth greater than 1200 m and confirmed the observations from reduced-to-Equator total magnetic intensity (RTE-TMI), tilt derivative, and Euler deconvolution. (6) We developed a lithologic profile of the Bétaré-Oya basin.

Subsurface structures and conceptual hydrothermal model of the area lying between Quseir and Safaga area, Red Sea Coast, Egypt

The study area is located in the Quseir–Safaga area of the Egyptian Red Sea Coast. This location considered one of the most promising areas for touristic villages depending on groundwater domestic uses as well as geothermal energy. The geothermal energy is considered one of the promising sources in the studied area. Nevertheless, few attempts have been carried out to evaluate the geothermal setting of the area. The present study aims to throw more light on studying the predominant structures in the area and their relation with geothermal manifestations, as well as defining the hydrothermal system type at the study area. Achieving this goal is dependent on using aeromagnetic data in the form of reduced to northern Pole (RTP) anomalies. These data were subjected to different techniques of processing and interpretation through both qualitative and quantitative analyses. Two-dimensional (2D) modeling of aeromagnetic data has been used to simulate the subsurface structure configuration along some selected profiles trending in NW-SE and E-W directions. In addition, a conceptual model of the hydrothermal system was built based on geophysical results of the aeromagnetic data analysis and processed numerically to obtain a 2D hydrothermal model that contains all simplifications and assumptions made on the conceptual model. The HYDROTHERM Interactive (HTI) program version 3 was used for two-dimensional simulation in the study area to study the temperature and pressure distributions beneath the study area. The results of the study showed that the depth to basement from the ground surface ranges from 20 to 1200 m. The hydrothermal simulation in the area indicated that the origin of thermal water is due to high heat flow and deep groundwater circulation controlled by structures in the subsurface reservoir. Under the thermal water, the water speeds up and flows through the fractures and faults. In general, the high heat flow in the Eastern Desert is associated with shallow basement depths. Thus, the modeled hydrothermal system is considered a dynamic type.

Evidence of Major Structural Features over the Pan-African Domain in the Bertoua-Mbangue Area (East Cameroon) from a Multiscale Approach of Modeling and Interpretation of Aeromagnetic Data

The aim of this study is to investigate crustal structures from East Cameroon, using aeromagnetic data. The modeling of aeromagnetic data is conducted using the Oasis Montaj 8.0 software. The total magnetic intensity map reduced to the equator (RTE-TMI) shows important anomalies features, namely, the Northern East magnetic anomalies of high amplitude, the Southwest where very low values of the magnetic intensity were observed, and a corridor with negative values relatively high, separating the anomalies. The horizontal gradient map shows on the one hand brittle and folded structures carried out in the area of study and on the other hand various rectilinear, narrow, and short-wave anomalies that can be classified as a family of little faults. The maxima observed on the RTE-TMI maps are correlated to intrabasement contacts; and the map derived from Euler’s solutions permitted to evaluate the depth of the geological accidents observed from the other filters. This map also reveals new faults with a depth greater than 5000 m. The lineaments identified in the Southwestern part could be linked to the Pan-African orogeny and seem to correspond to deep-seated basement structures, which are referred to the tectonic boundary between Congo Craton and the Pan-African orogeny belt. A 23/4-D modeling confirmed the observations derived from the RTE-TMI and HGM maps analyses. It shows intrusive bodies composed of gneiss and porphyroid granite and some domes with their roof situated at various depths not exceeding 1800 m from the surface. The structural map of the study area shows the trending of the structural features observed, namely, NE-SW, NW-SE, ENE-WSW, and WNW-ESE, respectively, while the E-W and N-S are secondary orientation of the observed tectonic evidence. Moreover, circular anomalies observed over the area are assimilated to intrusions of high magnetic materials or to granitic domes.

2.5D Modelling of Aeromagnetic Data and their Mining Implications over the Ngaoundere Area (Adamawa Province, Cameroon)

This study is based on the analysis and interpretation of aeromagnetic data using version 8.4 of the Geosoft Oasis Montaj Software, to map the subsurface or deep geological structures that affected the geological formations of the Ngaoundere area. The use of the standard aeromagnetic methods made it possible to draw up the maps of the residual magnetic field reduced to the equator (RTE), the horizontal gradient (HG), the analytical signal (AS) and that of the Euler solutions (ED) to find the main magnetic facies corresponding to these structures. The geological formations of the studied area thus appear to be intensely fractured by a NE-SW (N45°E) and ENE-WSW (N70°E) main orientation fault system, the depth of which has been estimated by combining the three-analytical methods HG, AS and ED. Advanced magmatic map analysis revealed dikes associated with vertical faults in the studied area. The development of an interpretative geological map taking into account the basic geology, the deep faults, the identified dikes and the mineralization index made it possible to extract a correlation between geological structures and mineralization of the studied area. The 2.5D modelling of two magnetic profiles plotted on the reduced residual map at the equator was performed to approximate the geometry and depth of the dikes sector, which are potential sources of mineralization here.

Interpretation of airborne geophysical data of Nsukka area, southeastern Nigeria

Airborne geophysical (aeromagnetic and gravity) data of Nsukka area was interpreted qualitatively and quantitatively with the aim of determining the susceptibilities of rock types, depth/mass of the anomalous bodies, possible cause of the anomalies and type of mineralization prevalent in the area. The estimated depths from forward and inverse modeling of aeromagnetic data for profiles 1, 2, 3, 4 and 5 were 1200m, 1644m, 1972m, 2193m and 2285m respectively. The respective susceptibility values were 0.0031, 0.0073, 1.4493, 0.0069 and 0.0016. These indicate dominance of iron rich minerals like limonite, hematite, pyrrhotite, and pyrite and forms lateritic caps on sandstones. SPI depth result ranges from 151.6m minimum (shallow magnetic bodies) to 3082.7 m maximum (deep lying magnetic bodies). Euler depths for the four different structural index (SI = 0.5, 1, 2, 3) ranges from 7.99 to 128.93m which are depths of shallow magnetic sources resulting from lateritic bodies in the outcrops in the study area. From the gravity data interpretation, Euler depth estimation reveals that depth to anomalous bodies ranges from 89.13 to 2296.92m. Density of the causative body obtained from modeling results for profile 1 was 1498kg/m3, which is in the range of clay material and the depth was about 923m. From models 2, 4 and 5, the densities of the causative bodies were 3523, 4127 and 3707kg/m3, while depths to the surface were about 604, 815 and 1893m respectively. These density ranges correspond to that of ironstone. From model three, the density of causative body obtained was 2508kg/m3, located at a depth of about 268m below the surface. This work has shown that Nsukka area is underlain by thick strata of shales, sandstones and ironstones, which together are suitable for ceramic production, and sufficiently thick sediments suitable for hydrocarbon accumulation.

Delineation of the northern limit of the Congo Craton based on spectral analysis and 2.5D modeling of aeromagnetic data in the Akonolinga-Mbama area, Cameroon

Aeromagnetic data of the Akonolinga-Mbama region are analyzed in order to elucidate the subsurface geology of the area. The available data in the form of a residual aeromagnetic map is interpreted as a vast magnetically quiet region, and complex zones which do not correlate with the surface geology of the region.Within the magnetically quiet zone, a high negative circular elongated anomaly zone seems to represent an intrusion of a plutonic rock into the metamorphic formations of the region. Spectral analysis and two-and-a-half dimensional (2½-D) modeling are used to estimate the depth of the causative bodies and determine the source rocks along three profiles crossing the suspected areas. Models from various zones of granitic intrusions are obtained, thereby proposing some shallow fault lines along zones of contact. This permits us to mark out the northern margin of the Congo Craton, thus enabling us to distinguish the cratonic formations from the Pan African fold belt. Part of the belt has been thrust over the northern portion of the Congo Craton in Cameroon.

Magnetic petrophysical results from the Hamersley Basin and their implications for interpretation of magnetic surveys

An extensive magnetic petrophysical investigation was carried out in the Hamersley Basin in Western Australia; which hosts large high-grade iron-ore deposits derived from banded iron formations (BIFs). Based on our new results, as well as those from previous studies, we demonstrate that the bulk susceptibility of BIF units has a bimodal distribution caused by the presence of chert-rich and magnetite-rich bands. The mean bulk susceptibility (MBS) of magnetite-rich bands is more than 10 times higher than that of chert-rich bands. Measurements of anisotropy of magnetic susceptibility (AMS) in BIF units show this to vary between 1.06 and 2.76. The natural remanent magnetisation (NRM) carried by iron ores varies with different deposits, and even within individual deposits. NRM measurements on non-ore units defined a northwesterly upward regional magnetic overprint (310°/–27°) present in the Fortescue and Hamersley Groups, and also in the lower Wyloo Group. Modelling of aeromagnetic data over the Hamersley Range demonstrates that the effects of AMS and NRM are significant and must be accounted for. The effects of AMS and NRM on magnetic anomalies due to BIFs depend on the volume fraction of magnetite-rich members in a BIF unit. Forward modelling of magnetic and gravity responses, using petrophysical results summarised in this study, over a mined hematite ore deposit in Tom Price indicates that TMI variations are expected to be significantly reduced in areas of mineralisation. However, magnetic data alone are not a reliable indicator of the presence of mineralisation because of other causes of reduced magnetism in BIFs.

The Offshore Palos Verdes Fault Zone near San Pedro, Southern California

High-resolution seismic-reflection data are combined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the San Pedro shelf, near Los Angeles, California. Prominent structures investigated include the Wilmington graben, the Palos Verdes fault zone, various faults below the west part of the San Pedro shelf and slope, and the deep-water San Pedro basin. The structure of the Palos Verdes fault zone changes markedly along strike southeastward across the San Pedro shelf and slope. Under the north part of the shelf, this fault zone includes several strands, with the main strand dipping west. Under the slope, the main fault strands exhibit normal separation and mostly dip east. To the southeast near Lasuen Knoll, the Palos Verdes fault zone locally is low angle, but elsewhere near this knoll, the fault dips steeply. Fresh seafloor scarps near Lasuen Knoll indicate recent fault movement. We explain the observed structural variation along the Palos Verdes fault zone as the result of changes in strike and fault geometry along a master right-lateral strike-slip fault at depth. Complicated movement along this deep fault zone is suggested by the possible wave-cut terraces on Lasuen Knoll, which indicate subaerial exposure during the last sea level lowstand and subsequent subsidence of the knoll. Modeling of aeromagnetic data indicates a large magnetic body under the west part of the San Pedro shelf and upper slope. We interpret this body to be thick basalt of probable Miocene age. This basalt mass appears to have affected the pattern of rock deformation, perhaps because the basalt was more competent during deformation than the sedimentary rocks that encased the basalt. West of the Palos Verdes fault zone, other northwest-striking faults deform the outer shelf and slope. Evidence for recent movement along these faults is equivocal, because we lack age dates on deformed or offset sediment. Manuscript received 26 February 2003.

Modeling of aeromagnetic data from the Precambrian Lake Owens mafic complex, Wyoming

Aeromagnetic anomalies overlying the Pre-cambrian Lake Owens layered mafic complex and adjacent bodies in the Medicine Bow Mountains of southeast Wyoming help constrain their geometric and magnetic properties. Modeling of these anomalies shows that the magnetic part of the Lake Owens Complex corresponds well with its surface outcrop and is only about 1 km thick. The remanent magnetization is approximately as strong as the induced magnetization. The total magnetization of the complex is 5.8 x 10-3 emu/cc. Rocks this strongly magnetized, were they located in the deep crust, could account for long-wavelength magnetic anomalies. The anomaly also shows the presence of a small, strongly magnetic body to the northeast of the Lake Owens Complex. This is probably a small, separate mafic body or a piece of the complex broken and disrupted during tectonic activity and/or during later granite intrusion. The Mullin Creek layered mafic complex, to the west of the Lake Owens Complex, has weaker magnetization dominated by induced magnetization or a viscous remanent overprint in the direction of the present Earth's magnetic field. Its surface outcrop corresponds poorly with boundaries of magnetic units. A magnetic unit between the two mafic complexes may be a quartz diorite or a separate, shallow mafic body.

Physical property measurements as an aid to magnetic interpretation in basement terrains

lnterpretation of magnetic anomalies from the southwest Arunta, and Cloncurry, Precambrian basement complexes has been possible only with information on the geochemistry and magnetic properties of the outcropping rocks. The lithological descriptions provided by regional geological maps are not adequate for the purpose of interpretation, owing to the paucity of Precambrian outcrop and also because the magnetic properties of rocks generally are not related to the factors used to classify rock types. Analyses of rock samples from the southwest Arunta Block indicate that the magnetic properties depend in part on rock composition, but also on the degree of metamorphism and other geological processes which are not always reflected in lithological descriptions. Modelling of aeromagnetic data using the measured magnetic properties as a primary control has yielded a new solid geology interpretation of the region. Magnetic susceptibilities and densities measured on samples from the eastern edge of the Cloncurry Complex have been used to interpret aeromagnetic and gravity data over a part of the complex where geological mapping of the basement is precluded by a thin veneer of younger rocks and alluvium. Both lithology and structure of the buried rocks have been interpreted in this way.