Sedimentary magnetic anomalies Part 1 .. TLE-Oct 2018-wLinks.pdf (original) (raw)
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M e t e r r e a d e r Abstract Possible sources of sedimentary magnetic anomalies include: • Detrital magnetite deposited in sediments that have a nearby source in igneous terrain; magnetization is induced and sometimes detrital or depositional remanent magnetism. • Diagenetic magnetic minerals (such as magnetite, pyrrhotite, greigite, and maghemite) that may be related to chemical changes of the sediments from microseepage of hydrocarbon reservoirs; magnetization is induced and sometimes chemical remanent magnetism. • Diamagnetism of salt and anhydrite, responsible for negative anomalies over salt structures. • Fault mineralization due to upward-migrating hydrothermal fluids along fractures and fault planes, which may or may not be related to hydrocarbons. • Combustion metamorphism (CM) of carbonaceous pyrite-rich and organic-rich sediments, producing strong magnetization as a result of exothermic oxidation of pyrite to magnetite, as well as thermal remanence acquisition. The organic material in the rocks is consumed as fuel, but host rock remains after combustion. The phenomenon can be related to source rock outcrops or seepage. • Clinkers (burned coal seams and lignite beds) is a special case of CM; however, host rock is consumed after combustion. Clinkers are diagenetic sources of strong magnetic anomalies over sedimentary rocks.
Magnetic Logging and In-Situ Magnetostratigraphy: A Field Test
Proceedings of the Ocean Drilling Program, 1994
During Ocean Drilling Program Leg 134 (Vanuatu), geological high sensitivity magnetic tools (GHMT) developed by CEA-LETI and TOTAL were used at two drill sites. GHMT combine two sensors, a proton magnetometer for total magnetic field measurements with an operational accuracy of 0.1 nanoteslas (nT), and a highly sensitive induction tool to measure the magnetic susceptibility with an operational accuracy of a few I0" 6 S1 units. Hole 829 A was drilled through an accretionary prism and the downhole measurements of susceptibility correlate well with other well-log physical properties. Sharp susceptibility contrasts between chalk and volcanic silt sediment provide complementary data that help define the lithostratigraphic units. At Hole 83 IB magnetic susceptibility and total field measurements were performed through a 700-m reef carbonate sequence of a guyot deposited on top of an andesitic volcano. The downhole magnetic susceptibility is very low and the amplitude of peak-to-peak anomalies is less than a few 10~5 S1 units. Based on the repeatability of the measurements, the accuracy of the magnetic logging measurements was demonstrated to be excellent. Total magnetic field data at Hole 83IB reveal low magnetic anomalies of 0.5 to 5 nT and the measurement of a complete repeat section indicates an accuracy of 0.1 to 0.2 nT. Due to the inclination of the earth's magnetic field in this area (~-40°) and the very low magnetic susceptibility of the carbonate, the contribution of the induced magnetization to the total field measured in the hole is negligible. Unfortunately, because the core recovery was extremely poor (<5%) no detailed comparison between the core measurements and the downhole magnetic data could be made. Most samples have a diamagnetic susceptibility and very low intensity of remanent magnetization (< I0" 4 A/m), but a few samples have a stable remanent magnetization up to 0.005 A/m. These variations of the intensity of the remanent magnetization suggest a very heterogeneous distribution of the magnetization in the carbonate sequence that could explain the magnetic field anomalies measured in these weakly magnetized rocks.
Estimation of Magnetic Contact Location and Depth of Magnetic Sources in a Sedimentary Formation
Materials and Geoenvironment, 2019
The aeromagnetic data of Idogo, Southwestern Nigeria, have been used to study the lithology and to determine the magnetic source parameters within Idogo and its environs. Idogo lies between latitudes 6°30′N and 7°00′N and between longitudes 2°30′E and 3°00′E. The magnetic anomaly map, the regional geology, the analytic signal and the local wavenumber were used to identify the nature and depth of the magnetic sources in the region. Data enhancement was carried out to delineate the residual features relative to the strong regional gradients and intense anomalies due to the basin features. The estimated basement depth using the horizontal gradient method revealed depths ranging between 0.55 km and 2.49 km, while the analytic signal amplitude and local wavenumber methods estimated depth to the magnetic sources to range from 0.57 km to 4.22 km and 0.96 km to 2.43 km, respectively. Depth computations suggested the presence of both shallow and deep sources. The total magnetic intensity val...
Spatial-domain filters for short wavelength sedimentary magnetic anomalies
The mapping of short-wavelength intra-sedimentary magnetic anomalies generally requires a band-pass filtering technique to isolate and enhance the wavelength band of interest. Triangular filters applied as wavelength convolution operators in the space domain are more efficient and stable as compared to frequency-domain filters. The filter is designed to remove the regional and long-wavelength basement anomalies and to attenuate the ultrahigh-frequency random noise. High resolution maximum entropy spectral analysis (MESA) technique can be useful and effective in the design of these filters, testing filter performance on the input data, and estimating average depth to the sources. Examples of the applications of both techniques to TMI profile data from the NPRA -North Slope and Gulf of Mexico basins are presented over areas with known hydrocarbon production or seeps, salt and shale structures, sedimentary faults, and sand channels.
Magnetic characterization of Cretaceous-Tertiary boundary sediments
Meteoritics & Planetary Science, 2007
Rock magnetic properties across several K-T boundary sections have been investigated to reveal any possible magnetic signature associated with the remains of the impact event at the end of the Cretaceous. Studied sections' locations vary in distance to the Chicxulub structure from distal (Agost and Caravaca, Spain), through closer (ODP Hole 1049A, Blake Nose, North Atlantic), to proximal (El Mimbral and La Lajilla, Mexico). A clear magnetic signature is associated with the fireball layer in the most distal sections, consisting of a sharp increase in susceptibility and saturation isothermal remanent magnetization (SIRM), and a decrease in remanence coercivity. Magnetic properties in these sections point to a distinctive ferrimagnetic phase, probably corresponding to the reported Mg-and Ni-rich, highly oxidized spinels of meteoritic origin. At closer and proximal sections magnetic properties are different. Although there is an increase in susceptibility and SIRM associated with a rusty layer placed on top of the siliciclastic deposit in proximal sections, and with a similar limonitic layer on top of the spherule bed that defines the boundary at Blake Nose, the magnetic properties indicate a mixture of iron oxyhydroxides dominated by fine-grained goethite. Based on previous geochemical studies at Blake Nose and new geochemical and PGE abundance measurements performed in this work at El Mimbral, this goethite-rich layer can be interpreted as an effect of diagenetic remobilization and precipitation of Fe. There is not enough evidence to assert that this Fe concentration layer at proximal sections is directly related to deposition of fine meteoritic material. Magnetic, geochemical, and iridium data reject it as a primary meteoritic phase.
Sedimentary basins reconnaissance using the magnetic Tilt-Depth method
Exploration Geophysics, 2010
We compute the depth to the top of magnetic basement using the Tilt-Depth method from the best available magnetic anomaly grids covering the continental USA and Australia. For the USA, the Tilt-Depth estimates were compared with sediment thicknesses based on drilling data and show a correlation of 0.86 between the datasets. If random data were used then the correlation value goes to virtually zero. There is little to no lateral offset of the depth of basinal features although there is a tendency for the Tilt-Depth results to be slightly shallower than the drill depths. We also applied the Tilt-Depth method to a local-scale, relatively high-resolution aeromagnetic survey over the Olympic Peninsula of Washington State. The Tilt-Depth method successfully identified a variety of important tectonic elements known from geological mapping. Of particular interest, the Tilt-Depth method illuminated deep (3km) contacts within the non-magnetic sedimentary core of the Olympic Mountains, where magnetic anomalies are subdued and low in amplitude. For Australia, the Tilt-Depth estimates also give a good correlation with known areas of shallow basement and sedimentary basins. Our estimates of basement depth are not restricted to regional analysis but work equally well at the micro scale (basin scale) with depth estimates agreeing well with drill hole and seismic data. We focus on the eastern Officer Basin as an example of basin scale studies and find a good level of agreement between previously-derived basin models. However, our study potentially reveals depocentres not previously mapped due to the sparse distribution of well data. This example thus shows the potential additional advantage of the method in geological interpretation. The success of this study suggests that the Tilt-Depth method is useful in estimating the depth to crystalline basement when appropriate quality aeromagnetic anomaly data are used (i.e. line spacing on the order of or less than the expected depth to basement). The method is especially valuable as a reconnaissance tool in regions where drillhole or seismic information are either scarce, lacking, or ambiguous.
Reviews of Geophysics, 1987
INTRODUCTION As was noted by Graham [1954], anisotropy of magnetic susceptibility (AMS) data has many applications to the study of geological processes. This type of anisotropy expresses directional variation in the magnetization induced in a rock, most notably in its iron oxides and especially in its magnetite and hematite. The AMS is commonly expressed by an ellipsoid, of which the principal axes are K., K•, and K , from greatest to least. 3 This ellip•oidZis generally interpreted in terms of the distribution and shape of grains of magnetic minerals in a rock although the ellipsoid determined depends also on magnetocrystalline anisotropy and on factors of instrumental technique [Ellwood et al., in press, 1987]. The challenge for the geoscientist in analyzing AMS data is first to identify the origin of the AMS geometry, then second to relate that geometry to the natural processes which produced the inferred or measured distribution of magnetic materials. This report for the International Union of Geodesy and Geophysics (IUGG) reviews recent developments in AMS science and applications to mid-1986, emphasizing work by U.S. workers and laboratories. The number of U.S. workers and INSTRuMENTATION AND TECHNIQUES Copyright 1987 by the American Geophysical Union. Paper number 7R0280. 8755-1209/87/007R-0280515. O0 Great variety exists among the instrumentation and techniques used for the analysis of anisotropy of magnetic susceptibility (Pearson, 1979;
Frontiers in Earth Science, 2020
Rifting of continental lithosphere leading to oceanic basins is a complex process conditioned by different factors such as the rheology and thermal structure of the underlying lithosphere, as well as underlying asthenospheric dynamics. All these processes, which finally lead to oceanic domains, can better be recognized in small oceanic basins. Powell Basin is a small oceanic basin bounded to the north by the South Scotia Ridge, to the east by the South Orkney Microcontinent, and to the west by the Antarctic Peninsula. It was formed between the Oligocene and Miocene, however, its age is not well defined, among other reasons due to the small amplitude of its spreading magnetic anomalies. This basin is an ideal framework to analyze the different rifting and spreading phases, which leads from continental crust to the formation of an oceanic domain through different extensional regimes. To identify the different boundaries during the formation of Powell Basin from the beginning of the rifting until the end of the spreading, we use different data sources: magnetic, gravity, multichannel seismic profiles and bathymetry data. We use seismic and bathymetry data to estimate the Total Tectonic Subsidence. Total Tectonic Subsidence has proven to be useful to delineate the different tectonic regimes present from early rifting to the formation of oceanic seafloor. This result together with magnetic data has been used to delimit the oceanic domain and compare with previous authors' proposals. This method could be applied in any other basin or margin to help delimiting its boundaries. Finally, we analyze the role that an asthenospheric branch intruding from the Scotia Sea played in the evolution of the magnetic anomaly signature on an oceanic basin.