High‐sensitivity aeromagnetic survey of the U.S. Atlantic continental margin (original) (raw)
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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.
Crustal geologic studies with Magsat and surface magnetic data
Reviews of Geophysics, 1987
to make a basement magnetization map for the San surface. Juan Basin, New Mexico, in a detail which would Grauch and Campbell (1984) showed that draping not have been possible with the original data as an aeromagnetic survey does not necessarily reduce flown. Blakely and Simpson (1986) developed an terrain effects, and in fact exaggerates them automated procedure for delineating magnetization relative to a high-level survey. Nevertheless, (or density) boundaries for machine application of they argue that a draped survey is still desirable the Cordell-Grauch method; this is simply an to keep the spectral content of the data uniform efficient way of finding the loci of maxima in the and to avoid overly attenuating short-wavelength horizontal gradient for a grid of data. Hildenbrand (1985) applied the drape-over-basement technique to the whole of the digital data set (Hildenbrand et al., 1983) for the U.S. midcontinent; the resulting map reveals much more apparent detail under the basin areas, as expected. Filtered, shaded-relief, and magnetization-density ratio maps were also produced via digital processing. In addition to Copyright 1987 by the American Geophysical Union. Paper number 7R0355. 8755-1209/87 /007R-0355 $15.00 anomalies in the data noise over valleys; such data can always be continued to a level surface if desired. Similar arguments were made by Cordell and Grauch (1985). Also see comments on this paper by Reford (1984) and Hansen (1984). Blakely and Grauch (1983) dealt with the effect of terrain by using Parker's Fourier transform method of calculating the magnetic anomaly due to uniformly magnetized terrain. The method is applied to registered 2-D elevation and magnetic data grids. Observed and computed fields can be compared visually and numerically by computing correlation coefficients, and anomalies due to topography can easily be distinguished from those due to buried 971 972 Mayhew and LaBrecque: Magsat and Surface Magnetic Data sources. The method was applied to the Oregon Cascades. On a much smaller scale, terrain anomalies due to arroyos cut into suprisingly magnetic (ca. 1 A/m) alluvial fan sediments were modeled by Mahrer et al. (1984). Keller et al. (1985) applied high and low pass, reduction to the pole, upward continuation, map presentation. This is in fact the purpose of all the techniques for manipulation of digital data sets reviewed above. They are not ends in themselves, but first steps in the process of model construction and hypothesis testing. Several other technique-development papers published during this quadrennium are worthy of strike, and vertical derivative filters to gridded note. Silva and Hohmann (1983) described an aeromagnetic and gravity data for west Texas. A optimization technique for magnetic modeling which major crustal transition associated with the involves a random sampling of an objective Ouachita system is revealed by the data. Yarger function hypersurface within a specified (1985) applied similar filters to the aeromagnetic "feasible" region of the parameter space; the mean data set for Kansas, and delineated Precambrian of the sample is a better estimate than any age boundaries, younger plutons, the southern individual estimate w_hen data noise is present. extension of the Central North American rift Ku and Sharp (1983) discuss the Werner
Aeromagnetic expression of faults that offset basin fill, Albuquerque basin, New Mexico
GEOPHYSICS, 2001
High-resolution aeromagnetic data acquired over the Albuquerque basin show widespread expression of faults that offset basin fill and demonstrate that the aeromagnetic method can be an important hydrogeologic and surficial mapping tool in sediment-filled basins. Aeromagnetic expression of faults is recognized by the common correspondence of linear anomalies to surficial evidence of faulting across the area. In map view, linear anomalies show patterns typical of extensional faulting, such as anastomosing and en echelon segments. Depths to the tops of faulted magnetic layers showing the most prominent aeromagnetic expression range from 0 to 100 m. Sources related to subtler fault expressions range in depths from 200 to 500 m. We estimate that sources of the magnetic expressions of the near-surface faults likely reside within the upper 500-600 m of the subsurface.
Geophysical Journal International, 2009
Marine magnetotelluric (MT) and marine controlled-source electromagnetic (CSEM) soundings can be used to study sedimentary structure offshore. In an example of this application, we collected MT and CSEM data in the 1-km deep water of the San Diego Trough, California. The Trough is a pull-apart basin and part of the complex Pacific/North American tectonic plate boundary, and is flanked by the Thirtymile Bank to the west and the Coronado Bank to the east. Our MT data are highly distorted by seafloor topography and the coast effect, which is largely 2-D and can be modelled using 2-D finite element codes. The distortion includes a strong (several orders of magnitude) static depression of TM mode resistivities (electric field perpendicular to structure), upward cusps in the TE mode resistivities (electric field parallel to structure) and negative TE mode phases. The depressed TM mode resistivity is a well-known consequence of galvanic interruption of coastperpendicular electric fields. The TE mode distortion is an inductive effect associated with currents flowing along the edge of the deep ocean basins, steepening the magnetic field and even causing a phase reversal in the horizontal field used for MT impedance calculations (and thus generating negative phases). The land-side enhanced vertical magnetic field is well known as the geomagnetic coast effect, but the ocean-side consequences have been less well documented. Although the MT data are dominated by coast effect and topographic distortion, inclusion of accurate bathymetry in the inversion model's finite element mesh allows the subseafloor geological structure to be recovered. This shows the Trough sediments to be about 3 km thick, bounded to the west by resistive basement, but to the east by conductive clastic sediments forming Coronado Bank. Amplitudes and phases of five frequencies of CSEM data (from 0.1 to 1.0 Hz) collected along the axis of the Trough are well fit with a simple, 1-D layered model, indicating that sediment resistivities increase with depth from 1.5 to 2.3-m and are no more than 3300 m thick, thinning to the north, in good agreement with the MT model. An existing density model generated by fitting surface and deep-towed gravity is in good agreement with the EM interpretations. In particular, combining sediment densities and CSEM resistivities allows us to estimate pore water conductivity and temperature, which follows a geothermal gradient of 25.4 ± 8 K km −1 .
Non-seismic methods: RAS, SPI and AS Magnetic Basement Depth Estimation
2018
The development of sedimentary basins is essentially controlled by responses in the crust to tectonically driven forces. The way in which the crust responds to those forces is controlled by the strength, composition and fabric of the crust at the time of the tectonic event. The properties of the basement can be characterized by magnetic and gravity data, as well as with seismic and well log data. The upper crust is considered as the major source of the magnetic anomalies and has been divided into a number of units characterized by constant densities and magnetization, which show a good correlation with the main structural elements of the Eastern Venezuela's Basins. In E&P prospecting, aeromagnetic information give us evidence from which we can determine depths to basement rocks and thus locate and define the extent of sedimentary basins. This work seeks to contribute to the study of Espino Graben (associated with Atlantic Ocean opening) through magnetic basement maps with gravity constraints. A combination of Source Parameter Images (SPI) and Analytical Signal (AS) were employed to evaluate the depth to source magnetic rocks. This research shows the critical role that non-seismic methods play for Interpreters, who can use themas tools to appreciate the tectonic features of the basins on geodynamic-scale. The seismic geometric attributes such as gradient and curvature, show the edges of the main geological / geophysical domains, and the depth of such domains were derived from SPI and AS approaches. Besides seismic data, a number of magnetic surveys compilations were used to gain insights into the deep crustal geometries. Furthermore, gravity anomalies were used to focus on the Espino graben crustal structure in a consistent way.
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.
NRIAG Journal of Geophysics, 195-209., 2008
Depth to subsurface sources stills playing a significant role in the interpretation of magnetic data, particularly in the drilling strategies of shallow and deep features. It is still no single method giving a unique solution for estimating the depth parameter because of the inherent ambiguity due to different subsurface sources. In this paper, we study the variation in depth estimation due to a single known source from two different measured components of the geomagnetic field. A shallow horizontal cylindrical pipe of known depth was imaged by measuring the total magnetic field and the vertical magnetic gradient in order to correlate the measured depth parameter with the computed one using Euler deconvolution, power spectrum and 2½ -D modeling methods. Euler deconvolution is constraint by the structural index of the source body, while the power spectrum is constraint by the spectral window of the FFT and the fitting method, and the 2½ -D modeling is constraint by the magnetic susceptibility of the subsurface layers. Two perpendicular profiles were demonstrated from each data set to show the variation in the depth parameter along the pipe.