Application of magnetic amplitude inversion in exploration for volcanic units in a basin environment

Geophysics ◽  
2012 ◽  
Vol 77 (5) ◽  
pp. B219-B225 ◽  
Author(s):  
Yaoguo Li ◽  
Zhanxiang He ◽  
Yunxiang Liu
Keyword(s):  
Remanent Magnetization ◽  
Volcanic Rocks ◽  
Wiener Filter ◽  
Inversion Method ◽  
Magnetization Direction ◽  
Amplitude Inversion ◽  
Final Interpretation ◽  
Local Geology ◽  
Magnetic Amplitude

We have performed a case study on the use of magnetic amplitude inversion in imaging volcanic rocks that are buried in a sedimentary basin and have strong remanent magnetization. The application arises in exploration for natural gas hosted in volcanic rocks in basin environments. The weak anomaly associated with the volcanic units and the presence of remanent magnetization therein pose major challenges in the magnetic interpretation. We first use a Wiener filter based on an ensemble analysis to examine the depth characteristics of the anomalies, and then we use an inversion-based signal separation to extract the anomaly for final interpretation. We apply an amplitude inversion method to recover the distribution of effective susceptibility in the absence of the knowledge about the total magnetization direction. The result effectively identifies the volcanic rock units at large depths and the imaged distribution of these units is consistent with information from drillholes and local geology.

Magnetic amplitude inversion for depth-to-basement and apparent magnetization-intensity estimates

Geophysics ◽  
2021 ◽  
Vol 86 (1) ◽  
pp. J1-J11
Author(s):  
Marlon C. Hidalgo-Gato ◽  
Valéria C. F. Barbosa ◽  
Vanderlei C. Oliveira
Keyword(s):  
Field Data ◽  
Regional Scale ◽  
Inversion Method ◽  
Magnetization Direction ◽  
Potential Field Data ◽  
Amplitude Inversion ◽  
Basement Rocks ◽  
Amplitude Data ◽  
Magnetization Intensity ◽  
Magnetic Amplitude

We have developed an inversion method to recover the depth and the total magnetization intensity of the basement under a sedimentary basin using the amplitude of the magnetic anomaly vector (amplitude data). Because the amplitude data are weakly dependent on the magnetization direction, our method is suitable for interpreting areas with remanent magnetization. Our method assumes constant magnetized basement rocks overlain by nonmagnetic sediments. To overcome the inherent ambiguity of potential field data, we assume knowledge of the average depth of the basement and use it as a constraint to regularize the inversion. A sensitivity analysis with synthetic data shows the weak dependency of the magnetic amplitude inversion on the magnetization direction. Different combinations of magnetization directions recover the interface separating sediments from basement rocks. Test on field data over the Foz do Amazonas Basin, Brazil, recovers the shape of the basement relief without any knowledge about the magnetization intensity and direction. The estimated basement relief reveals a smooth basement framework with basement highs in the central part of the area. In a regional-scale perspective, the deeper and constant estimated basement relief at the northernmost limit of the area may suggest changing in crustal domains from a hyperextended continental crust to homogeneous oceanic crust.

Application of 3D magnetic amplitude inversion to iron oxide-copper-gold deposits at low magnetic latitudes: A case study from Carajás Mineral Province, Brazil

Geophysics ◽  
2015 ◽  
Vol 80 (2) ◽  
pp. B13-B22 ◽  
Author(s):  
Marcelo Leão-Santos ◽  
Yaoguo Li ◽  
Roberto Moraes
Keyword(s):  
Iron Oxide ◽  
Gold Deposits ◽  
Magnetic Data ◽  
Magnetic Latitude ◽  
Amplitude Inversion ◽  
Carajás Mineral Province ◽  
Amplitude Data ◽  
Copper Gold ◽  
Magnetic Amplitude

Strong hydrothermal alteration modifies rock physical properties in iron oxide-copper-gold deposits (IOCGs) and may result in characteristic signatures detectable in geophysical surveys. Magnetic data are commonly used in characterizing orebodies, and 3D inversions are often used to assist in interpretations. In areas with strong remanence and self-demagnetization, the total magnetization can have directions different from the inducing field direction. This deviation precludes the use of traditional inversion methods. Magnetic amplitude inversion offers one solution to this challenge because the amplitude data are weakly dependent on the magnetization direction. In addition, the low magnetic latitude also imposes difficulty in amplitude data calculation due to the instability in the component conversion in the wavenumber domain. To formulate a practical approach, we present a case study on applying the magnetic amplitude inversion to the Furnas southeast IOCG deposit at the low magnetic latitude in Carajás Mineral Province, Brazil, and demonstrate that the approach can reliably recover an interpretable distribution of effective magnetic susceptibility and identify massive magnetite from hydrothermal alterations associated with the high-grade ore.

scholarly journals 3D magnetic amplitude inversion in the presence of self-demagnetization and remanent magnetization

Geophysics ◽  
2019 ◽  
Vol 84 (5) ◽  
pp. J69-J82 ◽  
Author(s):  
Boxin Zuo ◽  
Xiangyun Hu ◽  
Yi Cai ◽  
Shuang Liu
Keyword(s):  
Remanent Magnetization ◽  
Large Scale ◽  
Magnetic Data ◽  
Inversion Problem ◽  
Aeromagnetic Data ◽  
Inversion Algorithm ◽  
Rapid Convergence ◽  
Magnetization Direction ◽  
Amplitude Inversion ◽  
Nonlinear Conjugate Gradient

We have developed a general 3D amplitude inversion algorithm for magnetic data in the presence of self-demagnetization and remanent magnetization. The algorithm uses a nonlinear conjugate gradient (NLCG) scheme to invert the amplitude of the magnetic anomaly vector within a partial differential equation framework. Three quantities— the amplitude of the anomalous magnetic field, the analytic signal, and the normalized source strength, defined as the amplitudes of magnetic data that are weakly dependent on the magnetization direction — are inverted to recover the 3D distribution of the subsurface magnetic susceptibility. Numerical experiments indicate that our NLCG amplitude inversion algorithm has a rapid convergence rate that provides a reasonable inversion solution in the absence of knowing the total magnetization direction. High-resolution aeromagnetic data collected from the Pea Ridge iron oxide-apatite-rare earth element deposit, southeast Missouri, USA, are used to illustrate the efficacy of our amplitude inversion algorithm. This algorithm is generally applicable for tackling the large-scale inversion problem in the presence of self-demagnetization and remanent magnetization.

Inversion of magnetic anomaly on rugged observation surface in the presence of strong remanent magnetization

Geophysics ◽  
2014 ◽  
Vol 79 (2) ◽  
pp. J11-J19 ◽  
Author(s):  
Shu-Ling Li ◽  
Yaoguo Li
Keyword(s):  
Remanent Magnetization ◽  
Country Rock ◽  
Magnetization Vector ◽  
Magnetic Data ◽  
Magnetization Direction ◽  
Equivalent Source ◽  
Total Magnetization ◽  
Amplitude Inversion ◽  
Amplitude Data ◽  
And Inversion

We study the inversion of magnetic data acquired over a rugged observation surface and where the buried source bodies have strong remanent magnetization that leads to unknown total magnetization directions. These factors pose significant challenges for processing and inversion of such data. To tackle the challenges from both a rugged observation surface and an unknown magnetization direction, we propose a strategy through the joint use of the equivalent source technique and 3D amplitude inversion to obtain 3D magnetization strength. We use equivalent source processing to calculate the amplitude data in the space domain because the use of the wavenumber-domain method is invalid due to large variations in the data elevation. We then carried out an amplitude inversion to generate a 3D subsurface distribution of the magnitude of the total magnetization vector. The results from a synthetic example and aeromagnetic data in Daye Mine in China showed that this approach is effective and images the magnetic units whose contact zones with the limestone country rock host the mineralization. The method is general and can be applied to a variety of cases with similar challenges.

Constraining magnetic amplitude inversion with magnetotelluric data to image volcanic units: A case study

Geophysics ◽  
2020 ◽  
Vol 85 (3) ◽  
pp. B63-B75
Author(s):  
Kaijun Xu ◽  
Yaoguo Li
Keyword(s):  
Structural Information ◽  
Geophysical Methods ◽  
Magnetic Data ◽  
Structural Constraints ◽  
Structural Constraint ◽  
Magnetotelluric Data ◽  
Amplitude Inversion ◽  
Amplitude Data ◽  
Magnetic Amplitude

We present a case study on imaging volcanic units in gas exploration by constraining magnetic amplitude inversions using magnetotelluric (MT) sounding data at sparse locations. Magnetic data can be effective in mapping volcanic units because they have remanent magnetization and significant susceptibility contrast with surrounding rocks. Although magnetic data can identify the lateral distribution of volcanic units, they often have difficulties in defining the depth extent. For this reason, additional structural constraints from other geophysical methods can often help improve the vertical resolution. Among the independent geophysical methods, MT data can provide the needed structural information at a low cost. We have investigated an approach to combine a set of sparse MT soundings with magnetic amplitude data to image the distribution of volcanics in a basin environment. We first use a blocky 1D MT inversion based on Ekblom norm to obtain the structural constraint, and then we perform a constrained 3D magnetic amplitude inversion to recover the distribution of effective susceptibility by incorporating the structural information from MT soundings. We determine that even a small number of MT stations (e.g., 20) in a [Formula: see text] area is sufficient to drastically improve the magnetic amplitude inversion. Our results indicate that magnetic amplitude inversion with structural constraint from MT soundings form a practical and cost-effective means to map the lateral and vertical distribution of volcanics.

Paleomagnetic and geobarometric study of the Late Cretaceous Mount Lorne stock, southern Yukon Territory

1999 ◽  
Vol 36 (6) ◽  
pp. 905-915 ◽  
Author(s):  
M J Harris ◽  
DTA Symons ◽  
W H Blackburn ◽  
CJR Hart
Keyword(s):  
Late Cretaceous ◽  
Remanent Magnetization ◽  
Volcanic Rocks ◽  
Normal Fault ◽  
Yukon Territory ◽  
Magnetization Direction ◽  
The North ◽  
The Pacific ◽  
North American Craton ◽  
Oceanic Plates

The ~75 Ma Mount Lorne monzodiorite stock has been studied paleomagnetically to estimate the tectonic motion of the northern Canadian Cordillera since the Late Cretaceous. The stock is one of several paleomagnetic studies currently underway at the University of Windsor Paleomagnetic Laboratory which are funded by the Lithoprobe - Slave northern Cordillera Lithospheric Evolution (SNORCLE) Project. Al-in-hornblende geobarometry and plagioclase-amphibole geothermometry data suggest that the stock has not been tilted since emplacement but does provide evidence that the stock has a normal fault trending north-south through its centre with the east side uplifted relatively by ~1000 m. Paleomagnetic measurements from 12 of 19 granitoid sites yield a well-defined characteristic remanent magnetization direction that is south-southwest and up, and a further six sites yield a direction that is north-northeast and down. Data from a mafic dike yield a negative contact test, suggesting that the dike is coeval with the stock. Combining the 18 granitic site mean characteristic remanent magnetization directions yields a paleopole at 69.1°W, 78.3°N (dp = 4.1°, dm = 4.5°) which suggests that the host Stikine Terrane has been translated poleward by 10.5 ± 3.5° (1170 ± 390 km) and rotated clockwise by 57 ± 11° relative to the North American craton between 75 and 50 Ma. Except for the estimate from the nearby coeval Carmacks Group volcanic rocks, the Mount Lorne estimate is consistent with all other paleomagnetic results within the Stikine Terrane. These latter estimates are also consistent with plate-tectonic models that suggest the Pacific oceanic plates had an increased velocity northwards during the Late Cretaceous.

Constrained 3D inversion of magnetic data with structural orientation and borehole lithology: A case study in the Macheng iron deposit, Hebei, China

Geophysics ◽  
2019 ◽  
Vol 84 (2) ◽  
pp. B121-B133 ◽  
Author(s):  
Shida Sun ◽  
Chao Chen ◽  
Yiming Liu
Keyword(s):  
Remanent Magnetization ◽  
Magnetic Data ◽  
Iron Deposit ◽  
Reference Field ◽  
Total Field ◽  
Equivalent Source ◽  
Structural Orientation ◽  
Amplitude Data ◽  
Ore Bodies

We have developed a case study on the use of constrained inversion of magnetic data for recovering ore bodies quantitatively in the Macheng iron deposit, China. The inversion is constrained by the structural orientation and the borehole lithology in the presence of high magnetic susceptibility and strong remanent magnetization. Either the self-demagnetization effect caused by high susceptibility or strong remanent magnetization would lead to an unknown total magnetization direction. Here, we chose inversion of amplitude data that indicate low sensitivity to the direction of magnetization of the sources when constructing the underground model of effective susceptibility. To reduce the errors that arise when treating the total-field anomaly as the projection of an anomalous field vector in the direction of the geomagnetic reference field, we develop an equivalent source technique to calculate the amplitude data from the total-field anomaly. This equivalent source technique is based on the acquisition of the total-field anomaly, which uses the total-field intensity minus the magnitude of the reference field. We first design a synthetic model from a simplified real case to test the new approach, involving the amplitude data calculation and the constrained amplitude inversion. Then, we apply this approach to the real data. The results indicate that the structural orientation and borehole susceptibility bounds are compatible with each other and are able to improve the quality of the recovered model to obtain the distribution of ore bodies quantitatively and effectively.

scholarly journals Fractures in Volcanic Reservoir: a case study of Zhongguai uplift in Northwestern Margin of Junggar Basin, China

2018 ◽  
Vol 22 (3) ◽  
pp. 169-174 ◽  
Author(s):  
Cunhui Fan ◽  
Qirong Qin ◽  
Feng Liang ◽  
Zenghui Fan ◽  
Zhi Li
Keyword(s):  
Oil And Gas ◽  
Volcanic Rocks ◽  
Junggar Basin ◽  
Pressure Change ◽  
Reservoir Permeability ◽  
Northwestern Margin ◽  
Genetic Mechanisms ◽  
Relationship Of ◽  
Oilfield Development

Fractures in Carboniferous volcanic rocks located at Zhongguai Area (China) highly influence the accumulation and productivity of oil and gas. As such, the study of development periods and genetic mechanisms of tectonic fractures could throw useful information regarding the evaluation and development of that reservoir. Their tectonic origins caused high-angle and oblique shear fractures. The primary orientation of those fractures appears close to EW (270°±10°), NW (300°±15°), NE (45°±15°), and SN (0°±10°). At least four fracture generations can be found in Carboniferous volcanic rocks at Zhongguai Area. Combined with a tectonic evolution, they are based on the segmentation relationship of the fracture fillings, the thermometry measurement of the fracture filling inclusion, and the acoustic emission, as well. Affected by a new horizontal principal stress, the opening and permeability of nearly EW fractures are the best. In this way, a priority in the development of well's patterns should be considered close to EW fractures. The pressure change in the process of exploitation may damage the reservoir permeability of fractured volcano rocks severely. Accordingly, well patterns should be adjusted to dynamic changes of permeability happened during the oilfield development since some differences have been detected in distinct fracture sets. 

Infidélité paléomagnétique observée dans une formation lacustre varvée quaternaire dite: "argiles d'Eybens" (Grenoble, France)

1982 ◽  
Vol 19 (6) ◽  
pp. 1196-1217 ◽  
Author(s):  
Daniel Biquand
Keyword(s):  
Remanent Magnetization ◽  
Axial Direction ◽  
Field Studies ◽  
Particle Orientation ◽  
Easy Magnetization ◽  
Magnetization Direction ◽  
Continuous Field ◽  
Very High ◽  
Different Levels ◽  
Thermal Remanent Magnetization

We have investigated the properties of natural magnetization of a lacustrine interglacial Riss–Würm deposit near Grenoble (France). Made up of accumulated argillite 250 m thick, this deposit comprises annual varves with an average thickness of 0.5 cm.The magnetic viscosity of this sediment is not very high, and we have demonstrated that the characteristic magnetization, measured after a moderate thermal treatment (between 160 and 220 °C) followed by alternating field demagnetization with maximum intensity at 200 Oe (15.9 × 103 A/m), is possibly a detrital magnetization carried by the magnetite (thermomagnetic and strong continuous field studies).On a local scale (area of about 100 m2), the direction of this magnetization remains very homogeneous within each horizon; sampling restricted to about 10 sites 1 m apart indicates for each level a mean direction of magnetization with a high accuracy: α95 = 1–2°, k = 500–2500. The aveage direction calculated by this method for different levels indicates significant stratigraphic variations (attaining an inclination of 20° and a declination of 50°), which are reached rapidly (from 1 to few degrees per year). Such variations cannot be reasonably attributed to changes in the directions of the Earth's magnetic field.A study of the acquired anisotropy of the thermal remanent magnetization indicates a magnetic anisotropy that is related to the lithological structure of the sediment and shows a strong intensity: the maximum divergence between the field direction and the acquired thermal remanent magnetization direction in this field varies, with individual samples, between 9 and 50°. The study of the properties of this anistotropy demonstrates the "infidelity" of the detrital magnetization: for one of the small vertical sequences studied for anisotropy the characteristic magnetization of a specified level has a direction nearer the axial direction of easy magnetization when the anisotropic intensity is large. This infidelity may be caused by different factors influencing the deposition of the sediment, particularly the variable directions of water flows determining the particle orientation. [Journal Translation]

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