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Christopher J Potter - One of the best experts on this subject based on the ideXlab platform.
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interpreting the earthquake source of the wabash valley seismic zone illinois indiana and kentucky from seismic reflection gravity and Magnetic Intensity data
Seismological Research Letters, 2002Co-Authors: John H Mcbride, Thomas G Hildenbrand, William J Stephenson, Christopher J PotterAbstract:Reprocessing of seismic-reflection data reveals new images of upper- to middle-crustal structures beneath the Wabash Valley seismic zone, located north of the New Madrid seismic zone within the seismically active southern Illinois basin. Four intersecting deep seismic profiles (243 km total) indicate an anomalous, 5–10-km-wide zone of dipping reflections and diffractions below the western flank of the Wabash Valley fault system (WVFS). The zone corresponds in places to gently arched regions of Paleozoic strata. The reflector zone can be interpreted as a result of either (or a combination of) magmatic intrusion or structural deformation. The area encompassing the reflection profiles has experienced several moderate magnitude (3.0 ≤ mbLg ≤ 5.5) earthquakes during the past 50 years, defining the central part of the Wabash Valley seismic zone. The hypocenter of the largest 20th-century earthquake in the central USA midcontinent (9 November 1968, mbLg 5.5) corresponds to the most prominent zone of dipping middle-crustal reflections, just west of the WVFS. Both the focal mechanism (moderately dipping reverse fault) and the expected rupture zone size (∼2.9 km fault length) of this earthquake are consistent with the orientation and size of observed reflectors. Dipping reflector patterns in the Precambrian crust are not collinear with fault surfaces updip in the Paleozoic sedimentary section. This indicates that shallow Paleozoic structures are effectively “decoupled” from deeper, possibly seismogenic structure, which suggests that understanding Paleozoic structure is not the key to understanding the earthquake source. The complex dipping crustal reflectivity beneath the WVFS is typical of Paleozoic continental convergent zones observed elsewhere ( e.g. , Appalachian orogen) and thus may suggest a preserved Proterozoic suture, possibly associated with the distal Grenville orogeny or an older event. Although Magnetic Intensity, Bouguer gravity, and seismic-reflection data present different means of understanding the deep geology of the area, their integration aids in limiting the number of admissible interpretations. The reflection profiles indicate a variable zone of anomalous crustal structure, including the dipping reflector zone, along a trend of northeast-trending gravity and Magnetic highs locally defining the Commerce geophysical lineament (CGL), which is a suspected source of seismic hazard in the central USA midcontinent. Three-dimensional inverse modeling of the residual isostatic gravity anomaly values indicates that the upper part of the dipping reflector zone beneath the CGL lies near an important density boundary in the upper Precambrian crust. The results of our study suggest that the seismogenic source just north of the New Madrid seismic zone consists, in part, of a pre-existing fabric of blind thrusts localized along pre-existing igneous intrusions, locally coincident with the CGL. This suggests that the CGL may be seismogenic in places and thus a potential seismic hazard. The variation in the expression of the CGL using reflection and potential-field data sets is probably partly related to the differing geologic features by which it is expressed, but would be also consistent with its reactivation numerous times under varying stress regimes.
John H Mcbride - One of the best experts on this subject based on the ideXlab platform.
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interpreting the earthquake source of the wabash valley seismic zone illinois indiana and kentucky from seismic reflection gravity and Magnetic Intensity data
Seismological Research Letters, 2002Co-Authors: John H Mcbride, Thomas G Hildenbrand, William J Stephenson, Christopher J PotterAbstract:Reprocessing of seismic-reflection data reveals new images of upper- to middle-crustal structures beneath the Wabash Valley seismic zone, located north of the New Madrid seismic zone within the seismically active southern Illinois basin. Four intersecting deep seismic profiles (243 km total) indicate an anomalous, 5–10-km-wide zone of dipping reflections and diffractions below the western flank of the Wabash Valley fault system (WVFS). The zone corresponds in places to gently arched regions of Paleozoic strata. The reflector zone can be interpreted as a result of either (or a combination of) magmatic intrusion or structural deformation. The area encompassing the reflection profiles has experienced several moderate magnitude (3.0 ≤ mbLg ≤ 5.5) earthquakes during the past 50 years, defining the central part of the Wabash Valley seismic zone. The hypocenter of the largest 20th-century earthquake in the central USA midcontinent (9 November 1968, mbLg 5.5) corresponds to the most prominent zone of dipping middle-crustal reflections, just west of the WVFS. Both the focal mechanism (moderately dipping reverse fault) and the expected rupture zone size (∼2.9 km fault length) of this earthquake are consistent with the orientation and size of observed reflectors. Dipping reflector patterns in the Precambrian crust are not collinear with fault surfaces updip in the Paleozoic sedimentary section. This indicates that shallow Paleozoic structures are effectively “decoupled” from deeper, possibly seismogenic structure, which suggests that understanding Paleozoic structure is not the key to understanding the earthquake source. The complex dipping crustal reflectivity beneath the WVFS is typical of Paleozoic continental convergent zones observed elsewhere ( e.g. , Appalachian orogen) and thus may suggest a preserved Proterozoic suture, possibly associated with the distal Grenville orogeny or an older event. Although Magnetic Intensity, Bouguer gravity, and seismic-reflection data present different means of understanding the deep geology of the area, their integration aids in limiting the number of admissible interpretations. The reflection profiles indicate a variable zone of anomalous crustal structure, including the dipping reflector zone, along a trend of northeast-trending gravity and Magnetic highs locally defining the Commerce geophysical lineament (CGL), which is a suspected source of seismic hazard in the central USA midcontinent. Three-dimensional inverse modeling of the residual isostatic gravity anomaly values indicates that the upper part of the dipping reflector zone beneath the CGL lies near an important density boundary in the upper Precambrian crust. The results of our study suggest that the seismogenic source just north of the New Madrid seismic zone consists, in part, of a pre-existing fabric of blind thrusts localized along pre-existing igneous intrusions, locally coincident with the CGL. This suggests that the CGL may be seismogenic in places and thus a potential seismic hazard. The variation in the expression of the CGL using reflection and potential-field data sets is probably partly related to the differing geologic features by which it is expressed, but would be also consistent with its reactivation numerous times under varying stress regimes.
Michael S. Zhdanov - One of the best experts on this subject based on the ideXlab platform.
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Recovering Magnetization of Rock Formations by Jointly Inverting Airborne Gravity Gradiometry and Total Magnetic Intensity Data
Minerals, 2021Co-Authors: Michael Jorgensen, Michael S. ZhdanovAbstract:Conventional 3D Magnetic inversion methods are based on the assumption that there is no remanent magnetization, and the inversion is run for Magnetic susceptibility only. This approach is well-suited to targeting mineralization; however, it ignores the situation where the direction of magnetization of the rock formations is different from the direction of the induced Magnetic field. We present a novel method of recovering a spatial distribution of magnetization vector within the rock formation based on joint inversion of airborne gravity gradiometry (AGG) and total Magnetic Intensity (TMI) data for a shared earth model. Increasing the number of inversion parameters (the scalar components of magnetization vector) results in a higher degree of non-uniqueness of the inverse problem. This increase of non-uniqueness rate can be remedied by joint inversion based on (1) Gramian constraints or (2) joint focusing stabilizers. The Gramian constraints enforce shared earth structure through a correlation of the model gradients. The joint focusing stabilizers also enforce the structural similarity and are implemented using minimum support or minimum gradient support approaches. Both novel approaches are applied to the interpretation of the airborne data collected over the Thunderbird V-Ti-Fe deposit in Ontario, Canada. By combining the complementary AGG and TMI data, we generate jointly inverted shared earth models that provide a congruent image of the rock formations hosting the mineral deposit.
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Migration of 3-D Gravity, Gravity Tensor, and Total Magnetic Intensity Data
Inverse Theory and Applications in Geophysics, 2015Co-Authors: Michael S. ZhdanovAbstract:In this chapter, we discuss a novel approach to interpretation of 3-D gravity, gravity tensor, and total Magnetic Intensity data based on the principles of potential field migration. Migration can be mathematically described as the action of an adjoint operator on observed data. As applied to potential fields, such as gravity and Magnetic fields, migration manifests itself as a special form of downward continuation of the potential field and/or its gradients. This downward continuation is applied to the auxiliary field obtained by moving the sources of the true observed field into the upper half-space as the mirror images of the true sources. This transformation results in extrapolation of the field downward and, contrary to conventional downward continuation, away from the mirror images of the sources. We will show in this chapter that migration is a stable transformation similar to conventional upward continuation. At the same time, the migration field contains remnant information about the original source distribution, which is why it can be used for subsurface imaging. We will consider the basic principles of migration transformation and its application to imaging the sources of the potential field.
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3D migration for rapid imaging of total-Magnetic-Intensity data
GEOPHYSICS, 2012Co-Authors: Michael S. Zhdanov, Xiaojun Liu, Glenn A. Wilson, Le WanAbstract:Three-dimensional potential field migration for rapid imaging of entire total-Magnetic-Intensity (TMI) surveys is introduced,andrealtimeapplicationsarediscussed.Potential field migration is based on a direct integral transformation of the measuredTMI datainto a3Dsusceptibility model,which couldbedirectlyusedforinterpretationorasanapriorimodel forsubsequentregularizedinversion.Theadvantageofmigrationisthatitdoesnotrequireanyaprioriinformationaboutthe typeofthesourcespresent,nordoesitrelyonregularizationas perinversion.Migrationisverystablewithrespecttonoisein measured data because the transform is reduced to the downward continuation of a function that is analytical everywhere inthesubsurface.The3DmigrationofTMIdataacquiredover the Reid-Mahaffy test site in Ontario, Canada is used as a test study.Ourresultsareshowntobeconsistentwiththoseresults obtained from 3D regularized inversion as well as the known geology of the area. Interestingly, the migration of raw TMI data produces results very similar to the inversion of diurnally corrected and microleveled TMI data, suggesting that migration could be applied directly to real-time imaging during the acquisition.
Mike Evans - One of the best experts on this subject based on the ideXlab platform.
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Magnetic Intensity variations in red beds of the Lodève Basin (southern France) and their bearing on the magnetization acquisition process
Geophysical Journal International, 1992Co-Authors: J. M. Maillol, Mike EvansAbstract:SUMMARY An investigation of small-scale variations of magnetization of a red bed sequence in the Permian Lodbve Basin (southern France) has been carried out by means of 512 disc-shaped samples, each 25 mm in diameter and about 6 mm thick. It is found that Magnetic Intensity variations reaching an order of magnitude can occur over stratigraphic distances of only a few centimetres. These fluctuations show a clear lithostratigraphic control with Intensity peaks systematically located at bed tops. Rock Magnetic experiments and petrographic observations indicate that Intensity variations are not due to changes in the amount or properties of the Magnetic material. External causes, such as geoMagnetic field fluctuations or superposition of reversed and normal components, can be ruled out. Petrographical evidence suggests that the magnetization is carried by detrital haematite grains and it is proposed that the controlling factor of Intensity variations is the efficiency of alignment of these grains along the Earth’s Magnetic field. The characteristic Intensity patterns with peaks near bed tops can be explained by post-depositional alignment of the grains, with the highest efficiency near the exposed surface. Grain mobility can be enhanced by mechanical perturbations (bioturbations, hydraulic phenomena), increased water content and reduced load. All these factors tend to see their effect diminish away from the surface and could therefore produce the observed magnetization pattern. The proposed model points to an early acquisition of the remanence in the Lodbve red beds.
Thomas G Hildenbrand - One of the best experts on this subject based on the ideXlab platform.
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interpreting the earthquake source of the wabash valley seismic zone illinois indiana and kentucky from seismic reflection gravity and Magnetic Intensity data
Seismological Research Letters, 2002Co-Authors: John H Mcbride, Thomas G Hildenbrand, William J Stephenson, Christopher J PotterAbstract:Reprocessing of seismic-reflection data reveals new images of upper- to middle-crustal structures beneath the Wabash Valley seismic zone, located north of the New Madrid seismic zone within the seismically active southern Illinois basin. Four intersecting deep seismic profiles (243 km total) indicate an anomalous, 5–10-km-wide zone of dipping reflections and diffractions below the western flank of the Wabash Valley fault system (WVFS). The zone corresponds in places to gently arched regions of Paleozoic strata. The reflector zone can be interpreted as a result of either (or a combination of) magmatic intrusion or structural deformation. The area encompassing the reflection profiles has experienced several moderate magnitude (3.0 ≤ mbLg ≤ 5.5) earthquakes during the past 50 years, defining the central part of the Wabash Valley seismic zone. The hypocenter of the largest 20th-century earthquake in the central USA midcontinent (9 November 1968, mbLg 5.5) corresponds to the most prominent zone of dipping middle-crustal reflections, just west of the WVFS. Both the focal mechanism (moderately dipping reverse fault) and the expected rupture zone size (∼2.9 km fault length) of this earthquake are consistent with the orientation and size of observed reflectors. Dipping reflector patterns in the Precambrian crust are not collinear with fault surfaces updip in the Paleozoic sedimentary section. This indicates that shallow Paleozoic structures are effectively “decoupled” from deeper, possibly seismogenic structure, which suggests that understanding Paleozoic structure is not the key to understanding the earthquake source. The complex dipping crustal reflectivity beneath the WVFS is typical of Paleozoic continental convergent zones observed elsewhere ( e.g. , Appalachian orogen) and thus may suggest a preserved Proterozoic suture, possibly associated with the distal Grenville orogeny or an older event. Although Magnetic Intensity, Bouguer gravity, and seismic-reflection data present different means of understanding the deep geology of the area, their integration aids in limiting the number of admissible interpretations. The reflection profiles indicate a variable zone of anomalous crustal structure, including the dipping reflector zone, along a trend of northeast-trending gravity and Magnetic highs locally defining the Commerce geophysical lineament (CGL), which is a suspected source of seismic hazard in the central USA midcontinent. Three-dimensional inverse modeling of the residual isostatic gravity anomaly values indicates that the upper part of the dipping reflector zone beneath the CGL lies near an important density boundary in the upper Precambrian crust. The results of our study suggest that the seismogenic source just north of the New Madrid seismic zone consists, in part, of a pre-existing fabric of blind thrusts localized along pre-existing igneous intrusions, locally coincident with the CGL. This suggests that the CGL may be seismogenic in places and thus a potential seismic hazard. The variation in the expression of the CGL using reflection and potential-field data sets is probably partly related to the differing geologic features by which it is expressed, but would be also consistent with its reactivation numerous times under varying stress regimes.
