Electromagnetic induction (EM) studies are the only geophysical method available to investigate d... more Electromagnetic induction (EM) studies are the only geophysical method available to investigate deep Earth electrical conductivity structure. Owing to their adequate spatial and temporal coverage, satellite magnetic data play a major role in such type of studies. In this regard, a good understanding and characterization of the large scale external field is necessary. For the present analysis, we use CHAMP satellite data selected for the year 2001. Using these precise data, we investigate how well we can estimate/characterize the large scale external source field at different frequencies during geomagnetic storm periods. Furthermore, the estimates will be compared with the ground based observations. We investigate, possible local time (LT) dependency in our estimates. Agreement of our model with the ring current symmetric nature has been addressed. Preliminary results of our findings will be discussed.
Page 1. Chapter 4 Aeromagnetic and Marine Measurements Mohamed Hamoudi, Yoann Quesnel, Jérôme Dym... more Page 1. Chapter 4 Aeromagnetic and Marine Measurements Mohamed Hamoudi, Yoann Quesnel, Jérôme Dyment, and Vincent Lesur Abstract Modern magnetic measurements have been acquired since the 1940s over land and the 1950s over oceans. ...
Recently, simulations have been carried out to estimate the strength of electric and magnetic sig... more Recently, simulations have been carried out to estimate the strength of electric and magnetic signals generated by the steady motion (ocean circulation) of highly conducting oceans. The magnetic signals have been found to be on the order of several nT at the Earth's surface. The ocean circulation signals are especially pronounced in the high latitudes of the southern hemisphere, where the eastward current circulates without hitting continental landmass. In this study, we estimated the magnetic field generated by the ocean circulation and modelled it on a global scale using the spherical harmonics. We found that the energy spectrum of the expansion is dominated by rather high spherical harmonic degrees. Results of a spherical harmonic analysis are compared with recent spherical harmonic description of the lithospheric magnetic field. We seek to explain uncertainties in the lithospheric magnetic field models by oceanic signals, in order to improve current models of the geomagnetic field.
A time-dependent model of the Earth's magnetic field and its temporal change of the period 1957 t... more A time-dependent model of the Earth's magnetic field and its temporal change of the period 1957 to 2007
The oil industry uses geomagnetic field information to aid directional drilling operations when d... more The oil industry uses geomagnetic field information to aid directional drilling operations when drilling for oil and gas offshore. These operations involve continuous monitoring of the azimuth and inclination of the well path to ensure the target is reached and, for safety reasons, to avoid collisions with existing wells. Although the most accurate method of achieving this is through a gyroscopic survey, this can be time consuming and expensive. An alternative method is a magnetic survey, where measurements while drilling (MWD) are made along the well by magnetometers housed in a tool within the drill string. These MWD magnetic surveys require estimates of the Earth's magnetic field at the drilling location to correct the downhole magnetometer readings. The most accurate corrections are obtained if all sources of the Earth's magnetic field are considered. Estimates of the main field generated in the core and the local crustal field can be obtained using mathematical models derived from suitable data sets. In order to quantify the external field, an analysis of UK observatory data from 1983 to 2004 has been carried out. By accounting for the external field, the directional error associated with estimated field values at a mid-latitude oil well (55° N) in the North Sea is shown to be reduced by the order of 20%. This improvement varies with latitude, local time, season and phase of the geomagnetic activity cycle. By accounting for all sources of the field, using a technique called Interpolation In-Field Referencing (IIFR), directional drillers have access to data from a "virtual" magnetic observatory at the drill site. This leads to an error reduction in positional accuracy that is close to matching that of the gyroscopic survey method and provides a valuable independent technique for quality control purposes.
Thanks to the past decade of Geopotential Field Research, including the successful satellites CHA... more Thanks to the past decade of Geopotential Field Research, including the successful satellites CHAMP, Ørsted and SAC-C, recent core magnetic field models achieve an unprecedented accuracy. However, sophisticated models like GRIMM-2 or CHAOS-2 struggle in fitting some observatory secular variation (SV) estimates. These differences become evident when the short term SV is considered, i.e. very fast variations of a few months duration are not reflected by the models. The unmodelled signals can either be of internal or of external origin. After identifying the observatories for which SV time series are not properly fitted, we model the differences with the harmonic spline functions. Those basis functions are located at the selected observatory positions and permit an exact fit to the data, if required. Additionally, a model based on these harmonic spline functions can be easily translated in terms of SH, allowing for a direct comparison with the standard existing models. From the improved spatial and temporal description of these signals, we expect a better understanding of their underlying causes.
One possible method for imposing the frozen-flux approximation to a core magnetic field model, is... more One possible method for imposing the frozen-flux approximation to a core magnetic field model, is to co-estimate the field an a flow model at the top of the liquid outer core. The flow model is, in this approach, derived through the radial diffusion-less induction equation. It has been shown that the technique works remarkably well on seven years of CHAMP satellite data: An accurate magnetic field model has been obtained while constraints were applied exclusively on the flow. Although, during this seven years covered by the CHAMP data set, rapid time variations of the SV have been observed, none have an obvious global signature. Therefore, it is not clear if these variations are geomagnetic jerks. We apply the same technique on a observatory data set made of 50 years of SV estimates (1957.0 to 2006.4). The model is also constrained to fit models derived from satellite data in 1980.0 and 2004.0. Our goal is three folds: 1- Verify that the technique employed does not fail on long time period. 2- Verify that the known jerks can be explained in the framework of the frozen-flux approximation. 3- Derive a core field and a flow models, at the CMB, that can help understanding the mechanism associated with jerks. Preliminary results will be presented.
Magnetic field measurements are very valuable, as they provide constraints on the interior of the... more Magnetic field measurements are very valuable, as they provide constraints on the interior of the telluric planets and Moon. The Earth possesses a planetary scale magnetic field, generated in the conductive and convective outer core. This global magnetic field is superimposed on the magnetic field generated by the rocks of the crust, of induced (i.e. aligned on the current main field) or remanent (i.e. aligned on the past magnetic field). The crustal magnetic field on the Earth is very small scale, reflecting the processes (internal or external) that shaped the Earth. At spacecraft altitude, it reaches an amplitude of about 20 nT. Mars, on the contrary, lacks today a magnetic field of core origin. Instead, there is only a remanent magnetic field, which is one to two orders of magnitude larger than the terrestrial one at spacecraft altitude. The heterogeneous distribution of the Martian magnetic anomalies reflects the processes that built the Martian crust, dominated by igneous and cratering processes. These latter processes seem to be the driving ones in building the lunar magnetic field. As Mars, the Moon has no core-generated magnetic field. Crustal magnetic features are very weak, reaching only 30 nT at 30-km altitude. Their distribution is heterogeneous too, but the most intense anomalies are located at the antipodes of the largest impact basins. The picture is completed with Mercury, which seems to possess an Earth-like, global magnetic field, which however is weaker than expected. Magnetic exploration of Mercury is underway, and will possibly allow the Hermean crustal field to be characterized. This paper presents recent advances in our understanding and interpretation of the crustal magnetic field of the telluric planets and Moon.
Electromagnetic induction (EM) studies are the only geophysical method available to investigate d... more Electromagnetic induction (EM) studies are the only geophysical method available to investigate deep Earth electrical conductivity structure. Owing to their adequate spatial and temporal coverage, satellite magnetic data play a major role in such type of studies. In this regard, a good understanding and characterization of the large scale external field is necessary. For the present analysis, we use CHAMP satellite data selected for the year 2001. Using these precise data, we investigate how well we can estimate/characterize the large scale external source field at different frequencies during geomagnetic storm periods. Furthermore, the estimates will be compared with the ground based observations. We investigate, possible local time (LT) dependency in our estimates. Agreement of our model with the ring current symmetric nature has been addressed. Preliminary results of our findings will be discussed.
Page 1. Chapter 4 Aeromagnetic and Marine Measurements Mohamed Hamoudi, Yoann Quesnel, Jérôme Dym... more Page 1. Chapter 4 Aeromagnetic and Marine Measurements Mohamed Hamoudi, Yoann Quesnel, Jérôme Dyment, and Vincent Lesur Abstract Modern magnetic measurements have been acquired since the 1940s over land and the 1950s over oceans. ...
Recently, simulations have been carried out to estimate the strength of electric and magnetic sig... more Recently, simulations have been carried out to estimate the strength of electric and magnetic signals generated by the steady motion (ocean circulation) of highly conducting oceans. The magnetic signals have been found to be on the order of several nT at the Earth's surface. The ocean circulation signals are especially pronounced in the high latitudes of the southern hemisphere, where the eastward current circulates without hitting continental landmass. In this study, we estimated the magnetic field generated by the ocean circulation and modelled it on a global scale using the spherical harmonics. We found that the energy spectrum of the expansion is dominated by rather high spherical harmonic degrees. Results of a spherical harmonic analysis are compared with recent spherical harmonic description of the lithospheric magnetic field. We seek to explain uncertainties in the lithospheric magnetic field models by oceanic signals, in order to improve current models of the geomagnetic field.
A time-dependent model of the Earth's magnetic field and its temporal change of the period 1957 t... more A time-dependent model of the Earth's magnetic field and its temporal change of the period 1957 to 2007
The oil industry uses geomagnetic field information to aid directional drilling operations when d... more The oil industry uses geomagnetic field information to aid directional drilling operations when drilling for oil and gas offshore. These operations involve continuous monitoring of the azimuth and inclination of the well path to ensure the target is reached and, for safety reasons, to avoid collisions with existing wells. Although the most accurate method of achieving this is through a gyroscopic survey, this can be time consuming and expensive. An alternative method is a magnetic survey, where measurements while drilling (MWD) are made along the well by magnetometers housed in a tool within the drill string. These MWD magnetic surveys require estimates of the Earth's magnetic field at the drilling location to correct the downhole magnetometer readings. The most accurate corrections are obtained if all sources of the Earth's magnetic field are considered. Estimates of the main field generated in the core and the local crustal field can be obtained using mathematical models derived from suitable data sets. In order to quantify the external field, an analysis of UK observatory data from 1983 to 2004 has been carried out. By accounting for the external field, the directional error associated with estimated field values at a mid-latitude oil well (55° N) in the North Sea is shown to be reduced by the order of 20%. This improvement varies with latitude, local time, season and phase of the geomagnetic activity cycle. By accounting for all sources of the field, using a technique called Interpolation In-Field Referencing (IIFR), directional drillers have access to data from a "virtual" magnetic observatory at the drill site. This leads to an error reduction in positional accuracy that is close to matching that of the gyroscopic survey method and provides a valuable independent technique for quality control purposes.
Thanks to the past decade of Geopotential Field Research, including the successful satellites CHA... more Thanks to the past decade of Geopotential Field Research, including the successful satellites CHAMP, Ørsted and SAC-C, recent core magnetic field models achieve an unprecedented accuracy. However, sophisticated models like GRIMM-2 or CHAOS-2 struggle in fitting some observatory secular variation (SV) estimates. These differences become evident when the short term SV is considered, i.e. very fast variations of a few months duration are not reflected by the models. The unmodelled signals can either be of internal or of external origin. After identifying the observatories for which SV time series are not properly fitted, we model the differences with the harmonic spline functions. Those basis functions are located at the selected observatory positions and permit an exact fit to the data, if required. Additionally, a model based on these harmonic spline functions can be easily translated in terms of SH, allowing for a direct comparison with the standard existing models. From the improved spatial and temporal description of these signals, we expect a better understanding of their underlying causes.
One possible method for imposing the frozen-flux approximation to a core magnetic field model, is... more One possible method for imposing the frozen-flux approximation to a core magnetic field model, is to co-estimate the field an a flow model at the top of the liquid outer core. The flow model is, in this approach, derived through the radial diffusion-less induction equation. It has been shown that the technique works remarkably well on seven years of CHAMP satellite data: An accurate magnetic field model has been obtained while constraints were applied exclusively on the flow. Although, during this seven years covered by the CHAMP data set, rapid time variations of the SV have been observed, none have an obvious global signature. Therefore, it is not clear if these variations are geomagnetic jerks. We apply the same technique on a observatory data set made of 50 years of SV estimates (1957.0 to 2006.4). The model is also constrained to fit models derived from satellite data in 1980.0 and 2004.0. Our goal is three folds: 1- Verify that the technique employed does not fail on long time period. 2- Verify that the known jerks can be explained in the framework of the frozen-flux approximation. 3- Derive a core field and a flow models, at the CMB, that can help understanding the mechanism associated with jerks. Preliminary results will be presented.
Magnetic field measurements are very valuable, as they provide constraints on the interior of the... more Magnetic field measurements are very valuable, as they provide constraints on the interior of the telluric planets and Moon. The Earth possesses a planetary scale magnetic field, generated in the conductive and convective outer core. This global magnetic field is superimposed on the magnetic field generated by the rocks of the crust, of induced (i.e. aligned on the current main field) or remanent (i.e. aligned on the past magnetic field). The crustal magnetic field on the Earth is very small scale, reflecting the processes (internal or external) that shaped the Earth. At spacecraft altitude, it reaches an amplitude of about 20 nT. Mars, on the contrary, lacks today a magnetic field of core origin. Instead, there is only a remanent magnetic field, which is one to two orders of magnitude larger than the terrestrial one at spacecraft altitude. The heterogeneous distribution of the Martian magnetic anomalies reflects the processes that built the Martian crust, dominated by igneous and cratering processes. These latter processes seem to be the driving ones in building the lunar magnetic field. As Mars, the Moon has no core-generated magnetic field. Crustal magnetic features are very weak, reaching only 30 nT at 30-km altitude. Their distribution is heterogeneous too, but the most intense anomalies are located at the antipodes of the largest impact basins. The picture is completed with Mercury, which seems to possess an Earth-like, global magnetic field, which however is weaker than expected. Magnetic exploration of Mercury is underway, and will possibly allow the Hermean crustal field to be characterized. This paper presents recent advances in our understanding and interpretation of the crustal magnetic field of the telluric planets and Moon.
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Papers by Vincent Lesur