TY  - JOUR
A1  - Guillemoteau, Julien
A1  - Sailhac, Pascal
A1  - Behaegel, Mickael
T1  - Modelling an arbitrarily oriented magnetic dipole over a homogeneous half-space for a rapid topographic correction of airborne EM data
JF  - Exploration geophysics : the bulletin of the Australian Society of Exploration Geophysicists
N2  - Most airborne electromagnetic (EM) processing programs assume a flat ground surface. However, in mountainous areas, the system can be at an angle with regard to the ground. As the system is no longer parallel to the ground surface, the measured magnetic field has to be corrected and the ground induced eddy current has to be modelled in a better way when performing a very fine interpretation of the data. We first recall the theoretical background for the modelling of a magnetic dipole source and study it in regard to the case of an arbitrarily oriented magnetic dipole. We show in particular how transient central loop helicopter borne data are influenced by this inclination. The result shows that the effect of topography on airborne EM is more important at early time windows and for systems using a short cut-off source. In this paper, we suggest that an estimate be made off the locally averaged inclination of the system to the ground and then to correct the data for this before inverting it (whether the inversion assumes a flat 1D, 2D or 3D sub-surface). Both 1D and 2D inversions are applied to synthetic and real data sets with such a correction. The consequence on the ground imaging is small for slopes with an angle less than 25 degrees but the correction factor can be useful for improving the estimation of depths in mountainous areas.
KW  - airborne geophysics
KW  - electromagnetic imaging
KW  - inverse problem
Y1  - 2015
U6  - https://doi.org/10.1071/EG13093
SN  - 0812-3985
SN  - 1834-7533
VL  - 46
IS  - 1
SP  - 85
EP  - 96
PB  - CSIRO
CY  - Clayton
ER  - 
TY  - JOUR
A1  - Guillemoteau, Julien
A1  - Sailhac, Pascal
A1  - Boulanger, Charles
A1  - Trules, Jeremie
T1  - Inversion of ground constant offset loop-loop electromagnetic data for a large range of induction numbers
JF  - Geophysics
N2  - Ground loop-loop electromagnetic surveys are often conducted to fulfill the low-induction-number condition. To image the distribution of electric conductivity inside the ground, it is then necessary to collect a multioffset data set. We considered that less time-consuming constant offset measurements can also reach this objective. This can be achieved by performing multifrequency soundings, which are commonly performed for the airborne electromagnetic method. Ground multifrequency soundings have to be interpreted carefully because they contain high-induction-number data. These data are interpreted in two steps. First, the in-phase and out-of-phase data are converted into robust apparent conductivities valid for all the induction numbers. Second, the apparent conductivity data are inverted in 1D and 2D to obtain the true distribution of the ground conductivity. For the inversion, we used a general half-space Jacobian for the apparent conductivity valid for all the induction numbers. This method was applied and validated on synthetic data computed with the full Maxwell theory. The method was then applied on field data acquired in the test site of Provins, in the Parisian basin, France. The result revealed good agreement with borehole and geologic information, demonstrating the applicability of our method.
Y1  - 2015
U6  - https://doi.org/10.1190/GEO2014-0005.1
SN  - 0016-8033
SN  - 1942-2156
VL  - 80
IS  - 1
SP  - E11
EP  - E21
PB  - Society of Exploration Geophysicists
CY  - Tulsa
ER  -