@article{fdi:010044160,
  title = {{N}umerical study of the variations of magnetic resonance signals caused by surface slope},
  author = {{G}irard, {J}.{F}. and {L}egchenko, {A}natoli and {B}oucher, {M}. and {B}altassat, {J}.{M}.},
  editor = {},
  language = {{ENG}},
  abstract = {{W}hen performing forward modelling and inversion of {M}agnetic {R}esonance {S}ounding ({MRS}) data, the water-content distribution is typically assumed to be horizontal (1 {D} case). {T}his assumption is fully justified because {MRS} is often used for characterizing continuous aquifers in a nearly flat environment. {H}owever, {MRS} can also be used in areas with sharp topographical variations. {F}ollowing a review of the standard {MRS} equations when using a coincident transmitter/receiver loop, the mathematical terms potentially affected by tilting of the loop are discussed. {W}e present the results of a numerical modelling exercise, studying a case where the surface is not horizontal and the loop cannot be considered to be parallel to the top of the aquifer. {T}his shows that maximum variations in the {MRS}-signal amplitude are caused mainly by north- or south-dipping slopes. {S}lope effects depend on the loop size (a larger loop produces a larger error) especially in the presence of shallow water. {W}ith a geomagnetic-field inclination of 65 degrees and a slope angle <= 10 degrees, the topography causes a maximum variation in amplitude of less than 10%. {N}ear magnetic poles and equator, the slope effect is lower and undetectable in most cases. {I}t was found that within a 10% range of variation in the amplitude, errors introduced into inversions are within the typical uncertainty for {MRS} inversion and hence no topographic corrections are necessary. {T}hus, a significant effect from non-horizontal topography might be expected only when data uncertainty is lower than the slope effect (the slope effect is lower than equivalence when data quality is poor). {T}oday, most field data sets are inverted using the modulus of the {MRS} signal, but some new developments consider the complex signal (both modulus and phase). {H}owever, inversion of complex {MRS} signals, which would provide a higher sensitivity to groundwater distribution, may be affected by slope effect. {T}hus, the slope orientation and dip angle should be accurately measured in the field when the phase of {MRS} signals is inverted too.},
  keywords = {{MRS} ; {SNMR} ; {M}agnetic resonance sounding ; {T}opography ; {G}roundwater},
  booktitle = {},
  journal = {{J}ournal of {A}pplied {G}eophysics},
  volume = {66},
  numero = {3-4},
  pages = {94--103},
  ISSN = {0926-9851},
  year = {2008},
  DOI = {10.1016/j.jappgeo.2008.04.003},
  URL = {https://www.documentation.ird.fr/hor/fdi:010044160},
}