Magnetic Resonance in the Subsurface – 5th International ... - LIAG

Experimental verification of a 3D model for MRS
Legchenko 1* A., J-F. Girard 2 , S. Morlighem 3 and J-M. Baltassat 2
1 IRD / LTHE, Grenoble, France; 2 BRGM, Orléans, France; 3 VALE, New Calédonia.
anatoli.legtchenko@ird.fr
It is known that only one coincident
transmitting-receiving loop is required for the
1D magnetic resonance sounding (MRS)
measuring and inversion (Legchenko and
Shushakov, 1998). The method could be
extended to 2D (Girard et al., 2007) and 3D
(Legchenko et al., 2011) applications and use
of a multi-channel receiver allows a more
sophisticated 3D implementation (Hertrich et
al., 2009). Whatever would be the practical
implementation of a 3D MRS survey,
inversion is based on the forward modeling of
the magnetic resonance response.
Consequently, the mathematical model is a
crucial point for the inversion.
In the majority of reported applications the
MRS method is used in the FID mode when
the free induction decay signal is measured
(Legchenko et Valla, 2002). However, in some
rocks the geomagnetic field may be perturbed
and FID measurements may fail (Roy et al.,
2008). For improving the MRS performance in
the presence of magnetic rocks MRS could be
used in the spin echo (SE) mode (Legchenko et
al., 2010). In this case only the spin echo signal
is measured. Under some conditions both FID
and SE signals can be observed.
We have developed a 3D mathematical model
that allows computing MRS signal from 3D
targets both in FID and SE modes. This model
allows considering the field setup consisting of
either one transmitting loop with number of
separated receiving loops or the overlapped
coincident loops.
For experimental verification of the modeling
routine we used an artificial water reservoir
located in the southern part of the New
Caledonia Island. In this pool, of
approximately 80×30 m 2 large, the depth is
varying from 1.5 to 4 m. A rectangular loop of
100×50 m 2 was put around the pool. Rocks in
this area are characterized by the magnetic
susceptibility between 5e-4 and 5e-3 SIU thus
perturbing the geomagnetic field. However
within the pool these perturbations are
relatively small and we were able to measure
Experimental verification of a 3D model for MRS 44
both FID and SE signals. The geomagnetic
field was of 47246 nT and the Larmor
frequency was about 2013 Hz.
We have found that the relaxation times for the
signal from water in the pool are: T1=1950 ms,
T2=1400 ms, T2 * (FID)=100 ms and T2 * (SE)=125
ms. The maximum signal amplitude was
approximately 180 nV for the FID signal and
120 nV for the SE signal (measured with the
delay of 256 ms between the pulses).
Under these conditions numerical modeling of
the MRS signal from water in the pool showed
a good correspondence between observed and
theoretical signals for both FID and SE
measurements.
References
Girard, J-F, M. Boucher, A. Legchenko, and J-M.
Baltassat (2007): 2D magnetic resonance
tomography applied to karstic conduit imaging.
Journal of Applied Geophysics, 63, 103-116.
Hertrich, M., A.G. Green, M. Braun, and U.
Yaramanci (2009): High-resolution surface
NMR tomography of shallow aquifers based on
multioffset measurements. Geophysics, 74, 47-
59.
Legchenko, A.V., and O.A. Shushakov (1998):
Inversion of surface NMR data. Geophysics, 63,
75-84.
Legchenko, A., and P. Valla (2002): A review of
the basic principles for proton magnetic
resonance sounding measurements. Journal of
Applied Geophysics, 50, 3-19.
Legchenko, A., J.M. Vouillamoz, J. Roy (2010):
Application of the magnetic resonance sounding
method to the investigation of aquifers in the
presence of magnetic materials. Geophysics 75:
L91–L100.
Legchenko, A., M. Descloitres, C. Vincent, H.
Guyard, S. Garambois, K. Chalikakis and M.
Ezersky (2011): Three-dimensional magnetic
resonance imaging for groundwater. New
Journal of Physics, 13, 025022, doi:
10.1088/1367-2630/13/2/025022.
Roy, J., A. Rouleau, M. Chouteau, M. Bureau
(2008): Widespread occurrence of aquifers
currently undetectable with the MRS technique
in the Grenville geological province, Canada.
Journal of Applied Geophysics, 66, 82-93.
Magnetic Resonance in the Subsurface – 5 th International Workshop on Magnetic Resonance
Hannover, Germany, 25 – 27 September 2012