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

Modelling NMR signal for compact sensors
Aaron Davis and James Macnae
CSIRO and RMIT Universivty
aaron.davis@csiro.au and james.macnae@rmit.edu.au
Recent advances have increased the signal to
noise ratio of EM B and dB/dt sensors
remarkably. By tuning sensors for the low
frequencies appropriate to earth-field NMR
signal, it is theoretically possible to use them
in geophysical applications for the
invesitgation of groundwater in the subsurface
of the earth. In this paper, we model the
response of compact B and dB/dt sensors such
as the 'NMARMIT' sensor (Macnae, 2012).
We revisit the work of Weichman et al (2000)
to generalise the theory of surface NMR to
account for compact sensors arrayed on the
surface of the earth. Our theory is appropriate
for any loop shape and conductivity structure.
By simultanoeusly monitoring the current
pulse of the transmit loop and the NMR
response in B and dB/dt, we demonstrate the
theoretical feasability of these sensors to
Modelling NMR signal for compact sensors
measure NMR signal in three dimensions with
zero delay time.
Through forward modelling of the physical
response, we show that deployment of sensor
arrays can be used to detect complex
distribution of water in the earth; and that it
*
should be possible to measure both T1 and T 2
in a routine NMR tomography measurement in
the same time that it takes to do a conventional
sNMR sounding.
References
Macnae, J, 2012. A new generation of EM sensors,
ASEG conference extended abstracts, Brisbane.
Weichman, P.B., Lavely, E.M., and Ritzwoller,
M.H., 2000. Thoery of surface nuclear magnetic
resonance with applications to geophysical
imaging problems: Physical Review E, 62, 1,
1290–1312.
Magnetic Resonance in the Subsurface – 5 th International Workshop on Magnetic Resonance
Hannover, Germany, 25 – 27 September 2012
20