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

Recent advancements in NMR for characterizing the vadose zone 50
Recent advancements in NMR for characterizing the vadose zone
David Walsh 1 , Elliot Grunewald 1 , Hong Zhang 1 , Paul Ferre 2 and Andrew Hinnell 2
1 Vista Clara, Inc., Mukilteo, WA, USA, davewalsh@vista-clara.com, 2 University of Arizona, Tucson AZ, USA
Until recently, most applications of NMR
in hydrogeology have focused on detecting
and characterizing groundwater in the
saturated zone. Another target for
hydrogeophysical methods is water in the
vadose zone, the dynamics of which
control contaminant migration,
groundwater recharge, evapotranspiration,
and soil stability. The challenge of
characterizing water in the vadose zone by
NMR is that the signals are typically very
weak (due to low water content) and very
short (due to concentration of water on
grain surfaces or in the smallest available
pores). Here we present improvements in
NMR geophysics technology over the past
4 years, and demonstrate the application of
these technologies to vadose zone
characterization.
To address the challenges of using Earth’s
field surface NMR instrumentation to
detect and characterize water in the vadose
zone, we designed a modified GMR
instrument with faster switching and higher
powered electronics. This instrument has a
measurement dead-time of 2.8 ms, an
instantaneous power output of 7000 V and
800 A, and a noise floor of 0.5 nV/rt(Hz).
The new instrument was used to collect
and interpret surface NMR data at active
vadose zone investigation sites in the
western United States. First, the equipment
was used to detect, image and quantify
water infiltrating into the ground at a
managed aquifer storage and recovery
(ASR) facility in Tucson Arizona. Analysis
of this data indicates that the surface NMR
tool was able to measure the recharge
speed of the initial wetting front, and
quantify some, but not all of the water that
was recharged. Further analysis showed
that infiltrating water filled larger pores,
temporarily, and the water filled pore size
distribution in the top 15m returned to its
“dry“ condition within 3 weeks of the end
of the surface water infiltration.
Additional surface NMR measurements at
vadose zone investigations in Washington,
Nebraska and Kansas demonstrated that
the equipment could detect and image the
location of bound and perched water to
depths of at least 25m. Borehole NMR logs
at these sites indicated that the surface
NMR measurements were detecting only a
portion of the bound water. Additional
borehole NMR measurements in eastern
Kansas demonstrated the ability of NMR
logging to detect and quantify transient
water content in the vadose zone. In this
case decayed tree roots were temporarily
saturated with water, following a local
stream flood event.
Finally, a new geotechnical NMR
instrument has been developed and
demonstrated for detecting and
characterizing soil moisture in the top 1m
of the subsurface. This tool generates a
boosted static gradient field, and performs
non-invasive CPMG NMR measurements
in the low RF band (40 kHz – 500 kHz)
with an echo spacing of 1 ms or less.
Preliminary field tests showed that the tool
can accurately measure water content and
its T2 distribution at multiple depths, in
both dry and wetted conditions, with total
scan times of approximately 3 minutes.
Acknowledgement: This material is based upon
work supported, in part, by the US Department of
Energy under Grant numbers DE-FG02-
08ER84979 and DE-SC000423, and US Army
Contract # W912HZ-12-P-0018. Any opinions,
findings, and conclusions or recommendations
expressed in this material are those of the author(s)
and do not necessarily reflect the views of the US
Department of Energy or the US Army.
Magnetic Resonance in the Subsurface – 5 th International Workshop on Magnetic Resonance
Hannover, Germany, 25 – 27 September 2012