Soil carbon dioxide flux measurements were taken
over the Lightning Dock Known Geothermal
Resource Area (LDKGRA) in the Animas Valley of
southwestern New Mexico to determine if they could
aid in locating hidden geothermal faults. The
LDKGRA is located in an arid, sediment-filled basin,
and the geothermal system has no known surface
expression. There is a plume of degassed water at
temperatures greater than 100 o C in the valley fill
sediments, however the structures that control the
geothermal fluids are poorly understood. Our
working hypothesis is that CO2 released by boiling
fluid will rise vertically from buried faults and can be
detected by measuring soil CO2 flux over the area.
Over four hundred fifty soil CO2 flux measurements
at 50 to 100 meter East-West spacing were made near
the main surface heat anomaly over a period of more
than a year. One measurement site was monitored at
least once each field day to determine day to day and
seasonal flux variations.
The soil CO2 flux data show three populations, one
with a mean of 0.5 g m -2 day -1 and a standard
deviation of 0.3 g m -2 day -1 , a second with a mean of
3 g m -2 day -1 and a standard deviation of 1 g m -2 day -1
, and a third with a mean of 7 g m -2 day -1 and a
standard deviation of 2 g m -2 day -1 . Twenty-one
measurements performed at the same location
showed a mean of 2 g m -2 day -1 and standard
deviation of 0.2 g m -2 day -1 . The flux data show highs
along roughly linear trends. Soil CO2 flux anomaly
patterns lie along an inferred NE trending fault and
along a roughly located eroding Basin and Range
fault scarp in valley fill. Our data indicate that soil
CO2 flux can locate faults beneath Basin and Range
alluvium. Repeat measurements done at several times
during the year indicate seasonal variations in
temperature have little affect on soil CO2 flux
measurements. The predominance of single site
anomalies suggests sample spacing should be 50 m or
less. It is not known if this technique can be used in
areas with significant vegetation because plants can
generate significant soil CO2.

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