geothermal resource potential of the safford-san simon basin, arizona

subsurface temperatures and depths to zones producing artesian water flow.
Temperature gradients were estimated by subtracting mean annual temperature
(MAT)
from surface discharge temperatures of the wells and dividing this
difference by the well depths.
By multiplying this quotient by 1,000, an
estimated temperature gradient in °C/km is obtained, provided °c is used
for temperature and meters is used for well depths.
In this area, artesian
wells ranging 79 to 400 m depth have surface discharge temperatures ranging
20 to 36°C and have calculated gradients ranging 29 0 C/km to over l30 o C/km.
Histograms shown by Witcher (1979) indicate that most wells have estimated
temperature gradients between 50 and 60 o C/km.
In order to delineate water-producing intervals, Witcher (1979)
reviewed
drillers' comments reported by Knechtel (193S) and depths which drillers reported
increases in water flow.
These depths are shown in a summary table
of these data (Table 4) which clearly shows five different depth intervals
that produce artesian water flow.
gradients for wells below 140 m.
Also evident are similar temperature
This suggests a systematic increase in
temperature with depth.
Conductive heat flow through the basin-fill sediments
and their contained aquifers probably causes the observed consistency in
gradients.
Reinterpretation of the temperature and depth data (Figure 20) confirms
the linear change in temperature with depth.
When only wells in the Cactus
Flat area are used, a correlation of 0.93206 is obtained.
The slope or temperature
gradient is 45 0 C/km with an intercept (or surface temperature)
of 19.9 0 C, which is slightly above the MAT (lSoC). Because water will cool
during flow to the surface, the actual gradient is probably slightly higher.
If cooling effects are accounted, the surface intercept or the correct temperature-versus-depth
graph is closer to the MAT.
Anomalously warm wells at
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