PROJ00326

• Contract/PO #: C153498
• Contractor Name: U. S. Geological Survey
• Water District: 0
• County: Boulder
• Bill Number: SB 87-15
• Loan Projects - Doc Type: Contract Documents

0,5

0,4

LINE OF REGRESSION -
($Y = 0.0159 + 0.7677 $e)

I ONE STANDARD ERROR
OF ESTIMATE

:;: .
~ 0,3 . ...."
0 . . .....
...J ..... .
w .... . . .
:;: A
,.. ....
U .... ....
u: .... ....
U 0,2 ....
. ....
w ....
Il. .... . . ....
Vl .... ....
.... .... .
.... . ....
.... .... . .
.... .... .
0,1 .... . .....
.... .... .
..... . ...
.... .
....
....
....
....
....
00 0,' 0,2 0,3 0,4 0,5

EFFECTIVE POROSITY (~e)
Figure 6. Linear regression relation of specific yiElld on effective porosity.

Table 3. Mean specific yield from laboratory analyses of
core, effective-porosity regression, and apparent grain-
density regression

Mean speclllc yield

Number Effective- Apparent
Aquifer 01 Core porosity grain-
eemples analyses regres- density
slon regres-
sion
Dawson 19 0.15 0.16 0.15
Denver 20 .16 .16 .16
Arapahoe 25 ,22 .21 ,20

Apparent Grain-Density Log

Specific yield has been related to the grain size
of aquifer material in this report and in published lit-
erature (fig. 2). Therefore, a geophysical log that pro-
vides a measure of lithology in a clastic environment
also can provide a measure of specific yield. Schlum-
berger's Cyberlook package (Schlumberger Wen Ser-
vices, 1987) provides two logs that indicate the fine-
to coarse-grained character of formations. The first
log, the minimum shale index log, is a qualitative

interpretation of the sand-shale content of formations.
The log is useful in interpreting general lithology, but it
is not totally quantitative because some operator judg-
ment is involved in its production. The second log, the
apparent grain-density log, is quantitative and closely
corresponds in shape to the minimum shale index log.
The apparent grain-density log is calculated by the
equation:

Pma

Ph- tPraPf
l-tPta

(4)

where
Pma
Ph

tPta

= apparent grain density,
= bulk density from the gamma-density log,
= apparent total porosity derived from
neutron-density and neutron-sonic cross
plots, and
Pf = pore fluid density.
Regressions of apparent grain density on various
grain-size characteristics indicated that the two vari-
ables are correlated and apparent grain density is a
valid indicator of grain size.
The same set of core data (64 samples) used to
develop the regression of specific yield on effective
porosity was used to develop the regression of specific

SPECIFIC-YIELD ESTIMATES FROM GEOPHYSICAL LOGS 15