Preparation of Specific‐Yield Logs for Clastic Bedrock Aquifers

Abstract

AbstractSpecific yield is the principal aquifer characteristic needed to estimate the volume of recoverable ground water in storage in an aquifer. Determination of specific yield can be difficult and costly, particularly in deep, confined aquifers where core drilling and core analyses may be needed to define specific yield. A method has been developed for preparation of specific‐yield geophysical logs that could greatly ease the determination of specific yields in such aquifers.Three geophysical logs that were investigated as potential indicators of specific yield were the free fluid index log, the effective‐porosity log, and the apparent grain‐density log. The free fluid index log did not accurately represent conditions at the test site in central Colorado and may not be suitable for application in other shallow and permeable aquifers. The effective‐porosity and apparent grain‐density logs were each used in least‐squares linear regressions to correlate log response to specific yield measured in core samples. The resulting regression equations have coefficients of correlation (R) of 0.84 and 0.90, and were used to successfully prepare specific‐yield logs from the effective‐porosity and apparent grain‐density logs.