Comparison of calculated and observed hydrostatic pressure measurements at borehole DC-8

Static water-level and fluid-pressure measurements are commonly converted in hydrologic studies to formation pressure and hydraulic head, which are used to determine groundwater flow characteristics of aquifer systems. For hydrologic studies that use hydraulic head and formation pressure data obtained from surface-based and downhole instruments, the relationship between water-level and downhole-pressure measurements must be established. This report presents the results of a field evaluation of 30 quartz pressure probes, which were used in the reference repository location baseline monitoring program. The pressure probes were field tested at a depth of 2,700.23 ft within borehole DC-8 between December 1984 and April 1985. Water-level measurements corrected for instrumental and noninstrumental errors were used in the HEADCO program to calculate the hydrostatic pressure at the fixed probe depth setting. HEADCO-calculated pressure values were then compared to the fluid-column pressure measured with each probe. Results of the field evaluation at the borehole DC-8 test facility indicate that recently calibrated pressure probes give consistent and comparable pressure measurements. In all cases, the difference between measured and calculated pressure values was less than the stated manufacturer accuracy range of +-1.2 lbf/in2. For the majority of the probes evaluated (i.e., 77%), the difference between observed and calculated pressures was 0.3 lbf/in2 or less. The close comparison of calculated and measured pressures also indicates that accurate downhole pressures can be calculated using water-level measurements and the HEADCO program, provided that systematic errors, due to borehole deviation, and thermal expansion and elastic deformation have been accounted for in the water-level measurements