A Photometric Logging Probe for Dilution Logging in Fractured Bedrock Aquifers

Dilution logging is capable of precisely indicating inflow depth points while measuring extremely low rates of borehole flow, but requires cumbersome procedures to condition the fluid column, and introduction of distilled water or brine may not be acceptable to regulators on proposed environmental studies.  A photometric probe, the HRTFN Fotometer, performs dilution logging by measuring the concentration of an environmentally harmless food color dye.  The measurement section of the probe uses a LED light source tuned to the exact absorption of the dye so that concentrations as low as 10 mg/l can be used in dilution experiments.  The probe is also fitted with temperature and fluid resistivity sensors, and has a back-scatter optical detector for characterizing the concentration of suspended particles in the fluid column.  The photometric probe was tested in a bedrock borehole in New York known to contain weak ambient downflow expected to be slightly above the lower measurement limit (about 0.10 l/min) of the heat pulse flowmeter.   HP flowmeter measurements made less than an hour before the testing of the photometer indicated a downflow rate of 1.21 l/min with measurement scatter of about 0.10 l/min. An initial column dye concentration of about 60 mg/l was then established.  Repeat profiling of the column with the photometer showed the column diluted by inflow at 24 m in depth, eventually sweeping dye from the column down to an outflow depth at 34 m after 3 hours.   The flow model fit to the photometric data indicated a downflow rate of 1.25 l/min with an estimated error of +/- 0.03 l/min.   These results demonstrate that photometric dilution logging can be effectively used as a substitute for brine dilution in the detection and characterizing of inflow zones in bedrock boreholes where brine dilution is not acceptable and distilled water replacement is impractical.