Technical Notes - Fishing Tools for Retrieving Gamma-Ray Logging Components

Two special tools for recovering gamma-ray probes and logging cable from drillholes have been designed by Ohm and Bunker and constructed by Ohm. Though intended specifically for U. S. Geological Survey equipment, they may be used with other equipment of the same general type. Fishing Tool for Probes: A fishing tool of the overshot type (Fig. 1) was designed to retrieve logging probes (sondes) that become disconnected from the logging cable and are trapped in the drillhole. When the tool slides over the top of the gamma-ray probe, studs on the bottom engage the shoulder near the top of the probe (Fig. 2), which is then pulled to surface. The barrel can be constructed from brass or steel tubing. Wall thickness is not a critical dimension, but the tool should be heavy enough to sink through mud or water in the drillhole. The internal length of the tool must be enough to prevent interference with the upper end of the probe head as the tool slides over it. The design of the probe head used by the USGS required a minimum free space of 2 1/2 in. inside the fishing tool, which was made 7 1/2 in. long to increase the weight. The lower end, which engages the shoulder of the upper end of the probe, contains three studs placed at 120" around the circumference of the tool and held in the engaging position by the tension of a spring wire. The studs can be made of any metal, such as steel, having high tensile strength. As the tool slides over the probe the studs are pushed outward from the center against the spring tension, allowing the tool to slide into position below the shoulder of the probe head. The spring tension forces the studs inward into the engaged position after they pass the shoulder. When the tool is raised the studs stop against the shoulder, preventing the tool from sliding up over the top of the probe. Once it engages the shoulder of the probe it cannot be freed until both probe and fishing equipment are recovered from the drillhole.