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In moderate-to-high temperature environments (i.e., a sheeted dike complex), the measurement of borehole temperatures with either wireline or memory tools should precede any other logging operation in order to determine the temperature of the borehole fluids, estimate the geothermal gradient, and approximate the time of post-drilling temperature rebound. Schlumberger tools rated to 175°C can often be deployed with adequate hole cooling by circulating cold fluids for approximately 2-3 hours. If temperatures rebound quickly, however, these tools are at risk and logs may only be recorded in cases where the side-entry sub (SES) could be used. After circulating for several hours, a Schlumberger tool string should be lowered into the hole as quickly as possible and in combination with an Auxiliary Measurement Sonde (AMS) to monitor borehole fluid temperatures.
In high-temperature environments (i.e. hydrothermal systems or lower crustal settings), temperature logs can be recorded using the wireline slim-hole Hi-T Temperature Tool (HTT) developed at LDEO for operation at the TAG hydrothermal mound in 1994, or the University of Miami third-party GRC memory temperature tool.
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LDEO High-T Temperature Tool (HTT)
The HTT can be used in temperature conditions of up to 275°C although the Teflon insulation in the wireline will begin to degrade beyond 232°C. Generally, this can present a problem after extended use in temperature conditions exceeding 235°C; however, the HTT measuring system is only frequency dependant. Therefore, the tool can still transmit reliable measurements even after the cable has been considerably degraded. |
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| GRC Ultra-High T Temperature Tool
The GRC was developed with NSF funds by the University of Miami. Although this tool is available, arrangements need to be made for maintenance if it is desired for future use. This tool can be deployed on the sandline if temperatures exceed 232°C, as occurred during Leg 169 when the tool was successfully deployed in Hole 858G. However, since this is a memory tool, the loggers will be unable to monitor temperature or tool problems in real time. |
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Following drilling, circulation operations must occur to cool the hole. At this point, the Triple Combo with the AMS can be deployed. If the AMS records temperatures in the 175°C range, then the tool string must be retrieved immediately to avoid damage. In this case, additional hole cooling operations must occur, and a deployment using a modified string with only the Hostile Environment Natural Gamma Sonde (HNGS) and the Hostile-Environment Litho-Density Tool (HLDT) can be attempted. The tool's built-in temperature sensors must be monitored carefully in order to avoid exposing the electronics to harmful temperatures. In additions, downhole magnetic field measurements are also possible with the German (BGR) third-party three-component fluxgate magnetometer. This tool measures the three orthogonal components of the magnetic field up to 100 microTesla with a resolution of ± 0.1 nT. The tool also contains two inclinometers that measure tilt with a resolution of 0.1°. The probe is mounted inside a dewar flask and contains heat sinks that allow measurements at temperatures of up to 300°C. This magnetometer was previously used during ODP Leg 148 however, as it is the case with most third-party tools, any future deployments must obtain additional funds prior to the cruise for maintenance, shipping, and training.
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