Temperature Data Processing
ODP logging contractor: LDEO-BRG
Location: Chile Triple Junction (SE Pacific Ocean)
Latitude: 46° 14.219' S
Longitude: 75° 46.371' W
Logging date: January, 1992
Bottom felt: 2575.9 mbrf (used for depth shift to sea floor)
Total penetration: 742.9 mbsf
Total core recovered: 192.9 m (43.3 %)
Water Depth: 2564.2 mbsl
Temperature Tool Used: LDEO-TLT
Depth versus time recording available: NO
Logging string 1: DIT/SDT/NGT (pass 1)
Logging string 2: HLDT/CNTG/NGT
Wireline heave compensator was used to counter ship heave.
The LDEO-TLT tool is a self-contained, high precision, low-temperature logging tool that is attached to the bottom of the Schlumberger tool strings. The tool provides two temperature measurements (in degree Celsius, recorded by a fast-response and a slow-response thermistor. The fast-response thermistor, though low in accuracy, is able to detect sudden, small temperature excursions caused by fluid flow between the formation and the borehole. The highly accurate, slow-response thermistor can be used to estimate heat flow. Pressure and the two temperature measurements are recorded as a function of time: conversion to depth can be based on the pressure reading (Legs 123-157) or, preferably, on simultaneous recording (by Schlumberger) of depth and time (Legs 159-181).
A linear relationship of pressure versus depth has been calculated from the pressure reading at the mudline and at the total logging depth for each logging run. The pressure at the mudline corresponds to the pressure recorded by the tool during the calibration stop (about 5 minutes), which takes place at the mudline on each logging run. The pressure readings have been smoothed with a 10-sample running average before depth calculation. A 10-minute station was made at 500 mbsf on the way down during the HLDT run, during which the borehole temperature was allowed to rebound further towards equilibrium.
Depth = 735 * (Pressure - 277)/(364 - 277) (DIT run, pass 1)
Depth = 732 * (Pressure - 277)/(366.2 - 277) (HLDT run)
Depth = mbsf
Pressure = bars
735 mbsf, 732 mbsf = bottom of logged interval
277 bars = pressure at seafloor
364 bars, 366.2 bars = pressure at bottom of logged interval
This procedure does not fully account for the vagaries of the pressure readings that result in lots of ups and downs in the generated depth channel. Further problems arise because of pumping during logging operations, which affects the pressure, especially when the Side Entry Sub is used. Also, whenever heavy pills of mud are used, the pressure-depth calculation is affected, resulting in a non-linear effect that is difficult to account for. If the pressure conversion coefficient is recalculated for the mudline, the resultant total depth is often wrong.
Information about the temperature logging operations can be found in the Site Chapter (Operations, Downhole Measurements, and Heat Flow sections), ODP IR volume 141.
E-mail: Cristina Broglia