Temperature Data Processing
ODP logging contractor: LDEO-BRG
Location: Yermac Plateau (Arctic Ocean)
Latitude: 80° 15.894' N
Longitude: 6° 35.430' W
Logging date: September, 1993
Bottom felt: 2529 mbrf
Total penetration: 1061.8 mbsf
Total core recovered: 604.77 m (61%)
Temperature Tool Used: LDEO-TLT
Depth versus time recording available: NO
Logging string 1: DIT/SDT/HLDT/NGT (upper and lower sections)
The wireline heave compensator was used to counter the mild 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).
No original data was available at the time of reprocessing, in June 2000. The temperature and pressure data available is believed to have been smoothed with a mild filter and then decimated. During re-processing, the pressure data has been smoothed using a 25 and 50-point running average (upper and lower section, respectively) prior to the depth calculation. A linear relationship of pressure versus depth has been calculated from the pressure reading at the mudline and the maximum logged depth. 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.
Depth = 741 * (Pressure - 261.2)/(351.5 - 261.2) (upper section)
Depth = 1013 * (Pressure - 249)/(354 - 249) (lower section)
Depth = mbsf
Pressure = bars
351.5 and 354 bars = maximum pressures recorded
261.2 and 249 bars = pressures at mudline
741 and 1013 mbsf = maximum logged depths
The procedure described above does not fully account for the vagaries of the pressure readings that may 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 151.
For further information about the processing, please contact:
E-mail: Cristina Broglia