Temperature Data Processing
ODP logging
contractor: LDEO-BRG
Hole: 931B
Leg: 155
Location: Amazon Fan (equatorial NW Atlantic)
Latitude: 5° 8.520' N
Longitude: 46° 37.980' W
Logging date: April, 1994
Bottom felt: 3486.7 mbrf
Total penetration: 421.3 mbsf
Total core recovered: 296.05 m (70.3 %)
Water Depth: 3476 mbsl
Temperature
Tool Used: LDEO-TLT
Depth versus time recording available: NO
Logging
Runs
Logging string 1: DIT/SDT/HLDT/CNTG/NGT
Logging string 2: ACT/GST/NGT
The wireline heave compensator was used to counter the moderate 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).
The noisy pressure data have been initially smoothed using a 50-point running average. Then, a linear relationship of pressure versus depth has been calculated from the pressure reading at the mudline (361 bars) and at the bottom of the logged interval (389.5 bars at 251 mbsf and 390 bars at 250 mbsf for the DIT and GLT run, respectively). 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 = 251* (Pressure-361)/(389.5-361) (DIT run)
Depth = 250*(Pressure -360)/(390-360) (GLT run)
where
Depth = mbsf
Pressure = bars
Finally, the temperature data have been smoothed with a 5-point running average. There is an almost constant difference of 0.2 degC between the readings of the two thermistors. Because of its more accurate calibration, the value given by the slow thermistor, which is generally higher, is more reliable.
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 155.
For further information about the processing, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia