Temperature Data Processing
ODP logging contractor: LDEO-BRG
Location: Alboran Basin (Alboran Sea)
Latitude: 36° 12.313' N
Longitude: 4° 18.763' W
Logging date: June, 1995
Bottom felt: 1119.1 mbrf (used for depth shift to sea floor)
Total penetration: 928.7 mbsf
Total core recovered: 535.54 m (57.7 %)
Temperature Tool Used: LDEO-TLT
Depth versus time recording available: only during DIT/SDT/HLDT/CNTG/NGT run.
Logging string 1: DIT/SDT/HLDT/CNTG/NGT
Logging string 2: ACT/GST/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).
During this leg depth versus time data was recorded on a personal computer (part of the L-DEO digital logging data acquisition system in the ODP Downhole Measurements Laboratory aboard the JOIDES Resolution) with input from the Schlumberger MAXIS computer. The depths recorded through this system are the same used by Schlumberger to measure cable length as recorded in the logging data files. For the DIT/SDT/HLDT/CNTG/NGT run these depth data were merged with the data recorded in memory by the Temperature Logging Tool (TLT) by matching absolute time in the two data sets.
For the ACT/GST/NGT run, due to the lack of depth versus time data, a linear relationship of pressure versus depth has been calculated from the pressure reading at the mudline. 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 = Pressure * 10.005 - 1119.1
Depth = mbsf
Pressure = bars
10.005 = pressure conversion coefficient (m/bars)
1119.1 = water depth, mbrf
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 161.
For further information about the processing, please contact:
E-mail: Cristina Broglia