Temperature Data Processing
ODP logging
contractor: LDEO-BRG
Hole: 925A
Leg: 154
Location: Ceara Rise (Tropical NW Atlantic)
Latitude: 4° 12.256' N
Longitude: 43° 29.349' W
Logging date: February, 1994
Bottom felt: 3053 mbrf
Total penetration: 930.4 mbsf
Total core recovered: 490.05 m (52.7 %)
Water Depth: 3046.6 mbrf
Temperature
Tool Used: LDEO-TLT
Depth versus time recording available: NO
Logging
Runs
Logging string 1: DIT/SDT/HLDT/CNTG/NGT Pass 1
Logging string 2: DIT/SDT/HLDT/CNTG/NGT Pass 3
The wireline heave compensator was used to counter the moderate ship heave. The WHC was switched off at 240 mbsf during DIT/SDT/HLDT/CNTG/NGT pass 1 and 301.5 mbsf during DIT/SDT/HLDT/CNTG/NGT pass 3.
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).
Run 1. The pressure data has been initially smoothed using a 50-point running average 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 = 436 * (Pressure - 343)/(369 - 318) (run 1)
where
Depth = mbsf
Pressure = bars
369 bars = maximum pressure recorded
318 bars = pressure at mudline
436 mbsf = maximum logged depth
The temperature data has been smoothed with a 10-point running average.
Run 2. Original data for this run were not available at the time of revision of the processing performed in 1994. Originally, the pressure data had been smoothed using a 5-point running average prior to depth calculation and subsequently the data had been decimated by taking every fifth point. There is no documentation available about the relationship used to compute depth.
In June, 2000 the very noisy pressure data have been smoothed using a 20-point running average and a linear fit has been applied to the available pressure-depth data. The inferred depth/pressure relationship is:
Depth = 9.3655 * Pressure -3053
where
Depth = mbsf
Pressure = bars
3053 mbrf = "bottom felt" depth (drillers' sea floor)
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 154.
For further information about the processing, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia