Temperature Data Processing
ODP logging contractor: LDEO-BRG
Hole: 892C
Leg: 146
Location: Cascadia Margin (NE Pacific)
Latitude: 44° 40.44' N
Longitude: 125° 7.13'W
Logging date: November, 1992
Bottom felt: 685.5 mbrf (used for depth shift to sea floor)
Total penetration: 176.5 mbsf
Total core recovered: 7.4 m (4 %)
Water Depth: 674.4 mbsl
Temperature
Tool Used: LDEO-TLT
Depth versus time recording available: NO
Logging
Runs
Logging string 1: DIT/SDT/NGT
Logging string 2: HLDT/CNTG/NGT
No information available about use of wireline heave compensator.
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 are then converted to depth using a pressure/depth conversion that is linearly interpolated between the values determined at the mudline and total logging depth.
Depth = BD *(Pressure-MudlineP)/(Pmax- MudlineP)
where
Depth = mbsf
BD = bottom depth (mbsf)
Pressure = bars
Mudline P = bars
Pmax = pressure at bottom depth (bars)
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.
The following processing has been performed at Hole 892C:
Logging Run: DIT/SDT/NGT
Mudline P=297 bars
Pmax=98.3 bars at 125 mbsf
Depth = 125*(Pressure - 81)/(98.3 - 81)
Depth calculated from pressure at mudline and bottom of logged interval.
Temperature from slow thermistor only.
In order to eliminate some of the noise, the pressure data have been smoothed using a 11-sample window.
Logging Run: HLDT/CNTG/NGT
Mudline P=80 bars
Pmax=97.6 bars at 125 mbsf
Depth = 125*(Pressure - 80)/(97.6 - 80)
Depth calculated from pressure at mudline and bottom of logged interval.
Temperature from slow thermistor only.
Information about the temperature logging operations can be found in the Site Chapter (Operations, Downhole Measurements, and Heat Flow sections), ODP IR volume 146.
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia