Temperature Data Processing
ODP logging contractor: LDEO-BRG
Location: New Jersey Offshore (NW Atlantic)
Latitude: 38° 56.079' N
Longitude: 72° 46.375' W
Logging date: June, 1993
Bottom felt: 819 mbrf (used for depth shift to sea floor)
Total penetration: 740.1 mbsf
Total core recovered: 676.96 m (91.9%)
Water Depth: 808 mbsl
Temperature Tool Used: LDEO-TLT
Depth versus time recording available: NO
Logging string 1: DIT/SDT/NGT
Logging string 2: FMS/GPIT/NGT
The wireline heave compensator was used. Seas were calm.
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).
DIT/SDT/NGT run. The pressure data has been initially smoothed using a 25-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 = 216 * (Pressure - 63.6)/(83.5 - 63.6) (DIT/SDT/NGT run)
Depth = mbsf
Pressure = bars
83.5 bars = maximum pressure recorded
63.6 bars = pressure at mudline
216 mbsf = maximum logged depth
There is an almost constant difference of 0.5 degC between the readings of the two thermistors. Because of its more accurate calibration, the value given by the slow thermistor, which is higher, is the most reliable.
FMS/GPIT/NGT run. The pressure data has been initially smoothed using a 100-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 = 690 * (Pressure - 80)/(163 - 80) (FMS/GPIT/NGT run)
Depth = mbsf
Pressure = bars
163 bars = maximum pressure recorded
80 bars = pressure at mudline
690 mbsf = maximum logged depth
The fast thermistor did not function properly during this run. Use of the Side Entry Sub (SES) and extensive pumping due to the difficult hole conditions are responsible for the poor temperature data.
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 150.
For further information about the processing, please contact:
E-mail: Cristina Broglia