Temperature Data Processing

 

ODP logging contractor: LDEO-BRG

Hole: 841C

Leg: 135

Location: Tonga Trench (tropical SW Pacific)

Latitude: 23° 20.72' S

Longitude: 175° 17.879' W

Logging date: February, 1991

Bottom felt: 4821 mbrf (used for depth shift to sea floor)

Total penetration: 775 mbsf

Total core recovered: no coring performed. Hole dedicated solely to logging.

 

Water Depth: 4809.8 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: FMS/GPIT/NGT

 

No information available from logging reports about use of wireline heave compensate counter ship heave.

 

Tool Information

 

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).

 

Data Processing

 

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 841C:

 

Logging Run: DIT/SDT/HLDT/CNTG/NGT

Mudline P=484.5 bars

Pmax= 546bars at 580 mbsf

Depth = 580*(Pressure 484.5)/(546 484.5)

Depth calculated from pressure at mudline and maximum depth logged

 

Logging Run: FMS/GPIT/NGT

Mudline P=491.5 bars

Pmax=559.5 bars at 605 mbsf

Depth = 605*(Pressure 491.5)/(328 491.5)

Depth calculated from pressure at mudline and maximum depth logged

Pressure data were smoothed using a 15-sample window in order to reduce some of the noise.

 

Information about the temperature logging operations can be found in the Site Chapter (Operations, Downhole Measurements, and Heat Flow sections), ODP IR volume 135.

 

Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia