Temperature Data Processing

 

ODP logging contractor: LDEO-BRG

Hole: 903C

Leg: 150

Location: New Jersey Offshore (NW Atlantic)

Latitude: 38° 56.303' N

Longitude: 72° 49.024' W

Logging date: June, 1993

Bottom felt: 457.5 mbrf (used for depth shift to sea floor)

Total penetration: 1149.7 mbsf

Total core recovered: 323.2 m (60.4 %)

 

Water Depth: 446 mbsl

Temperature Tool Used: LDEO-TLT

Depth versus time recording available: NO

 

Logging Runs

 

Logging string 1: DIT/DSI/NGT

 

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

 

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 = 975 * (Pressure - 35)/(140 - 35)

 

where

 

Depth = mbsf

Pressure = bars

140 bars = maximum pressure recorded

35 bars = pressure at mudline

975 mbsf = maximum logged depth

 

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:

 

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