Dakota Aquifer Program--Subsurface Hydrology
Optical-fiber temperature logging, part 2 of 4
The Borehole Sites
Boreholes in the Paleozoic sedimentary sequence in Kansas (Midcontinent,
USA) were selected for this comparative study because of the large contrast
in thermal conductivity in the alternating sedimentary units and the
consequent distinctive thermal profile. The temperature measurements
were performed in two boreholes deep enough to test the entire length of
the fiber optic cable (nominally 1000 m). Table 1 gives the location and
logging dates of the two wells. Both boreholes were drilled by the U.S.
Geological Survey and studied extensively in the framework of the evaluation
of heat tlow and geothermal potential (Blackwell and Steele, 1989). The
boreholes are cased and filled with water almost to the surface. They have
been measured periodically for a variety of purposes, but in general left
undisturbed so that temperature equilibrium has been attained. Thus, the
borehole sites offer an unique opportunity to acquire highly accurate
temperature measurement comparisons.
Table 1. Location and logging dates for boreholes included in this study.
| Section/township/range | Borehole name | Date Logged |
| | | electric-line tool | DTS |
| NWNWSE sec. 13, T12S-Rl7E | Big Springs | 7 March 94 | 24 February 94 |
| SWSWSW sec. 32, T13S-R2W | Smokyhill | 27 June 95 | 19 June 95 |
Comparison of Temperature Measurements
For this study a comparison of the DTS results to data obtained by a
high-resolution conventional tool (base-line data) is made because the
latter technique is regarded as the state of the art in high-precision
thermal logging for heat-flow studies.
Distributed Optical Fiber Logging (DTS)
The DTS system used for the well logging consists of a gradient-index
fiber (temperature limit from -100 degrees C to +750 degrees C), a
transmitting and recording device for measuring the intensity of the
Raman backscattered light, and a PC acting as a data acquisition and
controlling unit for the DTS device. The laser pulse through the optical
fiber has a wavelength of 1064 nm, and the pulse duration of the laser
light is 10 ns. For details of the principle of measurement the reader
is referred to Hurtig and others (1994). The DTS unit is coupled with
an optical-fiber cable (the distributed sensor) of 1000 m length. The
cable consists of a high-grade steel tubule (2mm diameter) in which two
optical fibers coated by an acrylic material are embedded. The temperature
limit of the coating is 80 degrees C, The entire fiber system is embedded
in plastic material. To achieve a closed fiber loop the two fibers
embedded in the cable are connected in a specially designed waterproof
tool at the end of the cable. To connect the optical fibers of the
cable with the fibers inside the DTS device a link is set using two
pigtails. According to the manufacturer the DTS system allows temperature
measurements with a resolution of plus or minus 0.1 degree C and an
absolute temperature precision of about 0.3 degree C. The resolution
and precision of the system were confirmed by laboratory test of the
fiber used in this study. The system is calibrated using a
fiber-specific calibration function that is dependent on the fiber
properties and their temperature dependence. The calibration is
accomplished by heating a defined length of the fiber in a constant
temperature water bath. Because the temperature/Raman intensity ratio
is linear only two different temperatures are necessary to define the
calibration function.
In the field the cable is lowered first into the well to be logoed.
After the cable is installed in the borehole, it is allowed to equilibrate
for approximately 30 minutes before temperature recording is began. The
temperature measurement is simultaneous for the entire cable length. To
identify the borehole temperatures when the hole is shallower than 1000 m
from the data measured along the portion of the cable at the surface, a
hot spot (heated point) is set at the borehole collar. The present
experimental procedure is to make 5 recordings requiring a 1-minute
interval each. Investigations of the effect of different integration
times on the signal/noise ratio show that for the 1 m-distance intervals
used best results can be achieved with a 1-minute integration time. If
smaller logging intervals are needed (e.g. 0.5 m) an oversampling of data
would be necessary. But in this situation the signal/noise ratio
deteriorates and reduces the temperature accuracy. After completion of
temperature recording the five temperature curves then are stacked, and
the final temperature versus depth is obtained. In contrast to conventional
logging where temperatures are obtained sequentially, perhaps during a
time period of hours, the DTS data reflect instantaneous temperatures at
the selected depth interval along the fiber length (depth) in the borehole.
After completion of the 5 recordings and a total elapsed time of about 45
minutes the cable is removed from the borehole.
Conventional temperature logging
The conventional logging tool used for the comparative study was an
updated version of the electric-line tool described in Blackwell and
Spafford (1987). It uses an Analog Devices 590 integrated circuit
which outputs a current proportional to temperature. The logging
system has a resolution of plus or minus 0.001 degree C, precision of
plus or minus 0.1 degree C, and a (programmed) sample interval of 0.1 m.
The signal is passed to the surface through a 4000 m, single conductor
armored cable and displayed and recorded on a PC. The probe was lowered
in the borehole with a logging speed of about 3-5 m/min. For a typical
probe response times of 3-7 s, logging at this speed does not require
deconvolution to obtain the actual water temperature of the recorded
point. Although temperature recordings are made using a 0.1 m sample
interval, only data in 1-m distance intervals are used for the processing
described here in order to be comparable with the DTS data.
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Kansas Geological Survey, Dakota Aquifer Program
Updated Sept. 30, 1996
Scientific comments to P. Allen Macfarlane
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URL=http://www.kgs.ku.edu/Dakota/vol1/hydro/DTS2.htm