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REPEAT TEMPERATURE MEASUREMENTS IN BOREHOLE CC-I,
NORTHWESTERN UTAH: TOWARDS ISOLATING A
CLIMATE-CHANGE SIGNAL IN BOREHOLE TEMPERATURE PROFILES
Paper number 93GL01877
O094-8534/93/930L-0l 877S03.00
David S. Chapman and Robert N. Harris
Department of Geology and Geophysics, The University of Utah, Salt Lake City, Utah s. Send address
5ervice Departadvance notice.
)tice of address
'missing from te U.S., lost mail days of the tast ujon; for other
Abstract. Temperature-depth profiles in borehole CC-I, northwestern Utah, were measured in 1978, 1990, and 1992.
reproducible over the 14 year period indicating long term thermal stability. A slowly changing temperature field above
80 m depth has similiar characteristics to synthetic temperature profiles computed from a 100 year record of air temperature changes at Park Valley weather station 50 km northeast of the borehole site.
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Borehole temperature profiles contain a rich source of information about the Earths changing climate [extensive references are given in 19 papers on the subject in Lewis,
1992].
Temperature changes at the ground surface diffuse
conduction and are manifested at a later time by a perturbation to the background temperaturc profile over a particular depth range.
A fundamental assumption in reconstructing climate change pntterns from borehole temperatures is that departures from
heterogeneous thermal conductivity medium, departures from a constant heat flow condition are caused by changes in surface temperature with time. Curvature in temperature-depth profiles, however, can have several other causes including: systematic variation of thermal conductivity with depth.
radiogenic heat production, surface elevation effects, lateral variation ot' surface temperature caused by surface orientation and/or vegetation, uplift and erosion or subsidence and burial at the site, and vertical groundwater flow.
Although the
appendix in Chisholm and Chapman [1992]; Lewis and Wang
[1992]) and appropriate corrections to the temperature field made, it would be most useful to he able to isolate directly the climate change signal from these other effects.
One strategy is to make repeat measurements of temperature profiles in specific boreholes at appropriate time intervals
[Shen and Beck, 1991; Clow, 1992; Nielson and Beck, 1989;
Lewis and Wang, 1992]. Because the time scales for most of the perturhing factors listed above are either infinite, or very long, their perturbations may produce significant curvature in the borehole profile but the curvature would he stationary.
Temperature changes in the borehole through time, can then be ascribed to surface temperature histories. The purpose of this paper is to provide details of repeat temperature measurements
northwestern Utnh to understand better how one can isolate a climate change signal from other temperature signals in boreholes.
git5i nion.
Copyright 1993 by the American Geophysical Union,
Temperature Measurements in Borehole GC- 1
Borehole CC-i was drilled in August 1978 to investigate heat flow patterns in the northern Basin and Range tectonic province of the western USA. The site (latitude 410 32' N.
longitude 113° 42' W, elevation 1756 m) is on the south flank of the Grouse Creek Mountains of northwest Utah.
The location was specifically chosen within a granite pluton to avoid problems of rock heterogeneity and in a region of subdued terrain to minimize topographic disturbances to the temperature field.
Sparse but uniform juniper vegetation covers the area giving way to sagebrush and grass at lower elevation.
The 15cm diameter hole was drilled in less than 18 hours to a total depth of 152 in using an air-percussion technique and was completed by insetting a bottom-capped, 64 mm ID PVC pipe and backfilling the annulus with a slurry of dill cuttings.
The pipe was filled with water to facilitate rapid temperature measurements and capped.
Temperatures were measured in CC-i in the summers of
1978, 1990, and 1992, with slightly different techniques each time and with an evolving measurement strategy. In 1978 our objective was to determine heat flow; temperatures were measured at 5 m depth intervals, in 1990 we increased the sampling density to I m depth intervals to delineate small perturbations to the background temperature field. In 1992.
alerted to the possibility that borehole CC-I provided both a possible borehole thermal observatory and a test of extracting climatic information from borehole temperatures, we hegan a more systematic program of detailed and repeat measurements.
All measurements were made using a thermistor probe connected by a lightweight, four conductor cable to a digital resistance meter.
In the 1978 measurements, a Fenwahl
K212E thermistor probe was used with a Data Precision model
2530 multimeter. For the 1990 and 1992 measurements we also used a Fenwahl K212E thermistor probe, but a different probe than that used in 1978. By 1990 we had also replaced our LED Data Precision meter with a LCD Hewlett Packard model 3468A multimeter. The Fenwahl thermistor assembly is contained in a 2-mm diameter, 12 cm-long, stainless steel tube fit onto a 19-mm diameter brass probe connecting the thermistor probe to the cable. The time constant for the probe is 7 s in still water. Our probes are routinely calibrated against a Hewlett Packard model 2804A quartz thermometer. Misfits of individual calibration points to a calibration curve suggest
thermistor resistances are measured to ± In which, for our 10
K mK.
probes, translates into n temperature precision of ± 2.5
Depths in the hole were measured with a Veeder-Root measuring wheel and Veeder-Root model 1127 counter. The probe can be positioned to ± 0.1 wheel revolution, or ± 0.03
m; a positioning uncertainty of 3cm in CC-I is equivalent to a lempefatnie uncertainty of
hysteresis in the measuring wheel should not introduce significant errors to our temperature measurements. For the
1891

1892 logs taken in CC-i the maximum deviation from the average depth of 152.60 m recorded at the bottom of the hole was 0.08
m, and the depth meter always returned to within 0.06 m of zero at thc top of the hole upon completion of logging and rewinding the cable.
Measurements were made by lowering the probe slowly to the desired depth and recording the thermistor resistance after waiting 30 s or four time constants. In 1992 we additionally recorded the thermistor resistance in the first 5 s after reaching the target logging depth. Temperatures at 5 s waiting time are on average 10 mK cooler than temperatures at 30 s waiting time hut there are no systematic variations in the differences with borehole depth or position.
Observations
-
80
Ci
120
140
160
0
20
40
60
Temperature-depth profiles in CC-i for 1978, 1990, and
1992 are shown in Figure Ia. Temperatures above 20 m are influenced by large annual variations at the Earth's surface and thus are not plotted.
In general, the prot'iles are strikingly similar, suggesting a stable thermal environment at this site.
consistent with a heat flow of 98 mW m2 and average thermal conductivity (10 samples) of 3.14W m1K'
Whereas for heat flow studies one is concerned with the overall thermal gradient, in climate change studies one is more interested in departures from a reference thermal state. For this purpose we define a reduced temperature 'Fr given by
T1 (z) = T0(z) - [T0 + F z] where T0 is the reducing intercept
0
20
40
.i978
1992
(a)
60
-E a
C)
80
100
120
140
160
8 10 12 14
Temperature (°C)
16 18
1978
-,
0.1°C
1990
4
Reduced Temperature
1992
(b)
Fig. 1. Temperature and reduced temperature profiles for borehole CC-i. (a) Temperature profiles measured in 1978,
1990, and 1992; 1978 profile is offset by -2.0 °C, 1992 profile is offset by ±2.0°C for clarir. The 1992 profile is the average of the three 1992 logs.
(b) Reduced temperature profiles using a reducing gradient of 31.20 °C km-1 and a reducing intercept of 10.62 °C. Vertical dashed line indicates zero reduced temperature in each case.
Chapman and Harris: Repeat Borehole Temperature Measurements and F is the reducing gradient. Reduced temperature profiles for the 1978, 1990, and 1992 measurements in CC-i are shown in Figure lb. The expanded scale accentuates several
particularly evident in normal temperature-depth plots. First, there is a clear negative reduced-temperature anomaly between
20 m and 80 m in all three profiles. This anomaly has a maximum amplitude at 20 m of -300 mK; it extended to about
65 in depth in 1978 hut is visible to 80 m depth in both the
1990 and 1992 profiles. From MOrn to the bottom of the hole at 152 in the thermal gradient is effectively unchanged in the
14 year period spanned by these measurements. Root-meansquare reduced temperature below 80 m depth is 18, 15, and
16 mK respectively for the three logs shown in Figure lb.
We initially interpreted the scatter in the 1978 reducedtemperature plot as being caused by measurement imprecision, borehole thermal instability, or both. But the 1990 and 1992 reduced-temperature plots (Figure Ib) reveaL a coherent structure to the temperatures in the borehole that accounts for the amplitude of scatter in the 1978 sparsely sampled profile.
understand the fine structure of the temperature-depth profile in CC-I. The systematic pattern of reduced-temperature
characteristic length about 4 in.
The magnitude of local gradient deviation from the reducing gradient is up to 15 o km-t. Because thermal conductivity variation in the granite is small (see Figure Ai in Chishoim and Chapman [1992]), and the short wavelength temperature excursions at depth cannot be maintained by processes at the Earth's surface, we had focussed on the possibility of thermal instability in the borehole as a cause of the t'ine structure. We thought that a sequence of temperature logs could be used to average out the random fluctuations. Figure 2 shows convincingly that this hypothesis was incorrect.
A sequence of three temperature logs made over a three day
The measurements were made on July 7 from 16.00 to 20.30 hrs, on July 8 from 08.30 to 12.00 hrs, and on July 9 from 1530 to 20.00 hrs. A minimum waiting period between logs is necessary to allow the borehole to recover its equilibrium thermal state after the slight mixing by lowering and raising the temperature probe though the water column in the cased borehole. Mixing does not seem to be a problem, however, as the three temperature profiles are nearly identical. Root-meansquare differences, computed point by point, between logs 1 and 2, 1 and 3, and 2 and 3 are 11 mK, 15 mK and 17 mK respectively. Most important, however, is the replication of detailed features in all three logs, indicating that the process
stationary at least over a penod of three days.
To test whether the fine structure might be stationary over a longer period of time, we compare reduced-temperature profiles for 1992 and 1990 (Figure 2). Apart from systematic differences in the depth range 20 to 50 m, which we attribute as a climate signal, the profiles are again remarkably similar.
The root-mean-square difference between the two profiles in
Figure 2 (right) below 50 m is iS mK, no greater than the differences measured over a period of three days in 1992. The fine structure is evidently stationary in space and time.
Temporal Changes in Temperature
Acquisition of temperature logs over a span of 14 years allows us to compute whether the temperature-depth profile is changing with time, and if so, whether those changes can be attributed to past changes in the surface temperature with time.
120
140
160
Fig. 2. Re stability of and reducim logs meast average 19 three logs)
Figure 3 sh two plots te in pe rat u n third plot si three diffei
But hetwe significant shows bro; amplitude
1992 minu5 and the up
Warming. '1
Wam-ming trc
The Crou at Park Va kept for ahi records is
With Keltom gaps in the appropriate
Figure temperaturi
Synthetic
Station rec temperaturt at n times ç calculate a a preobser temperatti ri
20
60
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20
60
Chapman and Harris: Repeat Borehole Temperature Measurements
0
-0.3 -0.2 -0.1
0 -0.3 -0.2 -0.1
0 0.1
0
,.
0.1°C
40
60-.
E
80 -
V
120
140
1992
- 1978
1992
- 1990
1893
80
2
100
120
140
O
o
160
stability of fine structure. Reducing gradient is 31.20°C kmt and reducing intercept is 10.62 °C. (Left) Three temperature logs measured between July 7 and 9 in 1992. (Right) The average 1992 profile (ie. average of temperature measured on three logs) compared to the 1990 profile.
Figure 3 shows three temperature difference plots. The first two plots are of lower sampling density because in 1978 temperatures were measured only at 5 m depth intervals. The third plot shows temperature differences at I m intervals. All three difference plots show negligible change below 80 m.
significant hut different signals. The 1990 minus 1978 plot shows broad cooling from 20 to 75 m with a maximum amplitude of 1(X) mK. This cooling feature still exists in the
1992 minus 1978 plot, hut its amplitude is slightly diminished and the uppermost difference point at 20 m shows possible warming. The detailed 1992-1990 difference plot confirms the warming trend in the uppermost 50 m of the hole.
The Grouse Creek site is only 50 km from a weather station at Park Valley, Utah. where temperature records have been kept for about 60 years (Figure 4). An additional 40 years of records is obtained by combining the records of Park Valley
With Kelton, 20 km southeast of Park Valley, and filling in gaps in the records with data from Snowville and Wendover appropriately adjusted for station offsets.
temperatures for Kelton - Park Valley. We have computed synthetic borehole temperature profiles from the weather station record at Park Valley. assuming that the surface temperature history is composed of n individual step changes at a times prior to the borehole temperature measurement. To
Calculate a synthetic temperature profile, one niusl also assume a pit-observational mean temperature, ie. the reference surface
(emperature in the absence of climatic warming or cooling.
160
Temperature Difference
Fig. 3. Temperature changes in CC-i through time. Circles represent differences of measurements: 1990 minus 1978
(left), average 1992 minus 1978 (center), and average 1992 minus 1990 (right). Solid line is the corresponding synthetic temperature difference profile computed from the 100 year meteorological record of air temperatures at Park Valley weather station, 50 km to the northeast of CC-I.
12 to
2
C) p.
7_
POM
Snowvillc
5 Park Valicy
Kelton
4 -
\Vendover
ft
I lit
1860 1880 1900 1920 1940 1960 1980 2000
Year
Fig. 4. Surface air temperature record for Park Valley, Utah.
Inset shows source of data to make composite record adjusted for Park Valley. Dashed line prior to 1890 shows model temperature used to compute synthetic temperature-depth profiles for this record.
The dashed line prior to 1890 on Figure 4 shows the assumed early temperature history for Park Valley [Chishoim and
Chapman, 1992].
Three synthetic temperature profiles using the air temperature records were computed for the time series ending at times corresponding to our 1978, 1990. and 1992 measurements respectively. Differences between these synthetic profiles for
1990-1978, 1992-1978. and 1992-1990 were computed and are shown on Figure 3 as solid lines. The Park Valley surface
difference for 1990-1978 in the depth range 20 to 80 m as observed at CC-I although the amplitude between 20 and 40 iii exceeds the observed difference by ahout 100 mK. The
1992- 1978 difference predicted from Park Valley shows a diminished cooling anomaly from 30 to 50 m, a slightly larger negative anomaly from 50 to7O m, and a switch to a positive anomaly above 30 m. All these trends mimic the observations at CC-I. Finally, the 1992-1990 synthetic predicLs only very

1894 Chapman and Harris: Repeat Borehole Temperature Measurements slight cooling (c 5 mK) between 45 and 90 m hut significant warming above 40 m. Although the amplitude of the predicted warming at Park Valley does not precisely match the observed change at CC-I, we place considerable significance on the reversal of the sign of the anomaly in both the synthetic and measured temperature differences. We do not know whether the misfit in amplitudes of temperature differences is due to the failure of ground temperatures at CC-i to track accurately air temperatures at the site or is due to real differences in air or ground temperature histories at the two locations 50km apart.
Discussion noise, borehole stability, measurement repeatability, and ultimate signal resolution.
Finally, in addition to resolving the source of temperature curvature in CC-i our detailed repeat measurements also
temperature profiles. The fine structure is persistent but, in the upper 80 m of the borehole, is transparent to the smooth transient changes thai we aurihute to the climate change signal.
responsible for the fine structure (thermal conductivity variations, convective instabilities in the cased horehole or outside the casing, slow groundwater percolation through fractures in the granite or in the borehole annulus outside the casing) and are presently devising temperature monitoring and
elucidate this phenomena.
Temperature profiles in borehole CC-I (Figure 1) have a systematic negative temperature anomaly and significant curvature in the uppermost 60 m of the borehole. Cooling inferred from the GC-1 temperature profile, however, runs contrary to modest warm ing seen in this century from weather station records in much of the western US and to trends inferred from other borehole temperature profiles in western
Utah [Chisholm and Chapman. 1992]. Although the effects of thermal conductivity variations, heat production, topography.
groundwater flow for site CC-i were calculated or estimated
(see Appendix of Chisholm and Chapman [1992]) and shown to be small, a combination of these effects could not he ruled out as a source of the curvature in the CC-i temperature profile.
Our series of repeat measurements presents an opportunity to discriminate between these possible explanations. We have observed a slowly varying temperature field that is inconsistent with steady-state causes of borehole temperature curvature. A clear correspondence between the differences in measured temperatures between 1978 and 1992 and the differences in synthetic temperatures computed from a weather station time series of air temperatures at Park Valley, 50 km northeast of the borehole, now seem to favor a climate signal as the source of the negative temperature anomaly and curvature.
A more rigorous test of the ability of a borehole temperatures to respond predictably to changes in surface temperature
observatory mode with a weather station installed at the borehole. We are installing an observatory at borehole CC-i; other observatory mode studies are in progress in Alaska (C.
Clow, personal comm. 1993) or planned for the Great Plains
(W. Gosnold, personal comm. 1993). Our observatory mode installation will enable us to cheek the detailed local tracking of air and ground temperatures at multiple depths.
Because repeat measurement programs are likely to become common, especially for climate reconstruction studies, it is important to establish procedures and understand the problems associated with repeat logging. Our reduced temperature plots and associated statistics contain useful indicators of logging
Acknowledgments. This work was supported by NSF grants EAR-9104292 and EAR-920503i.
Riki Darling assisted with the temperature measurements in 1978. Tim
Chisholm in 1990.
References
Chisholm, 1. J., and D. S. Chapman, Climate change inferred from analysis of borehole temperatures: an example from western Utah, J. Ceophys. Res. 97 14155-i4175, 1992.
Clow, C. D., The extent of temporal smearing in surfacetemperature histories derived from borehole temperature measurements, Palaeogeogr.. Palaeoclimatol. Palaeoeeol.
(Global and Planet. Change Sect.),2R, 81-86, 1992.
Lewis, T. J., Ed. Spec. Issue, Climatic Change Inferred Front
Underground Temperatures. Palaeo°eogr.. Palaeoclimatoi.,
Palaeoecol. (Global and Planet. Change Sect.), 22i. 281 pp.,
1992.
Lewis, T. J.. and K. Wang, Influence of terrain on bedrock temperatures Palaeoc'eur.. Palaeoclimatol.. Palaeoecol.
(Global and Planet. Change Sect.), , 87-100, 1992.
Nielsen, S. B., and A. E. Beck, Heat flow density values and paleoelimate determined from stochastic inversion of four temperature-depth profiles from the Superior Province of the
Canadian Shield, Teetonophysies 164 345-359. 1989.
Shen, P. Y.. and A. E. Beck, Least squares inversion of borehole temperature measurements in functional space, L
Geophys. Res.
19965-19,979, 1991.
D. S. Chapman and R. N. Harris, University of Utah.
Geology & Geophysics, Salt Lake City, Utah 84i i2-1 183
(Received May 6, 1993; revised June 23, 1993; accepted July 8. 1993.)
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