Jökull - 01.12.1976, Page 23

or springs is a theoretical possibility. However,
from the instrumental point of view this is a
difficult task and the precision which can be
attained by available instrumentation would
rarely be satisfactory for the present purpose.
Because of the general difficulties of observ-
ing minor flow oscillations, it is of interest to
note that flowing geothermal systems provide
a special case where such oscillations may prob-
ably be observed rather easily with a relatively
high precision. This is based on the fact that
the thermal water issued by springs or bore-
holes of relatively small mass flow is subject
to quite considerable cooling during its ascent
to the surface. The water loses heat by con-
duction into the country rock. For example,
the temperature of thermal water issued by out-
lets which have a mass flow of less than one
kg/sec may have lost several or even several
tens of centigrades during its ascent. Some of
the relevant mathematical and physical pro-
blems have been discussed by Bodvarsson (1950,
1969), Lowell (1974) and Lowell and Bodvars-
son (1975). It can therefore be expected that
the surface temperature of boreholes and springs
of small flow will be affected by fluctuations
in the mass flow. Although the theory of this
phenomenon is not entirely simple, it is rather
obvious that the temperature will fluctuate
with the mass flow. A smaller flow will lead
to a larger temperature loss and hence lower
surface temperature and vice versa.
The temperature of water is a physical
quantity which usually can be observed with
a high precision. This opens up the possibility
of observing flow oscillations in thermal systems
by recording the temperature of boreholes and
springs of a relatively small mass flow. First
attempts to carry out such observations on
boreholes and springs are now being made in
Iceland by the junior author (A. B.). The pur-
pose of the present paper is to report on some
initial results and to derive an approximate
theory of hydroelastic cavity oscillations in sim-
ple systems.
TEMPERATURE FLUCTUATIONS
IN A FLOWING BOREHOLE
A thermal borehole of 1010 m depth and 340
mm inner casing diameter located at the out-
skirts of the city of Reykjavik was chosen for
the first field tests. This borehole has a mass
flow of about i/2 kg/sec entering at the depth
of 650 m and a surface temperature of 42.5 °C.
A thermistor temperature recorder in a thin
plastic cover was installed in the borehole at
a depth of 10 m. The instrument has a suf-
ficiently short response time and a precision
of about 1 millidegree C.
An 8 hour section of a temperature record
from this borehole is shown in Fig. 1. The raw
record shows temperature fluctuations with
amplitudes up to 0.02 °C and apparent periods
of the order of one or two tens of minutes. A
maximum entropy spectrum of this record is
shown in Fig. 2. The record exhibits a rather
significant spectral peak at a period of roughly
103 seconds, that is, 17 minutes.
The observational data from this borehole
confirm the existance of small oscillations which
can possibly be interpreted as hydroelastic os-
cillations of the type described in the intro-
duction above. At this juncture, we are unable
to present a solid support for this conjecture,
but we find the case interesting and further
data are now being recorded on other suitable
systems in Iceland. To supplement the present
discussion, we will below derive an approx-
imate theory of very simple borehole-cavity
hydroelastic systems with fracture type cavities.
Fig. 1. An eight hour
temperature record from
a thermal borehole in
Reykjavik.
Mynd 1. Hitasveiflur i
rennsli úr borholu við
Elliðaár í Reykjavík.
JÖKULL 26. ÁR 21