Jökull - 01.12.1976, Page 26

Using MKS units and inserting for conditions
in geothermal areas in Iceland för water p =
103kg/m3 and for flood-basalts p = 3 • 1010
Newton/m2 we find that
0) = (6.7 • 103/R)Va/hR , (16)
and hence the period
r = 9.4 • 10-4RVhR/a , sec (17)
where R, h and a are measured in meters and
meters2, respectively. The period t computed
on the basis of equation (17) is shown in Fig. 5
as a function of R and h/a for a range of
values of interest.
discussion
Provided that our tentative interpretation of
the small amplitude oscillations shown in Fig. 1
is correct, and using the results for the above
simple cavity-resonator model, we find that the
hypothetical radius of the oscillating cavity
should in this case be about 400 meters. This
result is not unreasonable in the flood-basalt
enviroment of Iceland. There are fracture-
like cavities at the contacts of lava layers and
along the walls of dikes. This shows that some
inferences of practical value may possibly be
drawn on the basis of observed cavity oscilla-
tions. On the other hand, since we know very
little about the geometry of the underlying real
system, we are unable to embark on a discus-
sion of the possible excitation mechanisms.
At this end, we would like to remark that
cavity resonances of the above type may be of
interest in volcanology. Unfortunately, no ob-
servational data are available to discuss this
interesting subject.
ACKNOWLEDGEMENT
This toork was supported by the U.S. Energy
Research and Development Administration
under contract E(45-l)-2227.
REFERENCE
Bodvarsson, G. 1950: Geophysical methods in
the prospecting for hot water in Iceland.
J. Eng. Assoc. (Iceland), 35, 49—59. (In
Danish).
— 1969: On the temperature of water flowing
through fractures. T. Geophys. Res., 74,
1987-1992.
Elmore, W. C. and M. A. Heald. 1969: The
Physics of Waves. McGraw-Hill, 477 pp.
Love, A. E. H. 1927: A Treatise of the Mathe-
matical Theory of Elasticity, 4th ed. Dover
Publications, 643 pp.
Lowell, R. P. 1975: Circulation in fractures, hot
springs and convective transport on mid-
ocean ridge crests, Geophys. J. R. Astr. Soc.,
40, 351-366.
— and G. Bodvarsson. 1975: Finite difference
models for flowing boreholes. Second U.N.
Symposium on the Development & Use of
Geothermal Resources, San Francisco,
1975.
ÁGRIP
Mældar hafa veriS smávægilegar hitasveiflur
í frárennsli borholu við Elliðaárnar í Reykjavík.
Tíðnigreining sýnir sveiflur með 17 mínútna
öldulengd. Þetta fyrirbrigði er skýrt með því,
að holan sé tengd holrúmi neðanjarðar, og fjað-
urmagn umlykjandi bergs valdi því, að vatns-
súla holunnar geti sveiflast lóðrétt. Fræðileg at-
hugun sýnir, að þvermál holrúmsins þarf að
vera um 800 m.
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