Jökull - 01.12.1982, Page 28

TABLE 1. Estimates ofheat transfer model dimensions for the Reykholtsdalur system.
TAFLA 1. Aœtlaðar stærðaríólur um varmajlutning í jarðhitakerjinu í Reykholtsdal.
Heat transfer
models State Assumed parameters Resulting estimates
steady D = 3km, AT = 175°C, d = 1 m Length: 1000 km
Single transient t = 3 X 10us, d = 1 m
pipe AT = 175°C 700 km
steady D = 3 km, e = 0.54, E = 0.70 Area: 2000 km2
Horizontal flow t = 3 X 10us
sheet
transient e = 0.54 240 km2
t = 3 x 10" Total horizontal
Convective downward AT = 200°C fracture length 40 km
migration (CDM) in transient C = 5 x 108 J/m3
a vertical fracture M = 3 km Total vertical
v = 10-8 m/s contact area 120 km2
A = Effective temperature diíferential
General assumptions: Thermal conductivity of formations
Thermal diífusivity of formations
Local temperature gradient
k = 2 W/m,°C
a . 10_6m2/s
g = 80°C/km
really a quite eífective heat transfer element as com-
pared to, for example, the horizontal flow sheet.
The rather drastic difference between these two
models results from the fact that the pipe is supplied
by a radial conduction flow, while the sheet is supp-
lied by a one-dimensional flow. However, the effec-
tiveness of the pipe is greatly reduced in a grid of
pipes where oonductive interaction develops. Equa-
tion (4) indicates that a sufliciently dense grid of
pipes is essentially equivalent to a flow sheet when
the spacing satisfies the inequality s«(7rat)1/2/2.
Reviewing the results in Table 1, and disregard-
ing the single pipe model, we find that the total area
estimated for the horizontal flow sheet in a steady
state situation is very large. In fact, it would appear
diflicult to place this area between the source region
and the outlets of the Reykholtsdalur system. More-
over, the very serious quesdon arises whether the
flood basalt series includes single open flow sheets
or interconnected networks of flow sheets of this
enormous extent.
The transient state flow sheet has a much smaller
estimated area and poses therefore fewer problems.
Nevertheless, even in this case it is appropriate to
question whether the flood basalts series includes
open horizontal sheets or networks of sheets ofa few
102km2 area.
The CDM model poses the least problems. As a
matter of fact, Fig. 1 indicates that 5 dikes outcrop
in the area, and we can only surmise that there may
be more. Its is quite conceivable that each dike has
an effective length of the order of 5 to 10 km where
CDM could take place. There would thus appear
little difliculty in accounting for the total CDM
length of40 km estimated inTable 1.
The present discussion can not be extended bey-
ond this stage. We can only conclude that the steady
state horizontal flow sheet is an unattractive model
while its transient counterpart and the CDM
model are much more realistic possibilities. On the
balance the CDM would appear to have a stronger
case.
As a matter of course, transient models can be
invoked only if there is a plausible cause or explana-
tion for the initiation of the processes involved.
Referring to the discussion above, this does not
appear to be a serious problem as far as LT geo-
thermal systems in Iceland are concerned. The
26 JÖKULL 32. ÁR