Jökull - 31.12.2001, Qupperneq 47

Seismic monitoring during a wastewater injection experiment in Svartsengi
system has a transecting fissure swarm which ex-
tends about 9 km offshore, along the Reykjanes ridge
(Sigurgeirsson, 1995). The Svartsengi field lies at the
junction of a small NE-SW striking rift valley and the
RSZ. The Svartsengi and Eldvörp geothermal systems
are linked as fluid pressure variations in the Svarts-
engi field have affected the Eldvörp field, 6km to the
west (Björnsson and Steingrímsson, 1991).
Figure 1. Seismicity on the Reykjanes Peninsula in
1993. A broken line shows the location of the plate
boundary. Seismic stations in operation during June-
August are denoted by black triangles in the mid-
dle figure. Fissure swarms are shaded. The loca-
tion of the Reykjanes (R), Eldvörp (E), Svartsengi (S),
and Krýsuvík (K) high-temperature geothermal fields
are also shown. The inset shows the location of the
Reykjanes Peninsula within Iceland. The earthquakes
(denoted by black circles) have a location error of
3 km or less, according to the SIL-network of the Me-
teorological Institute of Iceland. The largest event
reached M=3.6. The eastern earthquake cluster is as-
sociated with the Krýsuvík geothermal area and adja-
cent fissures. No surface geothermal activity is visible
at the mountain Fagradalsfjall (F) where the western
earthquake cluster originated in a single NS-trending
swarm, on September 18-19. - Jarðskjálftar (svartir
deplar) á Reykjanesskaga árið 1993 samkvœmt SIL-
staðsetningarkerfi Veðurstofu Islands. Stœrsti skjálft-
inn var 3,6stig á Richterskvarða. Ovissa í skjálfta-
staðsetningum er innan við 3 km en engir þeirra höfðu
upptök í jarðhitasvœðinu í Svartsengi. Brotna línan
táknar plötuskilin. Þau liggja þvert yfir eldstöðva-
kerfin (skyggðu svœðin). Helstu jarðhitasvœði eru
Reykjanes (R), Eldvörp (E), Svartsengi (S) og Krýsu-
vík (K). Svartir þríhyrningar á miðmyndinni tákna
staðsetningar fœranlegra jarðskjálftamœla sem rekn-
ir voru í Svartsengi í júní, júlí og ágúst, 1993.
A geothermal power plant has been in operation
in Svartsengi since 1976 (Figure 2). The exploita-
tion of geothermal fluids has caused more than 20 bar
pressure drawdown within the wellfield (Vatnaskil,
2002) and a cumulative surface subsidence of 237 mm
during 1976-1999 (Eysteinsson, 2000). As contin-
ued drawdown may diminish the wellfield productiv-
ity an injection of wastefluids (70-80°C) has been car-
ried out intermittently since 1984. Documented cases
of injection-induced earthquakes within geothermal
fields (i.e. Batini et al, 1985; Sherburn et al., 1990)
prompted the installation of a local seismic monitor-
ing network within and around Svartsengi during the
1993 injection experiment, results of which are re-
ported here.
THE SVARTSENGI GEOTHERMAL
FIELD
Hyaloclastites and basaltic lavas characterize the Neo-
volcanic zones of Iceland. Lithological cross sections
from boreholes in geothermal areas consist of alter-
nating sections of irregular hyaloclastite masses in
between interglacial and postglacial lava sequences.
A geological cross section of the Svartsengi field
derived from 11 boreholes contains six hyaloclastite
units down to 1 lOOm depth (Franzson, 1990).
The Svartsengi geothermal system consists of
three different aquifer systems. The uppermost aqui-
fer (at 30-300 m depth) contains a thin freshwater lens
on top of saline groundwater. This aquifer has tem-
peratures between 30°C and 100°C and lateral flow
towards SW. Beneath it is a steam dominated aquifer
which extends to the surface in the easternmost part
of the system and a liquid dominated brine water sys-
tem with temperatures of 230-240°C below 600 m
depth (Björnsson and Steingrímsson, 1991). Hyalo-
clastites with low permeability at 300-600 m depth
separate the upper-level, warm-water aquifer from
the main high-temperature reservoir (Franzson, 1990;
Björnsson and Steingrímsson, 1991). Intrusions are
found below 800 m depth and dominate the strata
from lOOOm to 1400m depth. The main aquifers of
the high-temperature system follow NNW-SSE and
NE-SW fractures and intrusion boundaries. The av-
erage porosity between 400 and 1000 m depth is ap-
proximately 10% within the Svartsengi reservoir but
around 18% outside the geothermal field (Franzson,
1990).
Various geophysical measurements have been
made within the wellfield since 1976 for the purpose
of monitoring the geothermal exploitation. However,
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