Deformation at Krafla and Bjarnarflag geothermal areas,

Northern Volcanic Zone of Iceland, 1993-2015

Vincent Drouin(1), Freysteinn Sigmundsson(1), Sandra Verhagen(2), Benedikt G. Ófeigsson(3), Karsten Spaans(4),

Sigrún Hreinsdóttir(5)

GEORG Geothermal Workshop

24 November 2016

(1) Institute of Earth Sciences, University of Iceland, 101 Reykjavík, Iceland(2)Delft University of Technology, department of Geoscience and Remote Sensing, 2628 CN Delft,

Netherland(3) Icelandic Meteorological Office, 101 Reykjavík, Iceland

(4)COMET, School of Earth and Environment, University of Leeds, Leeds, United Kingdom(5)GNS Science, Lower Hutt 5040, New Zealand

Model NVZ of Iceland, 2008-2014

Introduction

Introduction

InSAR

"Interferometric Synthetic Aperture Radar" from satellites

The first civil satellite was launched in 1991 : ERS-1 Measurements depend on the choosen frenquency bandLine-of-sight measurements Can operated at night and with cloud cover

An interferogram contains contributions from various signals that need to be removed to observe ground deformation:

Operation time of civil SAR satellites. X-band (red), C-band (green), L-Band (blue)

φintf φdeformation φtopography φorbital error φatmo stratified φatmo turbulent

φnoise

Overview

Krafla & Bjarnarflag

1993-2003

Data Krafla & Bjarnarflag

1993-2003

Data Krafla & Bjarnarflag

~ 5 km

1993-2003

Data Krafla & Bjarnarflag~ 5 km

~ 2 km

2009-2014

Data Krafla & Bjarnarflag

Deformation gradient

Krafla & Bjarnarflag

Model

The regional deformation model includes :

Plate spreading as modeled for the NVZ A deep inflation source North of KraflaA deep deflation source beneath Krafla A subsidence/contraction signal along the fissure swarm

The local deformation model includes : A shallow deflation source below

Leirnhnjúkur, at the location of a magma chamber active during the Krafla fires

Model AA shallow deflation source at 1.5 km below Leirbotnar to account for the array of boreholes of the Krafla power- plant.A shallow deflation source at 1.5 km below Bjarnarflag, to account for thearray of boreholes of the Bjarnarflagpower-plant.

Krafla & Bjarnarflag

●

Model BShallow deflation source for each boreholes utilized. we assume the extraction/injection of geothermal fluid happens at the similar level in the geothermal reservoir. The volume change of each source is set proportional to the water extracted/injected at the borehole it models. The best depth is about 900-1300 m and the best proportion about 6-10x10-6.

●

●

Assumptions Isotropic elastic half-spacea << d

C 1 ν

ΔVπ

●

●

●

●

ν :Poisson's ratio ΔV : volume changed : depth of the point sourcer : radial distance from the source

uz r

ur r

C . r2r d

C . d

2 3 2

Spherical pressure source (Mogi1958)

d

r 2 d23

2

r

aΔV

Model A

1993-2003

2009-2014

Krafla & Bjarnarflag

Model B

1993-2003

2009-2014

Krafla & Bjarnarflag

Model results Krafla & Bjarnarflag

Model A Model B

Inversion results: A) Closing rates of the sill. B) Volume change of the deep inflation source (red) and the deep deflation source (blue). C) Volume change of the shallow deflation sources: Leirhnjúkur (red), Leirbotnar (green), and Bjarnarflag (blue). Leirbotnar and Bjarnarflag volume changes are fixed for each time interval and indicate the sum of the volume change of the array of point sources at both area (see text). Error bars show time interval (X-axis) and 95% confidence interval (Y-axis, only for shallow pressure sources).

Rates of deflation at Krafla and Bjarnarflag geothermal areas have been relatively steady.

Ground subsidence and geothermal fluid extraction occur in a similar location at Krafla. At Bjarnarflag, the deformation maximum is localized to the west of the array of boreholes, possibly indicating a travel path for geothermal fluids.

Conclusion Krafla & Bjarnarflag

The model with a deflation source for each boreholes provides a slightly better fit than the model with a single deflation source for each geothermal areas

In each case, an average volume decrease of less than 105 m3/yr is found at sources located at about 0.9-1.5 km depth. This volume change represents about 0.1–1x10-5

of the volume of geothermal fluid extracted per year at each areas, indicating important renewal of the geothermal reservoir by water flow.

The deflation process could be affected by the result of a pressure drop and/or thermal contraction.Boreholes logging in Krafla and Bjarnarflag show a fairly stable pressure with time and slow temperature decrease.

2009-2016 velocities extracted from two TerraSAR-X time-series.A) Horizontal East-West velocities. B) Vertical velocities. Background shows Krafla caldera (comb line) and road 1(line).