Magma reflection imaging in Krafla, Iceland using microearthquake sources Doyeon Kim 1 , Larry D. Brown 1 , Knútur Árnason 2 , Kristján Águstsson 2 , and Hanna Blanck 2 1 Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA 2 Iceland Geosurvey (ISOR), Reykjavik, Iceland
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Magma reflection imaging in Krafla, Iceland using microearthquake sources
Doyeon Kim1, Larry D. Brown1, Knútur Árnason2, Kristján Águstsson2, and Hanna Blanck2
1Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA2Iceland Geosurvey (ISOR), Reykjavik, Iceland
Introduction
• Distribution and movement of magma in the earth has been a critical concern
• Magma chambers vs sill or dyke complexes• Role of viscosity & density in the magma transportation• Relative mixing of original magma with host rocks
during ascent• Recognition of precursors to major eruptive events• Energy source for geothermal system• Societal concerns
Imaging magma
Wei et al., 2001
Huang et al., 2015
Chmielowski et al., 1999
Seismic tomography in Yellowstone Receiver functions in Altiplano-Puna magma body
Magnetotelluric in Tibetan plateau
Seismic Reflection Imaging magma
Brown et al., 1996Bright spots beneath the Tibetan Plateau
deVoogd et al., 1986
Bright spots in death valley
Seismic Reflection Imaging magma
Matsumoto et al., 1996
Bright spots beneath Northeastern Japan arc
Sanford et al., 1977
Bright spots beneath Rio Grande Rift
Case real world:- NOT random nor sufficient sources
Developing the idea further by using more stations with near offset for stacking: Virtual reflection
Can become an artifact
Imaging microearthquake with seismic interferometry
Imaging microearthquake with seismic interferometry
In nature, earthquakes are not randomExtracting body waves with SI has been spotty at best
Our focus is based on redatuming of selected sources
Virtual Reflection Seismic Profiling (VRSP)
Drilled into the magma
Geological model of Krafla modified from Ármannsson et al. [2014].
DRG network, Krafla
Deep Roots of Geothermal systems (DRG) project, supported by ISOR, the GEOthermal Research Group (GEORG) and Icelandic power companies, 20 seismic stations were deployed at 200m spacing
Earthquake samples
Selected sources
Real sources close to the virtual sourceclose to the line we are imaging the same subsurface
145 and 137 microearthquakes were extracted for imaging beneath array 1 and 2 respectively.
Array 2Array 1
Example of virtual shot gather from selected sources
-virtual direct wave with apparent velocitiesthat are consistent with those measured bylocally (IMAGE-VSP survey)
-more traces which contains reproducible virtual reflections by crosscorrelation.
VSRP section
a) errors in the event locationsb) S wave contributions to the cross-correlation functions, c) variations in microearthquake focal which could result in polarity changes that degrade the stackd) contributions from converted phases (e.g. S to P).
3D VRSP in Krafla
NEED 2D DENSE ARRAY
R1: corresponds to a depth about 2.75km, comparable to the body intersected by IDDP-1R2: potential reflection from its base if it were 500 m thick.R3: represent energy arriving from out of the plain of the section (e.g. sideswipe)R4: could mark the top of the postulated deeper chamber/ another intrusion/brine or steam??
Landsvirkjun
Krafla Magma Testbed:potential deployment
2km
3km
Fairfield Nodal
4D?
Conclusion• Microearthquakes are valuable untapped source for high
resolution reflection imaging • Virtual reflection at Krafla observed at depth which corresponds to
the drilled magma• Really need dense 2D surface recording array • New seismic technology facilitates relatively low cost 3D/4D
reflection imaging with microearthquakes
• Potential applications– Geothermal systems– Active volcanoes– Aftershock sequences– Active faults (ongoing seismicity) – Hydraulic fracturing
We thank ISOR, GEORG, and the National Power Company, Iceland for providing the data
Acknowledgments
ISOR thanks Karin Berglund at Uppsala University and Pálmar Sigurðsson at ISOR for their dedication to the fieldwork in Krafla.
Special thanks to Gylfi Hersir for continuous support
Supplementary slides…
Virtual Reflection Profiling (VRP)
If a subsurface source is located directly below a receiver, 1. autocorreation of the transmission response ≈ reflection response from an
zero offset surface source2. a number of randomly distributed sources in depth will cancel out the
“artifact”
Virtual Seismic Reflection Profiling• Highest resolution: reflection seismology• Expensive for active sources• Effective bandwidth is limited for current passive techniques –
relatively low resolution image
Use reflections from local earthquakes with recent advance in seismic instrumentation (dense array) – Virtual Seismic Reflection Profiling (VSPR)
Imaging microearthquake with seismic interferometry
Treatment of ambient noiseP coda wave interferometryMultidimensional deconvolution
Extracting body wave with SI has been challenging
Nishitsuji et al., 2016Ito et al., 2012
Step3.
coherent energy bands increases as the number of events used in the stack increaseswe found relatively little improvement in the signal-to-noise as the number of events increase beyond 40
Results: VSRP
lateral coherent energy is more evident along Array 1 than Array 2 (Figure 11). We attribute this to the much smaller number of suitably located earthquakes available to produce Array 1 compared to Array 2.
Autocorrelations
Autocorrelating works the best if sources are located directly beneath every station
The effective spatial sampling is irregular, and there is little basis for discriminating reflections from virtual sources at the surface from coherent artifacts
Treat as ambient “noise”
(a) Entire earthquakes recorded (b) Regional earthquakes with coda wave interferometry with multidimensional deconvolution(c) Local earthquakes with coda wave interferometry with multidimensional deconvolution