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