C2 Doyeon Kim
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GEORG Geothermal Workshop 2016 Presentation Title: Magma Imaging beneath Krafla using Virtual Reflection Profiling
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C2 Doyeon Kim
- 1. 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, USA 2Iceland Geosurvey (ISOR), Reykjavik, Iceland
- 2. 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
- 3. 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
- 4. Seismic Reflection Imaging magma Brown et al., 1996Bright spots beneath the Tibetan Plateau deVoogd et al., 1986 Bright spots in death valley
- 5. Seismic Reflection Imaging magma Matsumoto et al., 1996 Bright spots beneath Northeastern Japan arc Sanford et al., 1977 Bright spots beneath Rio Grande Rift
- 6. 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
- 7. Imaging microearthquake with seismic interferometry In nature, earthquakes are not random Extracting body waves with SI has been spotty at best Our focus is based on redatuming of selected sources Virtual Reflection Seismic Profiling (VRSP)
- 8. Drilled into the magma Geological model of Krafla modified from Ármannsson et al. [2014].
- 9. 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
- 11. Selected sources Real sources close to the virtual source close 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
- 12. Example of virtual shot gather from selected sources -virtual direct wave with apparent velocities that are consistent with those measured by locally (IMAGE-VSP survey) -more traces which contains reproducible virtual reflections by crosscorrelation.
- 13. VSRP section a) errors in the event locations b) S wave contributions to the cross-correlation functions, c) variations in microearthquake focal which could result in polarity changes that degrade the stack d) contributions from converted phases (e.g. S to P).
- 14. 3D VRSP in Krafla R1: corresponds to a depth about 2.75km, comparable to the body intersected by IDDP-1 R2: 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??
- 15. Landsvirkjun Krafla Magma Testbed: potential deployment 2km 3km Fairfield Nodal
- 16. 4D?
- 17. 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
- 18. 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
- 20. 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 source 2. a number of randomly distributed sources in depth will cancel out the “artifact”
- 21. 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)
- 22. Imaging microearthquake with seismic interferometry Treatment of ambient noise P coda wave interferometry Multidimensional deconvolution Extracting body wave with SI has been challenging Nishitsuji et al., 2016Ito et al., 2012
- 23. Step3. coherent energy bands increases as the number of events used in the stack increases we found relatively little improvement in the signal-to-noise as the number of events increase beyond 40
- 24. 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.
- 25. 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
- 26. 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
Hinweis der Redaktion
- This work involves myself, Larry brown who is my advisor in Cornell, and our fellow collaborators in ISOR Knutur, Krishan, and Hanna. We’d like to present Magma reflection imaging in Krafla, using microeathquakes sources
- To begin with, distribution and movement of magma in the earth has been a critical interest for decades and this includes the most fundamental question about the structure of magma chamber whether it is a sill or dyke complexes, magma transportation, mixing with host rocks during ascent, and recognition of precursors to major eruptive events. Besides geological perspective, we are also interested in energy production for geothermal system (that’s why we are gathered up around here) and last but not least Sociietal concern highly associated with magma eruption itself.
- For seismologist like myself, I am most interested in whether we can image/or detect this marvelous structure in subsurface. Perhaps the answer to this would be yes we can detect them and here are some well-known examples. On the left hand side, seismic tomography reveals low velocity anomaly in subsurface and this particular example shows the magma related structures in Yellowstone. Another technique which people often use when trying to imaging the discontinuity of the earth, they use receiver function with teleseismic earthquakes. Also there is Magnetotelluirc method which is one of powerful measurement that can detect liquid body in subsurface. However, these examples that I show here rather gives you a low resolution picture of subsurface. Can we do better than this?
- Yes we can by using seismic reflection techniques with active sources on the surface. Here is a great example of imaging the magma in high resolution and the top figure shows bright spots beneath the Tibetan plateau. You can even point out where exactly is indicated as YDR… Figure at the bottom also shows another example using vibroseis truck at the surface and you can see a clear bright spot in death valley. There is also another however here again and its mainly because of the cost of using active sources itself not to mention other logistical obstacles of using them.
- The other end of the spectrum, people have been using natural sources. On the left, you can see the result from the classic paper by Sandford et al., 1977 and they used the direct measurement of the reflection phase from Microearhquakes. These reflections are from magma beneath Rio Grand Rfit. On the right hand side, a similar approach was applied by Japanese crews and they attempt to image bright spots benearth Northeastern Japan arc system.
- One of the popular technique that can be used with seismic reflection imaging nowadays is seismic interferometry. The idea is that if you have a source in subsurface and you have signals collected by the source… Xcorr direct wave with ghost reflection you get a virtual reflection and this process kinematically redatum the source to the surface. This type of imaging also involves an artifact which is a product of a direct wave and a downgoing wave (primary reflection from an interface) and it rises a nonphysical arrivals. However in theory if you have sources that are randomly distributed in space and you have a lot of them you can cancel out this artifact while enhancing the virtual reflection. The problem here is that the eathquakes does not happen random in space.
- And since the earthquakes are not random in real case, extracting body waves using SI has been spotty at best. So our approach is based on using SI to redatum the deep sources to the surface however we apply it to the selected sources in subsurface and we named this as virtual reflection seismic profiling in short VRSP
- To test this idea, Krafla is a unique and a obvious to place to do so since we have an experience of drilling into the magma. In other words, we know the known target at depth.
- The data we used is from DRG network in Krafla. Since Knutur gave us a great talk yesterday about this network I am not going to spend too much time on it. I used two arrays, array 1 and array2 in the figure and you can also see the microearthquakes that are recorded by this network in green circles.
- Here are some example of earthquake we collected. On the left you see a microearthquake its magnitude of 1. P and S waves are clear so as the reflection from 2km depth in between. On the right you see a regional eq .. You can also..
- As I mentioned, we selected these eqs before redatuming them to the surface and the criteria is as follows. Suppose you have an array of receivers in red with a source that has an offset to the side in green. If this source was an active source on the surface, the reflection point will be sampling the midpoint between the two as shown in gray dots. Now if this source is an earthquake you can see the reflection point from the ghost reflection approach slighted closer to the array based on Snell’s law. So our goal here is to select earthquakes that are close enough to the array so that we can place these blue dots into the CMP bins that are conventionally used in Reflection imaging technique. And the CMP bins for our data is on the righthand sided.
- Fix the figure
- Real value here is Always potential of doing time lapse 4D Slide
- Real value here is Always potential of doing time lapse 4D Slide Can use modeling result…
- micro
- Real value here is Always potential of doing time lapse 4D Slide
- optional
- optional
- Often used.. All of the result shown in Figure 8 provides little indication of the coherent energy in contrast to what we consistently observed from VRSP for the selected microearthquakes. optional