1. Cross-over Seismology : Blurring the line between passive and active source experiments Diego Quiros Cornell University

2. Dense Arrays Virginia 5 km 5000 m spacing200 m spacing

3. Aftershock Imaging with Dense Arrays (AIDA)-Virginia 5 km ~ Broadband DeploymentDense Array Deployment 5 km

4. Aftershock Imaging with Dense Arrays (AIDA)-Virginia Lower detection threshold More accurate hypocentral locations More accurate and detailed velocity structure High resolution 3D reflection imaging VSP Interferometry Dense Array Deployment

5. Seismic imaging with earthquakes – VSP* *Vertical Seismic Profiling

6. 3D Seismic imaging with individual earthquakes L1L2 L1 perspective L2 perspective

7. 3D Seismic imaging with several earthquakes Cross Line XL 65 with a reflector at ~7km depth also seen on cross lines XL 62, 63, 66, 67 Map view of 81 aftershocks 3D Volume What we need? More stations with the right geometry to “fill” the seismic volume

8. What do we need for 3D Seismic? …………………. dBG Earth Sciences B.V.

9. What is really dense? Avoiding spatial aliasing Δx < ½ λ, Array spacing less than half the wavelength Δx = ¼ λ, To be sure let’s sample λ at 4 points λ min = v min, Smallest λ that we are going to sample f max f max = 100 Hz, Maximum frequency we would expect to sample without frequency aliasing V min = 4000 m/s, this is an apparent velocity so v a * sin (θ) = v t λ min = 40 m, Minimum wavelength we expect to sample Δx = 10 m, So 10 m station spacing to avoid spatial aliasing This means that for a 20 km x 20 km survey we would need 4 million stations!! Let’s get realistic what could we do?

10. A more realistic survey!! f max = 50 Hz, Maximum frequency we would expect to sample without frequency aliasing V min = 5000 m/s, this is an apparent velocity so v a * sin (θ) = v t λ min = 100 m, Minimum wavelength we expect to sample Δx = 100 m, Station spacing to keep things doable! This means that for a 10 km x 10 km survey we would need 10 k stations!! How much data? Team of 5 deploys 100 stations in ~ 1 day 1 month continuous recording @ 100 Hz sampling? 1 Stn recording 1 hr = 1410 KB (this is for a single trace SEGY file) 10 k Stns recording 1 month ~ 9.5 TB sounds right?

11. Active or Passive ? Can’t tell! Continuous recording Time Stn # SEGY world Mseed, SAC Seamless two-way exchange between formats Why? i.e. USArray processed with industry methods

12. Acknowledgments  Cornell: Larry Brown, Anastasija Cabolova, Chen Chen, Rowena Lohman  Virginia Tech: Kathy Davenport, John Hole, Liang Han, Martin Chapman  USGS: Walter Mooney Software: Vista3D & Omni3D (Schlumberger), Matlab (MathWorks)

13. AIDA - Maine 10 km Hypocenter depth solutions for the single Aftershock recorded during AIDA-ME using subsets of stations (i.e. subarrays) to test improvement with increasing number of stations Upper Hemisphere F.M.

14. Ambient Noise 1 km Ithaca, NY Road noise experiment Sample of dataset Cross-correlation w/ virtual source at station 29 (red flag) Active Source Virtual Source NE SW

15. Montserrat – Active & Passive 5 km Designed as a controlled source experiment it recorded air gun shots located all around the island Originally we used all air gun shots treating them as ambient noise and cross correlated Inland Coast

16. Lessons from Dense Arrays Continuous recording means we collect controlled source, earthquakes (passive) and noise (cars) Apply industry techniques to passive data to generate 3D reflection images is feasible with appropriate surveys Need to seamlessly convert between mseed, SAC, SEGY (variants)