Cross-over Seismology : Blurring the line between passive and active source

experiments

Diego QuirosCornell University

Dense Arrays

Virginia5 km5000 m spacing200 m spacing

Aftershock Imaging with Dense Arrays (AIDA)-Virginia

5 km

~ Broadband Deployment Dense Array Deployment

5 km

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

VSPInterferometry

Dense Array Deployment

Seismic imaging with earthquakes – VSP*

*Vertical Seismic Profiling

3D Seismic imaging with individual earthquakes

L1 L2

L1perspective L2

perspective

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

What do we need for 3D Seismic?

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dBG Earth Sciences B.V.

What is really dense? Avoiding spatial aliasing

Δx < ½ λ , Array spacing less than half the wavelengthΔx = ¼ λ , To be sure let’s sample λ at 4 points

λmin = vmin , Smallest λ that we are going to sample fmax

fmax = 100 Hz , Maximum frequency we would expect to sample without frequency aliasingVmin = 4000 m/s , this is an apparent velocity so va * sin (θ) = vt

λmin = 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?

A more realistic survey!!

fmax = 50 Hz , Maximum frequency we would expect to sample without frequency aliasingVmin = 5000 m/s , this is an apparent velocity so va * sin (θ) = vt

λmin = 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?

Active or Passive ?

Can’t tell! Continuous recording

Tim

e

Stn #

SEGY world

Mseed, SAC

Seamless two-way exchange betweenformats

Why?i.e. USArray processed with industry methods

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)

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.

Ambient Noise

1 km

Ithaca, NY Road noise experiment

Sample of dataset

Cross-correlation w/ virtual sourceat station 29 (red flag)

Active Source

Virtual Source

NE SW NE SW NE SWNE SW

Montserrat – Active & Passive

5 km

Designed as a controlled source experimentit 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 Inland Coast

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)