1. Virtual Shear Checkshot with Airguns
A. Bakulin, A. Mateeva*, R. Calvert, and P. Jorgensen
Shell International E&P, Inc.
Houston, TX
a new technique for measuring interval velocities under complex overburden.
focus on Shear wave velocity and show you how to get it from VSP data, even if the VSP was acquired with P sources such as airguns. We call this technique Virtual Shear Checkshot because it’s based on the Virtual Source method.
File Title
Presented at SEG 2006
11/6/2018

2. Outline
Virtual Shear Source
Virtual Checkshots
Example from deepwater GOM
Conclusion

3. Virtual Source Concept Bakulin and Calvert, 2004
Complex near surface
Virtual Source
Simpler “middle” overburden
Well
The VS method was introduced by B&C and patented by Shell a couple of years ago. It’s a computational technique that takes the data recorded from surface shots into downhole receivers and converts them into data that would have been recorded in the same receivers if we had sources located in the borehole, at existing receiver locations. The benefit of creating a VS in the borehole is that you can see your target more clearly under complex overburden. You can think of the VS as a special kind of source redatuming. It’s special in two ways: 1. The new source positions are not arbitrary – they must coincide with existing receiver locations; 2 we don’t need to know anything about the velocity of the medium to do this redatuming. The process is completely data driven
Target

4. Virtual Source Concept Bakulin and Calvert, 2004
D (t) =  Dk(-t)*Dk(t) k k
Complex near surface
Dk 
Simpler “middle” overburden 
Dk
If you want to turn this receiver into a VS, you just take the trace recorded in it from a given surface shot, time reverse it, and convolve it with the trace recorded from the same shot into another receiver. You repeat this process for every surface shot and sum over shots. Thus you get a trace as if you had a source at A and a receiver at B. You can think of the VS as being fueled by whatever energy came to it from the real sources. In general, that would be a mixture of P and S waves. So, the VS would emit both P and S waves. But if you restrict the energy that enters the VS creation to only P or only S, you can manifacture a VS that radiates only P or only S. This is a very simple but powerful idea. It’s powerful because it allows you to create a Virtual Shear Source even when the physical sources at the surface are P sources.
D
Target

5. Virtual Shear Source Concept Bakulin and Calvert, 2005
Complex near surface S
Virtual Shear Source
Simpler “middle” overburden S
That’s because the shear energy fueling the VS doesn’t have to be emitted directly by the physical sources – it can be generated by P-S conversions in the overburden. The more heterogeneous the overburden, the more opportunities are there for P-S conversion. These conversions can be very complicated, occurring at many places at the same time. But we don’t need to know where they came from. The VS will take all of that scattered complicated Shear energy and collapse it into a single Shear impulse emitted from the VS location. This was first demonstrated by B&C last year at the SEG. Just to show you how it works, here is an example.
Target

6. Vs Vp Synthetic Example (North Sea) Airgun array depth (m)
depth (m)
These are P and S wave velocity profiles representative for a certain oil field in the North Sea. Near the water bottom there are many thin layers. Assuming horizontal layering, we simulated a walkaway VSP acquired with airguns in a deviated well. Here is what the data look like at the the top-most receiver on its horizontal component ….
Velocity (m/s)

7. Common Receiver Gather (X comp)
Synthetic Example
(North Sea)
Airgun array
Common Receiver Gather (X comp) PP Vs Vp
depth (m)
This is the first arrival. It’s a P wave. These later arrivals here are mostly P-S conversions generated at the thin layers above the receiver. We cannot tell which came from where because there are too many of them. But we expect them to be mostly shear because we are looking at the horizontal component. So, suppose we want to turn this receiver into a Virtual Shear Source. What do we do? We just mute the first arrival because we know it’s P, and use this gate here to cross-correlate with traces at other receivers. And here is the result… PS
Velocity (m/s)

8. vs. ‘real’ shear buried source
Synthetic Example
(North Sea)
Virtual Shear Source
vs. ‘real’ shear buried source
VSS Vs Vp
depth (m)
This is time, this is depth. We’ve plotted two data sets here – in red, we have the Virtual Shear Source data, and in black we have synthetic seismograms from a horizontal-force placed at the same location. The two data sets are practically coincident. If there is any difference worth mentioning, it is in this arrival here which is present on the black data set but not on the red. It’s a water bottom multiple generated by the up-ward emission of the horizontal force source. It’s absent on the Virtual Source data because the VS radiates mainly down-wards (because the physical sources that fuel it are above it). This down-ward radiation pattern of the VS is actually beneficial because we typically want to see reflectors below the receivers, and not events like this multiple. But these details are marginal to today’s presentation. Let’s focus on the first arrival. The FA from the VS arrives at the deep receivers at the same time as the that from the horizontal force source, testifying to the fact that we have created a VS that emits mainly shear waves. We can use the moveout of the first arrival to measure the shear wave velocity along this deviated borehole … Which brings us to the topic of Virtual Checkshots.
Velocity (m/s)

9. Example from deepwater GOM Conclusion
Outline
More in Bakulin and Calvert (SEG’05) and Bakulin et al. (to appear in Geophysics)
Virtual Shear Source
Virtual Checkshots
Example from deepwater GOM
Conclusion

10. Below the complicated overburden
Waveform distortions
what to pick ?
Raypath excursions
Below the complicated overburden
Short ray-path - along the well

11. Below the complicated overburden
Waveform distortions
what to pick ?
Raypath excursions
Below the complicated overburden
Short ray-path - along the well
P or S Virtual Source
can have S first arrival
P first arrival

12. Walk-away VSP, GOM airguns 3C receivers …..
* to create the Virt Shear Source we harvest P-S conversion. In this example, as perhaps in any other salt example, the strongest conversion is at the top of salt. You don’t to know this to create the VSS but it helps us develop intuition.

13. X component of VSP Direct P Raytracing P-S at Top Salt
This is a common receiver gather from the VSP – the inline horizontal component. You see these strong events here? They are down-going shear waves. We can tell from common shot gathers. We speculated that most of them are conversions at the top of salt. To check this we did some ray tracing. If you ray trace direct P arrivals to this receiver you get the traveltimes shown by yellow dots. As expected they line up with the first arrival. If you ray trace direct-P-converted-to-S-at-the-top-of-salt you get you get the traveltimes shown by green dots. They line up at the top of the shear event. The shear energy keeps coming for quite some time because there are plenty of short period multiples generated in the sediments above the salt that trail the down-going P. As each of them hits the top of salt, it’s partially converted to S. So, to create a Virt Shear Source in this case, we just muted everything above the green dots and used what was left.
~/Boreas/DocumentSVS/norm_x_zrec17500.gif
Traveltimes roughly placed by hand using /glb/data/eptr_resgeo/prj_wolf10/BOREAS/RayTracing/pp_traveltimes.ascii and ps_traveltimes.ascii
Had to convert local coordinates used by Patsy for raytracing to world coordinates (XSHT) to match her data to Boreas seismic. Did this in Vital->coordtrans using
/glb/data/eptr_resgeo/prj_wolf10/BOREAS/Vol40Flex_depth.key
And
/glb/data/eptr_resgeo/prj_wolf10/BOREAS/mars_all_m_2_all_ft.lkey
Patsy also showed me an example job for this (does not use key files)
/glb/data/eptr_resgeo/prj_wolf9/BOREAS/QC_FB/job_gloloc
Copied to /glb/data/eptr_resgeo/Albena/S_VS/gloloc.job

14. Virtual P Source Virtual S Source
receivers in salt
subsalt
Here is the result…

15. Excellent agreement with logsVs Vp
Vp:
mean (VS - smoothed log)  0 %
std (VS - smoothed log) = 2 %
max (VS – smoothed log) < 5 %
…that are in excellent agreement with the sonic logs smoothed to the resolution of the VSP. Note that we are at more than 7 km depth below salt, and we had nothing but a conventional marine VSP. Yet, extracting the S wave velocity just was as easy as getting the P wave velocity.
Our main interest in this example was in the sediment below the salt because that’s where velocities vary. But for the sake of completeness, …
Vs:
mean (VS - smoothed log) = 1 %
std (VS - smoothed log) = 5 %
max (VS – smoothed log) = 10 %

16. Average Salt Velocities:
Vp, m/s
Vs, m/s
Virtual Checkshots: 4,890  ,800  115
Sonic Logs: 4,880  ,780  12
We also measured the average P and S velocities in salt. And once again the Virtual Checkshots and the sonic logs are in excellent agreement.
Once again, excellent agreement between Virtual Checkshots and Logs.

17. Conclusion
Virtual Checkshots  Accurate P and S velocities under very complicated overburden
Shear Checkshot with airguns !!!
Velocity along deviated wells
Minor incremental cost to walk-away VSP
Further value in full VS wavefield
Clear image of target under heterogeneous overburden
S-S reflection data with airguns !