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

Outline Virtual Shear Source Virtual Checkshots Example from deepwater GOM Conclusion

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

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

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

Vs Vp Synthetic Example (North Sea) Airgun array depth (m) 700 600 500 400 300 200 100 0 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 …. 500 1000 1500 2000 Velocity (m/s)

Common Receiver Gather (X comp) Synthetic Example (North Sea) Airgun array Common Receiver Gather (X comp) PP Vs Vp 700 600 500 400 300 200 100 0 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 500 1000 1500 2000 Velocity (m/s)

vs. ‘real’ shear buried source Synthetic Example (North Sea) Virtual Shear Source vs. ‘real’ shear buried source VSS Vs Vp 700 600 500 400 300 200 100 0 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. 500 1000 1500 2000 Velocity (m/s)

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

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

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

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.

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

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

Excellent agreement with logs Vs 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 %

Average Salt Velocities: Vp, m/s Vs, m/s Virtual Checkshots: 4,890  110 2,800  115 Sonic Logs: 4,880  20 2,780  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.

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 !