1. Virtual Shear Source: a new method for shear-wave seismic surveys
Andrey Bakulin Rodney Calvert Shell Int E & P
Presented at SEG 2005, VSP 1.7
File Title
2/4/2019

2. Why shear waves (SS) did not fly?
Excitation of shear waves is costly
Abundant shear anisotropy even in near surface
Shear-wave source always excites substantial P -wave energy that acts as ''noise'' on shear records
Shear velocities are extremely low and vary greatly in the near-surface (both on land and on sea bed). Thus near-surface is a bigger problem in shear-wave seismic.

3. Converted (PS) waves
Polarization of excited shear waves is not controlled
Joint processing with PP data is required to estimate shear-wave properties
Converted wave processing is much more complicated
Fully suffers from near-surface problems (P and S)
Advent of 4D seismic monitoring has changed the economics of the seismic acquisition: it is recognized that repeated seismic surveys are necessary throughout the field life in order to optimize production. Such necessity makes it possible to spread the cost of preferably permanent installation of receivers and possibly sources over the life of the field and thus make it cheaper for each individual survey. Thus we can anticipate that role of first drawback of shear-wave seismology will be diminishing. The other three ones however remain.
Advent of ocean-bottom seismic (OBS) revealed the exciting possibility to measure converted $PS$ waves, thus extracting shear-wave information in more direct fashion that with AVO technique. Shear waves are excited right at the geological interfaces thus eliminating first and third drawbacks. However converted wave seismology still needs to address shear anisotropy and fully suffers from near-surface shear problems. It also brings three additional very challenging complexities:
Polarization of excited shear waves may not be controlled and is determined in very complex fashion through source-receiver azimuth, properties and geometries of the geological layers

4. Cheap slim wells potential for instrumented observation wells in the overburden
Smart wells
There is third important technology that may sound distant to shear-wave subject but to our opinion may contribute to complete
renaissance of shear-wave seismic. This technology is often referred as ``smart'', ``intelligent'' or ``instrumented'' wells. It offers unparalleled capabilities of multi-channel multicomponent {\it in-situ} recording along the existing wellbores.
It is the purpose of my presentation is to show you that seismic monitoring objectives combined with the ``smart'' well capabilities may offer unique combination that allows to combat all four drawbacks of shear-wave seismology and may lead to complete renaissance of shear-wave seismology. To achieve this objective we need to add third missing ingredient called ``Virtual Source'' (VS) method.

5. The Virtual Source method
Surface array of sources that simulates virtual source
Simpler “middle” overburden
Virtual source
(at R) R
Complex near-surface
Target Sk
Well
Sk
Skb
Db
VS method offers unique capability to overcome severe near-surface problems without knowledge of near-surface velocities.
In addition VS method relaxes the requirement to repeat the exact position of surface shots for repeatable seismic monitoring.
Geophones should be placed in the Earth below the most complex near-surface part (basalt layer, salt or just nasty near-surface layer). This for example could be horizontal or slanted well of producer/injector or dedicated smart sidetrack. As soon as we have done that we are able to directly measure the Green’s function or propagation effects caused by near-surface. Therefore we can avoid building velocity model above the receivers where we have most complex near-surface part. Instead, on computer, we simulate a new dataset with downhole sources located at receiver positions. As these downhole sources are simulated on computer and that is why we call them Virtual Sources. The bottom line is to get the red traces from blue traces. Once we obtain new virtual dataset with both downhole sources and receivers – we can perform conventional imaging of the lower part of the subsurface. To achieve that we only need bottom portion of the velocity model below the receivers which is usually simple to obtain.

6. Time reversal
Receivers
Source
Receiver
Sources

7. The Physics of Virtual Sources
reciprocity o t
time reversal
sum for a Virtual Source

8. Synthetic model with horrible overburden (full elastic finite-difference modeling)
m/s
(m)
Reservoir
Well with receivers
Sources
We first start with demonstrating the feasibility of VS approach on synthetic case study and then finish with the data example. On synthetics we walk through all the steps starting from VS processing, then imaging obtained VS data, estimating 4D response with VS data and determining repeatability of VS data. At each step we will compare that with results coming from surface data. Displayed here is the P-wave velocity model that resemples subsurface of Peace River field in Canada. You can see that bottom part is really simple. It consists of few layered formations with reservoir at about 600 m depth. The main challenge comes from complex near-surface part. The surface itself is represented by challenging Muskeg swamp environment. In addition upper 200m consists of very heterogeneous glacial fluvial deposits originating due to glacial channels. As we do not have good real model for that, we created this horrible synthetic model where velocity is wildly varying from 1000 m to 2400 m/s. We generated synthetic dataset for 80 receivers in horizontal well sitting below this horrible zone (depth of about 400 m) and for shot line at the surface.

9. Receiver gather (receiver X=900m, Z=430m, shot line at Z=15m) Explosion source to vertical component
Here you can see single receiver gather from dataset. It was generated by finite-difference modeling code (WFD). So it contains all multiples, conversions, diffractions, everything – it is full wavefield. It shows you the universe of overburden distortions that Rodney and Paul were talking about. You may clearly see that is not just simple static corrections – it is very severe phase distortions. With that data quality nobody it is almost impossible to build decent overburden velocity model that images the reservoir below. Let me know if anyone wishes to try.

10. Black – virtual receiver gather, red – real downhole gather
 Looking at the receiver gathers at few other locations along the line we can also see that we have done reasonably well.

11. PSDM comparisons VS data (aperture 300m)
Surface data migrated with exact velocity model of the overburden
First interface (505m)
How do we compare all that with the original surface-to-downhole data? Well, let us assume that we have built velocity model of the overburden and by virtue of magic it happens exactly to coincide with the true model.
Then we can indeed obtain really good depth image shown on the right. We have correctly imaged the plane interfaces including reservoir bottom and we the level of noise is reasonable. On the left you see the depth image of VS data. You can see that VS image is at least no worse than the one on the right. I can claim that certain things are slightly better on VS image but this is not the point. The point is that VS image was obtained without any knowledge of the near-surface velocity model while the one on the right is obtained with perfectly correct velocity model. To get VS image we only needed lower 1D portion of velocity model which is easily obtainable.
This is the end of the imaging part with simple conclusion that if you can do it from the surface – you are in good shape. If you can’t build perfect velocity models for your overburden – then you might use VS approach to overcome that.
Bottom reservoir (590m)

12. Shear-wave (SS) Virtual Source
Horizontal vibrator
X2X
Hor. geophone
Test selection of window for time-reversal
Quality against surface (SS) data
Quality against Virtual Source P -wave image

13. Large window with first arrivals

14. Large window with first arrivals (X2X)

15. Small window around strongest shear-wave arrival (X2X)

16. Window with first arrivals
Short window w/o first arrivals

17. PSDM VS data (full aperture)
Surface data migrated with exact velocity model of the overburden
First interface (505m)
How do we compare all that with the original surface-to-downhole data? Well, let us assume that we have built velocity model of the overburden and by virtue of magic it happens exactly to coincide with the true model.
Then we can indeed obtain really good depth image shown on the right. We have correctly imaged the plane interfaces including reservoir bottom and we the level of noise is reasonable. On the left you see the depth image of VS data. You can see that VS image is at least no worse than the one on the right. I can claim that certain things are slightly better on VS image but this is not the point. The point is that VS image was obtained without any knowledge of the near-surface velocity model while the one on the right is obtained with perfectly correct velocity model. To get VS image we only needed lower 1D portion of velocity model which is easily obtainable.
VS Shear is better !
Shear !
Bottom reservoir (590m)

18. PSDM migration X2X: horiz. force (vibrator) to horiz. component
VS S-wave image
(full aperture)
VS P-wave image
(full aperture)
To reservoir white loop (560m)
First interface (505m)
Shear !
Bottom reservoir (590m)

19. Buy one get one free? H2X V2X Explosion Hor. geophone
Vertical vibrator
V2X
Hor. geophone

20. H2X (no first arrivals)
X2X (no first arrivals)

21. V2X (no first arrivals)
X2X (no first arrivals)

22. Summary of VS data quality with multicomponent data

23. Shear-wave checkshot with airguns
Detailed S-wave velocity
model from surface wave
inversion on Tommeliten
field (North Sea)
by Alnor et al. (1997)
VSS S P
Signal to time- reverse
and send back

24. Shear-wave checkshot with airgunsVs Vp
Want to know more?

25. Virtual Shear Source
New method for imaging/monitoring below very complex near surface using downhole geophones
Can handle any complexity of near surface (no velocity model required)
Automatically takes care of regular and 4D statics and changes in the near surface
Better image than surface SS seismic (even with known velocity model!)
Comparable image with Virtual Source P –waves
Shear-wave checkshot is possible with P -wave sources
May relax requirements for exact repeat of surface shots positions for 4D
We have demonstrated you new VS method that fills the gap between existing techniques and can confidently image and monitor below very complex near surface using array of downhole geophones. We have shown that on realistic synthetic and on field data.
Since we directly measure the transmission response method does not require velocity between surface and well geophones and thus can be applicable for arbitrary complex near surface.
It automatically handles 4D static and in large corrects for changes in the near surface which is what we need for monitoring
And finally it may completely relax the requirement to repeat surface shot positions exactly that turn out to be very difficult in practice.

26. Conclusion regarding generation of pure shear-wave data
VS shear data (SS) can be obtained using recorded horizontal component and ALMOST any type of source:
Horizontal force (horizontal vibrator) – best quality data similar to P-wave results
Vertical force (vertical vibrator) – reasonably good quality – need to handle polarity reversals
Explosion source – poor data but still possible
Key learnings:
need to include window where SS energy arrives
need to exclude window around first PP arrivals