Getting More Bang for Your P-Wave

Utilizing the SV mode

When 3-D seismic technology became a must-have tool for the E&P crowd beginning in the mid-1990s, there was considerable scurrying to get up to speed on this then-daunting high-tech application.

Even today, it’s not uncommon to see a non-geophysicist’s eyes glaze over when a discussion turns to even basic seismic terminology like P-waves, which move through both solid rock and fluids, and S-waves, which travel only through rock.

(Both illuminating and reflective P-wave seismic data, or P-P, have long been used in the industry to evaluate the subsurface.)

But even geoscientists must constantly be on their toes given that new opportunities always lurk just ahead as advances in seismic technology are ongoing.

AAPG member Bob Hardage, senior research scientist at the Bureau of Economic Geology, Jackson School of Geosciences at the University of Texas at Austin and a past editor of the AAPG EXPLORER’s popular Geophysical Corner, is spot-on when it comes to innovation in this arena.

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When 3-D seismic technology became a must-have tool for the E&P crowd beginning in the mid-1990s, there was considerable scurrying to get up to speed on this then-daunting high-tech application.

Even today, it’s not uncommon to see a non-geophysicist’s eyes glaze over when a discussion turns to even basic seismic terminology like P-waves, which move through both solid rock and fluids, and S-waves, which travel only through rock.

(Both illuminating and reflective P-wave seismic data, or P-P, have long been used in the industry to evaluate the subsurface.)

But even geoscientists must constantly be on their toes given that new opportunities always lurk just ahead as advances in seismic technology are ongoing.

AAPG member Bob Hardage, senior research scientist at the Bureau of Economic Geology, Jackson School of Geosciences at the University of Texas at Austin and a past editor of the AAPG EXPLORER’s popular Geophysical Corner, is spot-on when it comes to innovation in this arena.

“A more rigorous geologic interpretation can be done if an analysis is based on a joint interpretation of P- and S-wave seismic data rather than limited to a study of only one wave mode,” Hardage commented.

“Our Exploration Geophysics Laboratory is focused on providing interpreters wider access to, and lower cost of, S-wave data so seismic stratigraphy studies can be based on multimode seismic data,” he emphasized.

“A downgoing P-wave will reflect from an interface as two waves: a P-wave known as P-P, and an SV, or P-SV, called converted shear by some,” Hardage noted. “You have to deploy three-component geophones to record (this) because the upgoing SV activates horizontal geophones rather than vertical.

“The new concept introduced by my lab is that ordinary P-wave sources create two downgoing illuminating wave modes: P and SV,” he said. “When this wave physics is accepted, we can then utilize a new illuminating wave, the SV mode.

“SV-P data are recorded by vertical geophones,” he added, “because the upgoing wave is P.”

Translation: More Data, Less Cost

To simplify, SV-P data reside in ordinary P-wave data acquired with vertical geophones, and the SV-P can be extracted. Because thousands of square miles of these common P-wave data are housed in myriad data libraries, geoscientists can evaluate prospects using the indigenous S-wave data without having to record expensive new seismic information.

In fact, the limited acceptance of S-wave reflection seismology has been based primarily on a couple of factors:

  • Added cost to acquire S-waves over and beyond the expense to acquire P-wave data alone.
  • S-wave sources are essentially limited to horizontal vibrators, which can’t be used on many surfaces, such as swamps, marshes, rugged mountainous terrain.

Because it’s now documented that S images can be made from either legacy or new P-wave data, the cost issue becomes moot.

As for the source limitations, SV-P technology allows some type of P-wave source to be used in locales having challenging terrains. For example, Hardage noted that shothole explosives produce significant downgoing SV in addition to downgoing P.

Vertical vibrators are another option.

Hardage referred to a study of the lower Permian Age Wolfberry unconventional reservoirs on the Fasken C-Ranch property located on the western platform of the Midland Basin. He noted that the laterally expansive, thin beds of the formation represent ages of turbidite action.

“The principle we observed in interpreting the Fasken seismic data was that SV-P data show the greatest population of turbidite-like depositional elements,” he said, “with P-P data showing the next highest population and P-SV data being a distant third in revealing turbidite reservoir architecture.

“This is the first-ever example of SV-P data extracted from pure P-wave data and used to evaluate a prospect,” Hardage emphasized.

“We have now done three SV-P studies and are ready to push the commercialization of this new concept for S-wave imaging.”

He noted that four patents have been issued to the UT Board of Regents, and UT’s Office of Technology Commercialization is pushing ahead to commercialize the technology.

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