Stop Treating Diffractions as Noise – Use them for Imaging of Fractures and Karst
Mark Grasmueck¹, Tijmen Jan Moser², and Michael A. Pelissier³
¹Center for Carbonate Research, University of Miami, Miama, FL, USA
²Moser Geophysical Services, Den Haag, The Netherlands
³Marathon Oil company, Houston, TX, USA
- Diffractions are the key to imaging of small-scale discontinuities of carbonate reservoirs and unconventional shale Gas/Oil plays.
- Current reflection seismic survey practice is optimized for continuous stratigraphy but suppresses and collapses diffractions. Quarter-wavelength acquisition and diffraction-friendly processing are needed.
- The combination of high-resolution 3D GPR outcrop imaging and Ray-Born synthetic modeling helps deciphering the signatures of unmigrated diffractions.
- Seemingly incomplete and asymmetric diffraction circles visible on timeslices actually contain the dip information of cross-cutting fracture systems.
The reflection seismic method is optimized towards imaging of continuous reflectors to delineate stratigraphic boundaries. However productivity of many carbonate reservoirs and unconventional shale reservoirs is governed by small scale discontinuities such as fractures or voids. As a consequence, reflection seismic is of limited use for characterization of discontinuous reservoirs and drilling success rates are lower than for continuous stratigraphic reservoirs. Fractures and voids are subwavelength discontinuities causing diffractions and producing scattered energy on seismic records. Such scatter is commonly regarded as noise and suppressed during acquisition and processing.
Until recently scattered energy has also prevented clear GPR imaging of fractured and karstified outcrop analogues. Ground Penetrating Radar uses electromagnetic waves but has very similar kinematics in terms of reflection, refraction and diffractions. However by acquiring very dense 3D GPR data with a grid spacing of less than quarter-wavelength in all directions and properly sampling diffractions we have been able to produce images of fractures and karst network with unprecedented resolution and clarity (Pomar 2010, Grasmueck et al., 2011). Key to producing these images was 3D migration processing to collapse the diffractions. The new work presented in this abstract analyzes the unmigrated signatures of diffractions. The objective is to better understand the origin of diffractions and investigate their information content in terms of fracture and karst geometry and distribution. Deciphering and verifying the signatures of raw diffractions is supported by Ray-Born synthetic modeling (Moser and Howard, 2008) reproducing the diffraction patterns observed in the 3D GPR field data. The combined findings from modeling and full-resolution 3D GPR data can be used to revise current seismic survey practices for improved imaging of discontinuous reservoirs.
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012