Introduction to February Focus: Anisotropy: Fractures and Faults

Welcome to this special issue of the RECORDER that presents five papers on fractures and anisotropy. While this theme has been in and out of the spotlight for many years, we are seeing novel approaches for extracting even more information from the data than we were a mere five years ago and we are seeing new datasets contributing to the story.

In our first Focus Article, “On Hookean solids in seismology: anisotropy and fractures”, Slawinski proposes the following. To infer the presence and orientations of subsurface fractures, seismologists might use directional properties of Hookean solids. In other words – using such a solid as a mathematical model – seismologists match its quantitative predictions with observations.

In Focus Article 2, “Results from diverse fracture analysis methodologies: consistent, contradictory, or complementary?”, Perz et al. examine four independent approaches for fracture detection using surface-seismic data. Fundamental algorithmic properties and limitations are studied in an attempt to explain observed differences between the attributes generated from the various approaches. Real data examples are presented, including one which demonstrates simultaneous analysis of all four techniques and which reinforces our belief that fracture characterization benefits from consideration of diverse attributes.

In Focus Article 3, “Case study: 3C and 9C prestack shear-wave splitting analysis at Big Sky” Grossman and Popov present a tutorial style case study in which they cross-validate two approaches to shear-wave splitting analysis and layer stripping: the first, and perhaps more familiar method utilizes converted-wave, or PS data, while the second utilizes pure shear-wave data. They also incorporate results from PP processing in the study. The objective is to compare essentially direct measurements derived separately from pure P- and S-mode datasets to the indirect mixed-mode measurements derived from PS data processing.

In Focus Article 4, “Anisotropic moment tensor inversion and visualization applied to a dual well monitoring survey”, Leaney at al. review the workflow used for anisotropic moment tensor inversion and visualization applied to a dual well monitoring survey from the West Pembina field in Alberta. The workflow makes use of an exact, dynamic, layered VTI ray-tracer that allows for constant dip. Included are a description of VTI model calibration, moment tensor inversion, moment tensor decomposition and visualization.

Finally, in Focus Article 5, “Evaluating fracture-induced anisotropy using borehole microseismic data", Kendall et al. discuss the broader potential for using microseismic data to image the reservoir. For example, anisotropy is a seismic attribute that is well suited to being studied using either small naturally occurring or stimulated earthquakes. Such events are effective in generating shear-wave energy and are therefore well suited to shear-wave splitting analysis – arguably the least ambiguous indicator of anisotropy. This splitting may be due to the intrinsic anisotropy of the rock or it may be an indicator of aligned fracture or crack sets (provided the fracture/crack spacing and size is much smaller than that of the dominant seismic wavelength; Crampin, 1984). Here they discuss how down-hole recordings of shear-wave splitting can be used to image fracture-induced seismic anisotropy.

Please enjoy this special issue on fractures and anisotropy and thanks to all the authors who contributed.