  Conventional fracture-characterization methods assume the presence of a
single set of aligned, vertical cracks in the subsurface. We relax this
assumption and demonstrate the feasibility of seismic characterization of
multiple fracture sets. Our technique relies on recent numerical findings,
indicating that multiple, differently oriented, possibly intersecting planar
cracks embedded in an otherwise isotropic host rock result in a nearly
orthorhombic (or orthotropic) effective medium. Here, the governing parameters
of crack-induced orthotropy are estimated from 3D, wide-azimuth, multicomponent
seismic reflection data acquired over the tight-gas Rulison Field in Colorado,
USA.
  We translate strong azimuthal variations of the normal-moveout velocities
into interval crack densities, fracture orientations, type of fluid infill, and
velocities of the P- and S-waves in an unfractured rock. Our inversion
procedure identifies a set of cracks aligned in approximately WNW-ESE direction
in the western part of the study area and multiple, likely intersecting
fractures in its eastern part. We validate both our underlying theoretical
model and the obtained estimates by two independent measurements: (i) the
estimated fluid-infill parameter indicates dry cracks as expected for the
gas-producing sandstones at Rulison; and (ii) the obtained crack orientations
are supported by measurements in a well. As a by-product of fracture
characterization, we build an anisotropic velocity model of the Rulison
reservoir that, we believe, is the first orthorhombic velocity field
constructed from surface seismic data.
