Inversion of microseismic data for triclinic velocity models

Abstract

Modern downhole microseismic surveys often employ geometries in which ray trajectories generated by a collection of locatable events provide full polar and azimuthal coverage, making it possible to estimate the in situ seismic anisotropy. We show that traveltimes and particle motions of the direct P‐ and shear‐waves acquired in such geometries can constrain stiffness tensors of triclinic media. While obtaining all 21 stiffness coefficients of a homogeneous triclinic space simultaneously with locating pertinent microseismic events from data recorded in a single vertical well is relatively straightforward, the same methodology does not necessarily succeed in layered formations because the combination of their vertical heterogeneity and azimuthal anisotropy might invalidate the commonly adopted approximation of the event azimuths by those of the P‐wave polarization vectors. When the event azimuths cannot be derived from the particle motions, traveltimes observed in two or more wells are required to locate the events and build layered triclinic or higher‐symmetry azimuthally anisotropic velocity models. As our numerical tests indicate, the multi‐well event‐location methods are expected to perform better than their single‐well counterparts because they rely solely on triangulation and eliminate the usually pronounced azimuthal uncertainties in the event locations that stem from noises adversely affecting hodogram analysis.

Approximate traveltime inversion in downhole microseismic monitoring

“…Ray tracing is a widely used tool for seismic modelling in anisotropic media (Červený ), employed in many microseismic inversion algorithms (Pei et al . ; Bardainne and Gaucher ; Grechka and Yaskevich ). Bending methods are convenient for solving a two‐point ray‐tracing problem in layered models (Wesson ).…”

“…The main motivation for anisotropic inversion is an improvement in microseismic event locations which usually results in the reduced traveltime misfit. The location errors caused by the wrong choice of anisotropic symmetry (VTI or ORT instead of triclinic) with synthetic data processing were found to be significant (about 30–60 m), and the traveltime misfit to be unacceptably large (>3.5 ms) (Grechka and Yaskevich ; Grechka and Heigl ). An interesting observation from a field‐data study of Grechka and Yaskevich () was that the microseismic event locations did not change much when changing the symmetry of anisotropic layers from VTI to ORT and further to triclinic.…”

Relative location of microseismicity

“…The velocity-model inversion for our data is better constrained than that discussed in Grechka (2015) because of the availability of a check shot that directly measures the vertical P-wave velocity. Both the traveltime surface in Figure 4 and the check-shot velocity were inverted (see Yaskevich, 2013, andGrechka, 2015, for details of implementation) to yield an effective tilted TI model characterized by (7) • the symmetry-axis azimuth N11.2°W and the tilt 2.3°from the vertical.…”

3D waveform inversion of downhole microseismic data for transversely isotropic media

“…due to natural or induced fracturing). Source locations can be estimated simultaneously with the velocity field using joint anisotropic inversion of direct‐arrival traveltimes of P‐waves and split S‐waves (Grechka and Yaskevich , ). Recently, Grechka et al .…”