Feasibility of joint 1D velocity model and event location inversion by the Neighbourhood algorithm

Abstract

A B S T R A C TUsing a set of synthetic P-and S-wave onsets, computed in a 1D medium model from sources that mimic a distribution of microseismic events induced by hydrofrac treatment to a monitoring geophone array(s), we test the possibility to invert back jointly the model and events location. We use the Neighbourhood algorithm for data inversion to account for non-linear effects of velocity model and grid search for event location. The velocity model used is composed of homogeneous layers, derived from sonic logging. Results for the case of one and two monitoring wells are compared. These results show that the velocity model can be obtained in the case of two monitoring wells, if they have optimal relative position. The use of one monitoring well fails due to the trade-off between the velocity model and event locations. *

Influence of a velocity model and source frequency on microseismic waveforms: some implications for microseismic locations

“…Ray based methods are commonly used to quantify the influence of velocity model on microseismic event locations and their error (e.g., Eisner et al, 2009;Maxwell, 2009;Jansky et al, 2010). However, ray based approaches neglect frequencydependent effects and non-geometrical arrivals (e.g., head waves), and are generally only suitable for smooth velocity models (i.e., when heterogeneity length scales are greater than the dominant seismic wavelength).…”

“…Location errors stem from limitations due to monitoring array geometry (e.g., Eisner et al, 2009;Jansky et al, 2010) as well as uncertainty in traveltime picks (e.g., Eisner et al, 2010), event azimuths (e.g. Jones et al, 2010) and velocity model.…”

Challenges for microseismic monitoring

“…The velocity model is a key factor for accurate unbiased locations especially in downhole monitoring with single vertical receiver array (e.g., Jansky et al 2010). Numerous studies show that realistic errors in velocity models (approximately 5% velocity changes or modest dip perturbation) can cause significant bias and mislocations of microseismic events.…”

Relative location of microseismicity

“…This recognition has led to three groups of approaches that differ from each other in how the velocities are handled: first, a velocity model might be fixed a priori and the hypocenters can be computed in that model (e.g., Geiger, 1912;Asch et al, 1996;Gambino et al, 2004;Chambers et al, 2010;Ito et al, 2012); second, initially estimated velocities and possibly anisotropy coefficients might be iteratively updated based on the data supplied by events themselves and simultaneously with obtaining their hypocenters (e.g., Thurber, 1986;Iyer and Hirahara, 1993;Thurber and Rabinowitz, 2000;Zhang et al, 2009;Jansky et al, 2010;Zhou et al, 2010;Grechka and Yaskevich, 2014;Li et al, 2014); and, third, the influence of velocities on the hypocenters of events within a selected cluster might be reduced by relying on the previously located events in the same cluster and finding the hypocenters of other events relatively to them (Waldhauser and Ellsworth, 2000;Waldhauser, 2001;Waldhauser and Schaff, 2008;Li et al, 2013).…”