Stability of source mechanisms inverted from P‐wave amplitude microseismic monitoring data acquired at the surface

Abstract

We study the stability of source mechanisms inverted from data acquired at surface and near‐surface monitoring arrays. The study is focused on P‐wave data acquired on vertical components, as this is the most common type of acquisition. We apply ray modelling on three models: a fully homogeneous isotropic model, a laterally homogeneous isotropic model and a laterally homogeneous anisotropic model to simulate three commonly used models in inversion. We use geometries of real arrays, one consisting in surface receivers and one consisting in ‘buried’ geophones at the near‐surface. Stability was tested for two of the frequently observed source mechanisms: strike‐slip and dip‐slip and was evaluated by comparing the parameters of correct and inverted mechanisms. We assume these double‐couple source mechanisms and use quantitatively the inversion allowing non‐double‐couple components to measure stability of the inversion. To test the robustness we inverted synthetic amplitudes computed for a laterally homogeneous isotropic model and contaminated with noise using a fully homogeneous model in the inversion. Analogously amplitudes computed in a laterally homogeneous anisotropic model were inverted in all three models. We show that a star‐like surface acquisition array provides very stable inversion up to a very high level of noise in data. Furthermore, we reveal that strike‐slip inversion is more stable than dip‐slip inversion for the receiver geometries considered here. We show that noise and an incorrect velocity model may result in narrow bands of source mechanisms in Hudson's plots.

Seismicity Induced by Hydraulic Fracturing in Shales: A Bedding Plane Slip Model

“…Noise present within this type of acquisition is successfully eliminated by stacking the signal over all the receivers (Anikiev et al, ; Duncan & Eisner , ). Using a large number of receivers and a sampling frequency of 2 ms for the radiated energy allows us to perform a very robust inversion for the source mechanisms (Staněk et al, ).…”

“…The velocity is increasing with depth, which affects the raypaths traveling from a source point to the receivers, as is shown in Figure . Using ray parameters, such as the traveltime, trajectory, and slowness vectors, we construct the impulse response of the media, the derivative of the Green's function (Staněk et al, ). We use two types of moment tensor inversions for the direct P wave amplitudes: the first type of inversion is for the full moment tensor, while the second type of inversion is for the pure shear mechanism.…”

“…The difficulty in determining the nonshear component of the source mechanisms is a consequence of the limited resolution of downhole microseismic monitoring arrays (Vavryčuk, ) and the low signal‐to‐noise ratio (SNR) of surface monitoring data sets (e.g., Chambers et al, ). Staněk et al () addressed this issue by testing the limits of a surface monitoring synthetic data set on the SNR of microseismic events with typically observed source mechanisms induced by hydraulic fracturing.…”

Simultaneous microseismic event localization and source mechanism determination

“…We choose vertical fault planes, a strike-slip pure shear source mechanism, to start with. This type of source mechanism was observed in many data sets, e.g., by Rutledge and Phillips (2003) and Snelling et al (2013). For simplicity, we use strike of 0°, dip of 90°, and rake of 0°; however, this choice of strike (dip and rake are well defined for strike-slip mechanism) does not affect the conclusions of this study.…”

Seismic arrival enhancement through the use of noise whitening

“…These events are observed in a variety of scenarios such as volcanic settings, earthquake hazard monitoring, assessing risk and containment in geo-industrial applications including geological storage of nuclear waste and carbon dioxide (CO 2 ), and monitoring of petroleum and mining procedures (Gambino et al, 2004;Schorlemmer and Wiemer, 2005;Maxwell, 2011;Oye et al, 2013). In general Signal to Noise Ratio (SNR) is maximised during the aquisition phase through survey design (Maxwell, 2010;Auger et al, 2013;Staněk et al, 2014). However a small number of noise suppression methods have been proposed for post aquisition, such as the application of multichannel Wiener filters (Wang et al, 2008), the use of matched filters to identify smaller events from a parent event (Eisner et al, 2008) and separating the seismic event from noise in the τ´p domain (Forghani-Arani et al, 2012).…”