11:00 〜 13:00
[SSS05-P04] Analytical source location with two-layer velocities for head waves measured as first arrivals in induced seismicity on multiple receivers
キーワード：induced seismicity、head waves、analytical solution、source location
Nowadays microseismic measurement and analysis play a crucial role in avoiding environmental risk caused by induced seismicity and estimating subsurface characterization on energy production, such as conventional/non-conventional hydrocarbon production, geothermal energy exploitation, and CO2 sequestration and gas storage. One of the significant steps in induced seismic analysis is to estimate source locations of the induced seismic events, because the location information is useful for understanding the physical processes governing induced and triggered seismicity. Direct arrivals of P-waves and S-waves have been commonly exploited for the event localization. Waveform-based methods, so-called reverse time migration (Baysal et al., 1983; McMechan, 1983; Whitmore, 1983) in active seismic processing, have also been presented (see e.g. Duncan, 2015; Nakata and Beroza, 2016). However, the first arrival of head waves (or refracted waves) can be measured earlier than that of direct waves of P-waves under limited conditions, like the existence of high velocity contrast, on downhole survey (see e.g. Fuller et al., 2010; Zimmer, 2010). The head waves have been generally regarded as a nuisance in microseismic analysis with event localization, because they cause an error with 100 meter order on source location in some subsurface condition if the first arrivals of head waves are wrongly picked in the analysis.
Several prior studies have been conducted to address this issue. The approaches to utilize head waves have also been proposed to improve the accuracy of event localization (Zimmer, 2010; Zimmer, 2011; Coffin et al., 2012; Zhang et al., 2015). Furthermore, Zhang et al., (2018) also developed an imaging scheme using head waves. These approaches are based on some numerical forward modeling, such as finite difference modeling. On the other hand, Masaya (2019) derived an analytical criterion to record head waves as first arrivals in a typical downhole model with two-layer velocities. Then, an analytical source location in the model was formulated by employing the first arrival traveltime of both head waves and direct waves on a single receiver. This method enables us to estimate the condition of head waves recorded as first arrivals and determine source location of induced seismicity without any large computation, like forward modeling or inversion. However, the challenge in this method is that some value of two-layer velocity model needs to be given in case of the geometry with a single receiver.
In this presentation, this analytical model (Masaya, 2019) is extended to multiple receivers for solving the velocity problem. A constraint between upper and lower velocity is formulated by adding a vertical receiver. Moreover, analytical solution for the two velocity models can be derived by additionally assuming an extra horizontal receiver. Then, numerical experiments are shown to discuss the validity of this extension.
Several prior studies have been conducted to address this issue. The approaches to utilize head waves have also been proposed to improve the accuracy of event localization (Zimmer, 2010; Zimmer, 2011; Coffin et al., 2012; Zhang et al., 2015). Furthermore, Zhang et al., (2018) also developed an imaging scheme using head waves. These approaches are based on some numerical forward modeling, such as finite difference modeling. On the other hand, Masaya (2019) derived an analytical criterion to record head waves as first arrivals in a typical downhole model with two-layer velocities. Then, an analytical source location in the model was formulated by employing the first arrival traveltime of both head waves and direct waves on a single receiver. This method enables us to estimate the condition of head waves recorded as first arrivals and determine source location of induced seismicity without any large computation, like forward modeling or inversion. However, the challenge in this method is that some value of two-layer velocity model needs to be given in case of the geometry with a single receiver.
In this presentation, this analytical model (Masaya, 2019) is extended to multiple receivers for solving the velocity problem. A constraint between upper and lower velocity is formulated by adding a vertical receiver. Moreover, analytical solution for the two velocity models can be derived by additionally assuming an extra horizontal receiver. Then, numerical experiments are shown to discuss the validity of this extension.