ALBERT

Description: 〈span〉〈div〉Summary〈/div〉Determining accurate microseismic event locations at the Groningen gas field in the Netherlands has important implications for understanding the ongoing induced seismicity and its associated seismic hazard. To improve the depth constraint of the microseismicity, downhole monitoring arrays have been deployed in the central region of the Groningen field. The observed seismicity at these receivers is characterised by significant complexity in the waveforms, due to the high velocity contrasts that exist and the acquisition geometry. Reliably identifying and picking phases for use in earthquake location algorithms is therefore particularly challenging. Using a well constrained and highly detailed 3D velocity model, we show how full waveform modelling can be used to understand the causes of the observed phase complexity. By identifying the different modelled phase arrivals in detail, we look to identify any systematic changes in waveform complexity with source location, to aid phase identification of the recorded downhole data. Theoretical travel-times are often the foundation of earthquake location algorithms. We highlight the associated problems of their computation for the downhole monitoring setup for the Groningen model by complementing the full waveform simulations with travel-time computations and their associated ray-paths from both an eikonal solver and using the wavefront construction method. We highlight large inconsistencies in the travel-times, demonstrate the limitations and sensitivity of ray-tracing in a layered velocity model, and show how the theoretical travel-times do not equate with the dominant phase arrivals observed within the modelled waveforms. Finally, we propose an approach based on computing P-wave and S-wave travel-times directly from the full-waveform modelling, such that phase picking is based on an amplitude threshold rather than individual phase identification, which can also be adjusted for any given moment tensor.〈/span〉