Using beamforming to maximise the detection capability of Kwintsheul’s seismic array

Abstract

To run Nature’s Heat geothermal operations (Kwintsheul, The Netherlands) in a safe and efficient manner, and with the support of the local population, it is important to mitigate potential hazards. A well known potential hazard related to geothermal operations is the occurrence of induced seismicity due to injection and extraction of fluids in and from the subsurface. Monitoring the subsurface allows detecting the induced seismicity due to the geothermal operations. To monitor the seismic activity around the geothermal field of Nature’s Heat project, a passive seismic network was installed by Delft University of Technology, Seismoctech (Greece), and Gastreat- ment Services BV. This network has detected and located one single weak event on July 14, 2019.

Within this bachelor thesis, a beamforming technique is adopted to provide a framework that can be used to detect seismic events recorded by the passive seismic network. The goal is to separate coherent signals from noise and characterize the signal (e.g., estimate propagation direction). The objective of this bachelor thesis is to assess whether beamforming is capable of lowering the detection threshold, and determining the signal’s apparent velocity and back azimuth, using data recorded by the Kwintsheul seismic array. For applying the beam- forming technique, Kwintsheul data was implemented in a python code, originated from Remote Online Sessions for Emerging Seismologists (ROSES).

Following applying the beamforming technique, the results of the detection, and the parameter estimation of the back azimuth and apparent velocity, are given for the P-wave and S-wave, 300.96 deg. 6.43km/s and 315 deg. 3.31km/s, respectively. The results of the Fisher statistics and the f-k analysis show that beamforming is capable to discriminate coherent signals from noise.
For beamforming analysis, it is necessary to have array elements that are distant from the event. As the ar- ray design is fixed, it is, therefore, necessary to eliminate data from array elements in the proximity of the event. Hence only data from array elements in the far-field is used for beamforming. This approach decreases the num- ber of array elements, and therefore the signal to noise ratio is decreased as well. In this case, to improve the resolution of the applied beamforming technique, the array design needs to be modified, with all array elements situated in the far-field.