We have analyzed microseismic monitoring data sets obtained from the surface and downhole-monitoring arrays recorded during the first experiment of hydraulic fracturing in Poland. Using the downhole-monitoring network, we were able to record and locate 844 microseismic events, including 10 perforation shots from six stages of the stimulation. We detected 2 perforation shots and no microseismic events using the surface array, which was operational only during the first two stages of the stimulation. To explain the poor detectability of the surface array, we analyzed the spectral content of the events from the downhole-monitoring array. We found that the detectability of the perforation shots on the surface array was consistent with the low-frequency part of the signal on the downhole recordings. Our observation is in agreement with the fact that microseismic events with low-frequency signal weaker than the two detected perforation shots were not detected by the surface-monitoring array. Using the low-frequency part of the spectra of the events recorded by the downhole array, we predicted the surface-array detection threshold. We found that some events from the later stages could have been detected if only the surface array had been operational during that time.

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