Detection of the Long Period Long Duration (LPLD) Events in Time-and Frequency-Domain

Anna Kwietniak

doi:10.2478/s11600-014-0262-1

Abstract

Long Period Long Duration (LPLD) signals are unusual seismic events that can be observed during hydraulic fracturing. These events are very similar in appearance to tectonic tremors sequences, which were first observed in subduction zones. Their nature is not well known. LPLD might be related to the productivity of the reservoir. Different methods of the LPLD events’ detection recorded during hydraulic fracturing are presented. The author applied two methods for LPLD detection–Butterworth filtering and Continuous Wavelet Transform (CWT). Additionally, a new approach to LPLD events detection–instantaneous seismic attributes–was used, common in a classical seismic interpretation but not in microseismic monitoring.

Highlights

The acronym LPLD comes from Long Period Long Duration events, which means that they last more than 10 s
LPLD events were registered during hydraulic stimulation in various geological settings, and it is possible that the formation properties have a role to play in the frequency content of these signals (Das and Zoback 2013)
According to previous studies dedicated to these unusual seismic signals (Das and Zoback 2011, 2013), they are similar in appearance to tectonic tremor sequences

Summary

Introduction

The acronym LPLD comes from Long Period Long Duration events, which means that they last more than 10 s (from 10 to 100 s). Low-frequency earthquakes (long-period events) occur around active volcanoes, and in different tectonic contexts compared with subduction zones near active fault systems. Considering the long duration and the mobility of tremor activity, the generation of tremors may be related to the movement of fluid in the subduction zones (Obara 2002) Another possibility is that tremor is generated directly by slow shear slip on the plane interface, and under this hypothesis, tremor is the weak seismological signature of slip that is otherwise too slow to generate detectable seismic waves (Shelly et al 2006)

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Discussion

Conclusion