Identification and localization of micro-seismic events using the cross-correlation technique for the Ketzin CO2 storage site

Abstract

The Ketzin CO2 storage site is a site that is intensively monitored for nearly a decade. An array, containing 3-C geophones and hydrophones, is located near the surface to monitor the pressure effects of the insertion of CO2 storage. This array has been passively monitored from 2009 onwards. The sandstone saline aquifer located at 650 m depth (Förster et al., 2009) below the K2 high velocity layer proved to be ideal for CO2 storage. The storage of CO2 causes pressure build up and release in the subsurface (Park et al., 2013, Paap et al., 2014). This build up and release results in seismic events. In this study cross-correlation is used to identify these induced events using a waveform template from a pre-determined event. This master event occurred in 2010, originated from the storage aquifer depth. The cross-correlation method identified two extra events in the 2010 data set. These two events were found using the hydrophone receivers which were used because they have a higher signal to noise ratio (SNR) than the 3-C geophones. Two methods were tested to locate these events using; 1. the time difference between P-and S-wave arrivals, 2. the time difference between the P-wave arrival and the earthquake onset time (t0). Method 2 proved to be most successful in locating the events. The main event and the two extra events found originate all three from the same location area, to the north east of the array. Because cross-correlation searches for the same waveform, events that are near identical can only be found. Events originating from a different location will not be detected. More events can still be found using the data sets of the remaining years. However, researching the 2010 data set revealed that even though events are found, they are of small magnitude and small in number.