Abstract
Locating microearthquake events below complex heterogeneous overburden requires robust location methodologies that can honor multipathing in the seismic wavefield. We have developed two full-waveform event location methods that form a complementary solution for locating earthquakes and simultaneously deriving focal mechanisms via moment tensor inversion. The methods are based on the application of 3D elastic wavefield modeling, which is used to generate waveforms and extract wavefield attributes, for comparison to the observed field data. Events are located and focal mechanisms are derived via a multiparameter inversion, which minimizes the differences between synthetic and observed data. The results have been applied to the induced seismicity observed within the giant Groningen gas field, onshore Netherlands, where recorded earthquakes are triggered by stress changes, induced in the reservoir through pressure depletion. Locating events below the field is compounded by the presence of strong guided waves, which are trapped in the lower velocity reservoir interval. This complex multivalued wavefield is problematic for traditional event location methods, which assume a single traveltime arrival. We overcome this limitation by using all event arrivals in a wave-based solution to improve the accuracy of locating earthquakes and overcome the ambiguity of solving for location and the focal mechanism simultaneously. The event location methods have been applied to shallow and deep monitoring networks, and 150 events have been located with high accuracy. The interpretation of the earthquake activity indicates that the events studied originate from the movement of larger graben bounding faults, which are oriented in a north-northwest–south-southeast direction.
Highlights
Many researchers and industry practitioners have clearly demonstrated that seismic imaging below complex overburden such as salt, for example, requires several key elements that include sufficient target illumination, an accurate velocity model, and an appropriate imaging algorithm (Regone, 2007; Etgen et al, 2009; Gerritsen et al, 2011; Jones and Davison, 2014)
Traditional raybased imaging methods have been superseded by wavefield imaging algorithms, which can inherently handle the multivalued arrivals and wavefront healing expected below rugose and high impedance overburden (Etgen et al, 2009; Jones, 2014). This step change in capability has been augmented by the acquisition of wide azimuth data, which provides improved target illumination and more accurate velocity models
Of some concern is the multiminima seen in the objective function
Summary
Many researchers and industry practitioners have clearly demonstrated that seismic imaging below complex overburden such as salt, for example, requires several key elements that include sufficient target illumination, an accurate velocity model, and an appropriate imaging algorithm (Regone, 2007; Etgen et al, 2009; Gerritsen et al, 2011; Jones and Davison, 2014). Traditional raybased imaging methods have been superseded by wavefield imaging algorithms, which can inherently handle the multivalued arrivals and wavefront healing expected below rugose and high impedance overburden (Etgen et al, 2009; Jones, 2014) This step change in capability has been augmented by the acquisition of wide azimuth data, which provides improved target illumination and more accurate velocity models. In the case of earthquake event location and source characterization, these same key elements are required, and one could argue that their inclusion is even more essential if we are to minimize the impact of the earthquakes on society To address this problem, we have developed two elastic full-waveform workflows, which when used together, provide a powerful solution for subsalt earthquake identification. These workflows are demonstrated by application to shallow and deep monitoring networks, Manuscript received by the Editor 19 March 2018; revised manuscript received 13 November 2018; published ahead of production 12 December 2018; published online 01 March 2019
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