Localization pitfalls of seismic events due to anisotropy – A KTB real data case study

The environment of the German Continental Deep drilling site (KTB) is known to be anisotropic. In this study we have analyzed the seismic events generated by the KTB long term injection experiment. More than 2500 seismic events were generated and about 260 events were recorded by the borehole geophone and by the 42 station surface array centered at the KTB. For this experiment we are in the favorable position to have a priori knowledge, where the acoustic emissions should originate, i.e., they should start at the injection point and slowly propagate away from it. Localizing the events using an isotropic model based on average velocities derived from check shots leads to a southward lateral shift of the center of the event cloud of about 500m away from the injection point. Since the total extension of the elongated event cluster is about 2 km in E-W direction and about 300-400 m in NS direction this is a significant contribution. Using the 3-D isotropic velocity model obtained from the KTB reflection data, no significant change in the localization is observed and the lateral shift is reproduced. Localizing the same events using an anisotropic model based on published data centers the event cloud around the injection point. The anisotropic localization not only removes the lateral shift of the events but it also significantly alters the shape of the event cloud from an elongated cluster to an almost circular distribution. This is important if the spatio-temporal evolution is interpreted with respect to the hydraulic properties of the rocks. Also, interpretations with respect to event clustering due to tectonics or hydro-fracking may be severely affected if the localization is based on the wrong model assumptions. In seismology, most often there is no a priori information on the subsurface available and we are in a less favorable position than for the KTB injection experiment. A dislocation of the events due to anisotropy may be overlooked leading to the mis-interpretation of the distribution of the events and its spatio-temporal evolution. INTRODUCTION The problem of earthquake location is one of the most basic problems in seismology. Although numerous applications exist worldwide, the inherent non-linearity prevents earthquake location and tomography from being a standardized routine tool. The general strategy of most applications is the minimization of traveltime differences between observed and measured data. Currently they all have in common that an isotropic subsurface model is assumed. It is, however, known from seismological studies and reflection seismology that the earth is not isotropic. This anisotropy affects the localization of seismicity. In this study we quantify this effect for the hydraulically-induced seismic events of the KTB injection experiment. A fluid injection experiment provides perfect conditions for a real data case study since a priori knowledge on the event location is available. The events should start close to the injection point and propagate away from it with increasing injection time. The subsurface of the KTB is known to be anisotropic (Rabbel (1994) and Rabbel et al. (2004)) and provides therefore an ideal environment for this study. We used a localization technique recently developed at the University of Hamburg. It allows to consider 3D-heterogeneous isotropic and anisotropic models. This technique is based on a grid search and was 286 Annual WIT report 2004 verified using synthetic data before it was applied on the KTB data for this study. It shows that the neglect of anisotropy can lead to serious mis-localizations and thus to mis-interpretation of the located events. We will first describe the KTB injection experiment before we briefly explain the localization method used. After this we discuss the anisotropy of the KTB and present results of localizations of the induced seismicity using isotropic and anisotropic subsurface models. Discussions and conclusions finalize the paper. EXPERIMENT During the KTB injection experiment performed in the year 2000, a total of more than 4000 m of water were injected into the KTB during a period of 60 days. The entire borehole was pressurized. The purpose was to generate micro-seismicity around the openhole section at a depth of about 9 km to investigate hydraulic properties of the KTB. More than 2500 seismic events were generated (Baisch et al., 2002). The events were observed by a surface network, which consisted of 42 stations, including a borehole geophone which was placed in the pilot hole located 200 m to the west of the KTB at a depth of about 4 km (Fig. 1). The borehole geophone was recording at 1000 Hz sampling frequency, while the surface network operated at 200 Hz. Due to a leakage of the casing near 5 km in depth about 75 % of the water escaped at this level. Using the Geiger method (Geiger (1910)) 237 events were localized by Baisch et al. (2002) assuming a