In order to improve the location of induced seismicity recorded by a triaxial downhole geophone, we present a method locating seismic events with respect to a reference event. This method requires similar seismic events called "doublets", usually assumed to be generated by a same geological discontinuity.
Pour ameliorer la localisation de la sismicite induite, enregistree par un capteur triaxe en profondeur, nous presentons une methode localisant les evenements en relatif par rapport à un evenement de reference. Cette methode necessite I'existence d'evenements sismiques d'une grande similitude appeles "doublets", produits à priori par la rnerne discontinuite gèologique.
Um die Lokalisierung der von einem in Tiefe gelegten Dreiachssensor registrierten induzierten Erdbebenhaufigkeit verbessern, haben wir eine Verfahrensweise entwickelt, die Relativerfassungen in gezug auf einen Referenzerfassung lokalisiert. Diese Methode erfordert sehr ahnliche seismische "dublette" geheißte Erfassungen, die von dieselbe geologische Unterbrechung produziert werden.
Hydraulic fracturing, geothermal energy extraction or salt leaching show an important induced seismicity. This seismicity is very often characterised by the existence of several families of similar events called doublets (figure 1). Taking into account the great simililarity between doublets inside the same family, it is assumed that they have all been generated by the same mechanism. The location of the doublets allows to specify the orientation of the geological discontinuity. Classically, with a triaxial downhole geophone, the seismic hypocentral location is performed using the hodogram method. This method derives the P wave incident angle (azimuth and nutation) from the analysis of the 3D particle motion, and the distance from the time difference between P and S arrivals. This method gives a location for each event independently of the previous locations. Applying this method to a family of doublets, it can be shown that the variable signal to noise ratio introduce a random error on the absolute location of each event, reducing the location accuracy, and blurring the swarm of seismicity. In order to improve the location accuracy, we studied a method using the simililarity between doublets (Moriya H., & al., 1994). This method consists in locating the doublets with respect to a reference event. The reference event is usually located by the classical hodogram method, but all the other doublets are localised with respect to it by computing their relative angle and distance differences. Among a class of doublets, the reference event is the event with the best signal to noise ratio. This ensures a reliable determination of incidence angle, and a good time picking of wave arrivals.
Hodogram method: the reference event location The reference event is located using the hodogram method (figure 2). By the projection of the 3D panicle motion on the three axis of the downhole transducer, we compute the three eigen vectors (inertia matrix). Using trigonometric functions, the three Eulerian angles (azimuth, nutation, and proper rotation) are determined (Cliet C, & al., 1988) The length of the time window is selected in order to minimize the time delay dispertion computed by the correlation (fig.3c) One can observe a variation of the time delay as a function of the length of the time correlation window. The best accuracy of the time delay corresponds to the minimum standard deviation between the three axis. The evolution of the time correlation window, from one to 10 periods (figure 3c), shows a time delay stability around a same average for the three axis, after 5 or 6 periods (minimum standard deviation). The error of the arrival time picking will also be minimum at this value of the correlation window.
