Location of seismicity : a migration based approach

We propose a new approach for precise location of seismic sources using a Gaussian beam type migration of multicomponent data. This approach requires only the preliminary picking of event time intervals and is much less sensitive to the picking precision than standard location procedures. Furthermore, this approach is characterised by a high degree of automation. The polarisation information of multicomponent data is used to perform initial-value ray tracing. By weighting the energy of the signal using Gaussian beams around these rays the stacking of energy is restricted to physically relevant regions only. Event locations correspond to regions of maximum energy in the resulting image. We show successful applications of the method to synthetic data examples with 20-30 per cent white noise and to real data of an hydraulic fracturing experiment. INTRODUCTION A precise location of seismic sources (i.e., attributing events to spatial coordinates of their hypocenter) is an important issue in a broad range of geophysical applications. This includes, e.g., earthquake seismology (Thurber and Rabinowitz, 2000) and seismicity based reservoir monitoring (Maxwell and Urbancic, 2001). Several location procedures require identification of seismic phases and picking of Pand S-wave arrival times as well as determination of the velocity structure between the hypocenter and the seismic station. One method is to calculate predicted arrival times for the seismic stations and relate arrival time residuals to the hypocenter and its origin time. The calculation of the predicted arrival times is repeated until arrival time residuals are sufficiently small (Thurber and Rabinowitz, 2000). Furthermore, the circle method is often used for location of the hypocenter and the origin time. The difference between Pand S-wave arrival times for all receivers are used to calculate hemispheres of travel-distances. The hypocenter is assigned to the intersecting region of these hemispheres (Lay and Wallace, 1995). These standard location procedures are characterised by a strong dependence on event picking accuracy and a low degree of automation. Manual picking in the case of large data sets makes the process of location very slow, expensive and not practicable. In contrast, we propose an automated location procedure that takes into account the full elastic wavefield of multicomponent data. Our method is based on the principles of wave field back-propagation and uses a Gaussian beam type migration in order to image seismic sources. CONCEPT OF THE LOCATION PROCEDURE The first step in our procedure is to find an event within the recorded data stream. This can be done by an approximate automatic detection algorithm (e.g., Baer and Kradolfer, 1987). Then a time interval around this event is selected, which contains at least the full waveform of the event. By analysing the three components of this signal we obtain the polarisation for each time sample. Then, initial-value ray tracing is performed using this polarisation information as the starting direction of the ray (see Figure 1(a)). In order to obtain the image value for an arbitrary grid point P (x, y, z) a Gaussian beam type back-propagation is performed. This means that the energy E(t) of the signal is weighted mainly with a Gaussian beam type Annual WIT report 2004 279 factor (Cerveny, 2001) and a summation of all weighted energies over all receivers is done to construct the image: M(x, y, z) = ∑