Seismic imaging using microseismic events: Results from the San Andreas Fault System at SAFOD

We have developed a new passive seismic imaging approach that consists of two steps. First, the hypocenter of the microseismic event is precisely located. Second, this event is treated as a pseudo-active seismic source and the reflections within the recorded wavefield are processed by using a directional migration algorithm in order to construct a high-resolution image of the illuminated subsurface region. In this paper we demonstrate the application of our approach to a number of microseismic events recorded by a borehole array in the San Andreas Fault Observatory at Depth main hole. Results obtained were high-resolution 3-D images of different SE-NW-oriented reflectors related to the San Andreas Fault (SAF) system in the close vicinity of the borehole. To support our approach, we compared our findings with other active and passive seismic images and analyzed the correlation with borehole lithology. We revealed a predominantly satisfactory agreement for both juxtapositions. Furthermore, the stacked image of several microearthquakes provides a spatial characterization of the complex internal structure of the SAF, with much higher resolution than can be obtained from surface seismic reflection data.