Gaussian Beam Imaging for Converted and Surface Reflected Waves

An overview of Gaussian beam imaging is given for converted and surface reflected seismic waves. The earthquake seismology community now regularly uses seismic waves from distant sources to illuminate structures beneath seismic arrays in so-called passive imaging experiments. Similarly, reservoir structures can be imaged with seismic waves incident from below using sources in boreholes. Gaussian beams are applied for passive imaging based on an over-complete frame-based Gaussian beam representation of the seismic wavefield. Paraxial Gaussian beams are then utilized for the propagation of the seismic waves back into the medium. The approach provides stable imaging of seismic data in smoothly varying background media where caustics and triplicated arrivals can exist. Gaussian beam imaging is found to be very flexible with respect to different experimental geometries and can be configured to allow for different types of converted or reflected waves. A synthetic example is first given for a collisional zone structure with an incident P-wave from below where several different scattered wave types are used to image the structure. Seismic data from the 1993 Cascadia experiment are then used to image the subduction zone beneath the Pacific Northwest in Oregon. The results from Gaussian beam imaging are found to compare favorably with imaging results obtained using ray/Born inversion.