Inversion Algorithm to Evaluate Velocity Profiles from Downhole Seismic Tests

The downhole seismic test is a widely used method to evaluate in-situ seismic wave velocity profiles. In this test, elastic waves are propagated from a stationary source point that is placed at the ground surface to the receivers, which are placed down in a borehole. The wave velocities are usually determined by evaluating the travel times either between the source and each of the receivers, called the direct method, or between receivers, called the true interval or pseudo-interval method. These conventional approaches of determining wave velocities make simplifying assumptions including a straight raypath between the source and the receivers and velocity isotropy. In a layered and anisotropic media, these assumptions can result in errors in the velocity profile. An inversion algorithm is presented to evaluate the velocity profile from downhole seismic test data. It models the soil profile as a stack of horizontal layers with elliptical velocity anisotropy. The raypaths are calculated using a nonlinear iterative algorithm based on the bending method, fulfilling Fermat's principle of minimum travel time and Snell's Law. The inversion algorithm models wave propagation in real soil media more accurately and thus enables a more robust estimate of the seismic wave velocity profile.