oa Combining Tomography and Reverse Time Migration for Improved Near Surface Seismic Imaging

Seismic traveltime Tomography is often used to attain velocity structure images but cannot resolve complex structures due to smoothing requirements in processing. Furthermore, imaging at depth may be restricted due to limited raypath density below a reflecting boundary. Consequently, imaging lateral discontinuities (e.g. steep dips, buried boulders) or beneath seismically reflective boundaries common in near surface environments can be difficult. Additionally, reflected signals of interest are regularly combined with direct arrivals so removing direct arrivals to observe only reflected signals is not possible. Reversed-time migration (RTM) uses the complete wave equation, making it possibly more accurate than the ray approximation Kirchhoff migration algorithm frequently used in Tomography. Furthermore, RTM can process arrivals from multiple waves and steep slopes. Though computationally expensive, multi-core machines commonly used today can process small data sets typical of near surface projects. We present an example of imaging below a seismically reflective boundary using Tomography followed by RTM. Tomography results show reflective layers at 10 and 17 meters below the ground surface. Most P-wave energy is reflected from these layers with little to no energy reflected from the deeper perched aquifer of interest. the Tomography results are used as the starting point for the high-quality velocity model required to produce an accurate image from the RTM method. the resulting RTM image has strong reflectors between 60-70 meters deep, which correspond with expected depth and range of the perched aquifer. Additionally, there appears to be a break in the continuity at a distance of x = 75 meters for depths in excess of 55 meters. This break may be related to the connection of the perched aquifer to the regional groundwater system, which is expected in this area. the case presented shows this two-step approach results in imaging complex structures and deeper depths than Tomography alone.<br>Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.