Joint Shear-Wave Analysis using MASW and Refraction Traveltime Tomography

We use the multichannel analysis of surface waves (MASW) and SH-wave refraction traveltime tomography to constrain each method’s starting layer-model at a site located at the Yuma Proving Ground, AZ. Our treatment resulted in a more stratified, higher-resolution MASW S-wave velocity (Vs) model that deviated from the tomographic SH-wave solution by approximately 15%. The top 12 m were our target at the test site, which is comprised of roughly 30 m of unconsolidated sediments overlying bedrock. A bungee-assisted drop-weight source was used for MASW and a hammer and block were used for the refraction survey. A preliminary MASW profile served as the starting model for refraction inversion. The resultant Vs tomogram contained slightly higher velocity ranges and imaged a high-velocity layer not apparent with initial MASW results. Using this a priori information, along with density values from well logs, more layers were added to an updated Vs model for MASW inversion. The subsequent higher-resolution MASW profile moved toward the tomographic solution. The use of two inversion schemes allowed a more constrained Vs model of the site, while limiting resolution uncertainties for the MASW Vs section. Moreover, the possible over-parameterization problem seen with increased layers is defended through refraction comparison. We recommend the combined approach as a means to constrain MASW inversion and increase confidence in future Vs appraisals.