Crosshole Sonic Logging is the most accurate and reliable technique for assessing the integrity of deep foundation elements constructed on-site from concrete or grout. The crosshole sonic logging method is normally applied as a quality assurance (QA) technique for newly placed drilled shafts and auger cast piles, but can also be applied to slurry walls, mat foundations, and mass concrete pours. CSL testing provides assurance that the foundation concrete is sound and contains no defects, such as soil intrusions, sand lenses, voids, etc. The extention, nature, depth, and approximate lateral location of defects can be determined with the CSL method.