AN OVERVIEW OF THE CALTRANS BOREHOLE GEOPHYSICAL LOGGING PROGRAM

Most geotechnical investigations involve drilling and coring to collect samples, measure subsurface properties, and develop lithologic logs. The California Department of Transportation (Caltrans) supports a statewide borehole geophysical logging program to provide supplemental in-situ borehole data. A typical Caltrans borehole logging suite consists of measurements obtained from three sondes, and include caliper, natural gamma, resistivity or induction, and in-situ Compression- and Shear-wave seismic velocity logs. Additional borehole tools that are available, or in development, include acoustic televiewer, borehole deviation, full eaveform sonic, cross-hole radar, and in-situ gamma density logging. Borehole geophysical investigations provide a cost-effective supplement to drilling due to low relative cost, the ability of geophysical methods to look beyond the sidewalls of the borehole, and the collection of in-situ data not available from a standard drilling program.