Guidelines on Geophysical Investigation for Bridges
2017 Edition

According to the guidelines, the most reliable surface geophysical techniques for assessing bedrock depth at bridge locations include seismic refraction, electrical resistivity surveys, and ground penetrating radar (GPR). Seismic refraction is extensively utilized for mapping bedrock depth and topography in stratified geological settings. Electrical resistivity aids in detecting bedrock depth and fractured zones by measuring moisture content variations, while GPR excels in high-resolution profiling of shallow bedrock layers. Subsurface methods such as seismic wave propagation complement these surface techniques for deeper investigations. The final method selection depends on factors like site access, target depth, required resolution, and budget considerations. Employing a combination of these methods typically yields the most accurate and cost-efficient results.

Ground penetrating radar (GPR) operates by transmitting high-frequency electromagnetic pulses into the bridge deck, which reflect off interfaces where dielectric properties differ, such as between concrete and reinforcing steel or areas with delamination. Variations in dielectric constants among materials like concrete, asphalt, and water create distinct reflected signals. GPR is capable of detecting the depth of reinforcement bars, identifying deterioration such as cracking and delamination, and measuring deck thickness. It also helps identify corrosion by detecting increased electrical conductivity caused by chloride contamination, which attenuates radar signals near rebar. GPR provides a rapid, non-invasive means of detailed deck evaluation, making it invaluable for quality assurance and ongoing condition monitoring, although interpretation requires expert analysis.

Crosshole seismic surveys provide precise in situ measurements of both compressional (P-wave) and shear (S-wave) wave velocities, enabling accurate determination of dynamic elastic properties such as Young's modulus, shear modulus, and Poisson's ratio. This method yields high-resolution velocity profiles capable of identifying thin anomalous layers and subtle subsurface variations. It is particularly valuable for characterizing soils, rocks, and engineered materials to support earthquake-resistant design, liquefaction evaluation, and deformation analysis. The technique reveals lateral and vertical heterogeneities in material properties that surface methods may miss. Conducted according to standardized procedures like ASTM D4428 M-84, crosshole seismic testing is essential for detailed, site-specific foundation investigations typically performed after initial surface surveys and drilling.

Electrical resistivity imaging (ERI) identifies subsurface cavities and fractures by measuring variations in the ground's electrical resistivity along a two-dimensional survey line. Cavities and fractures present much lower resistivity values than surrounding intact rock due to the presence of air gaps or water-filled voids. ERI produces detailed 2D resistivity pseudosections that highlight these contrasts, effectively mapping weak zones, discontinuities, and void spaces. This method provides fine-scale spatial resolution and can operate under conditions where seismic techniques might be less effective, such as beneath water bodies or shallow rivers. The technique offers deep penetration without requiring explosives, making it a valuable tool for structural assessments.

Although the standard does not explicitly detail quality assurance procedures, best practices include thorough pre-investigation planning to define objectives and select suitable geophysical methods. Regular calibration of all instruments—such as seismic, resistivity, and GPR equipment—is essential to maintain measurement accuracy. Employing qualified and experienced personnel ensures proper data acquisition and interpretation. Standardized data collection protocols, including appropriate grid spacing and penetration depths, help maintain consistency. Data validation through cross-verification with borehole logs or direct physical tests enhances reliability. Detailed documentation of methods, equipment settings, environmental conditions, and site parameters is critical. Repeatability tests involving multiple surveys verify data consistency. Monitoring key parameters like signal-to-noise ratio, survey density, and investigation depth supports maintaining high data quality throughout the project.