Introduction to 3D Bedrock Tomography in Near-Surface Geophysics
3D bedrock tomography is a sophisticated geophysical approach used to map the geometry, depth, and physical properties of bedrock surfaces in three dimensions. Unlike traditional 2D profiling, 3D tomography provides a volumetric representation of subsurface structures, offering critical insights for engineering projects, groundwater studies, and geohazard assessments. This method is particularly valuable in complex geological settings where bedrock surfaces are irregular or obscured by thick overburden. By integrating multiple geophysical datasets, 3D tomography can resolve ambiguities inherent in single-method surveys, producing more reliable and actionable models of subsurface conditions.
Seismic Refraction for 3D Bedrock Imaging
Seismic refraction is one of the primary methods employed in 3D bedrock tomography, leveraging the contrast in seismic wave velocities between unconsolidated overburden and competent bedrock. Advanced 3D refraction surveys use dense arrays of geophones and multiple shot points to capture first-arrival times from various angles, enabling the reconstruction of bedrock topography through tomographic inversion. This technique excels in mapping sharp velocity contrasts and identifying features such as paleochannels, fault zones, or weathered bedrock surfaces. Modern implementations often combine refraction data with surface wave analysis (MASW) to improve velocity model accuracy, particularly in heterogeneous materials where traditional refraction assumptions may break down.
HVSR as a Complementary Tool in Bedrock Characterization
The Horizontal-to-Vertical Spectral Ratio (HVSR) method provides a complementary approach to bedrock detection by identifying resonance frequencies associated with sediment-bedrock interfaces. When deployed in a grid pattern, HVSR measurements can be interpolated to create 3D representations of bedrock depth, particularly in areas where seismic refraction faces limitations due to minimal velocity contrasts. HVSR is especially useful for preliminary surveys or in urban environments where active-source methods are impractical. By calibrating HVSR-derived bedrock depths with sparse borehole data or targeted seismic profiles, practitioners can develop robust 3D models with reduced uncertainty, offering a cost-effective solution for large-area assessments.
Integrated Approaches and Emerging Technologies
The most advanced 3D bedrock tomography combines multiple geophysical methods—such as seismic refraction, HVSR, electrical resistivity, and ground-penetrating radar—to overcome the limitations of any single technique. This multi-method approach is particularly powerful in complex terrains where bedrock properties vary laterally or where overburden characteristics mask seismic or electrical contrasts. Emerging technologies like distributed acoustic sensing (DAS) and machine learning-enhanced inversion algorithms are pushing the boundaries of 3D tomography, enabling higher resolution and more accurate bedrock mapping. These integrated strategies provide engineers and geoscientists with comprehensive 3D models essential for foundation design, mineral resource evaluation, and infrastructure planning, representing the cutting edge of near-surface geophysical characterization.