Geotechnical and Geophysical Engineering: Seeing Below the Surface

Geotechnical engineers often rely on physical testing and boreholes to understand the subsurface. But sometimes, we need a broader picture—what’s happening between boreholes, over a larger area, or at depths that are difficult to reach.

That’s where geophysics comes in.

Here’s what geotechnical engineers should understand about working with geophysicists—and what geophysicists should know in return. 

1. Different Tools, Same Goal

Geophysics uses indirect methods—like seismic, resistivity, GPR, or electromagnetic surveys—to map subsurface conditions.
Geotechnical engineering uses direct testing—like CPTs, SPTs, and boreholes.

They’re not competing approaches. They’re complementary.

2. Where Geophysics Makes a Real Difference

Geophysics is particularly useful when:

• Bedrock depth varies across a site (e.g., sloping or dipping bedrock beneath a building or bridge)

• The contact between soil and rock is unclear, but critical for deep foundations

• There’s a need to locate voids, buried structures, or utility conflicts

• Large infrastructure spans an area where borehole coverage is limited

• You need to estimate dynamic soil properties (e.g., Vs30 for seismic design)

In these cases, geophysics can save time, reduce uncertainty, and guide borehole placement more effectively.

3. What Geotechnical Engineers Should Understand About Geophysics

• Geophysical results are indirect and require interpretation, not just measurement

• The data is resolution-limited and may smooth over sharp soil transitions

• Geophysicists need input from boreholes or known stratigraphy to calibrate models correctly

• Simply asking for a “bedrock map” without defining what you mean by "bedrock" can create confusion

4. What Geophysicists Should Understand About Geotechnical Practice

• Engineers are looking for design parameters, not just material boundaries

• A resistivity or seismic velocity change is interesting—but needs to translate to something usable: “This is where we hit refusal” or “This is where side resistance develops.”

• Resolution near the surface matters—especially for shallow foundations, pavement design, and utility coordination

• Close collaboration during survey planning improves alignment between what’s collected and what’s actually needed

5. Start the Discussion Early

Some of the best results happen when geotechnical engineers bring in geophysicists during the planning stage of site investigations—not as a troubleshooting tool after borehole data is unclear. This allows both sides to frame the right questions and use resources efficiently.